Synergistic combination of essential oils from Morocco and physical treatments for microbial inactivation

This work evaluates the bactericidal effect of essential oils (EOs) of Cyperus longus L, Eucalyptus globulus L, Juniperus phoenicea L, Mentha pulegium L, Rosmarinus officinalis L., and Thymus algeriensis L. from Morocco in combination with mild heat or pulsed electric fields (PEF) against Escherichia colt O157:H7 and Listeria monocytogenes EGD-e suspended in buffer of both pH 7.0 and 4.0, and apple juice. Heat and PEF treatments affected cell envelopes, causing sublethal injuries within survivors as a function of the treatment conditions. Outstanding synergistic lethal effects were shown using mild heat (54 degrees C, 10 min) or PEF (30 kV/cm, 25 pulses) combined with 0.2 mu l/ml of most EOs tested. The combinations of heat and M. pulegium L or T. algeriensis L. EOs were particularly effective: the time to inactivate 5 log(10) cycles of E. coli O157:H7 cells suspended in apple juice was decreased by 3.5 and 5.7 times, respectively, within the range of temperature assayed (54-60 degrees C). The reduction of the contamination of E. coli O157:H7 in apple juice to 2 x 10(4) cells/ml allowed us to diminish the addition of M. pulegium L or T. algeriensis L EOs from 0.2 to 0.083 and 0.061 mu l/ml, respectively, while keeping constant the synergistic effect. Industrial relevance: The bactericidal effect of EOs improves when combined with mild heat or PEF, especially when pH was acid. Thus, very low doses of EOs need to be proposed. The valuable synergistic effects observed between heat or PEF and a wide variety of EOs offer great potential to improve both traditional heat treatments by reducing treatment intensity and, thus undesirable effects on food quality, and novel PEF treatments by achieving a higher degree of microbial inactivation. (C) 2012 Elsevier Ltd. All rights reserved.