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Heat‐resistant foodborne pathogens have been a concern in low‐moisture foods and ingredients (LMFs). Due to low thermal conductivity of low moisture materials, thermal treatment is not efficient and may cause nutritional loss. This study investigated the enhancement of thermal treatment of meat and bone meal (MBM) at low water activity (aw) by inclusion of butylparaben (BP) as a model antimicrobial compound. Stationary phase Escherichia coli O157:H7 (Shiga toxin‐negative) or Salmonella enterica serotype Typhimurium was inoculated into MBM containing 0–2000 ppm BP and incubated at 55 or 60°C for up to 5 hr. A biphasic inactivation pattern was observed for both pathogens, indicating existence of potentially thermal resistant subpopulations. Addition of 1000 ppm BP to MBM (aw = 0.4) significantly lowered the D‐value at 55°C for E. coli O157:H7 (2.6 ± 0.5 hr) compared to thermal treatment alone (5.1 ± 0.6 h) during the treatment after the first 1 hr (p < 0.05), indicating that addition of BP accelerated the inactivation of thermal‐resistant subpopulation of E. coli O157:H7 in MBM. Interestingly, similar enhancement in thermal inactivation upon addition of BP was not observed in either the sensitive or resistant subpopulation of S. Typhimurium at aw of 0.4 or 0.7, which is likely caused by the higher thermal resistance developed by S. Typhimurium within a low aw environment (aw < 0.85). These results suggest that addition of certain antimicrobial compounds can improve the thermal processing efficiency in LMFs, while their efficiency against different pathogens may vary. Practical Application: Addition of appropriate food‐grade compounds may help to improve thermal treatment efficiency in low moisture foods with varied efficiency against different pathogens. This approach has the potential to reduce the required heat treatment intensity while minimizing food safety risk. [ABSTRACT FROM AUTHOR]

Our previous study found that water activity (a w)- and matrix-dependent bacterial resistance wasdeveloped in Salmonella Typhimurium during antimicrobial-assisted heat treatment in low moisture foods (LMFs) matrices. To better understand the molecular mechanism behind the observed bacterial resistance, gene expression analysis was conducted on S. Typhimurium adapted to different conditions with or without the trans -cinnamaldehyde (CA)-assisted heat treatment via quantitative polymerase chain reaction (qPCR). Expression profiles of nine stress-related genes were analyzed. The upregulation of rpoH and dnaK and downregulation of ompC were observed during bacterial adaptation in LMF matrices and the combined heat treatment, which likely contributed to the bacterial resistance during the combined treatment. Their expression profiles were partially consistent with the previously-observed effect of a w or matrix on bacterial resistance. The upregulation of rpoE, otsB, proV, and fadA was also observed during adaptation in LMF matrices and might contribute to desiccation resistance, but likely did not contribute to bacterial resistance during the combined heat treatment. The observed upregulation of fabA and downregulation of ibpA could not be directly linked to bacterial resistance to either desiccation or the combined heat treatment. The results may assist the development of more efficient processing methods against S. Typhimurium in LMFs. • Varied expressions of 9 genes were detected during adaptation and the treatment. • Expressions of rpoH, dnaK and ompC may contribute to treatment resistance. • RpoE, otsB, proV, fabA, fadA and ibpA likely not contribute to treatment resistance. [ABSTRACT FROM AUTHOR]

Increased thermal resistance of Salmonella at low water activity (a w) is a significant food safety concern in low-moisture foods (LMFs). We evaluated whether trans -cinnamaldehyde (CA, 1000 ppm) and eugenol (EG, 1000 ppm), which can accelerate thermal inactivation of Salmonella Typhimurium in water, can show similar effect in bacteria adapted to low a w in different LMF components. Although CA and EG significantly accelerated thermal inactivation (55 °C) of S. Typhimurium in whey protein (WP), corn starch (CS) and peanut oil (PO) at 0.9 a w , such effect was not observed in bacteria adapted to lower a w (0.4). The matrix effect on bacterial thermal resistance was observed at 0.9 a w , which was ranked as WP > PO > CS. The effect of heat treatment with CA or EG on bacterial metabolic activity was also partially dependent on the food matrix. Bacteria adapted to lower a w had lower membrane fluidity and unsaturated to saturated fatty acids ratio, suggesting that bacteria at low a w can change its membrane composition to increase its rigidity, thus increasing resistance against the combined treatments. This study demonstrates the effect of a w and food components on the antimicrobials-assisted heat treatment in LMF and provides an insight into the resistance mechanism. • Effect of the antimicrobial-assisted heat treatment reduces at lower water activity. • Bacterial resistance varies across different food matrices at the same water activity. • Treatment effect on bacterial metabolic activity varies across different matrices. • Changed membrane composition increases its rigidity at lower water activity. [ABSTRACT FROM AUTHOR]