The formation of spores by vegetative cells of natto bacilli, as occurs in the production of natto starter cultures, was investigated by subjecting the natto bacilli to environmental stress. This is an essential component of industrial natto production. We collected natto starter suspensions from sixteen natto factories and measured their spore ratios (%) according to the following equation : (spore/(spore+vegetative cell) ×100). The mean spore ratio was 94.0%, apart from two samples whose spore ratios were particularly low. The highest spore ratio, 97.3%, was achieved when stationary cells of Bacillus subtilis (natto) KFP 2, a commercial isolate, were cultured in NBP (NaCl-beef extract-phytone peptone) agar containing 0.1 mmol/l Mn²⁺, refrigerated, mixed with sea sand and dried under vacuum. We also induced sporulation and calculated spore ratios in KFP 1 strains isolated from commercial natto starter, and KFP 419, KFP 827 and KFP 828 strains from a culture collection held by our laboratory. Although KFP 4, one of the commercial natto strains, showed spore ratios as low as 71.4%, the ratios in the other strains exceeded 90%. Using spores prepared from KFP 2, we produced natto in a scaled-down version of the natto production process and used organoleptic testing to compare our product with natto made using commercial natto starter. Compared with natto made from commercial starter, the scores of our natto were higher for stickiness, the same for appearance and odor, and slightly lower for toughness and umami. Our new natto starter may be strongly recommended for use in commercial natto production.

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Microbial control is required for both quality maintenance and safety in foods. We examined the effectiveness of superheated steam (SHS) for sterilization of pathogenic and spoilage-causing bacteria in Chinese cabbage to be used for pickling (Asazuke). Raw Chinese cabbage harbored more than 5log CFU/g of viable cells, with the outer leaves exhibiting higher viable cell counts than the inner leaves. Most bacteria were found on the epidermal (surface) tissue and were not present in the inner tissue, which suggests that surface sterilization should be effective for microbial control in Chinese cabbage. Compared with the usual method of washing with sodium hypochlorite (NaClO), treatment with SHS was found to significantly reduce the population of microbes on Chinese cabbage without affecting texture or color tone. Populations of Gram-positive bacteria, Gram-negative bacteria, and nitrate-forming bacteria were found to decrease to less than 2.5log CFU/g. The effectiveness of SHS treatment in the sterilization of pathogenic bacteria was also evaluated. Treatment with SHS was found to reduce the populations of Escherichia coli O157 : H7 and Staphylococcus aureus in Chinese cabbage to below the detection limit. These results suggest that SHS treatment may be useful for microbial control when processing vegetables during pickle production.

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The effect of fermenting brown rice with lactic acid bacteria was examined. When grains of brown rice (cv. Koshihikari) were soaked in water inoculated with Lactobacillus casei subsp. casei 327 for 17h at 37°C, lactic acid bacteria were found to grow in the water and on the rice. Lactic acid bacteria isolated from the fermented brown rice were identified as L. casei subsp. casei 327 by comparing the nucleotide sequences of the 16S rRNA gene. When brown rice was soaked in water that did not contain lactic acid bacteria, the number of bacteria on the rice was found to increase, and the major bacterial species was identified as Enterobacteriaceae. As the pH of the rice fermented with L. casei subsp. casei 327 was approximately 6, the taste of the cooked rice was unaltered. When other varieties of brown rice (cv. Milky queen and cv. Koshiibuki) were used for fermentation, the number of lactic acid bacteria found after soaking increased in the same way as for Koshihikari. Five species of lactic acid bacteria were subjected to the same test, and numbers of all species were found to increase in the fermented brown rice. The largest increase was seen for L. casei subsp. casei 327, and this species was also the most effective in preventing the growth of other types of bacteria.

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Tubu-natto and hikiwari-natto (which are made from whole and split soybeans, respectively) are mainly produced in Japan. The production process consists of splitting (performed only for hikiwari-natto), soaking, and boiling of soybeans, followed by fermentation with Bacillus subtilis (natto). In this study, we examined changes in the contents of various types of soy isoflavone during the production of these two kinds of natto. There was little leakage of soy isoflavones throughout the process of tubu-natto production, but during production of hikiwari-natto, about 9.8% of isoflavones were lost as soybean powder generated during splitting. During processing of both kinds of natto, malonyl isoflavones were found to decrease after the soybeans were boiled, and succinylated isoflavone glycosides increased after fermentation with B. subtilis (natto).

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We evaluated angiotensin I-converting enzyme (ACE) inhibitory activities in raw soybeans, soaked soybeans, steamed soybeans, the substance exuded from soybeans during steaming and a series of natto samples which were produced with different fermentation times. We also compared the ACE inhibitory activities of natto produced using commercial bacterial strains and natto produced using strains obtained from dried rice plants. We found that the ACE inhibitory activity of natto varied depending on fermentation time. Furthermore, it was shown that fermentation with certain dried rice plant extracts resulted in natto with higher ACE inhibitory activity than that produced using commercial bacterial strains. It is suggested that some ACE inhibitory substances in natto are derived from the steamed soybeans used during production, while others are formed from steamed soybean components during fermentation.

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[in Japanese]

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