The optimal levels of LD type and nisin-produucing lactic starters for cheese curd making were determined in order to evaluate the degree to which nisin-producing lactic acid bacteria prevents butyric acid fermentation in Gouda cheese.
　Cheese curd making tests were repeated on a small scale with various contents of LD type and nisin-producing lactic starters, and the number of the latter bacteria and the production of nisin in cheese curd were determined. Nisin concentration in cheese curd was proportional to the number of nisin-producing lactic acid bacteria. To obtain nisin at concentrations above 100 IU/g after salting, at least 5.0×10⁸ cfu/g nisin-producing lactic acid bacteria must be present in the curd.
　When the ratio of nisin-producing starter to LD type starter was increased, nisin, which was transported via the starter culture and further produced by the nisin-producing starter bacteria, was found to suppress the growth of LD type lactic acid bacteria, and particularly that of nisin-sensitive Lactococcus lactis subsp. cremoris SC-3.
　On the other hand, when the ratio of nisin-producing starter to LD type starter was reduced, the growth of nisin-producing lactic acid bacteria was inhibited by LD type starter bacteria, and the reduced numbers of nisin-producing lactic acid bacteria produced smaller amounts of nisin.
　This study of cheese curd making tests showed that even with the addition of 0.5% nisin-producing starter, acid production remained almost normal during curd making.

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　The effect of nisin-producing lactic acid bacteria on late butyric acid fermentation during cheese ripening was evaluated by making Gouda cheese without the addition of nitrate but with the combination of conventional LD type and nisin-producing lactic starters.
　Control cheese, made with only the conventionail LD type starter, blew within 2 months of ripening, whereas the test cheese blew to an extent inversely proportional to its content of nisin. Butyric acid fermentation could be almost completely prevented in the presence of at least 50 IU/g of nisin.
　Regarding proteolysis in cheese repening, the maturity and the non-protein nitrogen ratio of cheese were sometimes higher in the test cheeses than in the control, and no apparent delay in ripening was observed when nisin-producing starter was used. The development of unwanted (bitter and unclean) flavors occurred for some of the test cheeses, although flavor was assessed as being rather good for others. The use of nisin-producing starter did not appear to directly affect cheese flavor.
　These findings suggest that the mixture of 1% conventional LD type starter and 0.5% to 1% nisin-producing starter, which is prepared by incubating a selected species of lactic acid bacteria in skim milk medium at 30℃ for 24 hrs, should be able, in the absence of nitrate, to prevent butyric acid fermentation in the late stage of cheese ripening without affecting the quality of cheese.

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