Info

N, no reliable information available; values in parentheses, estimated value.

Adapted from Holland et al. (1989) Milk Products and Eggs. Fourth supplement of McCance and Widdowsotl s The Composition of Foods, 4th edn. London: Royal Society of Chemistry/MAFF.

N, no reliable information available; values in parentheses, estimated value.

Adapted from Holland et al. (1989) Milk Products and Eggs. Fourth supplement of McCance and Widdowsotl s The Composition of Foods, 4th edn. London: Royal Society of Chemistry/MAFF.

improve the overall protein quality of a meal when consumed with foods of lower protein quality such as cereals and pulses.

Carbohydrate

The carbohydrate in milk is in the form of lactose, a disaccharide comprising a molecule of glucose and a molecule of galactose. This sugar is found naturally only in milk and is much less sweet than sucrose (packet sugar).

In the small intestine, lactose is digested by the enzyme lactase to its two component monosacchar-ides, in readiness for absorption. This enzyme is present in babies, young children, and most European adults. However, in some adults enzymatic activity can decrease, making milk in quantity less well tolerated. Such individuals are described as being lactose intolerant. Most can tolerate small quantities of milk, and fermented milk products appear to be better tolerated. Cheese is also usually well tolerated as it contains only trace amounts of lactose. In the UK, lactose intolerance is relatively rare in people of European descent but is more common in those of Asian, Far Eastern, and African descent, particularly first-generation members of these ethnic groups.

The fat in milk is in the form of minute droplets, which rise to the top when milk is left to stand. The principal component of milk fat is triacylglycerol, three fatty acids joined to a glycerol backbone. All triacylglycerols contain mixtures of three families of fatty acids: saturated, monounsaturated, and polyunsaturated. The contribution of these three types of fatty acid to milk fat in the UK is 61% saturated, 28% monounsaturated, and 3% polyunsaturated. The percentages do not add up to 100% because milk fat is not composed totally of fatty acids. When milk is skimmed, it is the amount rather than the type of fat that changes, and so the fatty acid profile remains the same. Milk fat contains small amounts of the two essential fatty acids, linoleic acid (1.4 g per 100 g fatty acids) and linolenic acid (1.5 g per 100 g fatty acids).

Vitamins

All of the known vitamins are to be found in whole milk (Table 1), although some are present in small quantities. Milk and milk products make a significant contribution to intakes of a number of these (Table 2). The fat-soluble vitamins - A, D, E and K - are removed with the fat when milk is skimmed. Consequently, they are present in only trace amounts in skimmed milk and in reduced amounts in semiskimmed milk. Whole milk is a good source of vitamin A, a pint (568 ml) providing 47% of the adult male and 55% of the adult female UK reference nutrient intake (RNI).

All of the three major types of cows' milk (whole, semiskimmed, and skimmed) are good sources of riboflavin (vitamin B2) and vitamin B12. Milk can also make an important contribution to intakes of thiamin (vitamin B1), niacin, and ascorbic acid (vitamin C), particularly where the overall diet is poor.

Some vitamins are sensitive to heat and light. Over 90% of milk for liquid consumption is pasteurized, a mild heat treatment that causes little loss of vitamins other than vitamin C. In general vitamin losses are less than 10%. Milk that has undergone the ultra heat treatment (UHT) process keeps for longer, but the higher temperature used in the processing results in slightly greater losses (10-20%) of some vitamins, particularly vitamins B6 and B12, vitamin C, and folate. The sterilization process used for milk has a somewhat greater effect: about one-third of the thiamin and half the vitamin C, folate, and B12 are destroyed.

Some loss of vitamins is inevitable when milk is stored. The extent of these losses is dependent on the translucency and permeability to light of the container, and the length and conditions of exposure. Milk exposed to bright sunlight on the doorstep will readily lose its vitamin C. Loss of riboflavin is slower but after 4 h, half will be lost and only a third remains after 6 h. Therefore, measures should be taken to limit such exposure. There will also be gradual losses of some vitamins from UHT and sterilized milks, even under ideal storage conditions, because of reactions with small amounts of oxygen remaining in the pack or bottle. Boiling milk also reduces its vitamin content, ranging from a 5% reduction in vitamin B12 to a 50% reduction in vitamin C.

Table 2 Contribution of milk and milk products to daily nutrient intake from all food and drink in Great Britain
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