Abstract
The effect of thermally processed rice koji extracts on survival of yeast Saccharomyces cerevisiae was examined in comparison with non-heated koji extract. In chronological life span (CLS) tests on high-sugar fermentation, the survivals of the yeast cells grown with heated koji extracts were higher than non-heated koji extract. Heat treatment of the extracts by autoclaving led to a loss in most of amino acids due to the Maillard reaction, although histidine contents slightly increased. In glucose-arginine mixtures, arginine was partly converted to histidine by autoclaving and the addition of histidine prolonged the CLS of yeast cells. The yeast cells grown with the non-heated extracts were more resistant to oxidative stress whereas the antioxidant activities were lower than those of the heated extracts. The yeast cells grown with the heated extracts were more tolerant to ethanol and had a higher reduction capacity in the late stationary phase when the cells were incubated in the presence of ethanol. Maillard reaction products elevated the levels of reactive oxygen species to yeast cells grown under ethanol stress in the late stationary phase. These results suggest that thermally processed koji extracts can act as a protectant against ethanol stress during the late stationary phase of yeast growth and extend the CLS due to the increase of histidine contents by autoclaving.
Highlights
Rice koji has a long history of use in the manufacturing processes of fermented foods
S. cerevisiae grown in media containing non-heated koji extract (KE0) was completely dead at 12 days, while the survivals of the yeast cells grown in media with KE15 and KE30 were 3.18log CFU/ml and 3.21log CFU/ml, respectively
The longer the heating time of koji extract was, the higher the survival of S. cerevisiae cell was. These results show that thermal processing by autoclaving of koji extracts was acknowledged to be beneficial in keeping the survival of yeast cells at the late stationary phase with high ethanol concentrations
Summary
Rice koji has a long history of use in the manufacturing processes of fermented foods. Rice koji which is exclusively cultivated on rice by yellow mold, Aspergillus oryzae, contributes to the saccharification of starch materials and alcohol fermentation simultaneously by yeast in sake mash. Rice koji has various enzymes such as α-amylase, glucoamylase, and carboxypeptidase. Following this characteristic, the materials cultivated with the addition of koji mold can be digested by these enzymes and the resultant enzymatic products become important nutrients containing sugars, amino acids, and minerals for yeasts. Heat-processed koji enhances its antioxidant activity in accordance with the Maillard reaction [3]
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