Selection of effective antifungal cultures in fermented dairy products and identification of antifungal metabolites

Introduction:Antifungal cultures are of growing interest as an alternative to chemical preservatives to reduce food fungal spoilage in the context of a growing consumers’ demand for clean label foods [1]. Our aim was to select strains that exhibit significant antifungal activity in dairy products and to identify the compounds most likely involved in their activity.Materials and methods:Lactic acid bacteria and propionibacteria strains were screened alone and in combinations in yogurt and cheese models against four major spoilage fungi (Penicillium commune, Mucor racemosus, Galactomyces geotrichum, and Yarrowia lipolytica). Two binary combinations of lactobacilli were then selected to be evaluated in situ as adjunct cultures in sour cream and semi-hard cheeses produced at a pilot scale to evaluate their antifungal activity during challenge tests and shelf life tests, and their impact on organoleptic properties. Four methods, including liquid and gas chromatography coupled to mass spectrometry, were combined to detect and quantify antifungal compounds. These methods targeted 56 antifungal compounds as well as volatile compounds and were followed by statistical analyses to identify the compounds associated to antifungal activity.Results and discussion:The more effective antifungal culture, composed of two lactobacilli strains, delayed the growth of P. commune, M. racemosus and R. mucilaginosa for 2–24 days on sour cream, depending of the antifungal culture inoculum, and the growth of P. commune in semi-hard cheese for 1–6 days. Antifungal cultures had no effect on organoleptic properties at low inoculum level (6 logCFU/mL) [2]. Overall, 53 antifungal compounds were detected, of which 33 were in significantly higher amount in at least one product inoculated with an antifungal culture compared to the controls. They were present at concentrations below their minimum inhibitory concentration and thus could act in synergy. Among them, the most commonly identified were acetic, hydroxyphenyllactic, phenyllactic, 3-phenylpropanoic, 3-(4-hydroxyphenyl)propanoic and 5-oxopyrrolidine-2-carboxylic acids, diacetyl, acetoin, and a yet unidentified volatile compound. Our screening approach allowed developing antifungal combinations that exhibit significant antifungal activity and providing future prospects for use as bioprotective cultures in dairy products. This extensive study also contributes to improve the knowledge about the action mode of antifungal lactobacilli.