Concentrations of volatile 4-alkyl-branched fatty acids in sheep and goat milk and dairy products

Goat and sheep milk and dairy products thereof are characterized by a strong and unique flavor. In this context, the volatile minor fatty acid 4-ethyloctanoic acid plays a prominent role along with 4-methyloctanoic acid when both are present in free form. Using a novel GC/MS method in the selected ion-monitoring mode, previously developed for sheep subcutaneous adipose tissue, we were able to analyze the total concentrations of these flavor-relevant minor fatty acids as methyl esters in goat and sheep milk as well as in their products. Differences between the concentrations and ratios of 4-methyloctanoic acid and 4-ethyloctanoic acid in goat milk (n = 4), goat cheese (n = 4), sheep milk (n = 2), and sheep cheese (n = 4) were observed. Goat milk and cheese resulted in higher concentrations for both fatty acids (190 to 480 μg/g milk fat) and smaller 4-Me-8:0 to 4-Et-8:0 ratios (1.4 to 2.7) compared to sheep milk and cheese (78 to 220 μg/g milk fat; 4-Me-8:0 to 4-Et-8:0 ratio: 15 to 42). In all samples, the concentration of 4-Me-8:0 exceeded the one of 4-Et-8:0. However, due to its lower flavor threshold value the contribution of 4-Et-8:0 to the flavor was generally >76%. The calculated flavor values were >1400 for goat milk and cheeses and >200 for sheep milk and cheeses. In goat milk and its products, only a proportion of <0.1% 4-alkyl-branched fatty acids present in free form in the goat milk and <0.5% in the sheep samples would be sufficient to generate the characteristic goaty flavor. Parameters that promote or prevent the release of 4-Me-8:0, and especially 4-Et-8:0, will be decisive for the flavor in the resulting dairy product.