Starter cultures inoculation procedure changes microbial community structure during low-salt moromi fermentation

Abstract

Soy sauce is a condiment made of soybeans fermented in high salt concentration (18-22% NaCl) called moromi. Efforts to prevent sodium overconsumption has led to salt reduction in moromi. However, it could alter the microbial community dynamics during fermentation, resulting in undesirable sensory changes. This study investigates the effect of moromi starter cultures, Tetragenococcus sp. and Zygosaccharomyces rouxii, on microbial population changes during reduced-salt moromi (6% NaCl) fermentation. Both microbes were inoculated at different times, concentrations, and ratios, and the changes in total bacterial and yeast population during fermentation were monitored by culture on tryptic soy agar (TSA) and potato dextrose agar (PDA), respectively. The results showed that the inoculum size could shorten the bacterial exponential growth as it increased. Moreover, increasing the proportion of Tetragenococcus or Z. rouxii by two-fold could enhance the yeast and bacterial population, respectively by ∼3 log CFU/mL. Simultaneous inoculation suppressed bacterial growth, while sequential inoculation of Z. rouxii could maximize bacterial growth up to 7.38 log CFU/mL. However, a synergistic growth in simultaneous inoculation was observed when 5% inoculum with an equal ratio was used. This study could help researchers and manufacturers determine appropriate inoculation procedures to control fermentation better.