D Values of Bacillus stearothermophilus Spores as a Function of pH and Recovery Medium Acidulant

The effect of the pH and the type of acidulant (citric acid or glucono-δ-lactone) of the recovery medium on the thermal resistance of Bacillus stearothermophilus ATCC 12980 was studied. The spores were heated in bidistilled water as a reference substrate and in acidified mushroom extract using citric acid or glucono-δ-lactone as acidulants (pH 6.2) and subcultured in reference (pH 7) and acidified (pH 6.2) media. A period of treatment-dependent pH sensitization was observed in heat-treated spores. In all cases, D values were lower in the acidified recovery medium than those obtained in the reference medium, but the type of acidulant used in the recovery medium had no influence on the D values. No influence on z values was observed as a consequence of the different recovery media, but they changed within a range of 7 to 10°C as a function of the different heating substrates. Glucono-δ-lactone proved to be as effective as citric acid in controlling the microbiological spoilage of foods. The pH has a great influence on decreasing the D values, mainly when acidification of the substrate and acidification of the recovery medium are combined. Consequently, it would be appropriate to take this effect into account in process calculations or validation.