Changes of Volatile Flavor Compounds during the Fermentation of Sufu

Objective: To clarify the dynamic evolution of volatile flavor compounds during the fermentation of sufu. Methods: Gas chromatography-mass spectrometry (GC-MS), electronic nose technology and principal component analysis (PCA) were combined to systematically analyze the physicochemical properties and volatile components of sufu at fermentation stages of 0, 3, 6, 9, 12 and 15 d. Results: At 0~3 d, aldehydes were the dominant flavor substances in sufu, with a total relative content ranging from 44.9% to 58.3%. Among them, the relative content of hexanal was 10.2%~15.8%, and that of 2-hexenal was 7.5%~12.3%, jointly contributing grassy and fruity aroma characteristics; At 9 d, the relative content of the alcohol 1-hexanol increased from 5.2% to 13.6%; The ester compounds ethyl acetate and ethyl butyrate rose from 1.8% to 11.5% and from 0.9% to 7.3%, respectively, accumulating significantly (P<0.01). The massive enrichment of esters established the fruity aroma profile. At 15 d, the relative content of ethyl acetate reached 13.8%, acetic acid among carboxylic acids increased from 12.5% to 33.5%, and the sulfur-containing compound dimethyl sulfide rose from 0.3% to 2.8%. The pH value continuously decreased from 6.5 to 4.0 accompanied by the gradual accumulation of acidic substances. Aldehydes dominated the grassy and fruity notes in the early fermentation stage; Alcohols and esters accumulated remarkably in the middle stage and formed the fundamental fruity aroma. In the late stage, esters, carboxylic acids and sulfur-containing compounds accumulated synchronously and abundantly, and the superposition of multi-component aroma maximized the flavor complexity of sufu at the end of fermentation. Electronic nose analysis showed that the response value of sensor R8, sensitive to alcohols/aldehydes-ketones, increased from 40 to 120 throughout the 0~15 d fermentation, while the response value of sensor R7 for inorganic sulfides reached 120 at 15 d, indicating an obvious enhancement of sulfur-containing flavor signals in the system. PCA results revealed that PC1 accounted for 98.98% of the total variance, which effectively distinguished samples from the early and middle fermentation stages, and the flavor characteristics tended to be stable in the later fermentation period. Sensors W2W (loading>0.45) and W5C (loading>0.40) were the key differential sensors that reflected the variation characteristics of volatile flavor substances of sufu at different fermentation stages. Conclusion: This study quantifies the staged variation rule of sufu flavor and clarifies the formation pattern of its characteristic flavor, which provides data support for process optimization of sufu production.