Growth of nisin-producing lactococci in cooked rice supplemented with soybean extract and its application to inhibition of Bacillus subtilis in rice miso

Lactic acid fermentation of cooked rice and rice koji by supplementation with soybean extract (SBE) and its application to rice miso fermentation were investigated. By supplementing the cooked rice with SBE, lactic acid bacteria (LAB) grew well without any unfavorable effects on the rice such as off-flavor or coloration. Lactococcus lactis subsp. lactis IFO12007 (Lc. lactis, a producer of the bacteriocin nisin) proliferated at 10(8 to approximately 9) cells/g after 24 h of incubation and produced high activity of nisin. The fermented rice with Lc. lactis strongly inhibited not only Bacillus subtilis ATCC19659 but also the other Bacillus strains. While some strains of LAB markedly inhibited the growth of Asp. oryzae, resulting in failure of koji fermentation, Lc. lactis did not affect the growth of these molds. When Lc. lactis was used for rice miso fermentation as a lactic acid starter culture, Lc. lactis rapidly proliferated and produced high nisin activity of 6,400 IU/g, in the steamed rice, resulting in complete growth inhibition of B. subtilis, which had been inoculated at the beginning of the koji fermentation. The rice miso after 12 weeks of aging had a suitable pH, and favorable taste and color. Furthermore, hyposalting of rice miso could be done without difficulty by lactic acid fermentation of both rice and soybeans.