Exploring the feasibility of biotransforming salted soy whey into a soy sauce-like condiment using wine yeast Torulaspora delbrueckii and soy sauce yeasts Zygosaccharomyces rouxii and Candida versatilis as single starter cultures.

Salted soy whey, a liquid by-product from salted tofu processing, is a source of valuable nutrients. However, it is often under-utilized due to its high salt content. The objective of this study was to investigate the feasibility of using different species of yeast to transform salted soy whey into a soy sauce-like condiment. Three yeasts were used for salted soy whey biotransformation: Zygosaccharomyces rouxii NCYC 1682, Candida versatilis NCYC 1433, and Torulaspora delbrueckii Biodiva. This study focused on the growth of the yeasts in soy whey added with different levels of NaCl and the physicochemical changes of salted soy whey after fermentation. The soy sauce yeasts (Z. rouxii and C. versatilis) grew by approximately 2 log CFU/mL in soy whey with 2% and 10% salt while the cell count of wine yeast T. delbrueckii increased by around 1.5 log CFU/mL at 10% salt and 1.78 log CFU/mL at 2% salt after 14 days of fermentation. Candida versatilis grew better at 10% salt with less glucose consumption (consumed 68.49% at 10% versus 94.05% at 2% salt) than at 2% salt. It was also found that all three yeasts converted isoflavone glycosides (bound isoflavones) into aglycones (free isoflavones), the latter having better bioavailability. C. versatilis showed the greatest ability to transform isoflavone glycosides in salted soy whey into higher amounts of aglycones (conversion efficiency of 23.04% at 2% salt and 15.05% at 10% salt). Salted soy whey fermented with different yeasts had different volatile profiles. Soy sauce yeasts produced more isobutyl alcohol, isoamyl alcohol and volatile phenols while T. delbrueckii generated a substantial amount of ethanol and esters. This study revealed the growth and flavour modulating potential of yeasts in salted soy whey fermentation, which provides a possible avenue to develop a soy sauce-like condiment using salted soy whey as a substrate.