Thermal inactivation of Listeria monocytogenes studied by differential scanning calorimetry

Abstract

The effect of NaCl on the thermal inactivation of Listeriu monocytogenes has been investigated by conventional microbiological techniques and by using differential scanning calorimetry (DSC). Addition of 1.5 M-NaCI to cells grown at lower NaCl concentrations significantly increases the tolerance of cells to mild heat stress (56-62 "C). DSC thermograms show five main peaks which are shifted to higher temperatures in the presence of 1.5 M-Naa. Measurement of loss of viability in the calorimeter gave good correlation between cell death and the first major thermogram peak at two NaCI concentrations. The time course of the loss of this first peak when cells were heated and held at 60 "C in the calorimeter matched the loss of viability, whereas the peak attributable to DNA showed little change during this process. The use of DSC to investigate the mechanisms involved in thermal inactivation is discussed.

Evaluation of thermotolerant capacity of lactic acid bacteria isolated from commercial sausages and the effects of their addition on the quality of cooked sausages

“…This adaptation would be reflected in the survival in the meat batter conditions after the cooking step. In the same manner, ∆H d values could explain the thermal inactivation of these strains, because the peak in the thermogram was probably due to the contribution of other macromolecules, such as DNA and RNA, besides the contribution of several cell constituent proteins (ANDERSON et al, 1991).…”

“…Besides, differential scanning calorimetry allows whole cells to be studied whilst applying a heat stress and the peaks in the region of the thermogram at which loss of viability was observed are irreversible and most probably due to protein unfolding and denaturation. Contributions from other macromolecules cannot be ruled out; the irreversible component of DNA and RNA melting has been considered to be a minor fraction of total enthalpy (ANDERSON et al, 1991). The main differences between the water bath test and differential scanning calorimetry were the maximum temperature reached and the heating speed.…”

“…Thermotolerant capacity of each of the 29 isolated lactic acid bacteria strains was determined in water baths at different temperatures, adapting the methodology reported by Petäjä (1992) and Anderson et al (1991). Each strain was subcultured in 10 mL of MRS broth and incubated at 35 °C during 24 ± 2 hours, until reaching an optical density (λ = 650 nm) of approximately one.…”

Heat killing of bacterial spores analyzed by differential scanning calorimetry

“…The To of vegetative cells (45°C) and the To of germinated spores (47°C) were both higher than the maximum growth temperature (44°C). Transition e was apparently associated with heat killing of the vegetative cells and germinated spores because of its similarity to a major DSC transition that was observed and directly correlated with measurement of cell death in another bacterium (2). Thermal inactivation of the vegetative cells and germinated spores during transition e was also indicated by its irreversibility on the DSC rescans (Fig.…”

“…Differential scanning calorimetry (DSC) is a thermal-analysis technique that has become a powerful tool for detecting and quantifying thermotropic transitions of various materials (10,13). The DSC method is usually used to characterize isolated biopolymers and macromolecular assemblies but has also been applied successfully to characterize more complex systems such as mammalian cells (24,25,27), bacterial vegetative cells (2,9,26,30,31,37,46), and bacterial spores (32)(33)(34)37). DSC can be used to determine the enthalpy of a thermally induced transition by measuring the differential heat flow required to maintain a sample and an inert reference at the same temperature.…”