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Sporulation and Heat Resistance of Psychrophilic Strains of Bacillus

Department of Food Science and Technology, University of California, Davis 95616

ABSTRACT

Sporulation and heat resistance were studied for nine strains of eight species of Bacillus that had been found able to grow at low temperatures, and activation of spores by heating was studied for one strain, B. cereus. Sporulation was best at the optimal growth temperature (25 to 30C) and on the surface of nutrient agar plus .05% MgSO₄ and .05% MnSO₄. D Values for spores of the nine strains deteimined in skimmilk at 90C were 4.4 to 6.6 min, and z values for four strains were 9.4 to 11.0C (16.9 to 19.8F). Vegetative cells in skimmilk were destroyed rapidly by heating at 65C. More than 99% of spores of the strain of B. cereus in skimmilk were destroyed by heating at 87.8C for 20 sec, storage at 32C for 4 hr, and reheating at 76.7C for 20 sec, whereas heating at 87.8C for 20 sec without the final heat treatment only destroyed 32%. Results indicate that extension of shelf life (about 3 days at 7.2C) provided by the two heat treatments with intervening storage is decreased slightly by addition of sucrose, corn syrup solids, carrageenan, sodium citrate, potassium dihydrogen phosphate, or sodium hexametaphosphate to skimmilk before heating.