Carvacrol suppresses high pressure high temperature inactivation of Bacillus cereus spores

The inactivation of bacterial spores generally proceeds faster and at lower temperatures when heat treatments are conducted under high pressure, and high pressure high temperature (HPHT) processing is, therefore, receiving an increased interest from food processors. However, the mechanisms of spore inactivation by HPHT treatment are poorly understood, particularly at moderately elevated temperature. In the current work, we studied inactivation of the spores of Bacillus cereus F4430/73 by HPHT treatment for 5 min at 600 MPa in the temperature range of 50–100 °C, using temperature increments of 5 °C. Additionally, we investigated the effect of the natural antimicrobial carvacrol on spore germination and inactivation under these conditions. Spore inactivation by HPHT was less than about 1 log unit at 50 to 70 °C, but gradually increased at higher temperatures up to about 5 log units at 100 °C. DPA release and loss of spore refractility in the spore population were higher at moderate (≤ 65 °C) than at high (≥ 70 °C) treatment temperatures, and we propose that moderate conditions induced the normal physiological pathway of spore germination resulting in fully hydrated spores, while at higher temperatures this pathway was suppressed and replaced by another mechanism of pressure-induced dipicolinic acid (DPA) release that results only in partial spore rehydration, probably because spore cortex hydrolysis is inhibited. Carvacrol strongly suppressed DPA release and spore rehydration during HPHT treatment at ≤ 65 °C and also partly inhibited DPA release at ≥ 65 °C. Concomitantly, HPHT spore inactivation was reduced by carvacrol at 65–90 °C but unaffected at 95–100 °C.