Growth Potential of Listeria monocytogenes in Feta Sauce Packed Under Aerobic Conditions and Stored at 4°C

Introduction: According to Commission Regulation (EC) No 2073/2005, food business operators (FBOs) that produce ready-to-eat (RTE) foods shall conduct studies, if necessary, so as to demonstrate whether their food products are able to support the growth of Listeria monocytogenes.

Purpose: The objective of the present study was to evaluate L. monocytogenes growth in cheese-based (Feta) sauce, by conducting a challenge test for the assessment of growth potential for the pathogen.

Methods: Samples of Feta sauce (ca. 1% NaCl, 900 ppm sorbic acid), were inoculated with a two-strain mixture of L. monocytogenes (ATCC 19115, wild type strain) and were stored in the air at 4°C for 30 days. Uninoculated samples served as controls and were used for enumeration of aerobic plate count (APC), lactic acid bacteria (LAB) and yeasts and moulds (Y/M) at the beginning of storage (day 0) and at the end of shelf-life (day 30). Sampling units (10 g) were analyzed in triplicate for pH and L. monocytogenes enumeration at days 0, 3, 5, 10, 15, 20, 25, 30, 35 and 40. The growth potential (i.e., difference between the log₁₀ CFU/g at days 0 and 30) was evaluated. Two batches of the product were tested.

Results: There was no natural L. monocytogenes contamination in Feta sauce. Average initial pH was 4.6 and APC, LAB and Y/M populations (in log₁₀ CFU/g) were 4.6, 4.5 and 2.6, respectively; pH dropped to 4.0 at day 30, while APC, LAB and Y/M were 5.8, 6.6 and 3.5 log₁₀ CFU/g, respectively. Starting from average concentrations of 2.3 and 2.2 log₁₀ CFU/g, L. monocytogenes count decreased to 1.1 log₁₀ CFU/g in both batches at the end of shelf-life; maximum growth potential of L. monocytogenes in Feta sauce was, therefore, -1.2 log₁₀ CFU/g (pathogen reduction). At days 35 and 40 pathogen counts were below the limit of detection (<0.5 log₁₀CFU/g).

Significance: Findings of this study should be useful to regulatory authorities and FBOs manufacturing acidic cheese-based spreadable products, as they consider L. monocytogenes contamination and safety of RTE products.