| Popis: |
Differential scanning calorimetry (DSC) was used to evaluate the effects of food preservation treatments on microorganisms based on the changes in thermal stability (Tm) of the cellular components and the total apparent enthalpy. DSC analysis of Escherichia coli and Lactobacillus plantarum whole cells showed that E. coli is more heat resistant than L. plantarum. A linear relationship existed between the fractional viability calculated from calorimetric data and plate count data for both organisms. Based on this relationship, calorimetric data was used to develop a mathematical model to predict the number of survivors as a function of heat treatment under linearly increasing temperature. The inactivation parameters, D and z values, based on first order inactivation kinetic model were calculated using both calorimetric and plate count data for E. coli cells heat treated in the DSC. Evaluation of the effect of acids, ethanol or NaCl on E. coli showed that the apparent enthalpy decreased, mainly due to reduction of ribosomal subunit peak as ethanol and NaCl concentration increased and pH decreased. The lower survivability of chemically treated cells during subsequent heat treatment (at 60, 62.5 and 65 °C), indicates the conformational changes in cellular components by chemicals may have sensitized bacteria to heat. The decrease in total apparent enthalpy was also observed for E. coli cells treated at high hydrostatic pressure (HHP) above 200 MPa. The total enthalpy reduction was mainly due to the decrease in the peak area attributed to the denaturaion of ribosome. The DNA transition was also affected above 300 MPa treatments. The evaluation of the effect of HHP and nisin treatment on two Salmonella Enteritidis strains showed that while nisin alone treatment did not decrease the viability of cells, both bacterial cells were inactivated by pressure at 500 MPa. When nisin (Nisaplin) at 200 IU/ml concentration level was added to the culture broth prior to pressurization, cells from both bacterial strains were inactivated at 400 MPa. |
