ACTIVATION OF Bacillus stearothermophilus SPORES AT LOW pH

Activation and decrease in heat resistance of B. stearothermophilus E‐2 spores were studied after mild heat treatments at low pH. Activated spores were measured by colony formation on Nutrient Agar + 0.03% MnSO 4 . Under the conditions used in this investigation activation of spores paralleled transformation to a heat‐sensitive form (D 86 = 7 min at pH 7.0). Activation was accelerated at pH 1.1–2.0 and 60–70°C. Prolonged low pH‐mild heat treatments resulted in decreased counts due to the inability of some spores to germinate. B. stearothermophilus spores present in selected food ingredients were activated in a similar manner. Under certain conditions, a low pH‐mild heat treatment may be used as a substitute for a severe heat‐shock to activate spores of B. stearothermophilus.