Vitamin A content in cow’s milk

All-trans retinol is the main form of vitamin A present in milk, the cis isomer of retinol being sometimes observed in a very low amount in cow’s milk.

The amount of vitamin A in milk varies with the carotene content of the feed. Normally, vitamin A potency is highest when the cow is on succulent pastures in the spring and lowest when the cow is hay-fed during the winter season.

The vitamin A potency of milk can be increased to a level that approaches summer milk by feeding rations high in carotene content.

Concentrations of vitamin A and carotenoids in milk are also dependent on animal species. Indeed caprine milk is richer (30%) in retinol and conversely beta-carotene is 30% higher in cow’s milk than goat’s milk.

Guernsey milk contained more carotene than preformed vitamin A, Jersey approximately equal proportions whereas the Holstein milk the carotene constituted approximately 30 percent and the vitamin A, 70 percent of the total biological active vitamin A.
Vitamin A content in cow’s milk

Carotene in milk

Carotene occurs in the forages consumed by cattle and is the precursor of physiological active vitamin A. The ability to convert carotene into vitamin A before it is secreted into the milk varies with different breeds.

The main site of conversion into vitamin A of ingested carotene is the intestinal wall, so that the absence of carotenoids from the tissues of these animals suggests that in them, carotenoids are either not absorbed into the circulation or are rapidly degraded in the tissues into colorless products.

Guernsey cow

The Guernsey cow (or Jersey) secretes a large proportion of vitamin A as carotene, while other breeds such as Holstein-Friesian and Ayrshire produce a milk low in carotene but equally high in vitamin A on a fat basis.

Consequently the latter milks have less color than either Jersey or Guernsey. The minimum maintenance requirement of cattle for carotene is approximately 3.5 mg per 100 kg of body weight.

At least 50 to 75 percent more is needed for normal growth and maintenance of adequate plasma and liver levels.
Carotene in milk

Deterioration of vitamin A in milk

Vitamin A is heat stable, therefore there is no loss of vitamin A when milk is pasteurized, evaporated or dried.

It is however, quite susceptible to oxidation especially in the presence of unsaturated fats under oxidizing conditions.

Light catalyzes oxidative deterioration of vitamin A in milk, which results in an off-flavor that has been describes as ‘haylike’, ‘strawlike’ and ‘raspberry’.

The reaction occurs frequently in lowfat milks and skim milk fortified with vitamin A.

Milk stores in green PET bottles experienced less lipid oxidation and vitamin A loss that milk stored in clear PET bottles or LDPE pouches and HDPE bottles. During the first week of storage, vitamin loss was lower in milk stored in green PET bottles than in milk stored in clear PET bottles and LDPE pouches.

It was reported that the off-flavor development was faster in skim and 2% milks fortified with an oil-based vitamin A than in an aqueous-based one, when each milk was exposed to the same intensity of fluorescent light.

Vitamin A also has been shown to be more stable in directly heated than is indirectly heated UHT milk, which is consistent with oxidation being partly responsible for loss of the vitamin.
Deterioration of vitamin A in milk 

Vitamin fortification in milk

The term fortification is usually used to designated the addition of nutrients not naturally present in a food, whereas the term enrichment generally means the addition of nutrients already present in a food. Often the terms are used interchangeably.

Many milks are fortified with vitamins A and D. Vitamin D is found naturally in very few foods and was initially added to milk, a staple food, to reduce the incidence of rickets, a bone softening condition in children that was at one time endemic in North America.

The fortification of milk and infant milk with 400 IU of vitamin D2 or vitamin D3 has eliminated rickets as a health problem in the United States and Canada.

Because vitamins A and D are fat soluble they are found in the milk fat of whole milk. For this reason, whole milk is not required to be fortified with either vitamin, although many milk manufacturers add both.

Vitamin A and carotene are in the fat portion of the milk, the vitamin A activity is removed in reduced fat (2% fat) and fat free milks, dried whole milk and evaporated skim milk, so vitamin A fortification is required.

The fortification of dried skim milk with vitamin A is viewed by WHO and FAO as an important measure to combat vitamin A deficiency in developing countries.

Fortification with vitamin D in reduced fat and fat free milks is optional. Vitamin D fortification is required for evaporated whole and fat free milks.
Vitamin fortification in milk