Heat has been used extensively in the food industry as a preservation method, especially due to its ability to inactivate microorganisms present in foods. However, many aspects regarding the mechanisms of bacterial inactivation by heat and the factors affecting this process are still not fully understood. The purpose of this review is to offer a general overview of the most important aspects of the physiology of the inactivation or survival of microorganisms, particularly vegetative bacteria, submitted to heat treatments. This could help improve the design of current heat processes methods in order to apply milder and/or more effective treatments that could fulfill consumer requirements for fresh-like foods while maintaining the advantages of traditional heat treatments.

The potential of the application of pulsed electric fields (PEF) to induce accelerate autolysis of a commercial strain of Saccharomyces cerevisiae for winemaking use was evaluated. The influence of PEF treatments of different intensity (5–25 kV/cm for 30–240 μs) on cell viability, cytoplasmic membrane permeabilization and release of mannoproteins and compounds absorbing at 260 and 280 nm has been investigated. After 8 days of incubation at 25°C the Abs600 of the suspension containing the control cells was kept constant while the Abs600 of the suspension containing the cells treated by PEF decreased. The measurement of the absorbance at 260 and 280 nm revealed no release of UV absorbing material from untreated cells after 8 days of incubation but the amount of UV absorbing material released drastically increased in the samples that contained cells treated by PEF after the same storage period. After 18 days of storage the amount of mannoproteins released from the untreated cell was negligible. Conversely, mannoprotein concentration increased linearly for the samples containing cells of S. cerevisiae treated by PEF. After 18 days of incubation the concentration of mannoproteins in the supernatant increased 4.2 times for the samples containing cells treated by PEF at 15 and 25 kV/cm for 45 and 150 μs. Results obtained in this study indicates that PEF could be used in winemaking to accelerate the sur lie aging or to obtain mannoproteins from yeast cultures.

Graphs for survival under high hydrostatic pressure (450 MPa; 25°C; citrate-phosphate buffer, pH 7.0) of stationary-growth-phase cells of eight Staphylococcus aureus strains were found to be nonlinear. The strains could be classified into two groups on the basis of the shoulder length. Some of them showed long shoulders of up to 20 min at 450 MPa, while others had shoulders of <3.5 min. All strains showed tails. No significant differences in the inactivation rate were found during the log-linear death phase among the eight strains. The entry into stationary growth phase resulted both in an increase in shoulder length and in a decrease in the inactivation rate. However, whereas shoulder length proved to depend on sigma B factor activity, the inactivation rate did not. Recovery in anaerobiosis decreased the inactivation rate but did not affect the shoulder length. Addition of the minimum noninhibitory concentration of sodium chloride to the recovery medium resulted in a decrease in shoulder length and in an increase in the inactivation rate for stationary-growthphase cells. In the tail region, up to 90% of the population remained sensitive to sodium chloride.The development of nonthermal methods for food preservation has been a matter of extensive study in the last 25 years. High hydrostatic pressure (HHP) is one of these proposed alternative processes that are being used commercially for the nonthermal pasteurization of different food products (17,22). This technology consists of the application of pressures in the range of from 100 to 1,000 MPa in order to inactivate pathogenic and spoilage microorganisms without affecting the quality of foods (40).A prerequisite for the definitive implementation of a new technology is reasonably detailed knowledge of its inactivation kinetics, which will allow calculation and adjustment of the intensity of the treatments (process criterion) and a certain number of log 10 cycles of bacterial inactivation to be secured (performance criterion). In addition, to define the performance criteria, it is also necessary to determine the microbial species that represents the main risk in a particular food because of its frequent appearance, its concentration, its low infectious dose, and/or its high level of resistance.The data accumulated over the last 25 years indicate that microbial inactivation by HHP typically does not follow exponential kinetics. Most of the published survival graphs show pronounced tails (14,23,28,35,41) and sometimes also shoulders (21, 25). Occasionally, both phenomena occur simultaneously, giving rise to graphs with sigmoid profiles (11,20,43). The occurrence of shoulders and tails complicates the comparison of the resistance of different bacteria and also makes it difficult to determine process criteria reliably. In order to solve both problems, diverse mathematical models, initially developed for other aims, have been applied to fit experimental survival data. The most widely used are the models based on the Weibull distribution, the log-logistic model, the mo...

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