Performance of a Growth–No Growth Model for Listeria monocytogenes Developed for Mayonnaise-Based Salads: Influence of Strain Variability, Food Matrix, Inoculation Level, and Presence of Sorbic and Benzoic Acid
A previously developed growth–no growth model for Listeria monocytogenes, based on nutrient broth data and describing the influence of water activity (aw), pH, and acetic acid concentrations, was validated (i) for a variety of L. monocytogenes strains
and (ii) in a laboratory-made, mayonnaise-based surimi salad (as an example of a mayonnaise-based salad). In these challenge tests, the influence of the inoculation level was tested as well. Also, the influence of chemical preservatives on the growth probability of L. monocytogenes
in mayonnaise-based salads was determined. To evaluate the growth–no growth model performance on the validation data, four quantitative criteria are determined: concordance index, % correct predictions, % fail-dangerous, and % fail-safe. First, the growth probability of 11 L. monocytogenes
strains, not used for model development, was assessed in nutrient broth under conditions within the interpolation region. Experimental results were compared with model predictions. Second, the growth–no growth model was assessed in a laboratory-made, sterile, mayonnaise-based surimi
salad to identify a possible model completeness error related to the food matrix, making use of the above-mentioned validation criteria. Finally, the effect on L. monocytogenes of common chemical preservatives (sorbic and benzoic acid) at different concentrations under conditions typical
of mayonnaise-based salads was determined. The study showed that the growth–no growth zone was properly predicted and consistent for all L. monocytogenes strains. A larger prediction error was observed under conditions within the transition zone between growth–no growth.
However, in all cases, the classification between no growth (P = 0) and any growth (P > 0) occurred properly, which is most important for the food industry, where outgrowth needs to be prevented in all instances. The results in the sterile mayonnaise-based salad showed again
that the growth–no growth zone was well predicted but that also, in real food systems, a transition zone between growth and no growth exists. This became even more obvious for lower inoculation levels. The maximum-allowed concentration of benzoic and sorbic acid in mayonnaise-based salads,
according to the European Union legislation, eliminated the growth of L. monocytogenes. Concentrations of 600 and 300 ppm were already sufficient to inhibit growth at 7 and 4°C, respectively, under conditions associated with mayonnaise-based salads (pH 5.6; aw, 0.985).
Document Type: Research Article
LFMFP, Laboratory of Food Microbiology and Food Preservation, Department of Food Safety and Food Quality, Ghent University, Coupure Links 653-9000 Ghent, Belgium
BioTeC, Chemical and Biochemical Process Technology and Control Section, Department of Chemical Engineering, Katholieke Universiteit Leuven, W. de Croylaan 46-3001 Leuven, Belgium
MeBioS, Division of Mechatronics, Biostatistics and Sensors, Department of Biosystems (BIOSYST), Katholieke Universiteit Leuven, W. de Croylaan 42-3001 Leuven, Belgium
Publication date: September 1, 2007
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