Triacylglycerols hydrolysis and hydroxy- and epoxy-fatty acids release during fermentation of walnuts as model system: inter- and intra-species capabilities of lactic acid bacteria

Background Although lactic acid bacteria (LAB) encompass a wide and heterogenous group of microbial genera, only few lactobacilli were previously associated to a specific lipolytic activity or to the conversion of fatty acids into hydroxy- or oxo-derivatives. The investigation of fatty acids metabolism by LAB was mainly limited to the fermentation of animal matrices (dairy and meat-based products) or to in vitro studies by using pure fatty acids as substrate. Objectives This study aimed to fill the current knowledge gaps on lipid metabolism during lactic fermentation of plant matrices, because of the complexity of their biochemical dynamics and the involvement of a plethora of microbial players and enzymes. Methods Liquid Chromatography-High-Resolution Mass Spectrometry (LC-HRMS) was employed for the detection and characterization of lipids, including native and oxidized fatty acids and mono-/di-acylglycerols, in lipid extracts of Persian walnut subjected to fermentation by a diversified pool of LAB and in related control samples. Results Most of the fermented walnut samples exhibited an increase of free fatty acids (linoleic, α-linolenic, palmitic, and oleic acids). The increase of diacylglycerols and, especially, monoacylglycerols levels in fermented walnuts confirmed that strain-specific bacterial lipolytic activities hydrolyzed triacylglycerols during walnuts fermentation. Twelve hydroxylated or epoxidized derivatives arising from oleic, linoleic, and linolenic fatty acids, divided into five groups of isomeric compounds, were also identified. A prominent role of Weissella cibaria, Enterococcus faecalis, and Leuconostoc mesenteroides emerged for the lipolytic activities and the capability to release hydroxy- and epoxy-fatty acids, in addition to the well-known role of some lactobacilli.