Yang Ren, Xu Jie, Lombardo Stephen P, Ganjyal Girish M, Tang Juming
Department of Biological Systems Engineering, Washington State University, P.O. Box 646120, Pullman, WA 99164-6120, USA.
Technical Innovation Center, McCormick & Company, Inc., Hunt Valley, MD 21031, USA.
Food Res Int. 2020 Nov;137:109519. doi: 10.1016/j.foodres.2020.109519. Epub 2020 Jul 10.
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.
长期以来，人们一直认为食用油对细菌具有抗热失活的保护作用，但其作用机制尚不清楚。我们最近的研究表明，油的水分活度（a）随温度升高呈指数下降。因此，在热处理过程中，油中细菌的水分活度也可能降低，从而使细菌更耐热。为了验证这一假设，我们测量了室温下初始水分活度不同（0.93、0.75、0.52和0.33）的花生油样品中细菌（粪肠球菌NRRL B2354，即粪肠球菌）在高达80°C的高温下的平衡水分活度。同时，还在80°C下测试了这些样品中粪肠球菌的耐热性。结果表明，油中细菌体系的水分活度与纯花生油的变化方式相同；它随温度呈指数下降，从室温下的0.93、0.75、0.52和0.33分别降至80°C时的0.36、0.30、0.21和0.13。这证实了热处理过程中细菌细胞在油中经历了干燥。花生油样品中粪肠球菌的热死亡率符合一级动力学。D值（在80°C下实现1个对数减少所需的时间）随80°C下水分活度的降低呈指数增加，从水分活度为0.36时的87分钟增加到水分活度为0.13时的1539分钟。图形比较（logD与高温下的水分活度）显示，油中粪肠球菌的耐热性与干燥空气中的耐热性相似，这支持了油通过干燥保护细菌免受热处理影响的假设。
