Heat Resistance of Bacillus coagulans Spores Isolated from Spoiled Canned Low-Acid Foods

Abstract

Bacillus coagulans is well known as a causative organism of flat-sour type spoilage of canned foods. The organism is generally considered to be an important target in heat processing not for low-acid foods (pH value is more than 4.6) but for acid foods (pH value is equal to or less than 4.6), because it can grow well in acidic foods and it had been isolated frequently from spoiled tomato products. Authors, however, isolated the organism from canned low-acid foods such as corn soup (pH 6.2), corned beef (pH 6.1), sukiyaki beef (pH 5.5) and formulated liquid meals for medical use (pH 6.3) representing flat-sour spoilage. One of the isolates grew well at 65°C. Three of four isolates examined produced spores having strong heat resistance. The D (decimal reduction time of survival spores) and z (the slope index of a thermal death time curve) values for the spores heated in 0.01M phosphate buffer solution (pH 7.0) were 1.4 to 1.6 minutes at 121°C and 7.2 to 7.8°C, respectively. The spores showed the similar strong heat resistance also in the food substrates. The D values for the spores heated and subcultured in food substrates were 11.2 minutes at 115°C in corn soup, 4.5 minutes at 110°C in corned beef, 6.5 minutes at 115°C in sukiyaki beef, and 11.3 minutes at 115°C in formulated liquid meals for medical use. Thus, it might be difficult to inactivate the organism, maintaining desirable food quality, solely by means of heat processing.