Sunlight-driven evolution of the aroma profile in fish sauce fermentation

The traditional “bask in daytime and dewed at night” process is essential for fish sauce flavor development; however, the precise role of sunlight in aroma formation remains unclear. In this study, fish sauce mash (FM) was prepared using insulation fermentation with koji inoculation. Two batches were subsequently fermented for 30 days under two conditions: a sunlight-treated group (ST) and a darkness-treated group (DT). Electronic nose (E-nose) analysis revealed that the ST sample exhibited stronger responses to W1W (sulfur-organic compounds) and W5S (broadrange compounds) than DT and FM. A total of 90 flavor compounds were identified. Sunlight significantly promoted the formation of 8 alcohols, 5 aldehydes, 7 oxygen heterocyclic compounds, and 4 other classes of compounds. Based on relative odor activity values (ROAV ≥1), 24 key odorants were identified, among which 3-methylbutanal (ROAV: 97.19–336.26), 2-ethylfuran (ROAV: 35.71–82.64), and 2-methylbutanal (ROAV: 32.04–71.70) were the dominant contributors. The metabolic pathways of key odorants were predominantly associated with amino acid catabolism and fatty acid oxidation. Sensory evaluation indicated that sunlight exposure shifted the fish sauce aroma from predominantly meaty (FM) to a more complex and multilayered profile, intensifying roast, fishy, caramel, fermented, sour, ammonia, and rancid notes. Seven key odorants, including 3-methylbutanol, contributed significantly to this transformation. This study elucidates the regulatory role of sunlight in aroma formation during fish sauce fermentation and provides a theoretical basis for improving flavor quality and modernizing traditional fermentation practices.