Growth potential assessment of listeria in milk fat products by challenge testing.

Abstract

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 onocytogenes 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 thebutter. 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