Effect of Acid Adaptation of Listeria monocytogenes on Its Mild Thermal Inactivation in a Simulated Fruit Juice Supplemented with Carvacrol

Abstract

Aims: The use of mild heat treatment in combination with carvacrol has been reported as a possible way to quickly inactivate L. monocytogenes in fruit juices. This study aimed at assessing the possible effect of a prior adaptation of this pathogen to fruit juice acidity on its inactivation under the effect of such combined process. Place and Duration of Study: Department of Microbiology of the University of Yaoundé I between November 2014 and April 2015. Methodology: Citric acid-adapted and non-adapted cells were first produced. Their inactivation were then followed at 60°C, with and without carvacrol supplementation (30 µL/L),  in simulated fruit juices adjusted to pH 4.5 and Brix 12, pH being adjusted in one case with hydrochloric acid, in another with citric and lastly with malic acid. Inactivation curves were fitted to the Weibull inactivation model.  Results: Acid adaptation of cells greatly increased their tolerance to the mild heat treatment. In fact, inactivation scales of acid-adapted cells were lower than those of non-adapted cells in all the tested conditions. The acidification of the model juice with citric or malic acid enhanced non-adapted cells inactivation. In contrast, with acid-adapted cells, only malic acid had a positive impact on inactivation with time.  In presence of citric acid, this inactivation was even lower than in model juice acidified with hydrochloric acid. A slight positive impact of carvacrol supplementation on non-adapted cells inactivation was noticed either comparing inactivation scale or inactivation with time in the tested conditions. This effect of carvacrol was minor with acid-adapted cells. Conclusion: Microorganism acid adaptation phenomenon should be taken into consideration while studying the antimicrobial efficiency of low thermal process treatment of fruit juices.