Process Analysis of Millet Bran Fermentation by Bacillus natto: Dynamic Changes in Enzyme Activities and Bioactive Components

To investigate the dynamic changes during millet bran fermentation by Bacillus natto, we systematically monitored microbial growth, key enzyme activities, and the contents of major bioactive components over time. The changes in viable bacterial count, spore count, and the activities of protease, amylase, cellulase, and nattokinase (NK) fibrinolytic activity were measured throughout the 0–84 h fermentation process. Concurrently, variations in the contents of total sugars, reducing sugars, soluble dietary fiber (SDF), β-glucan, arabinoxylan, peptides, and polyphenols were analyzed. The results indicated that the viable bacterial count in the fermentation broth peaked at 48 h (9.3 log CFU/mL) and subsequently declined, while the spore count significantly increased to 7.6 log CFU/mL by 84 h. The activities of protease, amylase, cellulase, and NK fibrinolytic activity all exhibited a trend of initial increase followed by a decrease, reaching their respective maximum levels at 48 h. The contents of SDF, peptides, and polyphenols attained their highest values at 60 h, corresponding to 2.4 times, 2.17 times, and 1.5 times those of the unfermented control, respectively. The β-glucan content peaked at 24 h (31.31 mg/g millet bran), whereas the arabinoxylan content reached its maximum at 60 h, which was 19.4 times higher than that of the unfermented sample. Based on a comprehensive evaluation of all indicators, 48–60 h was determined to be the optimal fermentation duration for millet bran using B. natto. This research elucidates the relationship between enzyme activities and the accumulation of active components during fermentation, providing a theoretical foundation for the high-value utilization of millet bran and the development of functional products.