There are five principal proteins in goat milk: as2-casein (as2-CN), p-casein (p-CN), k-casein (k-CN), p-lactoglob-ulin (p-Lg), and a-lactalbumin (a-La).[3-5] p-casein is the major casein fraction in goat milk, whereas as1-casein is the major one in cow milk. Differences in amino acid composition between casein fractions of goat milk are much greater than differences between species (goat versus cow).[4] The a-caseins contain greater aspartate, lysine, and tyrosine than p-casein, whereas the latter has higher leucine, proline, and valine than the former.[4] Casein micelles of goat milk are less solvated, are less heat stable, and lose p-casein more readily than bovine micelles.[9]

Commonalities in the overall amino acid pattern were reported among the milks of many species.[10] The most abundant amino acids were glutamate (plus glutamine, 20%), proline (10%), and leucine (10%). Among the three most abundant amino acids, goat and other nonprimate milk contained greater glutamate and proline and lower leucine than human milk. For sulfur-containing amino acids, cystine was higher and methionine was lower in primate milks than in goat and other nonprimate milks.[10]

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