Growth Potential Assessment of Listeria in Milk Fat Products by Challenge Testing

Milk fat products (MFP), including butter and low-fat dairy spreads, are a specific type of ready-to-eat food known as water-in-fat emulsions, in which the behavior of microbial foodborne pathogens such as Listeria monocytogenes is not clearly known. This study investigated the growth and survival of L. monocytogenes, and of Listeria innocua as a surrogate for L. monocytogenes, in these foods using challenge testing. Three commercial MFPs with various fat contents (butter, half butter and low-fat dairy spread) and two samples of traditional churned butter with various water droplet sizes were artificially contaminated with Listeria. Total mesophilic microflora including lactic acid bacteria, pH and Listeria were monitored throughout the shelf life. The growth potential of Listeria was calculated in the course of the shelf life and remains below the limit value of 0.5 log cfu/g during the whole shelf life in any of the butter. However, the concentration of Listeria remained stable during the shelf life in the tested MFPs (commercial and churned) except in the commercial low-fat dairy spread in which Listeria decreased gradually. Practical Applications European Union regulation No. 2073/2005 lays down the microbiological limit criteria for Listeria monocytogenes in ready-to-eat foods (RTE). According to this regulation, each RTE food has to be classified, with scientific justification, as able or unable to support the growth of L. monocytogenes during the shelf life. This challenge-test study on milk fat products (MFPs) provided some useful data needed to assess the behavior of Listeria in these particular RTE food products. This study showed that while MFPs do not promote growth, they do support Listeria survival. Water droplet size seems to be a key parameter in preventing the growth of both microflora and Listeria. The study also showed that, in order to determine whether a given MFP supports Listeria growth or not, it is crucial to investigate the impact of the variations in physico-chemical and microbiological parameters of each butter-making process used by companies.