Effect of l-Threonine Concentrations on Acetaldehyde Production and glyA Gene Expression in Fermented Milk by Streptococcus thermophilus

Abstract

Serine hydroxymethyltransferase encoded by the glyA gene, which has been reported to possess threonine aldolase activity, is able to catalyze the conversion of L-threonine to glycine and acetaldehyde. In this study, eight strains of Streptococcus thermophilus strains with good flavor-enhancing property were investigated to evaluate the acetaldehyde production as well as the expression of glyA gene. S. thermophilus strain MGD4–7, with high levels of acetaldehyde production, was selected for the determination of acetaldehyde and for the analysis of glyA gene using quantitative real-time polymerase chain reaction in the presence and absence of L-threonine. The results indicated that the increased acetaldehyde production was linear with increasing L-threonine levels in the fermented milk. Further more, quantitative real-time PCR analysis showed that the glyA gene was upregulated in the fermented milk in the presence of L-threonine. Understanding the degradation pathway from threonine to glycine and acetaldehyde can be applied to control and improve acetaldehyde production in fermented products with S. thermophilus as the starter culture.