The factors affecting the vitamin A and s-carotene contents of milk were investigated. A series of short- and long-term experiments were conducted using the University of Saskatchewan dairy herd and Saskatoon commercial fluid milk, respectively.
The vitamin A potency of the milk was significantly influenced by cow-to-cow variations when the experimental cows were receiving high levels (87,600 IU/kg feed) of vitamin A supplementation. The day-to-day variations significantly affected the vitamin A content of milk when early lactation cows received low levels (8,200 IU/kg feed) of vitamin A supplementation.
The milk of cows in late lactation contained more vitamin A per 100 ml but the milk of cows in early lactation contained more vitamin A per gm butter fat.
Pronounced seasonal variations were observed in Saskatoon market milks. The spring milk contained 28.87%, 20.09%, and 8.33% more vitamin A potency than that of winter, fall and summer milk, respectively. Overall, spring and summer milk had significantly more vitamin A (1.17 times) than that of winter and fall milk. Supplementation of feeds with about 10 times the
RDA of vitamin A increased the vitamin A content of the milk by 24% but did not affect either the milk yield or the B.F.% of the milk. Dietary hay suppressed, but dietary silage promoted, the recovery of dietary vitamin A in milk of cows fed the above forages, each supplemented with 250,000 and 600,000 IU vitamin A/cow/day. High levels of vitamin A intake depressed the milk
and blood levels of s-carotene. Dietary levels of the vitamin A did not affect the concentration of the vitamin in the blood of the experimental animals.
Pasteurization of milk had no deleterious effects on the vitamin A potency of milk. Skimming significantly reduced the vitamin A content of whole milk by 85.0% and 26.5% for skim and 2% B.F. milk, respectively. Fortification, at the processing plant, increased the vitamin A content
of the low fat milk to above average levels. Maximal photodegradation of vitamin A occurred between 340 to 440 nm. s-carotene significantly protected vitamin A from photodegradation. Storage stability of vitamin A in processed milks (skim, 2% B.F. and homogenized milk) was found to be much less than that of whole, raw milk.
The results from the study indicated that the vitamin A potency of milk can be optimized by fortification of winter feeds, particularly silage, with vitamin A. Once the milk has been produced, the vitamin A levels can be optimized by proper fortification of low fat milk and by proper packaging and storage of processed milk
