Desiccation in oil protects bacteria in thermal processing

Abstract Edible oils have long been acknowledged 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 (aw) of oil decreases exponentially with increasing temperature. Therefore, in thermal processing, the aw of the bacteria inside oil may decrease with the aw of the matrix and consequently make the bacteria more resistant to heat. To validate this hypothesis, in this study, the equilibrium aw of bacteria (Enterococcus faecium NRRL B2354, or E. faecium) in peanut oil samples, with different initial aw (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 aw of the bacteria-in-oil systems changed in like manner as 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 confirms that bacterial cells experience desiccation in oil during thermal treatments. The thermal death rates of E. faecium in peanut oil followed first-order kinetics in all cases. The D80 value (time needed to achieve 1-log reduction at 80 °C) exhibited an exponential relationship with the equilibrium aw at 80 °C; it increased sharply from 87 min at aw 0.36 to 1539 min at aw 0.13. A graphical comparison (logD80 vs. high temperature aw) 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.