Effect of rare sugars on soy sauce brewing and related microorganisms

For rare sugars to be used in fermented foods, their effect on microorganisms used in soy sauce brewing must first be determined. Here, when soy sauce-brewing microorganisms were cultured with a mixture of four rare sugars (D-allulose, D-allose, D-tagatose, and D-sorbose) with D-glucose, growth was not inhibited when the percentage of each rare sugar did not exceed 60 % of the carbon source. D-Tagatose was not inhibitory, even at 100 % of the carbon source, and it had the same effect as D-glucose on lactic acid fermentation by Tetragenococcus halophilus. To confirm soy sauce can be brewed with rare sugars, we conducted a small-scale brewing study with a rare sugar syrup (RSS). After aerobic fermentation by the yeast, the relative composition of each rare sugar remaining in the brew was 95 % allulose, 34 % allose, 68 % tagatose, and 84 % sorbose of the respective original concentration. Analyses of the final product found that acidic bitterness and acidic bitter aftertaste of the soy sauce were suppressed compared to the control, and this was attributed to a decrease in the amount of basic amino acids, which might be related to bitterness, during brewing.