Calorimetric and microbiological evaluation of bacteria after exposure to food preservation treatments

Thermal and non-thermal food preservation treatments affect cellular components of foodborne microorganisms that cause physiological changes in cells and eventually death of bacteria. Differential scanning calorimetry (DSC) thermograms of whole bacterial cells display thermally-induced transitions revealing the response of bacteria to heat under linearly increasing temperature condition. Therefore, DSC of the whole microbial cells can allow the detection in vivo of changes in their cellular components including ribosomes, nucleic acids, proteins and cell envelopes. The main purpose of this study was to evaluate the effects of physical and chemical treatments on microorganisms based on the changes in thermal stability (T_m) of the cellular components and the total apparent enthalpy (Δ H) from the calorimetric data. To compare with DSC data, the viability data from microbiological methods (plate counting) was also evaluated.; The viability and the change in the thermal stability of individual transitions of Escherichia coli and Lactobacillus plantarum were evaluated after pre-heating in the DSC to various temperatures. The fractional viability based on calorimetric data [(ΔH-Δ H_f)/(ΔH₀-Δ H_f)] and plate count data (N/N ₀) showed a linear relationship. Viability loss and the irreversible changes in DSC thermograms of whole cells pre-treated in DSC to various temperatures were highly correlated between 55 and 70°C. Comparison of DSC scans for isolated ribosomes showed that the thermal stability of ribosomes from E. coli is greater than the thermal stability of L. plantarum ribosomes, consistent with the greater thermal tolerance of E. coli observed from viability loss and DSC scans of whole cells.; The apparent enthalpy data obtained from DSC of E. coli cells were applied to a theoretical formalism to predict the number of surviving microorganisms as a function of linearly increasing temperature. The decimal reduction time (D) and thermal resistance constant ( z) values for E. coli determined from the calorimetric data were compared to the corresponding values from plate count data obtained after heat treatment in the DSC and after isothermal treatment to validate the new approach. The calculated D values using both apparent enthalpy and viability data for cells heat treated in the DSC were similar to the D values obtained from isothermal treatment. Temperatures for 1 through 10-log microbial population reductions, calculated from plate count and enthalpy data were in agreement within 0.5–2.4°C at a 4°C min−1 heating rate. (Abstract shortened by UMI.)...