Miso is a well‐known traditional Japanese fermented food, with a characteristic savory flavor and aroma, known predominately as the seasoning in miso soup. Miso production involves a two‐stage fermentation, where first a mold, such as Aspergillus oryzae, is inoculated onto a substrate to make koji. A subsequent fermentation, this time by bacteria and yeast, occurs when the koji is added to a salt and soybean mash, with the miso left to ferment for up to 2 years. The microbial community of miso is considered essential to the development of the unique taste, texture, and nutritional profile of miso. Despite the importance of microorganisms in the production of miso, very little research has been undertaken to characterize and describe the microbial process. In this review, we provide an overview of the two‐stage fermentation process, describe what is currently known about the microbial communities involved and consider any potential health benefits associated with the consumption of miso, along with food safety concerns. As the popularity of miso continues to expand globally and is produced under new environmental conditions, understanding the microbiological processes involved will assist to ensure that global production of miso is safe as well as delicious.

While established in Asia, rice koji and miso are fermented foods that are becoming more popular in western countries. They have been shown to contain a variety of microorganisms, consisting of bacteria, yeasts, and fungal species. Many contemporary miso varieties are not pasteurized as consumers are looking for more natural products, and/or have the desire to consume fermented foods containing live microorganisms. While correctly prepared fermented foods are rarely associated with food safety outbreaks, incidences have been recorded. On these occasions, pathogenic, or spoilage microorganisms were introduced into the products from external sources such as the raw material or the processing environment. Consequently, hygiene and fermentation conditions need to be carefully monitored to ensure food safety. Furthermore, many of the production steps during koji and miso manufacture do not fit into contemporary food safety guidelines for foods. Although pH is a required food safety hurdle for fermented foods, this does not apply to nonacidic foods such as koji or miso. This review focuses on control of microbial pathogens and discusses the processes of miso fermentation, and how fermentation of rice koji and miso fits with current food safety hurdles in western countries.

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.