Assessing Listeria monocytogenes growth during Spanish-style green table olive fermentation

Abstract

Table olives are one of the most important fermented vegetables in the Mediterranean countries. Although this fermented vegetable has a long record of microbial safety, various studies have shown the presence of Listeria monocytogenes during table olive processing. This study focused on evaluating the behaviour of a cocktail of L. monocytogenes strains during fermentation of Spanish-style green Manzanilla olives. For this purpose, a challenge test consisting of artificial inoculation of olive fermentation with high levels of the food-borne pathogen (7.0 log10 CFU/ml) was performed. The fermentation was carried out at the laboratory scale in sealed glass bottles (n = 168) under controlled conditions to avoid the initiation of lactic acid fermentation intentionally. This point was confirmed by the evolution of pH, which ranged from an initial 9.0 to final 6.0 units (decrease kinetic constant of −0.0016 h−1), as well as by the behaviour of lactic acid and mesophilic bacteria during the study (55 days). The fate of L. monocytogenes during fermentation was described using mathematical models, including the two-term Gompertz equation. During this time, the food-borne pathogen showed an initial growth phase which lasted until 250 h, reaching a maximum population level close to 9.0 log10 CFU/ml, even in the presence of high salt concentrations (>6.0% NaCl). Then, a rapid decline phase was observed but continuously detecting a residual tail of around 2 log10 CFU/ml. Data obtained in this work shows the importance of the control of olive fermentation, especially during the first weeks when pH is alkaline-neutral and environmental conditions could favour L. monocytogenes growth.