Mild heat treatment of lettuce enhances growth of Listeria monocytogenes during subsequent storage at 5oC or 15oC

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Abstract
Aims: The objective of this study was to determine the influence of mild heat treatment, storage temperature and storage time on the survival and growth of Listeria monocytogenes inoculated onto cut iceberg lettuce leaves. Methods and Results: Before or after inoculation with L. monocytogenes, cut iceberg lettuce leaves were dipped in water (20 or 50°C), containing or not 20 mg l–1 chlorine, for 90 s, then stored at 5°C for up to 18 days or 15°C for up to 7 days. The presence of 20 mg l–1 chlorine in the treatment water did not significantly (α=0·05) affect populations of the pathogen, regardless of other test parameters. The population of L. monocytogenes on lettuce treated at 50°C steadily increased throughout storage at 5°C for up to 18 days. At day 10 and thereafter, populations were 1·7–2·3 log10 cfu g–1 higher on lettuce treated at 50°C after inoculation compared with untreated lettuce or lettuce treated at 20°C, regardless of chlorine treatment. The population of L. monocytogenes increased rapidly on lettuce stored at 15°C. At 2 and 4 days, significantly higher populations were detected on lettuce that had been treated at 50°C, compared with respective samples that had been treated at 20°C, regardless of inoculation before or after treatment, or the presence of 20 mg l–1 chlorine in the treatment water. Conclusions: The results clearly demonstrated that mild heat treatment of cut lettuce leaves enhances the growth of L. monocytogenes during subsequent storage at 5 or 15°C. Significance and Impact of the Study: Mild heat treatment of cut lettuce may result in a prolonged shelf life as a result of delaying the development of brown discoloration. However, heat treatment also facilitates the growth of L. monocytogenes during storage at refrigeration temperature, thereby increasing the potential risk of causing listeriosis.