*2.1.3 Flavor compounds from LAB lipid metabolism*

The two major processes that produce flavor components in fermented foods are lipolysis and fatty acid oxidation. Most fermented foods contain free fatty acids as key aroma components produced by the breakdown of lipids (triglycerides, diglycerides, and monoglycerides) [11]. The strains of Lactobacillus exhibit lipases in quite high concentrations, which then produce these free fatty acids [11]. Free fatty acids, particularly saturated and unsaturated fatty acids, serve as catalysts for catabolic processes that result in the oxidation of lipids and the generation of a variety of volatile compounds, including alkanes, methyl ketones, esters, secondary alcohols, and lactones [13]. Unsaturated fatty acids are oxidized by two different pathways, one of which is the formation of hydroxyperoxides via β-oxidation of unsaturated fatty acids in the presence of free radicals. The synthesis of 4-5-hydroxy acids, which are transformed into α-δ-lactones that emit strong fruity aromas, might result from another pathway of unsaturated fatty acid metabolism [14]. In addition, a variety of esterases found in LAB may directly generate flavor ester from glycerides and alcohols through an alcoholysis process. For example, in order to create ethyl butanoate and ethyl hexanoate, LAB can esterify ethanol with butyric and hexanoic acids [11].
