Oil protects bacteria from humid heat in thermal processing

High-fat low-moisture foods are often implicated in outbreaks of Salmonellosis. Research has suggested that the fat content in peanut butter may play an important role in protecting Salmonella from thermal inactivation. Our recent studies indicate that the water activity (aw) of oil reduces sharply with increasing temperature, which causes desiccation to the bacteria and consequently enhances their thermal resistance. Therefore, a humid heating environment may help accelerate the thermal inactivation rate of bacteria inside oil. In this research, we selected Enterococcus faecium NRRL B-2354 (E. faecium) as a surrogate of Salmonella to study how the bacterium inside oil responds to thermal treatments under different relative humidities (RHs). Specially, we determined the decimal reduction time (D-value) of E. faecium in peanut oil which was exposed to different RHs (0–100%) at 80 °C. The data revealed that the D80 of E. faecium in 0.51 mm oil layers reduced exponentially from 6335.8 to 9.6 min with increasing RH (0–61%), then stayed constant (7.7 ± 1.5 min) at higher RH levels (61–100%). Further experiments with reduced sample thicknesses (0.26 & 0.10 mm) revealed significantly lower D80 values in thinner oil layers than in thicker oil layers at those high RH levels (61%–100%). The results of this study indicate that the water vapor equilibration was not reached in oil in short-time high-RH thermal treatments. This study suggests that oil protects bacteria from thermal inactivation not only through desiccation of bacterial cells but also by serving as a moisture barrier to hinder environmental water vapor from rehydrating the bacteria during short-time high-RH thermal processing. •High relative humidity accelerates the thermal death of bacteria inside oil layers.•A thicker oil layer protects bacteria from high relative humidities (>60%).•The slow mass diffusion of moisture in oil was the cause to the protective effect.