Growth of Listeria monocytogenes and Listeria innocua on fresh baby spinach leaves: Effect of storage temperature and natural microflora

Abstract

Leafy greens such as spinach may be contaminated with Listeria monocytogenes during pre-harvest and postharvest handling. Recent recalls issued for Listeria-contaminated leafy greens are driving the need for technologies to minimize safety issues in fresh and fresh-cut produce. This study assessed 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 when access to BL2 facilities is difficult. Each microorganism had a different (P<0.05) response to the type of washing treatment at room temperature (∼22°C) and the pathogen was harder to remove from the leaves than the surrogate was. Growth data for L. monocytogenes and L. innocua on fresh baby spinach leaves at 5–36°C were modeled 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. These models can provide useful input to quantitative risk assessment tools to evaluate the growth of pathogens in baby spinach during several stages of processing and distribution such as washing and cold storage. Although the natural microflora on fresh baby spinach leaves affected the growth parameters 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 in growth studies of L. monocytogenes.