Effects of pH and Agitation on the Growth of Listeria monocytogenes Scott A in Brain Heart Infusion Broth Containing Combined Potassium Lactate and Sodium Diacetate during Storage at 4 or 10°C

Abstract

The objective of this study was to compare the effects of pH on the growth kinetics of Listeria monocytogenes Scott A in static and agitated broths stored at 4 and 10°C with and without a combination of 1.85% potassium lactate (PL) and 0.13% sodium diacetate (SDA) (3.3% of a 60% commercial solution, PURASAL P Opti.Form 4). The pH of brain heart infusion broth without (control) or with 1.85% PL + 0.13% SDA was adjusted to 5.5, 6.0, 6.5, and 7.5. L. monocytogenes Scott A was inoculated (at 102 CFU/ml) into pH-adjusted broth, which was stored at 4 or 10°C with or without agitation. At pH 5.5, a listeriostatic effect was observed for the broth containing 1.85% PL + 0.13% SDA at 4 and 10°C both with and without agitation. At pH 6.0, 1.85% PL + 0.13% SDA fully controlled the growth of L. monocytogenes Scott A in static broth at 4°C for up to 20 days and significantly slowed the growth of the pathogen in agitated broth. At 10°C, the growth of L. monocytogenes Scott A was significantly reduced by 1.85% PL + 0.13% SDA in agitated and unagitated broths. At pH 6.5, 1.85% PL + 0.13% SDA significantly suppressed the growth of L. monocytogenes Scott A at both 4°C (P < 0.001) and 10°C (P < 0.01). At pH 7.5, 1.85% PL + 0.13% SDA had a limited effect on the growth of L. monocytogenes Scott A in broth stored at 4 and 10°C. At 4°C, agitation decreased the lag time and increased the growth rate of L. monocytogenes Scott A at all tested pHs. A similar but less obvious trend was observed for broths stored at 10°C. These results indicate that lactate-diacetate combinations effectively acted with low pH and temperature to inhibit the growth of L. monocytogenes Scott A.