Changes in the physicochemical characteristics, bioactive compounds, flavor volatiles and antioxidant properties of noni juice (Morinda citrifolia) during natural fermentation

Noni (Morinda citrifolia) is a tropical plant native to South-East Asia, the fruit of which is usually fermented to extract the juice as a drink, and for medicinal purposes. Despite its widespread consumption and good natural medicinal properties, little is known about its postharvest handling and compositional changes during processing. Here, changes in the physiochemical characteristics (pH value, titratable acidity (TA) and total suspended solid (TSS)), bioactive compounds (total phenolic compounds, flavonoids, lignins), flavor volatiles, and antioxidant capacity were monitored in the broth of naturally fermented of noni fruit using conventional analytical methods. The pH value of the fermented noni fruit showed an overall declining trend (from 4.35 to 4.04) while TA increased slowly (from 2.91 g/kg) in the first 24 weeks, reaching 9.25 g/kg at week 36. Fermentation had little impact on TSS, and values remained largely unchanged (1.34 mg/mL). The total phenolic compounds revealed and overall decreasing trend (from 1.52 mg/L to 0.87 mg/mL) reaching a maximum value (1.79 mg/mL) during the first week. Within this group, the total flavonoids decreased rapidly in the first week of fermentation (from 0.531 mg/mL to 0.295 mg/mL) while lignins decreased slowly in the first few weeks before increasing to 0.314 mg/mL by week 4. Both species increased from week 36 to reach 0.732 mg/mL and 0.366 mg/mL at the end of fermentation. The DPPH scavenging capacity of the extract reached its highest value (100%) at week 4. A total of 112 volatile compounds were identified in the noni juice at week 1, and their compositions changed during fermentation over 1-24 weeks. Methyl caproate and caprylic acid were the dominant compounds, with the latter contributing to the distinctive odor of the fermenting juice. During the first week, a high concentration of total esters and increasing TA was the dominant characteristics; total flavonoids reached highest level in week 2; alcohols were the major class of compound in week 4, reaching maximum levels at 8. The changes in physicochemical characteristics, active and odor compounds at different stages of the fermentation may provide a foundation to understand and control key relationships during the commercial production of noni fruit.