The thermostability parameters of three tetracycline antibiotics at high and ultrahigh temperatures (110-140 degrees C) as well as the influence of treatment medium pH and water activity on their thermotolerance have been investigated. The thermal degradation of the three antibiotics followed a first-order reaction kinetic within the 1.5-2 log(10) cycles investigated. A linear relationship was observed between the log of the DT values and the treatment temperature. The temperature dependence of the DT values was similar for the three molecules (z=28+/-2 degrees C). DT values of doxycycline were approximately 1.5 and 3 times higher than those of tetracycline and oxytetracycline, respectively. Changes in the treatment medium pH (7.0-4.0) and water activity (0.99-0.93) scarcely varied the antibiotics' thermal stability. Only when doxycycline was heat-treated at pH 4.0 did its thermal resistance increase by 3 times. The thermostability parameters obtained would allow the effect of different cooking and sterilization procedures to be estimated. Whereas low-temperature-long-time treatments (conventional sterilization) would destroy >98% of the initial concentration of the residues of the three antibiotics, high-temperature-short-time treatments (UHT) would leave unaltered residues in the 50-90% range.

The aim of this study was to find a model that accurately predicts the heat inactivation of Listeria monocytogenes (ATCC 15313) at constantly rising heating rates (0.5 to 9 degrees C/min) in media of different pH values (4.0 to 7.4). Survival curves of L. monocytogenes obtained under isothermal treatments at any temperature were nearly linear. Estimations of survival curves under nonisothermal treatments obtained from heat resistance parameters of isothermal treatments adequately fit experimental values obtained at pH 4.0. On the contrary, survivors were much higher than estimations at pH 5.5 and 7.4. The slower the heating rate and the longer the treatment time, the greater the differences between the experimental and estimated values. An equation based on the Weibullian-like distribution, log S(t) = (t/delta)p, accurately described survival curves of L. monocytogenes obtained under nonisothermal conditions within the range of heating rates investigated. A nonlinear relationship was observed between the scale parameter (delta) and the heating rate, which allowed the development of an equation capable of predicting the inactivation rate of L. monocytogenes under nonisothermal treatments at pH 5.5 and 7.4. The model predictions were a good fit to the measured data independent of the magnitude of the thermotolerance increase. This work might contribute to the increase in safety of those food products that require long heating lag phases during the pasteurization process.

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The sensitivity of pulsed electric fields (PEF)-treated E. coli O157:H7 cells to subsequent holding in apple juice has been evaluated. Escherichia coli O157:H7 cells in apple juice were resistant to PEF. A PEF treatment of 400 m s at any electrical field strength was not sufficient to inactivate one log 10 cycle of cells. However, PEF injured a large proportion of E. coli O157:H7 cells that became sensitive to a subsequent storage under refrigeration in apple juice. The total lethal effect of the combined process depended on the electrical field strength and storage time. The combination of a PEF treatment at 25 kV/cm for 400 m s and a subsequent storage of the apple juice under refrigeration for 48 h allowed five log 10 cycles of inactivation to be achieved. The combination of PEF and maintenance under refrigeration has been demonstrated to be an effective pasteurization method, by sufficiently reducing the presence of E. coli O157:H7 in apple juice in order to meet U.S. FDA recommendations.

Aims:  The aim was to assess the induced thermotolerance under nonisothermal treatments of two strains of Staphylococcus aureus in media of different pH. Methods and Results:  Staphylococcus aureus ATCC 25923 was more heat resistant than S. aureus ATCC 13565 at any pH investigated under isothermal conditions. At pH 7·4, the D58 value of the resistant strain was approx. 30 times greater. Both strains showed a higher heat resistance at pH 4·0 than at pH 7·4. In contrast, under nonisothermal treatments (0·5–2°C min−1), both strains were more heat resistant when treated at pH 7·4 than at pH 4·0 due to heat adaptation at the higher pH. At the slowest heating up rate tested at pH 7·4, the initially heat‐sensitive strain nearly reached the thermotolerance of the heat‐resistant strain. Conclusions:  The induced thermotolerance under nonisothermal treatments depended on the treatment medium pH and the microbial strain tested. The induced thermotolerance in a sensitive strain can be greater than in a heat‐resistant strain, showing similar resistance under nonisothermal conditions. Significance and Impact of the Study:  This work shows data of interest about mechanisms of microbial resistance and adaptation to heat. Moreover, it contributes to the development of more adequate combined processes for food preservation.