The impact of mixed amino acids supplementation on Torulaspora delbrueckii growth and volatile compound modulation in soy whey alcohol fermentation.

Soy (tofu) whey is a liquid side stream generated from tofu production and is often discarded as waste after it is generated. Direct disposal of soy whey can result in environmental issue in the long run. Soy whey has been previously successfully fermented using different types of wine yeasts, but the yeast available nitrogen (YAN) was found to be deficient. In this study, the soy whey YAN was estimated to be approximately 45.9 mg N/L. A mixture of four amino acids (valine, leucine, isoleucine and phenylalanine) was added into soy whey at a total concentration of +40, +80, +120 and +160 mg N/L and fermented with Torulaspora delbrueckii Biodiva for a period of 10 days. Increasing amino acid supplementation did not affect the yeast cell growth, but it sped up the sugar utilization proportionally. Increasing amino acid supplementation resulted in lower organic acid production and higher glycerol production. Amino acid supplementation also enhances the production rate of higher alcohols; increasing amount of higher alcohols and their respective esters were obtained with increasing amount of amino acid supplementation. However, higher levels of amino acid supplementation (particularly at +160 mg N/L sample) resulted in higher residual nitrogen contents which may lead to microbial instability. Supplementation of 120 mg N/L of amino acids was found to be the optimum concentration to enhance the metabolism of the yeast without leaving a high residual amino acid content. Therefore, with proper control of the amino acid addition dosage, the usage of mixed amino acid supplementation may be a strategy to regulate the fermentation kinetics and volatile compound modulation in soy whey alcohol fermentation.