Few foods naturally contain significant amounts of vitamin D, which is essential for bone health and may affect your susceptibility to certain medical conditions. To increase the amount of vitamin D in Americans' diets and prevent deficiency-related diseases, dairy processors began adding vitamin D to fluid milk in 1933. Although the U.S. Food and Drug Administration does not mandate fortification of all fluid milk, most processed milk in the United States is fortified with vitamin D.
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When Fortification Occurs
Milk processing is a multistep process. After filtering out foreign matter, milk fat is separated from the rest of the fluid, leaving skim milk. Measured amounts of fat are are added back to skim milk to produce 1 percent, 2 percent and whole milk. Most milk processors conduct the separation process before adding vitamin D because the vitamin naturally concentrates in milk fat. If vitamin D is added before separation, the skim milk may contain lower than desired levels and the fat-containing milk products may have too much. The FDA recommends that milk manufacturers add vitamin D after the separation of milk fat and before homogenization, the mixing process that keeps milk fats from rising to top of the liquid. Adding vitamin D before homogenization ensures that the vitamin is thoroughly mixed into the milk.
Milk manufacturers use different methods for adding vitamin D to milk. Batch fortification involves measuring the amount of milk in a large vat and adding the appropriate amount of vitamin D to yield a concentration of 400 IU per quart. The FDA mandates that all milk labeled "fortified" contain at least 400 IU of vitamin D per quart; up to 600 IU is acceptable. The necessity of precisely measuring the amount of milk in a batch to determine the correct amount of vitamin D concentrate to add is a drawback to batch fortification.
Continuous fortification, also known as metered fortification, is an inline system that measures the amount of milk flowing through the production pipes and automatically dispenses the correct amount of vitamin D to yield a final concentration of 400 IU per quart. High-volume milk processors typically use a continuous fortification system.
Stability of Added Vitamin D
The potency of vitamin D concentrates used to fortify milk may degrade over time. Milk manufacturers are responsible for proper handling and testing of vitamin D concentrates to ensure potency before use. Once vitamin D is added to milk, it is stable and does not lose any appreciable amount of activity during the usual shelf life.
Natural Vitamin D
All milk produced by cows and other animals contains a low concentration of vitamin D, which is affected by the animals' feed and sun exposure. The FDA reports that raw cow's milk typically contains 5 to 40 IU of vitamin D per quart. Because the level is low, it is not typically considered in determining the amount of vitamin D concentrate added to fortify milk. Therefore, processed milk may contain slightly more vitamin D than is listed in the nutrition facts.