Resistance of birch wood modified with sorbitol-citric acid to white-rot decay and weathering

Resistance of birch wood modified with sorbitol-citric acid to white-rot decay and weathering | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Resistance of birch wood modified with sorbitol-citric acid to white-rot decay and weathering Sheikh Ali Ahmed, Ramil Gainov, Kazuya Tamura, Reza Hosseinpourpia This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9615371/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The increasing interest in sustainable and bio-based wood modification strategies has led to growing attention in enhancing durability. This is particularly important for wood with low durability and thus requires improvement for exterior applications. This study evaluates the decay and outdoor weathering resistance of silver birch ( Betula pendula ) wood modified with a sorbitol-citric acid (SorCA) system. Birch wood samples were vacuum-pressure impregnated with aqueous solutions of SorCA at 20% and 40% w/w and cured at 140°C. The distribution of the modifying agent within the wood structure was examined using X-ray microtomography, which confirmed effective penetration with partial or complete filling of wood lumina by the in-situ formed polyester network. The modified samples were subjected to ten months of natural weathering, mold exposure under high humidity conditions, and also to white-rot fungi ( Trametes versicolor ). Outdoor weathering results indicated improved resistance to surface deterioration in modified wood, although increased color change (ΔE) was observed due to the formation of chromophoric groups during curing and subsequent photodegradation. SorCA modification significantly improved decay and mold resistance, especially at higher concentrations. In general, SorCA modification showed great potential to improve the durability of birch wood with enhancing the biological durability and weathering resistance. Silver birch white-rot fungi mold growth UV Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Birch (silver: Betula pendula and downy: B. pubescens ) is one of the most widespread hardwood species, accounting for about 13.1% of the standing forest volume in Sweden and is primarily used for pulping and firewood (Forest statistics 2024 , Woxblom and Nylinder 2010 ). This is mainly due to its poor dimensional stability and biological durability, especially in environments with fluctuating humidity (Hill 2006 ). Birch wood products have a low durability class of 5, meaning they are not durable (SS-EN 350 2016 ). Therefore, it requires specific treatment or modification for exterior use (Dubois et al. 2020 ). Even though the growing stock of birch in Sweden continues to increase (Lidman et al. 2024 ), the industrial utilization of birch wood remains limited due to its suitability for outdoor applications. Therefore, there is a clear need to develop strategies to enable the optimal, value-added use of Swedish birch wood, which is in accordance with the very recent initiative to enhance the use of Swedish Hardwood (NTT Woodnet 2026 ) Wood treatment is necessary to enhance resistance against decay, dimensional stability, and weathering. Conventional wood treatment with biocides often contains toxic chemicals and generally does not improve the inherent properties of wood, such as moisture resistance or mechanical performance. Furthermore, the leaching of biocides contaminates water, air, and soil, which has led to stricter regulations (Miranji et al. 2022 ). In chemical wood modification, permanent changes in the cell wall result in not only improved biological durability but also enhance intrinsic properties of the material (Ahmed and Hosseinpourpia 2025 , Hosseinpourpia and Mai 2016 , Ghavidel and Hosseinpourpia 2024). Thus, this type of modification requires the penetration of the modifying chemical into wood microstructures. As a result, wood species that are difficult to impregnate are less suitable for chemical modification. Penetration of liquid proceeds much easier into sapwood than into heartwood and thus sapwood exhibits higher weight gain after chemical modification when compared to heartwood (Mai and Militz 2023 ). Birch does not form any true heartwood (Hörnfeldt et al. 2010 ) and its diffuse pore structure can support effective liquid penetration and chemical distribution throughout its structure. Such a property of birch generally enhances the treatability for modification processes compared to wood species with low permeability. Studies on wood impregnation mechanisms showed that permeability is the key factor governing the uptake of modifying agents (Augustina et al. 2023 , Grinins et al. 2021 ). Consequently, birch wood exhibits considerable potential for various modification strategies. Esterification is among the most widely applied chemical reactions in wood modification (Mubarok et al. 2020 ). The reaction is commonly based on the reaction of anhydride/carboxylic acid/isocyanate/ester groups of a modifying agent and hydroxyl groups of wood polymer or one of the modifying agents and hydroxyl groups of other modifying agents/and wood polymer (Mai and Militz 2023 ). A polyesterification system based on sorbitol (Sor) and citric acid (CA) has attracted interest in recent years due to increased environmental awareness. This type of modification system has resulted in enhanced resistance to brown, white, and soft rot fungi in Scots pine, Norway spruce, and European beech (Kurkowiak et al. 2023 , Beck 2020 , Mubarok et al. 2020 , Larnøy et al. 2018 ). Investigation on field and laboratory studies has shown that SorCA-modified Scots pine sapwood and Norway spruce exhibit enhanced resistance to marine borers and subterranean termites after one year of exposure (Treu et al. 2020 ). Moreover, SorCA treatment has significantly enhanced the moisture-related properties of European beech and Scots pine (Ahmed and Hosseinpourpia 2025 , Hötte et al. 2025 , Mubarok et al. 2020 ). However, their experimental results showed that even if there is an increasing trend of modulus of elasticity, the modulus of rupture decreased depending on the modification intensity. The plausible reason for the decrease in strength could be the low pH of the modifying solution. The highly acidic property of the SorCA solution and curing temperature degrades the amorphous polysaccharide (especially hemicellulose) and results in lower bending strength, as it was reported in European beech and birch wood (Ahmed and Hosseinpourpia 2025 , Mubarok et al. 2020 ). Similar results were also reported for tannin-CA wood modification in Scots pine (Ahmed et al. 2025 ). Recent developments in self-neutralizing CA-based systems could mitigate such problems associated with high solution acidity and provide a promising direction for future work on self-neutralizing formulations (Zhou et al. 2025 , Xu et al. 2024 ). Both components used for SorCA modification are derived from a bio-based origin. Sorbitol is a commonly applied sweetener that is naturally present in a variety of fruits, such as berries. It is industrially manufactured from biomass sources, mainly from corn and wheat. The second component of the modifying agent is CA and is also obtained through bio-based production. The process involves the fermentation of glucose, primarily derived from beet and sugarcane molasses (Soccol et al. 2006 ). CA is widely used as a buffering agent and flavoring additive. Thus, the wood modification using the SorCA system has attracted considerable attention due to its high effectiveness, fully bio-based constituents, and cost efficiency. However, the performance of chemically modified wood is strongly influenced by the wood structure, which governs chemical uptake, distribution, and performance behavior (Xie et al. 2013 ). Penetration and micro distribution patterns of impregnation agents into the wooden structure vary depending on wood species and thus affect the performance of modified wood (Zimmer et al. 2012 ). While recent work on SorCA-modified silver birch ( B. pendula ) has demonstrated improvements in dimensional stability and modulus of elasticity (Ahmed and Hosseinpourpia 2025 ), its resistance to biological decay and durability under outdoor weathering conditions remain unexplored. These properties are critical for the utilization of silver birch in exterior applications. Thus, the aim of the present study is to evaluate the resistance of SorCA-modified silver birch against mold, white-rot decay and also to assess its performance after exposure to outdoor weathering conditions. Materials and methods Materials and sample preparation Kiln-dried silver birch ( Betula pendula Roth.) board (120 × 70 mm) with straight grain were purchased from KG List (Norrhult, Sweden). Specimens free of visible defects were extracted from those boards for various tests (Table 1 ). Specimens were then stored in a climate chamber at 20°C and 65% RH for acclimatization. Sorbitol (molecular weight: 182.17 g/mol) and citric acid (molecular weight: 210.14 g/mol) with ≥ 99% of purity (VWR International, Leuven, Belgium) were purchased in the form of solid powder from Avantor/VWR International AB, Kista, Sweden. Table 1 Specimen dimensions and oven-dry density of wood material used for various tests. Values in parentheses are the standard deviations. Tests Sample dimension (mm) (length × width × thickness) Density (kg/m 3 ) Outdoor weathering 376 × 77 × 20 647 (± 46) Decay 50 × 25 × 15 635 (± 10) Mold 150 × 77 × 20 645 (± 46) SorCA formulation and modification Aqueous solutions containing 20% and 40% (w/w) of sorbitol-citric acid (SorCA) were prepared by diluting 1 equimolar of Sor and 3 equimolar of CA in water until completely dissolved. This specific molar ratio (1:3) was selected because it enables nearly complete reaction of the acid functional groups with sorbitol, as reported by Doll et al. ( 2006 ). The pH of 20% and 40% (w/w) SorCA solutions was measured at 1.59 and 1.24, respectively. All specimens were oven-dried at 103 ± 2°C for 48 h. Initial oven-dry mass and dimensions were recorded prior to impregnation. Specimens were then impregnated with aqueous treatment solutions in an impregnation vessel (Scholz Maschinenbau SCHOLZ GmbH & Co. KG, Germany) under vacuum (91.2 kPa for 1 h), followed by pressure to 750 kPa for 2 h. Unmodified control specimens were impregnated with distilled water only. After impregnation, the samples were gradually dried and subsequently cured at 140°C for 24 h in accordance with the procedure described by Ahmed and Hosseinpourpia ( 2025 ). Characterizations X-ray microtomography Unmodified and modified birch at SorCA 40%) were scanned using an X-ray microtomography system (CT Lab HX-100, Rigaku Corporation, Tokyo, Japan). The micro-computed tomography (µ-CT) study was performed with a tube voltage of 40 kV and a tube current of 100 µA using a molybdenum X-ray target without any additional filter. The samples were scanned with a field of view (FOV) of 5 mm, resulting in a voxel size of 3.86 µm (standard scanning mode). The acquisition parameters included an exposure time of 35.7 ms per projection and 1680 projections collected during a long-geometry scan. The total scan time for each sample was approximately 15 minutes. These settings provided sufficient spatial resolution for imaging the anatomical features of wood-based materials. After the scanning process, the scanned data were reconstructed using the software VGSTUDIO 2025.3 (Volume Graphics GmbH, Heidelberg, Germany). This software enabled volume rendering, segmentation, and detailed analysis of the internal microstructure of the samples. Outdoor weathering and surface properties After modification, all specimens were acclimatized at 20°C and 65% relative humidity. Five replicates were used per treatment group for a 10-month outdoor weathering test at the ASA Experimental Forest and Research Station (part of the Swedish Infrastructure for Ecosystem Science, SITES) in Lammhult, located 37 km north of Växjö, Sweden. Specimens were randomly arranged on wooden racks approximately 60 cm above the ground at a 45° inclination (Fig. 1 ). Specimens were exposed south-facing, with a small plastic tube supporting each specimen from behind to prevent moisture deposition. Commercially impregnated Scots pine ( Pinus sylvestris L.) sapwood treated with Cu-based preservative- Celcure AC800 (copper tetra-amine-dihydrogencarbonate 38.0–40.0%, benzalkonium chloride 4.8%, and water 57.2–51.2%) according to the standard of the Nordic Wood Preservation Council was included as a reference. Sample color was measured before and after exposure to outdoor climate using a Chroma Meter CR-410 (Minolta Co. Ltd., Japan). Color was expressed in the CIELAB system with three-dimensional Lab coordinates, where L* represents lightness (0 = black, 100 = white), +a* the red direction, −a* the green direction, +b* the yellow direction, and − b* the blue direction. Total color change (∆Eab) was calculated using the following equation: $$\:{\varDelta\:\text{E}}_{ab}^{*}=\sqrt{{\left(\varDelta\:\text{L}\text{*}\right)}^{2}+{\left(\varDelta\:\text{a}\text{*}\right)}^{2}+{\left(\varDelta\:\text{b}\text{*}\right)}^{2}}$$ 1 where ∆L , ∆a*, and ∆b* represent the changes in lightness and chromatic coordinates following weathering. Measurements were taken at three fixed positions on the flat exposed surface of each specimen using reference marks to ensure measurements at the same positions before and after weathering. To characterize the surface topography after outdoor weathering, measurements were performed using a non-contact 3D optical profiling system (Veeco NPFLEX, Tucson, USA) operating in Vertical Scanning Interferometry (VSI) mode (p. 1). Data was acquired at 5.1× magnification with a 640 × 480 pixel array, resulting in a lateral sampling spacing of 1.95 µm. The measurement process involved a tilt correction to level the surface profile, while no additional digital filtering was applied to the raw height data to preserve the integrity of the surface features. Standard areal surface roughness parameters (S-parameters) were then extracted from the processed topography maps to quantify the amplitude, spacing, and hybrid characteristics of the sample. Three regions of each specimen were measured to account for the inherent heterogeneity of wood surfaces, and the average values were reported. Mold resistance Specimens measuring 70 mm (width) × 15 mm (thickness) × 150 mm (length) were prepared to assess mold resistance following the method as described by Ahmed et al. ( 2025 ). Five specimens per treatment group were tested in a Memmert HCP 246 climate chamber (Memmert GmbH, Germany) at 30°C and 95% relative humidity. Specimens were randomly suspended from the top of the chamber, with their flat surfaces oriented vertically and parallel to one another, maintaining a 10 mm gap. Three Scots pine sapwood ( Pinus sylvestris L.) samples naturally infested with Aspergillus , Rhizopus , Penicillium etc. were placed at the bottom of the chamber to serve as sources of mold inocula. After 4 weeks of incubation, the experiment was terminated due to abundant mold growth, which was observed on some surfaces. Both flat surfaces of each specimen were visually assessed and rated on a scale from 0 (no infestation) to 6 (extremely heavy infestation). Wood decay test The decay resistance of the modified wood against white rot fungus- Trametes versicolor (L.) Lloyd was evaluated using a malt agar incubation test in accordance with the standard SS-EN 113-1 ( 2020 ). Modified and unmodified specimens were incubated for 16 weeks at 22°C and 70% relative humidity. For comparison, commercially impregnated Scots pine ( Pinus sylvestris L.) sapwood treated with Celcure AC800 (Cu-based preservative) and untreated beech ( Fagus sylvatica L.) wood were used as controls. Prior to incubation, all specimens were oven-dried at 103 ± 2°C to constant mass and weighed to the nearest 0.001 g to determine oven-dry weight, four replications were used per treatment group. After steam sterilization in an autoclave at 120°C for 30 min, one modified and one unmodified control specimen were placed on fungal mycelium grown on 70 mL malt extract agar in 400 mL Kolle flasks. Correction values were determined from sterile non-inoculated control samples from all treatment groups. After the end of the incubation period, all specimens were cleaned of adhering fungal mycelium, weighed, oven-dried at 103 ± 2°C, and weighed again to determine mass loss as follows: $$\:ML\:\left(\%\right)=\frac{{\text{M}}_{0}-{\text{M}}_{0,inc}}{{\text{M}}_{0}}\:\times\:100$$ 2 where ML is the mass loss by fungal decay (%), M 0,inc is the oven-dry mass after incubation (g), and M 0 is the oven-dry mass before incubation (g). Microscopical analysis Small blocks measuring ca. 5 mm (radial) × 5 mm (tangential) × 10 mm (longitudinal) were prepared from samples and soaked in water. Thin sections approximately 10 µm thick were made using a sledge microtome (WSL, Birmensdorf, Switzerland). Sections were placed on a glass slide, a drop of 1% Safranin O (Merck KGaA, Darmstadt, Germany) was added. The slide was left for 5 minutes and then washed with 50% ethyl alcohol. After that, one drop of lactophenol blue (Sigma-Aldrich Sweden AB) was added to stain the fungal mycelium. The slide was left for 5 minutes and then washed with water. One drop of glycerin was added, and the sections were covered with coverslips. The prepared slides were examined under a motorized Olympus BX63F light microscope (Olympus, Tokyo, Japan) equipped with a DP73 color CCD camera (maximum resolution of 17.28 megapixels) and OLYMPUS cellSens Dimension software, version 4.2.1 (Olympus, Tokyo, Japan). Statistical analysis Statistical analyses were performed using IBM SPSS Statistics, Version 30.0.0.0 (IBM Corporation, New York, USA) to evaluate differences in color change, mold rating, and mass loss due to decay among different treatments. One-way analysis of variance (ANOVA) was conducted at a significance level of 0.05 to identify significant differences. When significant differences were detected, a Duncan multiple range test was applied to distinguish the treatments effects. Results and discussion Distribution of the modifying chemicals The µ-CT scan can show the internal anatomical structure of wood and can facilitate quantitative measurements of anatomical changes in wood caused by the chemical modification (Moghaddam et al. 2016). The natural porous structure is clearly visible in the transverse CT sections of the control and modified birch wood (Fig. 2 ). After modification, changes in intensity and contrast in the µ-CT images indicate that void spaces within the wood have been partly or fully filled by cured SorCA. The three-dimensional rendering further illustrates how impregnation and curing with SorCA affect internal structure (Fig. 3 ). The filled cavities revealed by µ-CT renderings correspond to regions where the SorCA reaction products have polymerized and occupied cell voids, resulting in a polymer network. In SorCA modification, CA reacts with the polyfunctional alcohol of sorbitol via in situ polyesterification and develops a cross-linked polyester network that both interacts with wood polymers and remains physically entrapped within the cell wall and lumen regions. Such modification is reported to improve dimensional stability, moisture tolerance, and biological durability of wood by reducing hygroscopicity and restricting pathways for water and microbial activity (Ahmed and Hosseinpourpia 2025 , Mubarok et al. 2020 ). Effective protection against wood-decaying fungi further depends on a uniform distribution of the fixated chemicals throughout the modified wood (Hill 2006 , Zelinka et al. 2010). In such a case, SorCA-modified wood showed uniform deposition of the modifying chemicals within the cell lumen, as observed in Figs. 2 and 3 . The extent of modification can be quantified by weight percent gain (WPG), reflecting the amount of polymer incorporated into the wood structure after impregnation and curing. Higher WPG values generally correlate with greater void space filling and improved performance properties. However, it requires thorough distribution of the reactants to avoid inhomogeneity (Kurkowiak et al. 2023 ). In this study, a homogenous distribution of the modifying chemicals was achieved. Outdoor weathering Figure 4 a shows the aesthetical characteristics of test samples before and after 10 months of outdoor exposure. Color of the modified wood depends on the concentration of the modifying liquid. It became darker at a concentration of 40%. This could be due to the higher acidity of the solution, which cleaved polysaccharides and formed furanic compounds during curing at the higher temperatures (Hosseinpourpia et al. 2017 ). During outdoor weathering under natural environmental conditions, the samples were exposed to solar ultraviolet (UV) radiation, and fluctuation of moisture and temperature, which induce photochemical and physicochemical changes in the wood surface. Such visible surface degradation is one of the most prominent macroscopic effects of weathering and reflects the breakdown of key wood constituents by sunlight and moisture. In the field test, the unmodified control birch samples exhibited more pronounced visual surface changes than the modified specimens (Fig. 4 a). This result is consistent with prior study on thermally-modified wood that outdoor weathering causes color degradation, surface checking, and loss of original hue in wood surfaces within months to years of exposure (Godinho et al. 2021 ). The total color change (ΔE) is a crucial parameter for evaluating the color stability of weathered samples. Figure 4 b presents the quantitative color change (∆E) of the different wood treatments after outdoor weathering. The commercial samples showed the lowest color change (15.38), which might be due to the photostabilization of wood by retarding the formation of carbonyl groups and reducing delignification during weathering (Temiz et al. 2005 ). In contrast, SorCA 20% showed the highest color change (33.75). Increasing the concentration of SorCA 40% significantly improved performance, reducing the color change to 24.65, which is slightly better than the unmodified control (25.81). This finding suggests that SorCA formulations significantly influence color change. Higher concentrations provided some protection against color change. The photodegradation of wood primarily happens due to the breakdown of its main constituents of cellulose, hemicellulose, lignin, and extractives (Dence 1992 ). Among these, lignin degradation is the principal driver of surface discoloration. Photochemical reactions are initiated by the absorption of UV-visible light, mainly by lignin, and form aromatic and other free radicals. These free radicals degrade lignin by reducing methoxyl content, photo-dissociation of carbon-carbon bonds and formation of carbonyl based chromophoric groups leading to color change and yellowing of the wood surfaces (Hon 2000 ). Degradation occurs on chromophoric structures and forms secondary chromophors, resulting in wood color from yellow to brown, then to silver, with a general deterioration in brightness (Cogulet et al. 2016 ). The higher color change (ΔE) observed for SorCA-modified wood compared to unmodified controls can be attributed to both chemical and photochemical effects associated with citric acid-based modification. During curing, SorCA acid treatment leads to esterification and polyesterification reactions, which introduce new chemical structures such as ester groups, carbonyls, and conjugated systems that can act as chromophoric groups (Kusnierek et al. 2024 ). These newly formed chromophores increase light absorption and photochemical sensitivity, resulting in more pronounced color changes during outdoor exposure. In addition, SorCA treatment causes an initially darker color resulting from the citric acid-sorbitol polymer and progressing thermal degradation of wood during curing at 140°C. Subsequent photodegradation, oxidation, and leaching processes during weathering produced larger shifts in color coordinates relative to the initial modified state and led to higher ΔE values. Similar behavior has been reported for chemically and thermally modified wood, where improved dimensional stability does not necessarily correspond to enhanced color stability (Godinho et al. 2021 , Cambazoglu et al. 2023 ). Furthermore, unmodified wood tends to undergo rapid lignin photodegradation and leaching. It leads to an early development of a relatively uniform grey surface and after which further color change progresses more slowly (Lampela et al. 2025 ). Higher ΔE values in SorCA-modified wood do not mean that the surface is degrading faster. Instead, the presence of chromophores in modified wood remains active and gradually changes upon exposure to outdoor weathering. The surface topography of unmodified and modified wood samples was analyzed using the NPFLEX 3D Optical Profilometer, a non-contact white-light interferometry (WLI) system capable of capturing three-dimensional surface height variations from the nanometer to micrometer scale with high repeatability. This technique enables areal roughness evaluation (e.g. Sa, Sq) by measuring height distributions over a defined scan area without physically contacting the wood surface, thus avoiding deformation of soft cellular structures. The reconstructed 3D maps (scan area ~ 1.2 mm × 0.9 mm) reveal distinct differences in surface morphology between the control, commercial, and chemically modified wood samples (Fig. 5 ). The control sample exhibited the highest surface irregularity, with a mean height (S a ) of 82.00 µm and a root-mean-square height (S q ) of 122.27 µm. This high roughness is characteristic of unmodified hardwoods, in which open vessel elements and loosely attached fibers on the surface create highly heterogeneous microtopography. In contrast, the commercial pine samples impregnated with Cu-based preservatives exhibited the lowest surface roughness values (S a = 12.55 µm and S q = 26.33 µm). This reduction in roughness can be attributed to differences in anatomical structure and chemical composition. Softwoods such as pine possess a more homogeneous structure constructed chiefly by tracheids. On the contrary, birch contains a more complex arrangement of vessels, fibers, and parenchyma, which leads to a greater surface heterogeneity. In addition, the presence of Cu-based preservatives may further modify the surface by stabilizing the cell wall components. Similar reductions in surface roughness of pine have been reported in the literature with Cu-based preservatives like Didecyldimethylammonium chloride, Celcure AC 500 (copper carbonate hydroxide, benzalkonium chloride, and boric acid), Micronized copper quat, and Copper (II) sulfate pentahydrate (Ozgenc and Yildiz 2014 ). The SorCA-modified samples showed a significant reduction in roughness compared to the control, though they remain rougher than the commercial pine. In the SorCA 20% samples, the S a is reduced to 40.76 µm, whereas in the SorCA 40% samples, it increases slightly to 47.59 µm. This reduction relative to the control could be due to the esterification process, where the SorCA polyester acts as a bulking agent, filling the cell wall and partially occluding the lumina, and thereby flattening the micro-scale voids. The slight increase in roughness observed when moving from 20% to 40% concentration likely results from higher polymer loading, which can lead to the formation of localized polymer clusters or uneven surface deposits during the high-temperature curing phase. Mold resistance The mean mold growth grades of the different specimens shown in Fig. 6 indicate a clear influence of samples type and SorCA concentration on mold resistance. The unmodified control and pine sapwood exhibit the highest mold grades, reflecting the natural susceptibility of unprotected wood to fungal colonization under favorable moisture conditions. The commercially impregnated wood showed a reduced mold grade and the result is consistent with the known antifungal efficacy of copper-based preservatives (Myronycheva et al. 2018 , Ahmed et al. 2013 ). SorCA-modified samples display a concentration-dependent improvement in mold resistance. Wood modified with 20% SorCA showed statistically similar mold growth to control and pine sapwood, whereas treatment with 40% SorCA yielded the lowest mold grade among the samples. This enhanced resistance could be attributed to the SorCA modification, which reduces wood hygroscopicity, lowers the availability of nutrients, and creates a less favorable environment for fungal growth through esterification and polymer formation within the wood structure. Similar reductions in fungal staining were observed in SorCA-modified Scots pine wood, where low pH might be a reason for the limited mold growth (Larnøy et al. 2018 ). Generally, most fungi grow over the pH range 3–7 (Zabel and Morrell 1992 ). The pH of SorCA 20% and SorCA 40% solutions was 1.59 and 1.24, respectively. Although the pH of the surface of the modified samples was not determined, the lower pH might have restricted the mold growth, especially at the higher concentration level. Wood decay Figure 7 illustrates the mean mass loss of different wood samples after 16 weeks of incubation with the white-rot fungus Trametes versicolor , demonstrating a clear effect of treatment on fungal resistance. The unmodified birch and beech samples exhibit the highest mass losses, reflecting the validity of the test and inherent susceptibility of unmodified hardwoods to white-rot fungi, which preferentially degrade lignin and polysaccharides in the wood cell wall (Schmidt 2006 ). In contrast, the commercially impregnated pine treated with a copper-based preservative shows significantly lower mass loss, consistent with the well-established antifungal efficacy of copper compounds against basidiomycetes (Myronycheva et al. 2018 ). SorCA-modified samples display a concentration-dependent reduction in mass loss, with the SorCA 40% treatment providing significantly greater resistance than the 20% treatment. These results are also consistent with previous findings (Mubarok et al. 2020 , Beck 2020 ). The improvement of enhanced decay can be attributed to citric-acid-based modification, which reduces wood hygroscopicity, limits nutrient availability, and introduces a cross-linked polyester network within the wood structure, thereby restricting fungal colonization and enzymatic degradation (L’Hostis et al. 2018 ). The durability of SorCA-modified wood could be attributed to two key factors, viz., the cell wall bulking and the cross-linking of SorCA polyesters with wood polymers. This type of modification substantially reduces the available water molecules within the wood cell wall, thereby restricting the diffusion of low-molecular-weight compounds, enzymes, and ions released by fungal hyphae in the lumen (Kurkowiak et al. 2023 ). In addition, chemical modification at high curing temperatures may alter the glass-transition behavior of hemicelluloses, which could further contribute to enhanced resistance to fungal decay. The presence of acids in SorCA-modified wood may also contribute to decay protection, as citric and other organic acids have been shown to hinder fungal growth (Hassan et al. 2015 ). However, residual acid in the wood may present a potential hazard of this technology whereby acidity drives cellulose depolymerization over the long term, destroying the mechanical integrity of the wood (Ahmed and Hosseinpourpia 2025 ). Trametes versicolor is a typical white-rot fungus capable of degrading all major wood polymers, including lignin, cellulose, and hemicelluloses, through the combined action of oxidative and hydrolytic enzymes such as laccases, manganese peroxidases, and cellulases (Martínez et al. 2005 ). In unmodified wood, these enzymes readily diffuse into the cell wall matrix, where lignin depolymerization facilitates subsequent polysaccharide degradation. However, SorCA modification alters the chemical and physical environment of the wood cell wall, thereby affecting fungal colonization and enzymatic efficiency. In SorCA-modified wood, esterification and polyesterification reactions introduce a cross-linked polymer network within the cell wall, leading to reduced cell wall moisture content and restricted diffusion pathways for fungal enzymes and low-molecular-weight metabolites (L’Hostis et al. 2018 , Hill 2006 ). Since white-rot fungi such as T. versicolor require sufficient moisture and accessibility to initiate ligninolytic reactions, this reduction in cell wall accessibility can slow the decay process. Furthermore, the lower hygroscopicity of SorCA-modified wood limits the formation of favorable microenvironments required for sustained enzyme activity (Kurkowiak et al. 2023 ). In addition to physical barriers, SorCA modification may also influence fungal metabolism. Citric acid, sorbitol, and their esterified products can act as chelating agents, potentially interfering with the availability of metal ions essential for oxidative enzyme systems involved in lignin degradation (Karimi and Goli 2021 , Cigala et al. 2020 ). After 16 weeks of incubation, distinct differences were observed between the unmodified and modified wood samples. In the control birch samples (Fig. 8 d-f), extensive degradation of the cell walls was observed after fungal exposure. The structure appears severely decayed wood cell walls. This indicates active colonization and enzymatic breakdown of wood components by the fungus. Such degradation is characteristic of white-rot decay where lignin and other structural polymers are progressively degraded, leading to loss of cell wall integrity. Cu-based preservative impregnated pine (Fig. 8 c) and SorCA-modified birch (Fig. 8 g-i) exhibited enhanced resistance to wood decay as observed by undamaged cell walls compared to unmodified samples. White-rot fungi are capable of degrading lignin as well as other wood cell-wall components, although the extent and sequence of degradation vary among species. Based on the pattern of cell-wall component removal, white-rot fungi are commonly classified as selective (preferential) or simultaneous degraders. Trametes versicolor is considered a non-selective or simultaneous white-rot fungus, as chemical analyses have shown that it degrades lignin, hemicelluloses, and cellulose concurrently (Mohebby 2005 , Eriksson et al. 1990 ). During fungal attack, various hydrolyzing enzymes may be released to cleave linkages in cell wall components (Mohebby 2005 ). Although T. versicolor relies primarily on enzymatic mechanisms rather than chelator-mediated Fenton chemistry for lignin degradation, metal ion availability remains essential for the activity of its ligninolytic enzymes. Laccases are multicopper oxidases that require copper ions for their catalytic function, while manganese peroxidases depend on manganese ions as substrates in oxidative lignin depolymerization (Hatakka and Hammel 2011 ). Consequently, any alteration in metal ion availability may influence the efficiency of enzymatic lignin degradation by T. versicolor (Baldrian 2006 ). The presence of acidic components in SorCA-modified wood may also contribute to enhanced decay resistance. Citric acid and other organic acids have been reported to inhibit fungal growth and metabolic activity (Hassan et al. 2015 ). However, the retention of residual acidity within the wood matrix may be a limitation of this modification approach, as prolonged acidic conditions can promote cellulose depolymerization, leading to a gradual loss of mechanical integrity (Ahmed and Hosseinpourpia 2025 ). A self-neutralizing CA-based system might mitigate such problems, while biological and weathering resistance also need to be thoroughly explored to understand the efficacy of the modification. Conclusions This study demonstrated that SorCA modification significantly improves the biological durability and weathering performance of silver birch wood. The vacuum–pressure impregnation process ensured effective penetration of the modifying solution. The in situ polyesterification resulted in a cross-linked polymer network within the wood structure, confirmed by µ-CT analyses. SorCA-modified wood showed markedly enhanced resistance to white-rot fungal decay and mold growth compared with unmodified controls. Improved biological resistance was observed at a SorCA 40% which could be primarily due to the reduced hygroscopicity, cell wall bulking, and restricted diffusion of fungal enzymes and nutrients within the modified wood structure. Outdoor weathering results further indicated that SorCA treatment mitigated surface degradation and the development of roughness, although it did not fully prevent color changes. Increased ΔE values in modified samples could be due to the formation of chromophoric structures during curing and subsequent photochemical reactions during exposure. Compared with unmodified wood, SorCA modification provided improved surface stability. In general, SorCA modification represents a promising bio-based approach for enhancing the outdoor performance of birch wood. Despite these benefits, the acidic nature of the SorCA system remains a potential limitation, as residual acidity may contribute to long-term degradation of wood polymers and gradual loss of mechanical strength. Future research should focus on optimizing formulation chemistry, particularly through self-neutralizing systems. Declarations Acknowledgement The authors gratefully acknowledge J. Gust. Richert Stiftelse for supporting the InnoBjörk project. Reza Hosseinpourpia also acknowledges the Knowledge Foundation for supporting the project “Competitive timber structures- Resource efficiency and climate benefits along the wood value chain through engineering design.” Authors also thank Jonatan Stridh for assistance with the impregnation process. Author contributions Study conception and design: Reza Hosseinpourpia. Material preparation and data collection: Sheikh Ali Ahmed, Ramil Gainov and Kazuya Tamura. Data analysis: Sheikh Ali Ahmed. Interpretation of results: Sheikh Ali Ahmed and Reza Hosseinpourpia. Projects management: Reza Hosseinpourpia. The first draft of the manuscript was written by Sheikh Ali Ahmed, and all authors reviewed the manuscript. All authors read and approved the final manuscript. Funding Open access funding was provided by Linnaeus University. This work was financially supported by J. Gust. Richert Stiftelse (project number: 2022-00777) and the Knowledge Foundation (project number: 2023-0005). Data availability The raw data files are available upon reasonable request from the corresponding authors. Conflict of interest The authors have no competing interests to declare that are relevant to the content of this article. 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Holzforschung 71(2):119–128 Mohebby B (2005) Attenuated total reflection infrared spectroscopy of white-rot decayed beech wood. Int Biodeterior Biodegrad 55:247–251 Mubarok M, Militz H, Dumarçay S, Gérardin P (2020) Beech wood modification based on in situ esterification with sorbitol and citric acid. Wood Sci Technol 54:479–502 Myronycheva O, Sehlstedt-Persson M, Karlsson O, Sandberg D (2018) Growth of mold and rot fungi on copper-impregnated Scots pine sapwood: Influence of planing depth and inoculation pattern. BioResources 13(4):8787–8801 Ozgenc O, Yildiz UC (2014) Surface characteristics of wood treated with new generation preservatives after artificial weathering. Wood Res 59(4):605–616 Schmidt O (2006) Wood and tree fungi: Biology, damage, protection, and use. Springer-, Berlin, Heidelberg Soccol CR, Vandenberghe LPS, Rodrigues C, Pandey A (2006) New perspectives for citric acid production and application. 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IRG/WP 20–40901 NTT Woodnet (2026) Nationell plan ska femdubbla sågverkens lövträproduktion. https://www.woodnet.se/article/view/1223806/nationell_plan_ska_femdubbla_sagverkens_lovtraproduktion . Accessed on 2026-04-27 Woxblom L, Nylinder M (2010) Industrial utilization of hardwood in Sweden. Ecol Bull (53):43–50 Xie Y, Fu Q, Wang Q, Xiao Z, Militz H (2013) Effects of chemical modification on the mechanical properties of wood. Eur J Wood Prod 71:401–416 Xu D, Li C, Pizzi A, Xi X, Wang Z, Du G, Chen Z, Lei H (2024) Self-neutralizing citric acid-corn starch wood adhesives. ACS Sustainable Chem Eng 12(35):13382–13391 Zabel RA, Morrell JJ (1992) Wood microbiology: Decay and its prevention. Academic, London Zelinka SL, Altgen M, Emmerich L, Guigo N, Keplinger T, Kymäläinen M, Thybring EE, Thygesen LG (2022) Review of wood modification and wood functionalization technologies. Forests 13(7):1004 Zhou Y, Xu D, Li C, Pizzi A, Du G, Lei H (2025) Self-neutralizing tannin-citric acid wood adhesives. Int J Biol Macromol 310(2):143419 Zimmer K, Larnøy E, Høibø O (2012) Assessment of fluid flow paths and distribution in conifers. Wood Res 57(1):01–14 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9615371","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":634601926,"identity":"38b541b4-e920-4147-bb06-00613b43e0f8","order_by":0,"name":"Sheikh Ali Ahmed","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3klEQVRIiWNgGAWjYDACCQaGg40NDAwGIPaHAlK1SM4wIFILI0yLNA8xWuRnNx88OHMHg7y52OGDt20M7iQ28B8+gFeLwZ1jCQc3nmEw3Dk7Ldk6x+BZYoNEWgJ+LRI5BgcftjEwbridYyadY3AYqIWA8+Rn5H8AabHfcDv/m7QFSAv/+Q/4PXMjh+HgxjaGRKAtbNIMIC0MOfh1GNxIMzg4s00iecPtNGPLHoPDxm0SaYQclvz4Y2+bje2G28kPb/yoOCzbz3/4AX5rIEACwWQjRv0oGAWjYBSMAvwAAKkfTisfxyLhAAAAAElFTkSuQmCC","orcid":"","institution":"Linnaeus University","correspondingAuthor":true,"prefix":"","firstName":"Sheikh","middleName":"Ali","lastName":"Ahmed","suffix":""},{"id":634601927,"identity":"424c6e9b-f4c6-4d1b-bc98-8e54b3425cfc","order_by":1,"name":"Ramil Gainov","email":"","orcid":"","institution":"Rigaku Europe SE","correspondingAuthor":false,"prefix":"","firstName":"Ramil","middleName":"","lastName":"Gainov","suffix":""},{"id":634601928,"identity":"e4934d1c-7384-444e-9915-99852c5d8147","order_by":2,"name":"Kazuya Tamura","email":"","orcid":"","institution":"Rigaku Corporation","correspondingAuthor":false,"prefix":"","firstName":"Kazuya","middleName":"","lastName":"Tamura","suffix":""},{"id":634601929,"identity":"67180f0e-828b-431d-afe3-953a1b479f1d","order_by":3,"name":"Reza Hosseinpourpia","email":"","orcid":"","institution":"Linnaeus University","correspondingAuthor":false,"prefix":"","firstName":"Reza","middleName":"","lastName":"Hosseinpourpia","suffix":""}],"badges":[],"createdAt":"2026-05-05 07:40:31","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9615371/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9615371/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108589956,"identity":"b87d1c39-d8b7-48f0-8acd-4cd514ebeebd","added_by":"auto","created_at":"2026-05-06 09:35:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":12860269,"visible":true,"origin":"","legend":"\u003cp\u003eSamples on the rack for outdoor weathering test.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/174c6c3b86b949284dcc52cc.png"},{"id":108805045,"identity":"70153b36-366f-433b-926b-f1b5a4cc9a7a","added_by":"auto","created_at":"2026-05-08 15:24:37","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1685845,"visible":true,"origin":"","legend":"\u003cp\u003eMicro-computed tomography (µ-CT) images of the transversal sections of birch control (left) and after modification (right) with sorbitol-citric acid (SorCA).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/68ee5b94c06ecbf1bbf1ecf6.png"},{"id":108589957,"identity":"7575e220-60cc-4b84-96b7-53908711585d","added_by":"auto","created_at":"2026-05-06 09:35:48","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":237862,"visible":true,"origin":"","legend":"\u003cp\u003eThree-dimensional rendering of a modified birch wood sample obtained by micro-computed tomography (µ-CT). Filled cavities of birch wood after impregnation and curing with sorbitol-citric acid (SorCA).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/d4e5e9e14bd47f0e9ffd1ea0.png"},{"id":108804525,"identity":"dde66bcc-82fc-4612-93ce-dc2c62ab2245","added_by":"auto","created_at":"2026-05-08 15:21:19","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":872014,"visible":true,"origin":"","legend":"\u003cp\u003eTest samples before and after outdoor weathering for 10 months (a) and resulting total color change (b). Treatment represents sorbitol-citric acid (SorCA) at 2 different concentrations of 20% and 40%, unmodified control, and commercially impregnated pine with Cu-based preservative.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/45c95d423441580c6b42a59a.png"},{"id":108805210,"identity":"f0d70b1d-f7b6-4488-ac97-c5550984dd40","added_by":"auto","created_at":"2026-05-08 15:25:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1297131,"visible":true,"origin":"","legend":"\u003cp\u003eSurface topography of different samples after 10 months exposure to outdoor weathering conditions analyzed by a profile meter. Treatment represents sorbitol-citric acid (SorCA) at 2 different concentrations of 20% and 40%, unmodified control, and commercially impregnated pine with Cu-based preservative.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/3c6b3a25ef0d23be7c5f3d9b.png"},{"id":108589960,"identity":"0cb9cd0d-f23e-4f94-955a-3eb08c05d5e8","added_by":"auto","created_at":"2026-05-06 09:35:48","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":123923,"visible":true,"origin":"","legend":"\u003cp\u003eMold growth of different samples after 4-week of incubation. Treatment represents sorbitol-citric acid (SorCA) at 2 different concentrations of 20% and 40%, unmodified control, commercially impregnated pine with Cu-based preservative, and pine sapwood. Mean values followed by different letters within each group indicate that there is a significant difference (\u003cem\u003eP\u003c/em\u003e ≤ 0.05) as determined by ANOVA and Tukey’s multiple comparisons test. The error bars in the columns indicate the standard deviations.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/9deaf4aeaa1cf29b770c43c3.png"},{"id":108589962,"identity":"f88874d4-3523-44f6-b680-751dcf7acf0b","added_by":"auto","created_at":"2026-05-06 09:35:48","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":165529,"visible":true,"origin":"","legend":"\u003cp\u003eMean mass loss of different samples with the white rot fungus \u003cem\u003eTrametes versicolor \u003c/em\u003eafter 16 weeks of incubation. Treatment represents sorbitol-citric acid (SorCA) at 2 different concentrations of 20% and 40%, unmodified control, beech, and commercially impregnated pine with Cu-based preservative. Mean values followed by different letters within each group indicate that there is a significant difference (\u003cem\u003eP\u003c/em\u003e ≤ 0.05) as determined by ANOVA and Tukey’s multiple comparisons test. The error bars in the columns indicate the standard deviations.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/9ed898de3151dbb4ad4a49b9.png"},{"id":108589963,"identity":"46ebacbc-f3c1-443d-9e66-3dbaef53c52c","added_by":"auto","created_at":"2026-05-06 09:35:48","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":924588,"visible":true,"origin":"","legend":"\u003cp\u003eOptical microscopy images of wood samples after decay exposure to the white rot fungus \u003cem\u003eTrametes versicolor\u003c/em\u003e. Cross-sections showing undecayed birch (a), decayed unmodified beech (b), commercially impregnated pine with Cu-based preservative (c), control birch at cross (d), radial (e), tangential section (f), sorbitol-citric acid (SorCA)- modified birch at 20% (g), and 40% concentration at cross (h) and tangential section (i). Arrowheads indicate fungal mycelia inside the cell lumen. Scale bar = 50 µm.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/62dc1e89c422f1bf728a7044.png"},{"id":108809854,"identity":"30922957-35f3-4c8f-a111-25b7d475e009","added_by":"auto","created_at":"2026-05-08 15:55:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":18472409,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9615371/v1/e87b7160-9dbd-4ac0-bb39-cddc7dfde5c3.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eResistance of birch wood modified with sorbitol-citric acid to white-rot decay and weathering\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eBirch (silver: \u003cem\u003eBetula pendula\u003c/em\u003e and downy: \u003cem\u003eB. pubescens\u003c/em\u003e) is one of the most widespread hardwood species, accounting for about 13.1% of the standing forest volume in Sweden and is primarily used for pulping and firewood (Forest statistics \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, Woxblom and Nylinder \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). This is mainly due to its poor dimensional stability and biological durability, especially in environments with fluctuating humidity (Hill \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Birch wood products have a low durability class of 5, meaning they are not durable (SS-EN 350 \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Therefore, it requires specific treatment or modification for exterior use (Dubois et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Even though the growing stock of birch in Sweden continues to increase (Lidman et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), the industrial utilization of birch wood remains limited due to its suitability for outdoor applications. Therefore, there is a clear need to develop strategies to enable the optimal, value-added use of Swedish birch wood, which is in accordance with the very recent initiative to enhance the use of Swedish Hardwood (NTT Woodnet \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2026\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eWood treatment is necessary to enhance resistance against decay, dimensional stability, and weathering. Conventional wood treatment with biocides often contains toxic chemicals and generally does not improve the inherent properties of wood, such as moisture resistance or mechanical performance. Furthermore, the leaching of biocides contaminates water, air, and soil, which has led to stricter regulations (Miranji et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In chemical wood modification, permanent changes in the cell wall result in not only improved biological durability but also enhance intrinsic properties of the material (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Hosseinpourpia and Mai \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e, Ghavidel and Hosseinpourpia 2024). Thus, this type of modification requires the penetration of the modifying chemical into wood microstructures. As a result, wood species that are difficult to impregnate are less suitable for chemical modification. Penetration of liquid proceeds much easier into sapwood than into heartwood and thus sapwood exhibits higher weight gain after chemical modification when compared to heartwood (Mai and Militz \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Birch does not form any true heartwood (H\u0026ouml;rnfeldt et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) and its diffuse pore structure can support effective liquid penetration and chemical distribution throughout its structure. Such a property of birch generally enhances the treatability for modification processes compared to wood species with low permeability. Studies on wood impregnation mechanisms showed that permeability is the key factor governing the uptake of modifying agents (Augustina et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2023\u003c/span\u003e, Grinins et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Consequently, birch wood exhibits considerable potential for various modification strategies.\u003c/p\u003e \u003cp\u003eEsterification is among the most widely applied chemical reactions in wood modification (Mubarok et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The reaction is commonly based on the reaction of anhydride/carboxylic acid/isocyanate/ester groups of a modifying agent and hydroxyl groups of wood polymer or one of the modifying agents and hydroxyl groups of other modifying agents/and wood polymer (Mai and Militz \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). A polyesterification system based on sorbitol (Sor) and citric acid (CA) has attracted interest in recent years due to increased environmental awareness. This type of modification system has resulted in enhanced resistance to brown, white, and soft rot fungi in Scots pine, Norway spruce, and European beech (Kurkowiak et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e, Beck \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, Mubarok et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, Larn\u0026oslash;y et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Investigation on field and laboratory studies has shown that SorCA-modified Scots pine sapwood and Norway spruce exhibit enhanced resistance to marine borers and subterranean termites after one year of exposure (Treu et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Moreover, SorCA treatment has significantly enhanced the moisture-related properties of European beech and Scots pine (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, H\u0026ouml;tte et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Mubarok et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). However, their experimental results showed that even if there is an increasing trend of modulus of elasticity, the modulus of rupture decreased depending on the modification intensity. The plausible reason for the decrease in strength could be the low pH of the modifying solution. The highly acidic property of the SorCA solution and curing temperature degrades the amorphous polysaccharide (especially hemicellulose) and results in lower bending strength, as it was reported in European beech and birch wood (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Mubarok et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Similar results were also reported for tannin-CA wood modification in Scots pine (Ahmed et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Recent developments in self-neutralizing CA-based systems could mitigate such problems associated with high solution acidity and provide a promising direction for future work on self-neutralizing formulations (Zhou et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Xu et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBoth components used for SorCA modification are derived from a bio-based origin. Sorbitol is a commonly applied sweetener that is naturally present in a variety of fruits, such as berries. It is industrially manufactured from biomass sources, mainly from corn and wheat. The second component of the modifying agent is CA and is also obtained through bio-based production. The process involves the fermentation of glucose, primarily derived from beet and sugarcane molasses (Soccol et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). CA is widely used as a buffering agent and flavoring additive. Thus, the wood modification using the SorCA system has attracted considerable attention due to its high effectiveness, fully bio-based constituents, and cost efficiency. However, the performance of chemically modified wood is strongly influenced by the wood structure, which governs chemical uptake, distribution, and performance behavior (Xie et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Penetration and micro distribution patterns of impregnation agents into the wooden structure vary depending on wood species and thus affect the performance of modified wood (Zimmer et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). While recent work on SorCA-modified silver birch (\u003cem\u003eB. pendula\u003c/em\u003e) has demonstrated improvements in dimensional stability and modulus of elasticity (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), its resistance to biological decay and durability under outdoor weathering conditions remain unexplored. These properties are critical for the utilization of silver birch in exterior applications. Thus, the aim of the present study is to evaluate the resistance of SorCA-modified silver birch against mold, white-rot decay and also to assess its performance after exposure to outdoor weathering conditions.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials and sample preparation\u003c/h2\u003e \u003cp\u003eKiln-dried silver birch (\u003cem\u003eBetula pendula\u003c/em\u003e Roth.) board (120 \u0026times; 70 mm) with straight grain were purchased from KG List (Norrhult, Sweden). Specimens free of visible defects were extracted from those boards for various tests (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Specimens were then stored in a climate chamber at 20\u0026deg;C and 65% RH for acclimatization. Sorbitol (molecular weight: 182.17 g/mol) and citric acid (molecular weight: 210.14 g/mol) with \u0026ge;\u0026thinsp;99% of purity (VWR International, Leuven, Belgium) were purchased in the form of solid powder from Avantor/VWR International AB, Kista, Sweden.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSpecimen dimensions and oven-dry density of wood material used for various tests. Values in parentheses are the standard deviations.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTests\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSample dimension (mm)\u003c/p\u003e \u003cp\u003e(length \u0026times; width \u0026times; thickness)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDensity (kg/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOutdoor weathering\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e376 \u0026times; 77 \u0026times; 20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e647 (\u0026plusmn;\u0026thinsp;46)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDecay\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e50 \u0026times; 25 \u0026times; 15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e635 (\u0026plusmn;\u0026thinsp;10)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMold\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c2\"\u003e \u003cp\u003e150 \u0026times; 77 \u0026times; 20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e645 (\u0026plusmn;\u0026thinsp;46)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSorCA formulation and modification\u003c/h3\u003e\n\u003cp\u003eAqueous solutions containing 20% and 40% (w/w) of sorbitol-citric acid (SorCA) were prepared by diluting 1 equimolar of Sor and 3 equimolar of CA in water until completely dissolved. This specific molar ratio (1:3) was selected because it enables nearly complete reaction of the acid functional groups with sorbitol, as reported by Doll et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The pH of 20% and 40% (w/w) SorCA solutions was measured at 1.59 and 1.24, respectively. All specimens were oven-dried at 103\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C for 48 h. Initial oven-dry mass and dimensions were recorded prior to impregnation. Specimens were then impregnated with aqueous treatment solutions in an impregnation vessel (Scholz Maschinenbau SCHOLZ GmbH \u0026amp; Co. KG, Germany) under vacuum (91.2 kPa for 1 h), followed by pressure to 750 kPa for 2 h. Unmodified control specimens were impregnated with distilled water only. After impregnation, the samples were gradually dried and subsequently cured at 140\u0026deg;C for 24 h in accordance with the procedure described by Ahmed and Hosseinpourpia (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003eCharacterizations\u003c/h3\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eX-ray microtomography\u003c/h2\u003e \u003cp\u003eUnmodified and modified birch at SorCA 40%) were scanned using an X-ray microtomography system (CT Lab HX-100, Rigaku Corporation, Tokyo, Japan). The micro-computed tomography (\u0026micro;-CT) study was performed with a tube voltage of 40 kV and a tube current of 100 \u0026micro;A using a molybdenum X-ray target without any additional filter. The samples were scanned with a field of view (FOV) of 5 mm, resulting in a voxel size of 3.86 \u0026micro;m (standard scanning mode). The acquisition parameters included an exposure time of 35.7 ms per projection and 1680 projections collected during a long-geometry scan. The total scan time for each sample was approximately 15 minutes. These settings provided sufficient spatial resolution for imaging the anatomical features of wood-based materials. After the scanning process, the scanned data were reconstructed using the software VGSTUDIO 2025.3 (Volume Graphics GmbH, Heidelberg, Germany). This software enabled volume rendering, segmentation, and detailed analysis of the internal microstructure of the samples.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eOutdoor weathering and surface properties\u003c/h3\u003e\n\u003cp\u003eAfter modification, all specimens were acclimatized at 20\u0026deg;C and 65% relative humidity. Five replicates were used per treatment group for a 10-month outdoor weathering test at the ASA Experimental Forest and Research Station (part of the Swedish Infrastructure for Ecosystem Science, SITES) in Lammhult, located 37 km north of V\u0026auml;xj\u0026ouml;, Sweden. Specimens were randomly arranged on wooden racks approximately 60 cm above the ground at a 45\u0026deg; inclination (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Specimens were exposed south-facing, with a small plastic tube supporting each specimen from behind to prevent moisture deposition. Commercially impregnated Scots pine (\u003cem\u003ePinus sylvestris\u003c/em\u003e L.) sapwood treated with Cu-based preservative- Celcure AC800 (copper tetra-amine-dihydrogencarbonate 38.0\u0026ndash;40.0%, benzalkonium chloride 4.8%, and water 57.2\u0026ndash;51.2%) according to the standard of the Nordic Wood Preservation Council was included as a reference.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSample color was measured before and after exposure to outdoor climate using a Chroma Meter CR-410 (Minolta Co. Ltd., Japan). Color was expressed in the CIELAB system with three-dimensional Lab coordinates, where L* represents lightness (0\u0026thinsp;=\u0026thinsp;black, 100\u0026thinsp;=\u0026thinsp;white), +a* the red direction, \u0026minus;a* the green direction, +b* the yellow direction, and \u0026minus;\u0026thinsp;b* the blue direction. Total color change (∆Eab) was calculated using the following equation:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{\\varDelta\\:\\text{E}}_{ab}^{*}=\\sqrt{{\\left(\\varDelta\\:\\text{L}\\text{*}\\right)}^{2}+{\\left(\\varDelta\\:\\text{a}\\text{*}\\right)}^{2}+{\\left(\\varDelta\\:\\text{b}\\text{*}\\right)}^{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003e∆L\u003c/em\u003e, ∆a*, and ∆b* represent the changes in lightness and chromatic coordinates following weathering. Measurements were taken at three fixed positions on the flat exposed surface of each specimen using reference marks to ensure measurements at the same positions before and after weathering.\u003c/p\u003e \u003cp\u003eTo characterize the surface topography after outdoor weathering, measurements were performed using a non-contact 3D optical profiling system (Veeco NPFLEX, Tucson, USA) operating in Vertical Scanning Interferometry (VSI) mode (p. 1). Data was acquired at 5.1\u0026times; magnification with a 640 \u0026times; 480 pixel array, resulting in a lateral sampling spacing of 1.95 \u0026micro;m. The measurement process involved a tilt correction to level the surface profile, while no additional digital filtering was applied to the raw height data to preserve the integrity of the surface features. Standard areal surface roughness parameters (S-parameters) were then extracted from the processed topography maps to quantify the amplitude, spacing, and hybrid characteristics of the sample. Three regions of each specimen were measured to account for the inherent heterogeneity of wood surfaces, and the average values were reported.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eMold resistance\u003c/h2\u003e \u003cp\u003eSpecimens measuring 70 mm (width) \u0026times; 15 mm (thickness) \u0026times; 150 mm (length) were prepared to assess mold resistance following the method as described by Ahmed et al. (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Five specimens per treatment group were tested in a Memmert HCP 246 climate chamber (Memmert GmbH, Germany) at 30\u0026deg;C and 95% relative humidity. Specimens were randomly suspended from the top of the chamber, with their flat surfaces oriented vertically and parallel to one another, maintaining a 10 mm gap. Three Scots pine sapwood (\u003cem\u003ePinus sylvestris\u003c/em\u003e L.) samples naturally infested with \u003cem\u003eAspergillus\u003c/em\u003e, \u003cem\u003eRhizopus\u003c/em\u003e, \u003cem\u003ePenicillium\u003c/em\u003e etc. were placed at the bottom of the chamber to serve as sources of mold inocula. After 4 weeks of incubation, the experiment was terminated due to abundant mold growth, which was observed on some surfaces. Both flat surfaces of each specimen were visually assessed and rated on a scale from 0 (no infestation) to 6 (extremely heavy infestation).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eWood decay test\u003c/h3\u003e\n\u003cp\u003eThe decay resistance of the modified wood against white rot fungus- \u003cem\u003eTrametes versicolor\u003c/em\u003e (L.) Lloyd was evaluated using a malt agar incubation test in accordance with the standard SS-EN 113-1 (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Modified and unmodified specimens were incubated for 16 weeks at 22\u0026deg;C and 70% relative humidity. For comparison, commercially impregnated Scots pine (\u003cem\u003ePinus sylvestris\u003c/em\u003e L.) sapwood treated with Celcure AC800 (Cu-based preservative) and untreated beech (\u003cem\u003eFagus sylvatica\u003c/em\u003e L.) wood were used as controls. Prior to incubation, all specimens were oven-dried at 103\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C to constant mass and weighed to the nearest 0.001 g to determine oven-dry weight, four replications were used per treatment group. After steam sterilization in an autoclave at 120\u0026deg;C for 30 min, one modified and one unmodified control specimen were placed on fungal mycelium grown on 70 mL malt extract agar in 400 mL Kolle flasks. Correction values were determined from sterile non-inoculated control samples from all treatment groups. After the end of the incubation period, all specimens were cleaned of adhering fungal mycelium, weighed, oven-dried at 103\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C, and weighed again to determine mass loss as follows:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:ML\\:\\left(\\%\\right)=\\frac{{\\text{M}}_{0}-{\\text{M}}_{0,inc}}{{\\text{M}}_{0}}\\:\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere ML is the mass loss by fungal decay (%), M\u003csub\u003e0,inc\u003c/sub\u003e is the oven-dry mass after incubation (g), and M\u003csub\u003e0\u003c/sub\u003e is the oven-dry mass before incubation (g).\u003c/p\u003e\n\u003ch3\u003eMicroscopical analysis\u003c/h3\u003e\n\u003cp\u003eSmall blocks measuring ca. 5 mm (radial) \u0026times; 5 mm (tangential) \u0026times; 10 mm (longitudinal) were prepared from samples and soaked in water. Thin sections approximately 10 \u0026micro;m thick were made using a sledge microtome (WSL, Birmensdorf, Switzerland). Sections were placed on a glass slide, a drop of 1% Safranin O (Merck KGaA, Darmstadt, Germany) was added. The slide was left for 5 minutes and then washed with 50% ethyl alcohol. After that, one drop of lactophenol blue (Sigma-Aldrich Sweden AB) was added to stain the fungal mycelium. The slide was left for 5 minutes and then washed with water. One drop of glycerin was added, and the sections were covered with coverslips. The prepared slides were examined under a motorized Olympus BX63F light microscope (Olympus, Tokyo, Japan) equipped with a DP73 color CCD camera (maximum resolution of 17.28 megapixels) and OLYMPUS cellSens Dimension software, version 4.2.1 (Olympus, Tokyo, Japan).\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003eStatistical analyses were performed using IBM SPSS Statistics, Version 30.0.0.0 (IBM Corporation, New York, USA) to evaluate differences in color change, mold rating, and mass loss due to decay among different treatments. One-way analysis of variance (ANOVA) was conducted at a significance level of 0.05 to identify significant differences. When significant differences were detected, a Duncan multiple range test was applied to distinguish the treatments effects.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eDistribution of the modifying chemicals\u003c/h2\u003e \u003cp\u003eThe \u0026micro;-CT scan can show the internal anatomical structure of wood and can facilitate quantitative measurements of anatomical changes in wood caused by the chemical modification (Moghaddam et al. 2016). The natural porous structure is clearly visible in the transverse CT sections of the control and modified birch wood (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). After modification, changes in intensity and contrast in the \u0026micro;-CT images indicate that void spaces within the wood have been partly or fully filled by cured SorCA.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe three-dimensional rendering further illustrates how impregnation and curing with SorCA affect internal structure (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The filled cavities revealed by \u0026micro;-CT renderings correspond to regions where the SorCA reaction products have polymerized and occupied cell voids, resulting in a polymer network. In SorCA modification, CA reacts with the polyfunctional alcohol of sorbitol via \u003cem\u003ein situ\u003c/em\u003e polyesterification and develops a cross-linked polyester network that both interacts with wood polymers and remains physically entrapped within the cell wall and lumen regions. Such modification is reported to improve dimensional stability, moisture tolerance, and biological durability of wood by reducing hygroscopicity and restricting pathways for water and microbial activity (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Mubarok et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Effective protection against wood-decaying fungi further depends on a uniform distribution of the fixated chemicals throughout the modified wood (Hill \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2006\u003c/span\u003e, Zelinka et al. 2010). In such a case, SorCA-modified wood showed uniform deposition of the modifying chemicals within the cell lumen, as observed in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe extent of modification can be quantified by weight percent gain (WPG), reflecting the amount of polymer incorporated into the wood structure after impregnation and curing. Higher WPG values generally correlate with greater void space filling and improved performance properties. However, it requires thorough distribution of the reactants to avoid inhomogeneity (Kurkowiak et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In this study, a homogenous distribution of the modifying chemicals was achieved.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eOutdoor weathering\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows the aesthetical characteristics of test samples before and after 10 months of outdoor exposure. Color of the modified wood depends on the concentration of the modifying liquid. It became darker at a concentration of 40%. This could be due to the higher acidity of the solution, which cleaved polysaccharides and formed furanic compounds during curing at the higher temperatures (Hosseinpourpia et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). During outdoor weathering under natural environmental conditions, the samples were exposed to solar ultraviolet (UV) radiation, and fluctuation of moisture and temperature, which induce photochemical and physicochemical changes in the wood surface. Such visible surface degradation is one of the most prominent macroscopic effects of weathering and reflects the breakdown of key wood constituents by sunlight and moisture. In the field test, the unmodified control birch samples exhibited more pronounced visual surface changes than the modified specimens (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). This result is consistent with prior study on thermally-modified wood that outdoor weathering causes color degradation, surface checking, and loss of original hue in wood surfaces within months to years of exposure (Godinho et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe total color change (ΔE) is a crucial parameter for evaluating the color stability of weathered samples. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb presents the quantitative color change (∆E) of the different wood treatments after outdoor weathering. The commercial samples showed the lowest color change (15.38), which might be due to the photostabilization of wood by retarding the formation of carbonyl groups and reducing delignification during weathering (Temiz et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). In contrast, SorCA 20% showed the highest color change (33.75). Increasing the concentration of SorCA 40% significantly improved performance, reducing the color change to 24.65, which is slightly better than the unmodified control (25.81). This finding suggests that SorCA formulations significantly influence color change. Higher concentrations provided some protection against color change. The photodegradation of wood primarily happens due to the breakdown of its main constituents of cellulose, hemicellulose, lignin, and extractives (Dence \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1992\u003c/span\u003e). Among these, lignin degradation is the principal driver of surface discoloration. Photochemical reactions are initiated by the absorption of UV-visible light, mainly by lignin, and form aromatic and other free radicals. These free radicals degrade lignin by reducing methoxyl content, photo-dissociation of carbon-carbon bonds and formation of carbonyl based chromophoric groups leading to color change and yellowing of the wood surfaces (Hon \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Degradation occurs on chromophoric structures and forms secondary chromophors, resulting in wood color from yellow to brown, then to silver, with a general deterioration in brightness (Cogulet et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe higher color change (ΔE) observed for SorCA-modified wood compared to unmodified controls can be attributed to both chemical and photochemical effects associated with citric acid-based modification. During curing, SorCA acid treatment leads to esterification and polyesterification reactions, which introduce new chemical structures such as ester groups, carbonyls, and conjugated systems that can act as chromophoric groups (Kusnierek et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). These newly formed chromophores increase light absorption and photochemical sensitivity, resulting in more pronounced color changes during outdoor exposure. In addition, SorCA treatment causes an initially darker color resulting from the citric acid-sorbitol polymer and progressing thermal degradation of wood during curing at 140\u0026deg;C. Subsequent photodegradation, oxidation, and leaching processes during weathering produced larger shifts in color coordinates relative to the initial modified state and led to higher ΔE values. Similar behavior has been reported for chemically and thermally modified wood, where improved dimensional stability does not necessarily correspond to enhanced color stability (Godinho et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Cambazoglu et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Furthermore, unmodified wood tends to undergo rapid lignin photodegradation and leaching. It leads to an early development of a relatively uniform grey surface and after which further color change progresses more slowly (Lampela et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Higher ΔE values in SorCA-modified wood do not mean that the surface is degrading faster. Instead, the presence of chromophores in modified wood remains active and gradually changes upon exposure to outdoor weathering.\u003c/p\u003e \u003cp\u003eThe surface topography of unmodified and modified wood samples was analyzed using the NPFLEX 3D Optical Profilometer, a non-contact white-light interferometry (WLI) system capable of capturing three-dimensional surface height variations from the nanometer to micrometer scale with high repeatability. This technique enables areal roughness evaluation (e.g. Sa, Sq) by measuring height distributions over a defined scan area without physically contacting the wood surface, thus avoiding deformation of soft cellular structures. The reconstructed 3D maps (scan area\u0026thinsp;~\u0026thinsp;1.2 mm \u0026times; 0.9 mm) reveal distinct differences in surface morphology between the control, commercial, and chemically modified wood samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe control sample exhibited the highest surface irregularity, with a mean height (S\u003csub\u003ea\u003c/sub\u003e) of 82.00 \u0026micro;m and a root-mean-square height (S\u003csub\u003eq\u003c/sub\u003e) of 122.27 \u0026micro;m. This high roughness is characteristic of unmodified hardwoods, in which open vessel elements and loosely attached fibers on the surface create highly heterogeneous microtopography. In contrast, the commercial pine samples impregnated with Cu-based preservatives exhibited the lowest surface roughness values (S\u003csub\u003ea\u003c/sub\u003e = 12.55 \u0026micro;m and S\u003csub\u003eq\u003c/sub\u003e = 26.33 \u0026micro;m). This reduction in roughness can be attributed to differences in anatomical structure and chemical composition. Softwoods such as pine possess a more homogeneous structure constructed chiefly by tracheids. On the contrary, birch contains a more complex arrangement of vessels, fibers, and parenchyma, which leads to a greater surface heterogeneity. In addition, the presence of Cu-based preservatives may further modify the surface by stabilizing the cell wall components. Similar reductions in surface roughness of pine have been reported in the literature with Cu-based preservatives like Didecyldimethylammonium chloride, Celcure AC 500 (copper carbonate hydroxide, benzalkonium chloride, and boric acid), Micronized copper quat, and Copper (II) sulfate pentahydrate (Ozgenc and Yildiz \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The SorCA-modified samples showed a significant reduction in roughness compared to the control, though they remain rougher than the commercial pine. In the SorCA 20% samples, the S\u003csub\u003ea\u003c/sub\u003e is reduced to 40.76 \u0026micro;m, whereas in the SorCA 40% samples, it increases slightly to 47.59 \u0026micro;m. This reduction relative to the control could be due to the esterification process, where the SorCA polyester acts as a bulking agent, filling the cell wall and partially occluding the lumina, and thereby flattening the micro-scale voids. The slight increase in roughness observed when moving from 20% to 40% concentration likely results from higher polymer loading, which can lead to the formation of localized polymer clusters or uneven surface deposits during the high-temperature curing phase.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eMold resistance\u003c/h2\u003e \u003cp\u003eThe mean mold growth grades of the different specimens shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e indicate a clear influence of samples type and SorCA concentration on mold resistance. The unmodified control and pine sapwood exhibit the highest mold grades, reflecting the natural susceptibility of unprotected wood to fungal colonization under favorable moisture conditions. The commercially impregnated wood showed a reduced mold grade and the result is consistent with the known antifungal efficacy of copper-based preservatives (Myronycheva et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, Ahmed et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). SorCA-modified samples display a concentration-dependent improvement in mold resistance. Wood modified with 20% SorCA showed statistically similar mold growth to control and pine sapwood, whereas treatment with 40% SorCA yielded the lowest mold grade among the samples. This enhanced resistance could be attributed to the SorCA modification, which reduces wood hygroscopicity, lowers the availability of nutrients, and creates a less favorable environment for fungal growth through esterification and polymer formation within the wood structure. Similar reductions in fungal staining were observed in SorCA-modified Scots pine wood, where low pH might be a reason for the limited mold growth (Larn\u0026oslash;y et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Generally, most fungi grow over the pH range 3\u0026ndash;7 (Zabel and Morrell \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1992\u003c/span\u003e). The pH of SorCA 20% and SorCA 40% solutions was 1.59 and 1.24, respectively. Although the pH of the surface of the modified samples was not determined, the lower pH might have restricted the mold growth, especially at the higher concentration level.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eWood decay\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e illustrates the mean mass loss of different wood samples after 16 weeks of incubation with the white-rot fungus \u003cem\u003eTrametes versicolor\u003c/em\u003e, demonstrating a clear effect of treatment on fungal resistance. The unmodified birch and beech samples exhibit the highest mass losses, reflecting the validity of the test and inherent susceptibility of unmodified hardwoods to white-rot fungi, which preferentially degrade lignin and polysaccharides in the wood cell wall (Schmidt \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). In contrast, the commercially impregnated pine treated with a copper-based preservative shows significantly lower mass loss, consistent with the well-established antifungal efficacy of copper compounds against basidiomycetes (Myronycheva et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). SorCA-modified samples display a concentration-dependent reduction in mass loss, with the SorCA 40% treatment providing significantly greater resistance than the 20% treatment. These results are also consistent with previous findings (Mubarok et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2020\u003c/span\u003e, Beck \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The improvement of enhanced decay can be attributed to citric-acid-based modification, which reduces wood hygroscopicity, limits nutrient availability, and introduces a cross-linked polyester network within the wood structure, thereby restricting fungal colonization and enzymatic degradation (L\u0026rsquo;Hostis et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The durability of SorCA-modified wood could be attributed to two key factors, viz., the cell wall bulking and the cross-linking of SorCA polyesters with wood polymers. This type of modification substantially reduces the available water molecules within the wood cell wall, thereby restricting the diffusion of low-molecular-weight compounds, enzymes, and ions released by fungal hyphae in the lumen (Kurkowiak et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In addition, chemical modification at high curing temperatures may alter the glass-transition behavior of hemicelluloses, which could further contribute to enhanced resistance to fungal decay. The presence of acids in SorCA-modified wood may also contribute to decay protection, as citric and other organic acids have been shown to hinder fungal growth (Hassan et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). However, residual acid in the wood may present a potential hazard of this technology whereby acidity drives cellulose depolymerization over the long term, destroying the mechanical integrity of the wood (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eTrametes versicolor\u003c/em\u003e is a typical white-rot fungus capable of degrading all major wood polymers, including lignin, cellulose, and hemicelluloses, through the combined action of oxidative and hydrolytic enzymes such as laccases, manganese peroxidases, and cellulases (Mart\u0026iacute;nez et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). In unmodified wood, these enzymes readily diffuse into the cell wall matrix, where lignin depolymerization facilitates subsequent polysaccharide degradation. However, SorCA modification alters the chemical and physical environment of the wood cell wall, thereby affecting fungal colonization and enzymatic efficiency. In SorCA-modified wood, esterification and polyesterification reactions introduce a cross-linked polymer network within the cell wall, leading to reduced cell wall moisture content and restricted diffusion pathways for fungal enzymes and low-molecular-weight metabolites (L\u0026rsquo;Hostis et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, Hill \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Since white-rot fungi such as \u003cem\u003eT. versicolor\u003c/em\u003e require sufficient moisture and accessibility to initiate ligninolytic reactions, this reduction in cell wall accessibility can slow the decay process. Furthermore, the lower hygroscopicity of SorCA-modified wood limits the formation of favorable microenvironments required for sustained enzyme activity (Kurkowiak et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In addition to physical barriers, SorCA modification may also influence fungal metabolism. Citric acid, sorbitol, and their esterified products can act as chelating agents, potentially interfering with the availability of metal ions essential for oxidative enzyme systems involved in lignin degradation (Karimi and Goli \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Cigala et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter 16 weeks of incubation, distinct differences were observed between the unmodified and modified wood samples. In the control birch samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed-f), extensive degradation of the cell walls was observed after fungal exposure. The structure appears severely decayed wood cell walls. This indicates active colonization and enzymatic breakdown of wood components by the fungus. Such degradation is characteristic of white-rot decay where lignin and other structural polymers are progressively degraded, leading to loss of cell wall integrity. Cu-based preservative impregnated pine (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec) and SorCA-modified birch (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eg-i) exhibited enhanced resistance to wood decay as observed by undamaged cell walls compared to unmodified samples. White-rot fungi are capable of degrading lignin as well as other wood cell-wall components, although the extent and sequence of degradation vary among species. Based on the pattern of cell-wall component removal, white-rot fungi are commonly classified as selective (preferential) or simultaneous degraders. \u003cem\u003eTrametes versicolor\u003c/em\u003e is considered a non-selective or simultaneous white-rot fungus, as chemical analyses have shown that it degrades lignin, hemicelluloses, and cellulose concurrently (Mohebby \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2005\u003c/span\u003e, Eriksson et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). During fungal attack, various hydrolyzing enzymes may be released to cleave linkages in cell wall components (Mohebby \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Although \u003cem\u003eT. versicolor\u003c/em\u003e relies primarily on enzymatic mechanisms rather than chelator-mediated Fenton chemistry for lignin degradation, metal ion availability remains essential for the activity of its ligninolytic enzymes. Laccases are multicopper oxidases that require copper ions for their catalytic function, while manganese peroxidases depend on manganese ions as substrates in oxidative lignin depolymerization (Hatakka and Hammel \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Consequently, any alteration in metal ion availability may influence the efficiency of enzymatic lignin degradation by \u003cem\u003eT. versicolor\u003c/em\u003e (Baldrian \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe presence of acidic components in SorCA-modified wood may also contribute to enhanced decay resistance. Citric acid and other organic acids have been reported to inhibit fungal growth and metabolic activity (Hassan et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). However, the retention of residual acidity within the wood matrix may be a limitation of this modification approach, as prolonged acidic conditions can promote cellulose depolymerization, leading to a gradual loss of mechanical integrity (Ahmed and Hosseinpourpia \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). A self-neutralizing CA-based system might mitigate such problems, while biological and weathering resistance also need to be thoroughly explored to understand the efficacy of the modification.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study demonstrated that SorCA modification significantly improves the biological durability and weathering performance of silver birch wood. The vacuum\u0026ndash;pressure impregnation process ensured effective penetration of the modifying solution. The \u003cem\u003ein situ\u003c/em\u003e polyesterification resulted in a cross-linked polymer network within the wood structure, confirmed by \u0026micro;-CT analyses. SorCA-modified wood showed markedly enhanced resistance to white-rot fungal decay and mold growth compared with unmodified controls. Improved biological resistance was observed at a SorCA 40% which could be primarily due to the reduced hygroscopicity, cell wall bulking, and restricted diffusion of fungal enzymes and nutrients within the modified wood structure. Outdoor weathering results further indicated that SorCA treatment mitigated surface degradation and the development of roughness, although it did not fully prevent color changes. Increased \u003cem\u003eΔE\u003c/em\u003e values in modified samples could be due to the formation of chromophoric structures during curing and subsequent photochemical reactions during exposure. Compared with unmodified wood, SorCA modification provided improved surface stability. In general, SorCA modification represents a promising bio-based approach for enhancing the outdoor performance of birch wood. Despite these benefits, the acidic nature of the SorCA system remains a potential limitation, as residual acidity may contribute to long-term degradation of wood polymers and gradual loss of mechanical strength. Future research should focus on optimizing formulation chemistry, particularly through self-neutralizing systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch3\u003eAcknowledgement\u003c/h3\u003e\n\u003cp\u003eThe authors gratefully acknowledge J. Gust. Richert Stiftelse for supporting the InnoBj\u0026ouml;rk project. Reza Hosseinpourpia also acknowledges the Knowledge Foundation for supporting the project \u0026ldquo;Competitive timber structures- Resource efficiency and climate benefits along the wood value chain through engineering design.\u0026rdquo; Authors also thank Jonatan Stridh for assistance with the impregnation process. \u003c/p\u003e\n\u003ch3\u003eAuthor contributions\u003c/h3\u003e\n\u003cp\u003eStudy conception and design: Reza Hosseinpourpia. Material preparation and data collection: Sheikh Ali Ahmed, Ramil Gainov and Kazuya Tamura. Data analysis: Sheikh Ali Ahmed. Interpretation of results: Sheikh Ali Ahmed and Reza Hosseinpourpia. Projects management: Reza Hosseinpourpia. The first draft of the manuscript was written by Sheikh Ali Ahmed, and all authors reviewed the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eOpen access funding was provided by Linnaeus University. This work was financially supported by J. Gust. Richert Stiftelse (project number: 2022-00777) and the Knowledge Foundation (project number: 2023-0005).\u003c/p\u003e\n\n\u003cp\u003eData availability\u003c/p\u003e\n\u003cp\u003eThe raw data files are available upon reasonable request from the corresponding authors. \u003c/p\u003e\n\n\u003cp\u003eConflict of interest\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAhmed SA, Hosseinpourpia R (2025) Dimensional stability and bending properties of silver birch (\u003cem\u003eBetula pendula\u003c/em\u003e Roth.) modified wood with sorbitol and different types of polycarboxylic acids. 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Wood Res 57(1):01\u0026ndash;14\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Silver birch, white-rot fungi, mold growth, UV","lastPublishedDoi":"10.21203/rs.3.rs-9615371/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9615371/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe increasing interest in sustainable and bio-based wood modification strategies has led to growing attention in enhancing durability. This is particularly important for wood with low durability and thus requires improvement for exterior applications. This study evaluates the decay and outdoor weathering resistance of silver birch (\u003cem\u003eBetula pendula\u003c/em\u003e) wood modified with a sorbitol-citric acid (SorCA) system. Birch wood samples were vacuum-pressure impregnated with aqueous solutions of SorCA at 20% and 40% w/w and cured at 140\u0026deg;C. The distribution of the modifying agent within the wood structure was examined using X-ray microtomography, which confirmed effective penetration with partial or complete filling of wood lumina by the \u003cem\u003ein-situ\u003c/em\u003e formed polyester network. The modified samples were subjected to ten months of natural weathering, mold exposure under high humidity conditions, and also to white-rot fungi (\u003cem\u003eTrametes versicolor\u003c/em\u003e). Outdoor weathering results indicated improved resistance to surface deterioration in modified wood, although increased color change (ΔE) was observed due to the formation of chromophoric groups during curing and subsequent photodegradation. SorCA modification significantly improved decay and mold resistance, especially at higher concentrations. In general, SorCA modification showed great potential to improve the durability of birch wood with enhancing the biological durability and weathering resistance.\u003c/p\u003e","manuscriptTitle":"Resistance of birch wood modified with sorbitol-citric acid to white-rot decay and weathering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-06 09:35:43","doi":"10.21203/rs.3.rs-9615371/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"bcfc0a5d-26d5-45b4-963f-577062e626be","owner":[],"postedDate":"May 6th, 2026","published":true,"recentEditorialEvents":[{"type":"checksComplete","content":"","date":"2026-05-06T05:37:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Wood Science and Technology","date":"2026-05-05T07:23:38+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-06T09:35:43+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-06 09:35:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9615371","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9615371","identity":"rs-9615371","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}