Recovery Of Heat-injured Cells Research Articles

Determination and validation of D-values for Listeria monocytogenes and Shiga toxin–producing Escherichia coli in cheese milk

Certain cheeses can be legally produced in the United States using raw milk, but they must be aged for at least 60 d to reduce pathogen risks. However, some varieties, even when aged for 60 d, have been shown to support growth of Listeria monocytogenes or survival of Shiga toxin-producing Escherichia coli (STEC). Thermization, as a subpasteurization heat treatment, has been proposed as a control to reduce the risk of pathogens in raw cheese milk while retaining some quality attributes in the cheese. However, the temperature and time combinations needed to enhance safety have not been well characterized. The objective of this research was to determine and validate decimal reduction values (D-values) for L. monocytogenes and STEC at thermization temperatures 65.6, 62.8, and 60.0°C; a D-value at 57.2°C was also determined for L. monocytogenes only. Nonhomogenized, pasteurized whole-milk samples (1 mL) were inoculated with 8-log cfu/mL L. monocytogenes or STEC (5- or 7-strain mixtures, respectively), vacuum-sealed in moisture-impermeable pouches, and heated via water bath submersion. Duplicate samples were removed at appropriate intervals and immediately cooled in an ice bath. Surviving bacteria were enumerated on modified Oxford or sorbitol MacConkey overlaid with tryptic soy agar to aid in the recovery of heat-injured cells. Duplicate trials were conducted, and survival data were used to calculate thermal inactivation rates. D65.6°C-, D62.8°C-, and D60.0°C-values of 17.1 and 7.2, 33.8 and 16.9, and 146.6 and 60.0 s were found for L. monocytogenes and STEC, respectively, and a D57.2°C-value of 909.1 s was determined for L. monocytogenes. Triplicate validation trials were conducted for each test temperature using 100 mL of milk inoculated with 3 to 4 log cfu/mL of each pathogen cocktail, A 3-log reduction of each pathogen was achieved faster in larger volumes than what was predicted by D-values (D-values were fail-safe). Data were additionally compared with published results from 21 scientific studies investigating L. monocytogenes and STEC in whole milk heated to thermization temperatures (55.0-71.7°C). These data can be used to give producers of artisanal raw-milk cheese flexibility in designing thermal processes to reduce L. monocytogenes and STEC populations to levels that are not infectious to consumers.

Transcriptional analysis on heat resistance and recovery from thermal damage in Salmonella under high salt condition

Sodium chloride maintains osmotic pressure of living cells including bacteria. Heat treatment is common for decontamination of bacteria in food. In this study, effects of NaCl on heat resistance of Salmonella Typhimurium were investigated. After cultivation in TSB containing 0.5% (TSB), 4% (4SC) and 8% (8SC) NaCl, S. Typhimurium cells were heated at 60 °C for 20 min. Total viable counts including intact cells and injured but recoverable cells determined by the plating method using TSA of S. Typhimurium cultured in 4SC and 8SC were significantly higher than those of the cells cultured in TSB. Meanwhile, changes of gene transcription were analyzed by DNA microarray. Transcription of genes involved in the colanic acid synthesis largely increased after cultivation in 4SC and 8SC. The amount of colanic acid significantly increased in the cells cultured in 4SC compared to that in M9-glucose medium. After recovery culture for 3 h, the genes involved in the phage shock response strongly up-regulated, suggesting contribution of these gene products in recovery of heat injured cells. The outcome of this study contributes to understand the mechanism of cross protection in Salmonella.

Heat resistance of Salmonella senftenberg 775W at various sucrose concentrations in distilled water

The heat resistance of Salmonella senftenberg 775 W, NCTC 9959, has been determined in distilled water pH 6.5 at sucrose concentrations up to 2.20 mol l−1 at temperatures between 63 and 70°C. Surviving cells were counted on minimal and enriched agar media to investigate the influence of the various nutrients on the recovery of heat injured cells. At various sucrose concentrations and temperatures multiphasic exponential parts of inactivation curves were found. Systematic differences between the recovery media depended on sucrose concentration, temperature and phase of exponential inactivation. At 60°C and sucrose concentrations between 0.52 and 1.82 mol l−1 the relationship between inactivation rate and sucrose concentration could be described by the equation ln k5=ln k0-αT [sucrose]. The activation energy of thermal inactivation reactions, substantially decreased when sucrose (1.82 mol l−1) was added to the heating menstruum. The activation energies in different recovery agars were of the same order, which suggests that the critical sites in heat inactivation are not key enzymes of the synthetic pathways of amino-acids and nucleotides. The differences between activation energies, calculated for cells of the various exponential phases of inactivation in both non-sucrose and 1.82 mol sucrose per 1 heating media, were also small, further suggesting that these critical sites are the same in cells from the various phases. Compared to published data on the heat resistance of S. senftenberg 775 W, we found a decreased resistance in a non-sucrose medium but an equal or increased resistance, depending on the phase of exponential inactivation, at a sucrose concentration of 1.82 mol l−1.