Effects of indigenous yeasts on physicochemical and microbial properties of Korean soy sauce prepared by low-salt fermentation

This study deals with understanding the effects of salt reduction on both the physicochemical and microbiological properties of soy sauce fermentation and also the application of indigenous yeast starters to compensate for undesirable changes occurring in salt-reduced processes. Fermentation was tested in situ at a Korean commercial soy sauce processing unit. Salt reduction resulted in higher acidity as well as lower pH and contents of residual sugar and ethanol. Moreover, undesired flavor characteristics, due to a lack of distinctive compounds, was observed. In addition, putrefactive Staphylococcus and Enterococcus spp. were present only during salt-reduced fermentation. To control these adverse effects, a single or mixed culture of two indigenous yeasts, Torulaspora delbrueckii and Pichia guilliermondii, producing high ethanol and 3-methyl-1-butanol, respectively, were tested. Overall, all types of yeast applications inhibited undesirable bacterial growth despite salt reduction. Of the starter cultures tested, the mixed culture resulted in a balance of more complex and richer flavors with an identical flavor profile pattern to that obtained from high salt soy sauce. Hence, this strategy using functional yeast cultures offers a technological option to manufacture salt-reduced soy sauce while preserving its typical sensory characteristics without affecting safety.