Black Garlic: Evolution of the Chemical Composition and Broad Biological Activities.

Currently, challenges persist in obtaining a product with consistent qualitative characteristics and biological effects, primarily due to the lack of a standardized manufacturing method and a comprehensive understanding of the desired compounds formation mechanisms, aside from the well-established involvement of the Maillard reaction. , The Maillard reaction ( Figure ) is a nonenzymatic browning reaction that occurs between the carbonyl groups of reducing sugars and the amino groups of amino acids, peptides, and proteins. This process develops through three main stages. Initially, reducing sugars react with amino acids, leading to the formation of Amadori or Heyns products, depending on whether the sugar is an aldose or a ketose, respectively. Yuan et al. observed a 40- to 100-fold increase in the main Amadori and Heyns compounds in BG compared to FG. In the second stage, sugar fragmentation and amino acid degradation occur, resulting in the formation of various intermediates, such as 5-hydroxymethylfurfural (HMF). In the final phase, these compounds polymerize, leading to the production of high-molecular-weight brown polymers known as melanoidins. Schematic representation of the different stages of the Maillard reaction involved in black garlic ripening (modified from Yoon & Baek ). Created in BioRender. BORGATTI, M. (2025) https://BioRender.com/sz5f9z4 . The Maillard reaction contributes to changes in the nutritional profile, color, texture, and flavor of garlic. , These transformations are strongly influenced by temperature and relative humidity, both of which play a decisive role in determining black garlic quality attributes. , Higher temperatures accelerate the ripening process and intensify the final products’ color and flavor, but excessive heat (e.g., 90 °C) can lead to a bitter taste due to the rapid depletion of reducing sugars, which are consumed to sustain the Maillard reaction. Instead, humidity critically determines the product texture, with optimal conditions achieved when the water content reaches 400–500 g/kg. Conversely, when it falls below 350 g/kg, BG becomes too hard to be consumed. Processing conditions also alter the concentration of bioactive compounds in black garlic. , For example, subjecting fresh garlic to a temperature of 60 °C enhances the levels of SAC, the primary antioxidant compound in BG. However, the accumulation of HMF, another significant antioxidant molecule, occurs at a considerably slower rate at this temperature. HMF production also depends on the duration of the ripening period; indeed, its concentration increases more than 6-fold when the period has been extended from 25 to 90 days. A detailed overview of the mechanisms behind BG ripening could help identify optimal production conditions to enhance organoleptic properties, nutritional value, and bioactivity. Although the changes underlying BG production are largely attributed to nonenzymatic reactions driven by heat and humidity, emerging evidence suggests that endophytic microorganisms may also contribute to the ripening process. Only a few studies have examined the microbial species found in garlic, which primarily belong to the Bacillus genus, a bacterial strain commonly found in soil, water sources and plants. More specifically, Qiu and colleagues isolated 78 endophytic strains during black garlic processing and found that Bacillus subtilis remained dominant throughout, with B. methylotrophicus and B. amyloliquefaciens also contributing significantly to the microbial community. Additionally, bacteria from the genera Thermus , Corynebacterium , Streptococcus , and Brevundimonas have been identified. These microorganisms can adapt to various carbon sources and exhibit significant heat resistance. Therefore, they could play a role in the development of compounds that contribute to the flavor and bioactivity of BG. , In a subsequent study, Qiu et al. selected the most relevant endophytes identified in black garlic, based on their relative abundance and preliminary experimental findings, to examine their contributions during the aging process. The investigation involved four B. strains, including the three previously mentioned, with the addition of B. licheniformis , and confirmed their ability to proliferate across a broad temperature range (20–50 °C) and pH spectrum (5–9). Notably, when the temperature reaches 50 °C, the growth of both B. subtilis and B. amyloliquefaciens undergo a marked decline, whereas B. licheniformis and B. methylotrophicus growth appear less sensitive. Moreover, the inoculation of the four strains with garlic polysaccharide and garlic juice media demonstrated their capacity to hydrolyze garlic polysaccharides, thereby increasing the percentage of reducing sugars. Finally, by inoculating the endophytes with fresh garlic cloves, the authors illustrated that, in comparison to controls, B. methylotrophicus , B. amyloliquefaciens , and B. subtilis can slightly accelerate the formation of black garlic (0.8–2.8%), in contrast to B. licheniformis , which delays the browning process. Collectively, these findings underscore the potential impact of endophytes on aging dynamics, although further research is necessary to provide deeper insights into this phenomenon. In addition to ripening conditions and microbial influences, the intrinsic characteristics of fresh garlic contribute significantly to the physicochemical and bioactive properties of the final product. In particular, garlic variety affects moisture content, polyphenol concentration, total soluble solids, pH, antioxidant activity, texture, and color. Nevertheless, most fresh garlic traits are not reliable predictors of BG quality. Consequently, additional studies are required to clarify which specific attributes of fresh garlic are decisive in determining the final characteristics of black garlic. ,

The health benefits of black garlic have been extensively documented. However, the specific bioactive constituents responsible for its various biological effects, as well as the molecular mechanisms underlying their activity, remain only partially understood. Furthermore, the potential synergistic interactions among these compounds have received limited attention. Hence, further research is necessary to elucidate the contribution of individual key constituents and their potential interactions.

The health-promoting effects of black garlic are primarily attributed to its rich profile of bioactive compounds ( Figure ), which exert a broad spectrum of biological functions that have been investigated through both in vitro and in vivo studies ( Table ). Chemical structures of key bioactive compounds identified in black garlic. Abbreviations: 4-HNE, 4-hydroxy-2-nonenal; ACE, angiotensin-converting enzyme; Akt, protein kinase B; ATGL, adipose triacylglyceride lipase; Bad, Bcl-2-associated death promoter; Bax, Bcl-2 associated X-protein; Bcl-2, B-cell lymphoma 2; Bim, Bcl-2-interacting mediator of cell death; CAT, catalase; CDK, cyclin dependent kinase; COX, cyclooxygenase; CYPs, cytochrome P450; DNMTs, DNA methyltransferases; EMT, epithelial-mesenchymal transition; FAS, fatty acid synthase; GPx, glutathione peroxidase; GSH, glutathione; GST, glutathione S-transferase: HATs, histone acetyltransferases; HbA1c, hemoglobin A1c; HDACs, histone deacetylases; HIF-α, hypoxia inducible factor-alpha; HIV, immunodeficiency virus; HMF, 5-hydroxymethylfurfural; HSL, hormone sensitive lipase; ICAM-1, intercellular adhesion molecule-1; IL, interleukin; iNOS, inducible nitric oxide synthase; JAK, Janus kinase; LOX, lipoxygenase; MAPK, mitogen-activated protein kinase; MDA, malondialdehyde; MMP, metalloproteinase; mTOR, mammalian target of rapamycin; NF-κB, nuclear factor kappa-B; NLRP3, NLR family pyrin domain containing 3; NO, nitric oxide; NOX, NADPH oxidase; PDGF, platelet-derived growth factor; PGE2, prostaglandin E2; PI3K, phosphoinositide 3-kinase; PLA 2 , phospholipase A2; Rb, retinoblastoma protein; RNS, reactive nitrogen species; ROS, reactive oxygen species; SAC, S-allyl- l -cysteine; SAMC, S-allylmercaptocysteine; STAT, signal transducer and activator of transcription; TBARS, thiobarbituric acid reactive substances; TGF-β, transforming growth factor-beta; TLR, toll-like receptor; TNF-α, tumor necrosis factor-alpha; TREM-1, triggering receptor expressed on myeloid cells-1; VCAM-1, vascular cell adhesion molecule-1; VEGF, vascular endothelial growth factor. Among the principal bioactive compounds in black garlic, S-allyl- l -cysteine has attracted particular interest. In fact, this sulfur-containing amino acid exhibits multiple biological activities. First of all, the antioxidant properties of SAC have been deeply investigated both in vitro and in vivo . The in vitro studies demonstrate its ability to scavenge ROS and hypochlorous acid, thereby protecting LLC-PK1 kidney cells from potassium dichromate-induced oxidative damage. The in vivo studies corroborated these findings by assessing the activities of the antioxidant enzymes SOD, CAT, and GPx: following the administration of SAC (150 mg/kg) for 45 days, diabetic Wistar rats exhibited enhanced activity of these enzymes in liver and kidney tissues. Beyond its antioxidant action, SAC also demonstrates anticancer activity in vitro through several mechanisms of action, such as the induction of carcinogen detoxification, inhibition of cell proliferation, induction of apoptosis, − and suppression of epithelial-mesenchymal transition and invasion , of cancer cells. Similar evidence has been observed in animal models, where SAC consumption was shown to suppress the growth of lung carcinoma in xenografted BALB/CAnN-Foxn1 nude mice. Further in vivo studies extend these findings, highlighting how SAC is also effective in lowering blood glucose in streptozotocin-induced diabetic rats, reducing serum triglyceride and cholesterol levels, and exerting neuroprotection. Regarding this last aspect, it has been demonstrated that SAC reduces lipid peroxidation, ROS production, and dopamine loss in the striatum, thereby improving motor deficits in mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a toxin used to induce Parkinson’s disease in animal models. Likewise, Ashafaq et al. demonstrated SAC’s ability to reduce oxidative damage and improve neurological deficits in a rat model of focal cerebral ischemia. S-allyl- l -cysteine also exhibits hepatoprotective properties. For example, it has been shown to protect BRL-3A rat liver cells against alcohol-induced apoptosis. Furthermore, SAC reduces the levels of pro-inflammatory cytokines such as IL-1β, IL-6, and TNF-α, demonstrating anti-inflammatory activity in mice. , Additional reported benefits in vivo include nephroprotective, cardioprotective, and antihypertensive activities. S-allylmercaptocysteine is a water-soluble organosulfur compound with demonstrated antioxidant properties both in vitro and in vivo . Specifically, it scavenges hydroxyl radical and singlet oxygen, inhibits lipid peroxidation in vitro , and mitigates kidney damage in rats treated with gentamicin, an antibiotic known to induce nephrotoxicity via oxidative stress. These nephroprotective effects are associated with the prevention of decreases in antioxidant enzymes such as glutathione reductase and manganese superoxide dismutase. SAMC also exhibits hepatoprotective activity in vivo , as evidenced by its ability to protect the liver of rats affected by nonalcoholic fatty liver disease against chronic injury through inhibition of apoptosis and enhancement of autophagy. Its anti-inflammatory effects were demonstrated by Yang et al., who observed reduced levels of the pro-inflammatory cytokines IL-1β, IL-6, and TNF-α in the serum of mice treated with posaconazole, suggesting its capacity to attenuate this antifungal drug-adverse effects. The anticancer potential of SAMC has also been widely studied. It has been shown to reduce the onset and progression of various tumors through multiple mechanisms, in vitro and in vivo . , For instance, SAMC prevents benzo­(a)­pyrene-induced carcinogenesis in human lung A549 cells by reducing ROS formation, increasing SOD activity, inhibiting NF-κB, suppressing cell proliferation, and regulating the cell cycle. Additionally, SAMC has also proven to be effective against cancer cells derived from multiple organs, including the colon, prostate, liver, breast, stomach, bladder, thyroid, and ovary. − Finally, in xenografted mice SAMC could effectively suppress the growth and metastasis of colorectal cancer cells. β-carboline alkaloids are known for their wide range of biological activities, including anticancer, antiviral, antimicrobial, antiparasitic, and anxiolytic effects. − Among them, 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (THβC) has been identified in black garlic. This compound likely forms during the ripening process through a condensation reaction between acetaldehyde – a byproduct of the Maillard reaction – and tryptophan. , A salient property of THβC is its substantial antioxidant activity in vitro , which encompasses the scavenging of hydrogen peroxide and the inhibition of lipid peroxidation. To date, direct in vivo confirmation is still lacking. Pyruvate is abundant in black garlic and contributes significantly to its antioxidant properties. Indeed, it not only suppresses ROS generation but also reduces NO and PGE2 production induced by LPS in RAW264.7 cells. These findings suggest that pyruvate has anti-inflammatory effects. Also in vivo, multiple studies have demonstrated that exogenous pyruvate exerts diverse biological effects, including antioxidant, anti-inflammatory, and neuroprotective activities. However, the properties observed in black garlic appear to be less prominent than those exerted by pyruvate alone, indicating that other BG constituents might interfere with its activity. 5-Hydroxymethylfurfural is a furanic compound formed as an intermediate in the Maillard reaction. The process of formation is of pivotal significance in the characteristic color transition of garlic during thermal treatment. In particular, when HMF levels reach approximately 4 g/kg, BG acquires its distinctive dark appearance. Although it remains unclear whether HMF exposure poses a health risk, it seems that it possesses weak genotoxic and mutagenic potential only at high concentration. , Despite these concerns, a mounting body of evidence suggests that HMF concurrently engenders multiple beneficial effects. For instance, Zhao et al. reported that HMF exhibits a strong antioxidant activity. It reduces ROS production and lipid peroxidation while enhancing the activity of the antioxidant enzymes GPx, SOD, and CAT in human erythrocytes treated with 2,2′-azobis­(2-amidinopropane) dihydrochloride, a compound employed to induce oxidative damage. These observations indicate a protective effect against oxidative stress in vitro . Furthermore, HMF also displays anti-inflammatory properties through the suppression of NO, PGE 2 , TNF-α, IL-1β, and IL-6 production in LPS-stimulated RAW264.7. In addition, it downregulates the expression of iNOS and COX-2, key mediators of inflammation. The anti-inflammatory effect of HMF appears to be mediated by the inhibition of the MAPK, NF-κB, and Akt/mammalian target of rapamycin (mTOR) signaling pathways. Additionally, HMF has demonstrated anticancer activity through G 0 /G 1 phase arrest and induction of apoptosis, as evidenced by its antiproliferative effects on human melanoma A375 cells. However, in vivo validation is still limited and partly contradictory. For example, Zhang et al. demonstrated that the intraperitoneal injection of HMF in mouse models of acute-lung injury ameliorated disease conditions by exerting anti-inflammatory and protective effects. Conversely, a study conducted on Brown Norway rats highlighted the nonallergic anaphylaxis induced by HMF, underlying its related immunotoxic risks. However, most evidence remains restricted to cell-based analysis and further investigations are necessary to clarify these aspects. Melanoidins are heterogeneous, nitrogen-containing brown polymers. Similarly to 5-hydroxymethylfurfural, these pigments are synthesized during the final stages of the Maillard reaction and contribute to the characteristic dark color of thermally processed garlic. , Beyond their role in color development, melanoidins have attracted considerable interest due to their diverse biological activities. Notably, melanoidins have exhibited antihypertensive properties, which are attributed to their capacity to inhibit ACE activity in vitro . Additionally, they have demonstrated antimicrobial effects against both Gram-positive ( Staphylococcus aureus and Listeria monocytogenes ) , and Gram-negative ( Salmonella enteritis and Escherichia coli ) bacteria. Interestingly, melanoidins act as bacteriostatic agents at low concentrations and display bactericidal activity at higher doses. Furthermore, melanoidins derived from black garlic have shown promising antiobesity effects. In vivo studies have shown that melanoidin supplementation significantly reduces body weight and white adipose tissue accumulation, while also decreasing blood glucose levels and improving lipid profile. Finally, melanoidins exhibit significant antioxidant capacity, mainly through metal-chelating and radical-scavenging mechanisms demonstrated by in vitro studies. , Polyphenols are naturally occurring compounds derived from the secondary metabolism of plants, where they serve a critical function in mitigating various environmental stressors, including ultraviolet radiation and pathogen aggression. Structurally, these phytochemicals feature one or more aromatic rings substituted with hydroxyl groups. Polyphenols are broadly classified into two groups: flavonoids and nonflavonoids. Each class comprises several subcategories, defined by the number of phenolic units in their molecular structure, the nature of substituent groups, and/or the linkage type between phenolic units. Flavonoids share a common diphenylpropane (C6–C3–C6) skeleton, consisting of two benzene rings connected by a three-carbon unit that typically forms an oxygen-containing heterocyclic ring. Variations in the hydroxylation pattern and the oxidation state of the central ring allow further classification into flavanols, anthocyanidins, isoflavones, flavones, flavonols, flavanones, flavanonols, neoflavonoids, and chalcones. , In contrast, nonflavonoids generally exhibit simpler structures, often consisting of a single aromatic ring. This group includes phenolic acids, stilbenes, and lignans. Among these, phenolic acids represent the principal subgroup and are primarily derived from benzoic and cinnamic acids. Polyphenols are common constituents of plant-based foods and beverages, and their content is influenced by numerous factors such as environmental conditions, harvest ripeness, storage methods, and culinary processing. − Garlic subjected to various thermal treatments has been found to contain significantly higher total polyphenol content compared to fresh garlic. According to Kim et al., flavanols (catechin, epicatechin, and epigallocatechin gallate) are the most abundant flavonoids in BG, followed by flavonols (myricetin, morin, and quercetin). Regarding phenolic acids, derivatives of hydroxycinnamic acid (caffeic acid, p-coumaric, m-coumaric, o-coumaric, and ferulic acid) are the most prevalent, although hydroxybenzoic acid derivatives (gallic and vanillic acid) have also been identified. The health-promoting potential of polyphenols is well-documented. Diets rich in polyphenol-containing foods are associated with a reduced incidence of chronic diseases, including cancer, cardiovascular disorders, and neurodegenerative conditions. Oxidative stress has been implicated as a common etiological factor among many of these diseases. Within this framework, polyphenols have demonstrated potent antioxidant properties by scavenging free radicals, acting as reducing agents, hydrogen donors, and singlet oxygen quenchers. Furthermore, they chelate transition metals such as ferrous ion (Fe 2+ ), thereby preventing the formation of additional free radicals via the Fenton reaction, which occurs between Fe 2+ and hydrogen peroxide. Moreover, polyphenols contribute to redox homeostasis by regenerating vitamin E. , − In vivo studies have shown that polyphenols also increase serum levels of antioxidant enzymes such as SOD, GPx, and CAT, while reducing lipid peroxidation. − Neuroprotective effects have also been attributed to polyphenols, potentially reducing the incidence of Parkinson’s and delaying the onset of Alzheimer’s disease, primarily due to their antioxidant capabilities. , − Beyond their antioxidant activity, polyphenols exhibit notable immunomodulatory and anti-inflammatory effects, in vitro and in vivo . They influence immune cell populations, regulate cytokine production, and modulate the expression of pro-inflammatory genes. , − For instance, polyphenols interfere with the NF-κB and MAPK signaling pathways, reducing the formation of pro-inflammatory cytokines. , They also modulate the expression and activity of cyclooxygenase and 5-lipoxygenase, leading to a reduction in the synthesis of prostaglandins and leukotrienes – two major mediators of inflammation. , , Due to this broad range of bioactivities, polyphenols have garnered increasing attention for their chemopreventive potential. Their protective effects stem primarily from their capacity to mitigate oxidative stress, a critical factor in carcinogenesis and cancer progression. Furthermore, they inhibit procarcinogens activation by reducing the activity of phase I metabolizing enzymes, while facilitating detoxification from carcinogenic substances through the induction of phase II metabolizing enzymes. These aspects were confirmed in vivo , as reported in a study on green tea polyphenols, which upregulated the expression of detoxifying enzymes such as heme oxygenase 1 and NAD­(P)H quinone oxidoreductase, while reducing transaminases and total bilirubin levels in the liver of Kunming mice. Beyond these detoxifying properties, polyphenols also influence epigenetic regulation, which is pivotal in cancer development as it modulates gene expression without altering the underlying DNA sequence. Specifically, they are capable of inhibiting DNA methyltransferases and histone deacetylases, as well as modulating histone acetyltransferases. This leads to the reactivation of tumor suppressor genes and the downregulation of oncogenes transcription in vitro , and in vivo . , Although widely recognized for their antioxidant activity, polyphenols can also exhibit prooxidant effects under certain conditions, particularly at high concentrations, elevated pH, and in the presence of transition metals. Such behavior is attributed to the formation of an unstable aroxyl radical, which may react with oxygen to generate superoxide anion (O 2 •‑ ). Beyond direct ROS generation, some polyphenols promote oxidative stress by stimulating intracellular ROS production via NADPH oxidase or through the reduction of metal ions involved in redox-cycling. This dual redox behavior has particular relevance in the context of cancer. Compared to normal cells, cancer cells frequently display elevated oxidative stress and disrupted redox homeostasis. This imbalance has the potential to stimulate cell proliferation and activate adaptive responses that may contribute to tumorigenesis, metastasis, and treatment resistance. However, further exposure to ROS has been demonstrated to trigger cell death in cancer cells. Conversely, normal cells are typically less sensitive to ROS-inducing stimuli, as they maintain redox homeostasis through efficient adaptive mechanisms. Accordingly, the prooxidant activity of polyphenols may contribute to apoptosis induction and cell cycle arrest in cancer cells. In addition, they suppress specific signaling pathways involved in cell proliferation, which are typically hyperactivated during tumorigenesis. Further in vitro studies have indicated that some polyphenols also possess the ability to inhibit DNA replication, transcription, and repair in cancer cells. , They also counteract angiogenesis by downregulating pro-angiogenic molecules such as vascular endothelial growth factor and exert antimetastatic effects through the suppression of metalloproteinase expression and the modulation of epithelial-to-mesenchymal transition. , In another domain, polyphenols exhibit antimicrobial activity through multiple mechanisms, including disruption of bacterial membrane integrity and inhibition of certain enzymes. Although the exact pathways remain incompletely elucidated, it has been hypothesized that polyphenols can selectively induce the death of pathogenic species while promoting the growth of beneficial microorganisms. , Some polyphenols have also demonstrated antiviral activity. For instance, epigallocatechin gallate exhibit activity against human immunodeficiency virus, influenza virus, and hepatitis C virus. Phenolic compounds are additionally recognized for their potential to reduce the risk of cardiovascular disease, particularly through their antihypertensive properties. These include the enhancement of NO-mediated vasodilation, inhibition of ACE, and attenuation of oxidative stress. Furthermore, polyphenols have demonstrated antiobesity effects by decreasing lipogenesis, suppressing triglyceride accumulation, promoting lipolysis, and stimulating fatty acid β-oxidation. Multiple studies conducted on animal models and human subjects, in fact, highlight how these compounds reduce multiple obesity-related parameters, including the adipose tissue weight and the fat accumulation. − In addition, a growing body of evidence from both in vitro and in vivo studies supports their role in the prevention and management of type 2 diabetes. Indeed, polyphenols enhance insulin secretion and sensitivity, thereby improving glycemic control. In light of this compelling evidence, it can be concluded that polyphenols represent an extremely diversified array of bioactive molecules with extensive health-promoting effects. Many of these biological properties are consistent with those attributed to black garlic, further supporting its relevance as a potential functional food.

The chemical profile of fresh garlic changes significantly due to several factors, including variety, cultivation location and practices, season, and climate. , FG mainly comprises carbohydrates (26–30%, with 1.5% of dietary fiber), proteins (1.5–2.1%), lipids (0.1–0.2%), sulfur compounds (1.1–3.5%), phenols (17.16–42.53 mg of gallic acid equivalent (GAE)/g), and more complex substances such as saponins (0.04–0.11%). It also contains vitamins (0.015%) and minerals (0.7%), such as C, E, B-group vitamins, calcium, sodium, potassium, magnesium, phosphorus, zinc, copper, iron, sulfur, manganese, and selenium. − Fresh garlic is particularly rich in γ-glutamylcysteine, which undergoes hydrolysis and oxidation to form alliin. Actions like cutting, crushing, or chewing garlic can disrupt its cellular structure, resulting in the release of alliinase, an enzyme stored in vacuoles. This enzyme catalyzes the conversion of alliin into allicin, which imparts the characteristic pungent odor of garlic. This reaction also produces pyruvic acid as a byproduct. Allicin and other thiosulfinates are rapidly converted into several compounds, such as diallyl sulfide, diallyl disulfide, diallyl trisulfide, dithiins, and ajoene. Simultaneously, γ-glutamylcysteine is converted into SAC. , The conversion from FG to BG induces substantial changes in its chemical profile ( Table ), which are influenced by processing conditions. Data are presented as mean ± SD or %. Abbreviations: BG, black garlic; FG, fresh garlic; GAE, gallic acid equivalent; HMF, 5-hydroxymethylfurfural; SAC, S-allyl- l -cysteine. During the ripening of black garlic, polysaccharides are degraded into oligosaccharides, disaccharides, and monosaccharides. Fructans progressively degrade under high-temperature conditions and the action of fructan exohydrolase. Specifically, Lu et al. highlighted that this phenomenon is largely attributable to the thermal treatment, while enzymatic hydrolysis plays a secondary role, as the enzyme is rapidly inactivated at the temperatures employed. Consequently, BG contains more reducing sugars than FG, imparting a sweeter taste to the final product. , The content of these sugars also depends on their consumption during the Maillard reaction. The predominant reducing sugars in black garlic are fructose (57.14%), sucrose (7.62%), and glucose (6.78%). Furthermore, Nassur et al. observed a minor increase in protein content in BG compared to FG. Nevertheless, protein degradation may also occur from enzymatic or nonenzymatic hydrolysis, leading to an initial increment in amino acids content. Although the amino acid profile varies significantly depending on the ripening conditions, Kang documented a change in the total amount of 14 free amino acids from 843.11 ± 3.75 to 167.65 ± 1.08 mg/100 g of substance. An accumulation of certain amino acids – such as leucine, isoleucine, and phenylalanine – has been observed, accompanied by a reduction in others. In particular, the depletion of cysteine and tyrosine may be attributed to their involvement in the Maillard reaction. In conjunction with the degradation of hexoses in an acidic environment, this process contributes to the formation of HMF. Additionally, it produces melanoidins, which cause garlic browning. , Kang reported a rise in the melanoidin content during the thermal process. In addition, an almost 4-fold increase in crude lipid content was observed when the bulbs were subjected to the aging process. Nonetheless, further studies are required to elucidate the changes in the lipid profile. The transformation of FG into BG also results in a 1.15- to 1.92-fold rise in water-soluble vitamin content. Nevertheless, thermal treatment under high-humidity conditions and increased acidity causes a reduction in certain vitamins. This includes thiamine (vitamin B1), biotin (vitamin B7), cobalamin (vitamin B12), vitamin C, and a wide array of fat-soluble vitamins. Conversely, an augmented concentration of niacin (vitamin B3) and pantothenic acid (vitamin B5) has been recorded. The former may be attributed to its release following the disruption of cell membranes, while the latter might arise due to the concentration effect resulting from reduced moisture content. This process also leads to a concomitant rise in the quantity of minerals, particularly sodium, potassium, iron, and calcium. Additionally, BG contains high levels of β-carboline alkaloids, which are derived from tryptophan. These compounds are only found in trace amounts in FG, yet during ripening 1,2,3,4-tetrahydro-β-carboline derivatives are formed, thus contributing to its antioxidant activity. , As Zhang et al. observed, the organic acid content varies from 4.6 to 33.61, 37.50, 30.96, and 36.37 g/kg when FG is transformed into BG at 60 °C, 70 °C, 80 °C, and 90 °C, respectively. Particularly, levels of acetic and formic acids increase, which affects the flavor of garlic. Furthermore, Bae et al. reported a decrease in pH from 6.42 to 5.00 and 3.05 after exposing FG to 40 and 85 °C for 45 days. Among dietary vegetables, garlic is particularly rich in phenolic compounds, which are known for their antioxidant properties. , The aging of FG into BG increases their concentration, enhancing its antioxidant activity. Choi et al. documented a rise in total polyphenols from 13.91 mg GAE/g to 25.81–58.33 mg GAE/g, depending on processing conditions. This may be attributed to the release of bound phenolics and enhanced extractability resulting from the disruption of cellular structures during thermal treatment. Kim et al. identified hydroxycinnamic acid derivatives as the primary phenolic acids in black garlic, with flavanols being the predominant flavonoid class. However, extended exposure to high temperatures can reduce certain phenolic compounds. In addition, pyruvate levels also increase during garlic ripening, contributing to BG antioxidant activity. This compound is typically produced by the alliin-allicin pathway. Furthermore, BG exhibits lower allicin levels than FG, as this compound is unstable and rapidly produces other organosulfur compounds. Allicin also reacts with l -cysteine to form S-allylmercaptocysteine. Lastly, thermal treatment leads to a 4- to 6-fold rise in S-allyl- l -cysteine content, depending on the temperature applied. Indeed, Bae et al. found that SAC reached 124.67 μg/g of dry matter when garlic was subjected to a temperature of 40 °C for 45 days, but dropped to 85.46 μg/g at 85 °C. This variation occurs since at lower temperatures (30–50 °C) SAC is primarily produced through the enzymatic hydrolysis of γ-glutamyl-S-allylcysteine (GSAC) by γ-glutamyl transpeptidase (GGT), whereas at higher temperatures SAC formation occurs through the nonenzymatic hydrolysis of GSAC and, to a lesser extent, by the reduction of alliin, as GGT becomes inactive under these conditions. , An overview of the effects of the ripening process on black garlic quality attributes and chemical composition is provided in Table . Abbreviations: HMF, 5-hydroxymethylfurfural; SAC, S-allyl- l -cysteine; SAMC, S-allylmercaptocysteine; T, temperature.

In recent decades, scientific research has been increasingly focusing on investigating edible plants health-promoting benefits and potential therapeutic applications. These species are rich in phytochemicals that may help prevent or delay the onset of various diseases and provide valuable support in their treatment. Among these plants, garlic ( Allium sativum L. ) has garnered considerable attention, partly due to its long-standing use in traditional medicine for treating various ailments. Nevertheless, fresh garlic (FG) consumption has declined due to its strong flavor and pungent odor, along with the gastrointestinal discomfort it may cause in certain individuals. , To address this issue, various garlic-based products have been developed to enhance its organoleptic attributes. Among the commercially available garlic-derived products, black garlic (BG) is one of the most studied. Although the origin of BG remains unclear, historical evidence suggests that it has been consumed in Asian countries since ancient times. BG is obtained by aging fresh garlic bulbs at controlled high temperature (60–90 °C) and relative humidity (70–90%) for 15 to 90 days, without the use of any additives. , During this transformation, garlic cloves acquire a dark color ( Figure ), a sweeter flavor, and a chewy texture. , The color change results from various chemical transformations, including the Maillard reaction, caramelization, and the oxidation of phenols. Changes in the color of black garlic during the ripening process. In contrast, changes in flavor and texture are primarily associated with the accumulation of reducing sugars and the degradation of cell wall polysaccharides under high-temperature conditions, ultimately resulting in a loss of tissue hardness. During this process, molecules responsible for the distinctive aroma of FG, such as allicin, are converted into other compounds, including S-allyl- l -cysteine (SAC) and S-allylmercaptocysteine (SAMC), thereby reducing the unpleasant odor. The transformation of FG into BG not only modifies its organoleptic characteristics but also enhances the content of bioactive substances, as polyphenols and organosulfur compounds. Fresh garlic is widely recognized for its beneficial properties, including antibacterial, antiviral, antidiabetic, antioxidant, anti-inflammatory, antihypertensive, cardioprotective, hypolipidemic, and immunomodulatory effects. Nevertheless, black garlic displays distinct biological activity that differs from that of fresh garlic. In particular, BG possesses higher antioxidant activity and exerts anti-inflammatory, anticancer, immunostimulatory, antiallergic, hepatoprotective, antidiabetic, and antiobesity effects. Therefore, black garlic could be proposed as a functional food, offering health benefits that extend beyond basic nutrition when regularly included in a balanced diet and consumed in appropriate amounts. This review provides an overview of the main mechanisms underlying BG production and their impact on its physicochemical properties and bioactivity. Relevant studies were retrieved through targeted searches in PubMed and ScienceDirect, with particular attention to research addressing processing conditions, chemical transformations, and the biological activities of the resulting bioactive compounds. By integrating current evidence, this work aims to clarify the functional potential of black garlic and outline areas requiring further investigation.