Fatty acid profile, conjugated linoleic acid content and antioxidant activity of probiotic fermented milks

Development of probiotic fermented milks is of great interest as they combine improved nutritional values, particular sensory properties, health benefits and extended shelf life. Among all of them, kefir an ancient milk beverage, has a creamy consistency, mild acid flavor, a natural carbonated effervescence and may contain between 0.08 to 2% alcohol [1]. Its production involves the use of kefir grains as starter. They contain a complex mixed population of lactic acid bacteria (LAB), acetic acid bacteria (AAB) and yeast that are able to modulate the chemical composition of the fermented product. It is subject to variations, depending on several factors including the type of milk. The present study investigates the influence of the type of milk on fatty acid profile, phenolic content, antioxidant activity and also rheological behaviour of kefir. Cow, buffalo, goat, camel, donkey and sheep milks are used for the growing of kefir grains, having the same microbial composition. After 24 hours, the chemical composition and rheological behaviour of milks before and after fermentation are evaluated. The fatty acid profile is subjected to changes according to the type of milk used for the fermentation. kefir from cow, goat and sheep milks show an increase in the saturated fatty acid content, especially in myristic (C14:0) and palmitic acids (C16:0). Among monounsaturated fatty acids (MUFA), oleic acid content is 17.9, 2.0 and 2.9 folds higher in fermented cow, goat and sheep milks, respectively, compared to their corresponding unfermented milk. Among the polyunsaturated fatty acids (PUFA), the initial linoleic acid content in cow milk (25.3±1.1mg∙g-1) increases during fermentation to 453.7±1.81mg∙g-1. The main conjugated linoleic acid (CLA) isomer found is cis-10 trans-12, C18:2, which increases during fermentation, especially in sheep milk where it quadruples. Fermentation leads to a significant increase in phenolic content of fermented milks. The best results are found for fermented buffalo (260.4±5.5 μg GAE/mL), camel (204.7±2.5 μg GAE/mL), and sheep milks (218±1.0 μg GAE/mL), which increase by 46, 53 and 54 %, respectively, compared to the starting milks. The increase in phenolic content was also confirmed by the FRAP (ferric reducing antioxidant power) and ABTS (2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assays. ABTS assay shows increased activity in all fermented milk being highest in goat milk (76.2± 0.5%) followed by cow milk (74.1± 0.6%). The effect of rheological properties on kefir quality is largely important, so the apparent viscosity of all samples, before and after fermentation is evaluated [2]. According to flow curves, viscosity decreases with increasing shear rate, meaning that the kefir behaves as a pseudoplastic fluid. At low shear rate, the highest viscosity value is observed in the sheep sample, while the lowest viscosity value is found in the buffalo one. The results highlight that fermentation enhances the nutritional and therapeutic values of the products, to a variable extent according to the type of milk.