PROTEIN-QUALITY OF SPANISH MURCIANO-GRANADINA GOAT MILK DURING LACTATION

The evolution of contents of total protein, caseins (alpha, beta, kappa) and whey proteins (alpha-lactalbumin, beta-lactoglobulin and 'remaining' whey proteins) in the milk of 44 Spanish goats (Murciano-Granadinabreed) was studied during part of the lactation (70 to 210 d after kidding) using polyacrylamide gel electrophoresis-SDS. Mean values of protein groups (g/l) and their fractions were: total protein (40.9), total casein (32.1), whey proteins (8.7), alpha-casein (8.5), beta-casein (21.0), kappa-casein (2.5), beta-lactoglobulin (5.8), alpha-lactalbumin (1.5) and 'remaining' whey protein (1.4). All proteins groups and fractions increased significantly (P<0.01) during the lactation except for 'remaining' whey proteins which decreased (P<0.001). Stepwise discriminant analysis showed that alpha-casein and beta-lactalbumin differed significantly between sample dates (P<0.001), with more variation than the other proteins (F = 48.0 and F = 17.3, respectively, P < 0.001).