Increased Water Activity Reduces the Thermal Resistance of Salmonella enterica in Peanut Butter

Abstract

c Increased water activity in peanut butter significantly (P < 0.05) reduced the heat resistance of desiccation-stressed Salmonella enterica serotypes treated at 90°C. The difference in thermal resistance was less notable when strains were treated at 126°C. Using scanning electron microscopy, we observed minor morphological changes of S. enterica cells resulting from desiccation and rehydration processes in peanut oil. Salmonellosis outbreaks linked to contaminated peanut butter products have brought worldwide attention to the microbial safety of these popular food items. Salmonella enterica serotype Tennessee caused a salmonellosis outbreak in 2006 -2007 that was linked to peanut butter; it sickened 425 persons and resulted in 71 (20%) hospitalizations in 44 states in the United States (1). This and other food-borne outbreaks (2, 3) have highlighted the need for a reexamination of S. enterica behavior in low-water-activity (a w ) peanut butter products.The water activity of peanut butter is typically 0.35 or less (2, 4-9), which precludes the growth of spoilage and pathogenic microorganisms. When present in peanut butter, S. enterica becomes heat resistant, possibly due to adaptation to the desiccation stress and the protective effects of the fat content in the product (2,(4)(5)(6)(7)(10)(11)(12)(13)(14). We recently demonstrated that heat treatment at 72°C for 1 hour resulted in a less-than-2-log reduction of desiccationstressed S. enterica in artificially contaminated peanut butter with an a w of 0.4 (15). In this study, we evaluated the effects of desiccation and subsequent rehydration on the relative heat resistance of three S. enterica serotypes: S. Tennessee K4643 (a human isolate from the 2006 -2007 peanut butter outbreak in the United States) (1), S. Enteritidis BSS-1045 (an isolate from the 2000 -2001 raw almonds outbreak in the United States and Canada) (16-18), and S. Typhimurium LT2 (19,20). We compared two commercial peanut butter formulations (regular and low fat) to assess the influence of carbohydrate and fat contents on the heat resistance of S. enterica. Most published thermal challenge studies of S. enterica in peanut butter have focused on heat treatments at either 72°C or 90°C (11,21,22) and not at the higher temperatures commonly used in commercial peanut butter processing, such as dry roasting at 126°C (22). In this study, we thermally challenged S. enterica serotypes in artificially contaminated peanut butter at both 90°C and 126°C.Individual serotypes and a three-serotype cocktail were grown separately as previously described (15), followed by suspension in 5 ml of peanut oil prior to inoculation of peanut butter samples (a w , 0.2). Bacterial cell suspensions were transferred to 500 g of peanut butter and vigorously stirred for 20 min by using a sampler spatula. Homogenous distribution of the cells was verified as previously described (15). Inoculated samples were stored at 25°C for 4 weeks, then serially diluted and plated on brain heart infusion (BHI) agar for calculating bacterial d...

Inactivation of Salmonella Senftenberg, Salmonella Typhimurium and Salmonella Tennessee in peanut butter by 915 MHz microwave heating

“…In this study, we used 25 g of peanut butter which is not a suitable size for industrial processing. Other research studies which confirmed the effect of conventional heating on peanut butter pasteurization reported that less than 1 min of come-up time was required for samples to reach 90 C. But these studies used small amounts of peanut butter (Li et al, 2014;Ma et al, 2009) and flattened peanut butter samples to increase heat transfer (He et al, 2011(He et al, , 2013Li et al, 2014;Ma et al, 2009). We used a 915 MHz microwave which provided much greater penetration depth than a 2450 MHz microwave.…”

“…This is due to the increase in the consumption of peanut butter from 1999 to 2008 while peanut candy and snack consumption has not (USDA, 2010). Peanut butter has low a w (less than 0.35) which precludes the growth of foodborne pathogens (He et al, 2013). But unfortunately, multistate outbreaks caused by peanut butter have been reported.…”

“…There have been many research studies which have confirmed the effect of conventional heat treatment on inactivation of pathogens in peanut butter (He et al, 2011(He et al, , 2013Ma et al, 2009;Shachar and Yaron, 2006). However, conventional thermal treatment is not effective for reducing pathogens in peanut butter because of the long treatment time required.…”

Microbiological food safety in Malaysia from the academician’s perspective

“…A great example is the Salmonella outbreak in peanut butter. It was a shocking evidence to discover that Salmonella had higher heat tolerance (Shachar and Yaron, 2006;He et al, 2013) in low water activity and high-fat content food. 600 cases were reported in the Salmonella outbreak in peanut butter (Crum-Cianflone, 2008).…”

Thermal inactivation of Salmonella spp. in commercial tree nut and peanut butters in finished packaging

“…Thermal processing has been evaluated as an option for inactivating Salmonella spp. in peanut butters, spreads, and pastes using conventional thermal inactivation study approaches (He et al, ; He, Guo, Yang, Tortorello, & Zhang, ; Keller et al, ; Li, Huang, & Chen, ; Shachar & Yaron, ). Time and temperature were consistently verified as critical parameters; however, heat resistance of Salmonella spp.…”