Leafy vegetables such as spinach may be contaminated with Listeria monocytogenes during pre-harvest and postharvest management. Recent Listeriosis outbreaks associated to contaminated leafy vegetables have marked the need for technologies to minimize safety issues in fresh and fresh-cut produce. US scientists at Texas A&M University have studied the effectiveness of washing treatments as a postharvest practice to minimize the growth of the pathogen and L. innocua on fresh baby spinach leaves under different storage temperatures and to evaluate the feasibility of using L. innocua as a surrogate to the pathogen. The objectives of the study were:
1. to determine the response of L. monocytogenes and L. innocua to different washing treatments with or without chlorine (200 mg/L) at room temperature (∼22°C);
2. to assess the effect of natural microbiota load on growth of both microorganisms at different storage temperatures, from 5 to 36°C;
3. to validate the use of L. innocua as a surrogate of L. monocytogenes for further studies with fresh baby spinach leaves.
Scientists developed predictive models to investigate the effect of simulated storage temperature on the growth patterns of L. monocytogenes and L. innocua.
Results showed that each microorganism had a different significant response to the type of washing treatment at room temperature and the pathogen was harder to remove from the leaves than the L. innocua was.
Although, the natural microflora on fresh baby spinach leaves affected the growth parameters (Maximum grow rate, lag time, maximum population density) for both bacteria, the effect was not significant. Thus, in the specific case of spinach leaves, the study shows that L. innocua may be a suitable surrogate for L. monocytogenes in growth studies.
Growth data for L. monocytogenes and L. innocua on fresh baby spinach leaves at 5–36 °C were modelled using the Baranyi and Ratkowsky (secondary) models which were validated by comparing the root mean square error (RMSEs) and biases between the growth data and model predictions. The secondary models showed good agreement between observed and predicted values.
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The validation results show that these models could provide reliable estimates for growth of L. monocytogenes and L. innocua as a function of temperature. These models may be used by processors to evaluate the impact of postharvest practices such as storage and washing on the growth of Listeria in baby spinach leaves evaluated in this study. These models can provide useful input to quantitative risk assessment models.
Source: Basri Omac, Rosana G. Moreira, Alejandro Castillo, Elena Castell-Perez, "Growth of Listeria monocytogenes and Listeria innocua on fresh baby spinach leaves: Effect of storage temperature and natural microflora", 2015, Postharvest Biology and Technology, Vol. 100, 41–51.