Mare milk nitrogen fractions during lactation and detrmination by reversed-phase HPLC of the major whey proteins

The aim of this research was to describe the changes of the nitrogen fractions in milk of Haflinger nursing mares and in particular to determine the whey protein content and distribution, and their evolution throughout the first 6 mo of lactation. Samples were collected by hand-milking on days 4, 20, 40, 60, 80, 120,150 and 180 post-partum. Nitrogen fractions were determined by Kjeldahl on 80 samples from 10 mares, while HPLC separation of whey proteins was conducted on 40 samples from 5 mares. The total N, casein N, and true whey protein N contents showed a statistically significant decrease from day 4 to day 40, and then remained unvaried. In general, the nitrogen distribution of mare's milk significantly changed between day 4 and day 20 and then remained almost unchanged until day 180 (except for the day 150 value, which showed a statistically significant increase for CN x 100/TN). beta-Lactoglobulin and serum albumin contents showed a marked reduction, of 27 and 45.5%, respectively, between day 4 and day 20, and then remained unchanged; a-lactalbumin and immunoglobulins contents had a reduction of 24.7 and 38.3%, respectively, between day 4 and day 20, and another decrease between day 20 and day 40, of 20 and 32.8%, respectively. All whey proteins, expressed as a percentage on the total sum of the four whey proteins considered, did not vary significantly during lactation. However, the sum on the two whey proteins of mammary gland origin (beta-Lg + alpha-La) increased between day 4 and day 20 by 6.3%, and between day 20 and day 40 by 3.8%; the sum of the whey proteins of blood origin (SA + Ig) showed an opposite trend, with a decrease by 14.5% between day 4 and day 20 and by 11% between day 20 and day 40.