Microbial ecology of Australian commercial rice koji and soybean miso

Background

Rice koji and miso are growing in popularity globally. This is because of both the unique sensory properties of the foods, and the belief that fermented foods may be beneficial for our health. This study aimed to increase the collective understanding of the physiochemical properties and microorganisms found in rice koji and miso.

Results

The microbial communities within rice koji and soybean miso were analyzed using physiochemical, microbiological and metagenomic methods. Metagenomic analysis found Bacillus and Weizmannia were the dominant bacterial genera in rice koji samples, and Tetragenococcus, Staphylococcus and Bacillus predominant in the miso samples. Aspergillus was the predominant fungal genera in all three rice koji samples, and in five of the seven miso samples. Debaryomyces was the dominant genera in one miso sample, while no fungal microorganisms were identified in the other miso sample. The physiochemical analysis found rice koji water activity (A_w) between 0.90 and 0.91 and pH from 5.3 to 5.6. The miso samples showed A_w ranging from 0.73 to 0.88, pH between 4.2 and 5.3, and salt concentration from 6.4% to 15.5%.

Conclusion

The predominant genera detected and physiochemical properties in each sample varied according to supplier, length of fermentation and ratio of ingredients used. Details of the microorganisms identified in varieties of unpasteurised rice koji and miso can be used to assist producers with food safety processes, manipulate the flavor of the product and enable accurate labeling of the foods.