Influence of feeding different types of roughage on the oxidative stability of milk

The variation in susceptibility to light-induced lipid and protein oxidation was studied in relation to the concentration of riboflavin, fatty acid composition, α-, γ-, and δ-tocopherol and β-carotene, lutein and zeaxanthine in milk from cows fed diets with roughage containing either grass silage or corn silage mixed with concentrates. The results show that milk from cows fed grass silage had a higher concentration of the antioxidants measured. The contents of unsaturated C18 fatty acids were significantly different in the two types of milk, showing that the compositions of antioxidants and fatty acids were influenced by the feeding. Despite a higher antioxidative capacity in the milk from cows fed grass silage, lipid oxidation was higher in this milk compared to milk from cows fed corn silage. The results indicate that quenching of singlet oxygen by the antioxidants did not prevent lipid oxidation. The degree of unsaturation in linolenic acid, C18:3, is proposed to be important for the higher accumulation of lipid hydroperoxides in milk from cows fed grass silage. In contrast to milk from cows fed grass silage being more vulnerable to lipid oxidation, milk from cows fed corn silage is more vulnerable to protein oxidation. The results indicate that protein oxidation occurs independently of lipid oxidation. It is concluded that the higher concentration of antioxidants in milk from cows fed grass silage is able to extend the lag phase of protein oxidation and delay the formation of dityrosine.