Desiccation in oil protects bacteria in thermal processing.

Edible oils have long been considered to have a protective effect on bacteria from thermal inactivation, but the mechanism for this effect remains unclear. Our recent study suggests that the water activity (a) of oil decreases exponentially with increasing temperature. Therefore, in thermal processing, the a of the bacteria inside oil may also decrease making the bacteria more resistant to heat. To validate this hypothesis, the equilibrium a of bacteria (Enterococcus faecium NRRL B2354, or E. faecium) in peanut oil samples, with different initial a (0.93, 0.75, 0.52 & 0.33) at room temperature, were measured at elevated temperatures up to 80 °C. Meanwhile, the thermal resistances of E. faecium in these samples were also tested at 80 °C. Results indicate that the a of the bacteria-in-oil systems changed in the same manner as that of pure peanut oil; it decreased exponentially with temperature from 0.93, 0.75, 0.52 & 0.33 (at ~23 °C) to 0.36, 0.30, 0.21 & 0.13 (at 80 °C), respectively. This confirmed that bacterial cells experienced desiccation in oil during the thermal treatments. The thermal death rates of E. faecium in peanut oil samples followed first-order kinetics. The D value (time needed to achieve 1-log reduction at 80 °C) increased exponentially with the reduced a at 80 °C, from 87 min at a 0.36 to 1539 min at a 0.13. A graphical comparison (logD vs. high-temperature a) showed a similarity between the thermal resistance of E. faecium in oil and that in dry air, which supports the hypothesis that oil protects bacteria from thermal treatments through desiccation.