High Heating Rates Affect Greatly the Inactivation Rate of Escherichia coli

Abstract

Heat resistance of microorganisms can be affected by different influencing factors. Although, the effect of heating rates has been scarcely explored by the scientific community, recent researches have unraveled its important effect on the thermal resistance of different species of vegetative bacteria. Typically heating rates described in the literature ranged from 1 to 20°C/min but the impact of much higher heating rates is unclear. The aim of this research was to explore the effect of different heating rates, such as those currently achieved in the heat exchangers used in the food industry, on the heat resistance of Escherichia coli. A pilot plant tubular heat exchanger and a thermoresistometer Mastia were used for this purpose. Results showed that fast heating rates had a deep impact on the thermal resistance of E. coli. Heating rates between 20 and 50°C/min were achieved in the heat exchanger, which were much slower than those around 20°C/s achieved in the thermoresistometer. In all cases, these high heating rates led to higher inactivation than expected: in the heat exchanger, for all the experiments performed, when the observed inactivation had reached about seven log cycles, the predictions estimated about 1 log cycle of inactivation; in the thermoresistometer these differences between observed and predicted values were even more than 10 times higher, from 4.07 log cycles observed to 0.34 predicted at a flow rate of 70 mL/min and a maximum heating rate of 14.7°C/s. A quantification of the impact of the heating rates on the level of inactivation achieved was established. These results point out the important effect that the heating rate has on the thermal resistance of E. coli, with high heating rates resulting in an additional sensitization to heat and therefore an effective food safety strategy in terms of food processing.

State of the art of nonthermal and thermal processing for inactivation of micro-organisms

“…; Huertas et al . ). The medium in which the organism has grown, the growth temperature, and the phase of growth or age of the culture are important factors with respect to their ability to resist heat.…”

“…Some bacteria exhibit a higher heat resistance after being exposed to temperatures which only stress them (Nicholson et al 2000;Huertas et al 2016). The medium in which the organism has grown, the growth temperature, and the phase of growth or age of the culture are important factors with respect to their ability to resist heat.…”

Limonene nanoemulsified with soya lecithin reduces the intensity of non-isothermal treatments for inactivation of Listeria monocytogenes

“…In every case, there are relatively large deviations between the model predictions based on isothermal data and the dynamic observations. This is a common observation in microbial inactivation [30][31][32][33][34][35]48 . It has been reported that L. monocytogenes can develop a physiological response to mild stresses, that increases its resistance to posterior treatments 36,37 .…”

“…It has also been reported that high heating rates can also have an impact on the microbial response of microorganisms. Several studies have shown that heating rates above 10 °C/min can reduce the resistance of microbial cells to thermal stress 35,51 . This effect results in a lower microbial count to the one predicted based on isothermal experiments.…”

“…There is empirical evidence that dynamic treatments may enable microbial cells to develop a physiological response that may increase their resistance to the thermal stress [30][31][32][33][34] . On the other hand, some research works have observed that a rapid heating of the cells may result in an increased sensitivity, reducing its resistance to posterior stresses 35 . These physiological mechanisms are not yet well understood at a molecular level and is currently an active field of research 36,37 .…”