Impact of Moderate Heat, Carvacrol, and Thymol Treatments on the Viability, Injury, and Stress Response of Listeria monocytogenes.

The microbial safety and stability of minimally processed foods are based on the application of combined preservative factors. Since microorganisms are able to develop adaptive networks to survive under conditions of stress, food safety may be affected, and therefore understanding of stress adaptive mechanisms plays a key role in designing safe food processing conditions. In the present study, the viability and the sublethal injury of Listeria monocytogenes exposed to moderate heat (55 °C) and/or essential oil compounds (carvacrol and thymol, 0.3 mM) treatments were studied. Synergistic effects were obtained when combining mild heat (55 °C) with one or both essential oil compounds, leading to inactivation kinetics values three to four times lower than when using heat alone. All the treatments applied caused some injury in the population. The injury levels ranged from around 20% of the surviving population under the mildest conditions to more than 99.99% under the most stringent conditions. Protein extracts of cells exposed to these treatments were analysed by two-dimensional gel electrophoresis. The results obtained revealed that stressed cells exhibited differential protein expression to control cells. The proteins upregulated under these stressing conditions were implicated, among other functions, in stress response, metabolism, and protein refolding.

Modelling the effects of temperature and osmotic shifts on the growth kinetics of Bacillus weihenstephanensis in broth and food products.

Bacillus weihenstephanensis is a psychrotolerant bacterium belonging to the Bacillus cereus group. Some strains may be cytotoxic although they have not been described as food-poisoning agents so far. The objective of this work is to model the effects of temperature and a(w) downshifts on the lag time of B. weihenstephanensis and the dependence of µ(max) on the growth conditions (temperature and a(w)). Effects of temperature downshifts were studied on 30 experimental conditions (shifts magnitude ranging from 2 to 20 °C, temperature after shift from 10 to 20 °C and a(w) ranging from 0.977 to 0.997). Osmotic shifts were studied for 13 conditions (shift magnitude ranging from 0.008 to 0.020 units of a(w), temperature from 10 to 30 °C, a(w) after shift from 0.977 to 0.997). Experimental results show that temperature downshifts were able to induce considerable lag times (up to 20 days) when occurring near the growth limits. At lower temperatures, osmotic shifts had also a significant effect. Validation experiments in food subjected to changing conditions of temperature showed that the model provided valid predictions in diluted creamed pasta but overestimated bacterial growth in carrot soup (fail safe predictions).