Effects of temperature on the changes of enzymatic activities and metabolite during wheat nuruk fermentation

Nuruk is a fermentation agent, which has been used for the production of traditional Korean alcoholic beverages. The objective of this study was to investigate the effects of temperature on nuruk fermentation. One wheat nuruk sample was fermented at 36°C for 30 days (TN-A) and another at 45°C for 10 days followed by 36°C for 20 days (TN-B). The activities of a-amylase, glucoamylase, and acidic protease, as well as metabolite contents were measured. Initially, the enzymatic activities increased rapidly regardless of the fermentation temperature. After 3 days of fermentation, the enzymatic activities were maintained in TN-A, but gradually decreased in TN-B until the end of fermentation process. Metabolite analysis using ¹H-NMR showed that the levels of glucose, glycerol, fructose, mannitol, and lactose initially increased quickly and then decreased in TN-A. However, they initially decreased and then were maintained over the fermentation period in TN-B. The contents of glycine, proline, and serine were higher in TN-A than in TN-B. This study suggests that a constant temperature of approximately 36°C is appropriate for achieving high amylolytic and proteolytic activities in the production of wheat nuruk.