Effect of forage preservation method on fatty acid composition and oxidative stability of organic sheep milk

The aim of this study was to evaluate the effect of different forage preservation method in mountainous areas on the relationship between the fatty acid (FA) profile, fat-soluble antioxidants contents and oxidative stability of organic sheep milk. Twenty-four multiparous Turcana ewes were randomly assigned to 3 treatments: grazed grass (G), hay grass (H), and grass silage (GS). Indoor ewes were offered ad libitum grass hay or grass silage. All animals received 300 g DM grain mix (triticale and barley,1:1) per day. Conservation of grass by drying compared with ensiling resulted in lower forage 18:2n-6, 18:3n-3 and fat-soluble vitamin concentrations. Milk from ewes grazed, has a considerably higher concentration of n-3 FA and n-6 FA but and a higher content of nutritionally beneficial trans-fatty acids (e.g. CLA; conjugated linoleic acid, VA; vaccenic acid) than milk from ewes fed hay or grass silage. In spite of lower content, milk fat 18:2n-6 and 18:3n-3 content was higher (P < 0.05) for hay than for silage diets (2.34 vs. 1.64 and 2.11 vs. 1.70 % of total FA, for hay, and silages, respectively). Forage conservation method had no clear effects on milk VA or CLA content. Compared with silage, hay diets resulted in milk containing lower (P < 0.001) alpha-tocopherol and retinol concentrations, but had no effect on gamma-tocopherol. There was no clear association between increased levels of the alpha-tocopherol and retinol in milk and a lower risk of lipid oxidation (lower concentration of malondialdehyde). There was a marked effect of composition of the lipids on the oxidative stability of milk, where high concentrations of PUFA (polyunsaturated FA), especially the concentration of n-3 FA were associated with an increasing risk of lipid oxidation.