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 _w ) of oil decreases exponentially with increasing temperature. Therefore, in thermal processing, the a _w of the bacteria inside oil may also decrease making the bacteria more resistant to heat. To validate this hypothesis, the equilibrium a _w of bacteria (Enterococcus faecium NRRL B2354, or E. faecium) in peanut oil samples, with different initial a _w (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 _w 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 ₈₀ value (time needed to achieve 1-log reduction at 80 °C) increased exponentially with the reduced a _w at 80 °C, from 87 min at a _w 0.36 to 1539 min at a _w 0.13. A graphical comparison (logD ₈₀ vs. high-temperature a _w ) 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.
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