Spore Formation Caused by Environmental Stress in Natto Bacilli

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  • 環境ストレス下における納豆菌の胞子形成
  • カンキョウ ストレス カ ニ オケル ナットウキン ノ ホウシ ケイセイ

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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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