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 -delta-lactone) of the recovery medium on the thermal resistance of Ba cillus stearothermophilus ATCC 12980 was studied. The spores were heat ed in bidistilled water as a reference substrate and in acidified mush room extract using citric acid or glucono-delta-lactone as acidulants (pH 6.2) and subcultured in reference (pH 7) and acidified (pH 6.2) me dia. A period of treatment-dependent pH sensitization was observed in heat-treated spores. In all cases, D values were lower in the acidifie d 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 t he different recovery media, but they changed within a range of 7 to 1 0 degrees C as a function of the different heating substrates. Glucono -delta-lactone proved to be as effective as citric acid in controlling the microbiological spoilage of foods. The pH has a great influence o n decreasing the D values, mainly when acidification of the substrate and acidification of the recovery medium are combined. Consequently, i t would be appropriate to take this effect into account in process cal culations or validation.