Characterisation of dynamic changes in nutritional components and aroma compound formation in natto during fermentation and post-ripening

Fermentation and post-ripening plays a significant role in shaping the nutritional value, taste, and aroma of natto. This study aimed to analyse nutritional trends in natto during fermentation and ripening, identify its characteristic volatile organic compounds (VOCs), and elucidate their formation pathways. VOCs were detected using HS-GC-IMS combined with sensory evaluation in the study. Dynamic changes in key nutritional components, protein secondary structure, and texture characteristics were also examined and correlated. During the fermentation phase observed every 4 h till 20 h, peptide and amino acid content increased due to proteolysis, accompanied by a continuous decrease in hardness, springiness, and chewiness, and an increase in adhesion. During the post-ripening stage over 20 h, the overall nutrient composition and textural properties remained relatively stable, except hardness, which continued to decrease. The study identified 41 VOCs and based on the calculated relative odour activity value (ROAV), 7 compounds had ROAVs above 1 throughout the process. Moreover, compared to the fermentation stage, the concentrations of 2,3-dimethylpyrazine and 2-ethyl-3-methylpyrazine were significantly increased (p < 0.05) in the post-ripening. Furthermore, the study revealed that the distinctive aroma compounds in natto, which were significantly correlated with the contents of Phe, Tyr, Val, Ile, and Thr, included butyl acetate, 2-methoxy-3-methylpyrazine, 2-pentylfuran, ethyl propanoate, and 2,3-butanedione (p < 0.05). In conclusion, this study provides valuable insights into the formation of diverse aroma compounds in natto and lays a theoretical foundation for producing high-quality natto with enhanced nutritional value, a soft texture and a rich, mellow aroma.