Fermentation as a strategy to increase conjugated linoleic acid in dairy products
Conjugated linoleic acids (CLA) are a group of non-methylene interrupted octadecadienoic acids, naturally present in different foods, among which foods derived from ruminants, as milk and dairy products. The rumenic acid (9cis, 11trans-CLA) is the predominant isomer in milk fat, followed by 10trans, 12cis-CLA and other isomers in traces. In order to characterize the profile of CLA isomers found in dairy products, the silver-ion high performance liquid chromatography (Ag+-HPLC) equipped with multi-columns provides satisfactory resolution. To evaluate the fatty acid (FA) composition of dairy products, the high resolution gas chromatography (HRGC) analysis of derivatized FA, as fatty acid methyl esters, is generally used. Actually, there is always more interest of researchers in increasing the content of CLA in foods, including milk and dairy products. In fact, a lot of studies, carried out on animals and humans, have suggested that CLA may have important human health benefits, among which antiatherogenic, antiadipogenic, and antidiabetic properties. Among the strategies to increase CLA content during the manufacturing of dairy products, fermentation is one of these. CLA content in fermented dairy products is extremely variable (generally from 3.4 to 8.8 mg/g fat) and strongly linked to the strain type, to the aerobic or anaerobic conditions and to other parameters, as pH, time and temperature of growth culture. Strains of Bifidobacterium, Enterococcus, Lactobacillus, Lactococcus, Propionibacterium, and Streptococcus can be considered potential CLA-producers. The manipulation of the feed and dietary regimen of the animals is an alternative strategy to increase CLA content. Linseed, soybean, olive, canola, sunflower and fish oils, added to feed, can improve the unsaturated FA contents, including CLA, in milk and dairy products. Alternatively, interest in the enzymatic modification of FA composition of dairy products in order to produce healthier fats, is increasing. In this review, an overview of the main strategies to increase CLA content in dairy products, with particular focusing on fermentation, is reported.