THE COMPOSITION OF MILK

Many factors influence the composition of milk, including breed, genetic constitution of the individual cow, age of the cow, stage of lactation, interval between milkings, and certain disease conditions. Since the last milk drawn at each milking is richest in fat, the completeness of milking also influences a sample. In general, the type of feed only slightly affects the composition of milk. Human milk is relatively low in both proteins and minerals compared with that of cows and goats.

Goat milk has about the same nutrient composition as cow milk, put it differs in several characteristics. Goat milk is completely white in colour because all the beta-carotene is converted to vitamin A.

Sheep milk is rich in nutrients, having 18 percent total solids (5.8 percent protein and 6.5 percent fat). Reindeer milk has the highest level of nutrients, with 36.7 percent total solids (10.3 percent protein and 22 percent fat). These high-fat, high-protein milks are excellent ingredients for cheese and other manufactured dairy products.

The major components of milk are water, fat, protein, carbohydrate (lactose), and minerals (ash). However, there are numerous other highly important micronutrients such as vitamins, essential amino acids, and trace minerals. Indeed, more than 250 chemical compounds have been identified in milk.

The protein in about a litre of milk is approximately equivalent to that in 140 grams of meat or fish, large eggs, about 100 grams of American or Cheddar cheese, or 16 slices of bread. It is important that due to its amino acid composition, milk provides man with high-quality protein, that is protein containing all of the essential amino acids and in appreciable amounts.

There are two main proteins in milk — casein and lactalbumin, traces of other proteins such as lactoglobulin being also present. Casein comprising about 82 percent of the total amount of milk proteins.

Under certain conditions the casein complexes are disrupted, causing curdling of the milk. Curdling results in the separation of milk proteins into two distinct phases, a solid phase (the curds) and a liquid phase (the whey).

Lactose (milk sugar). Lactose is the principal carbohydrate to be found in milk, the latter being the only source of lactose in nature.

Lactose was found to be a disaccharide composed of one molecule leacljx>f the monosaccharides (simple sugars) glucose and galactose.

Galactose is known to be the main constituent of the central nervous system and brain tissue, so milk lactose is likely to be brain food, as well as a special nutrient for growth and development of the central nervous system of mammalian young.

Another favourable feature of lactose is its hygienic value. Thus, being an important food source for several types-of fermenting bacteria, lactose stimulates growth of microorganisms producing organic acids and synthesizing many B-complex vitamins. The process of converting the lactose into lactic acid in the presence of certain bacteria is the basis for obtaining several types of dairy products.

It is now known that the presence of lactose enhances the absorption of calcium, phosphorus, magnesium, and barium from the intestine. This unique quality of lactose also makes milk an excellent anti-rachitic food even when milk is low in vitamin D.

Milk fat. The historical aspects of the nutritional importance of milk fat are generally known. The fat in milk is secreted by specialized cells in the mammary glands of mammals. It is released as tiny fat globules "or droplets5, which are stabilized by phospholipids and proteins presented in milk.

Fat aids in calcium absorption, and since milk contains abundant calcium, the complementary effect of fat in whole and lowfat milks is especially important to the nutrition and health of man.

Vitamins. Milk contains all of the known vitamins. Most fluid milks are enriched with vitamins A and D13 so that milk and milk products provide man with a rich and well-balanced supply of the fat-soluble 'vitamins (A, D, E, and K) and of the water-soluble vitamins (except • ascorbic acid, or vitamin C).

Milk also provides many of the B vitamins..It is riboflavin that milk especially rich in, while it provides lesser amounts of thiamine (Bl) and niacin. Other B vitamins found in trace amounts are pantothenic acid, folic acid, biotin, pyridoxine (B6), and vitamin B12. It is important that consumers obtain about one-half of their riboflavin from dairy products and certain other B vitamins.

It is required that senior citizens should increase their consumption of milk products to compensate their needs in certain vitamins, which play important role in many life processes.

Minerals. Milk is noted for its abundant supply of minerals. Moreover, they occur in milk in the right proportions or ratios for optimal absorption into blood from the digestive tract. Milk is the best nutritional source of calcium, not only because of its richness in calcium, but also because of the favourable calcium/phosphorus (Ca/P) ratio (about 1.4:1). It also contains trace amounts of potassium, chloride, sodium, magnesium, sulfur, copper, iodine, and iron.

Approximately 99 percent of the calcium and 90 percent of the phosphorus in the body is in the skeleton. The dramatic supplementary value of milk minerals for growth of children was noted by the study involving 20,000 schoolchildren (5 to 12 years of age) in Scotland, one group received about 0.35 litre of milk daily in addition to food consumed at home. The researchers concluded that "the influence of the addition of milk to the diet of schoolchildren is reflected in a definite increase in the rate of growth both in height and weight". This growth-accelerating effect of milk results largely from the richness and availability of milk calcium and phosphorus. Skim milk (nonfat milk) has been reported to increase growth in height as much as does whole milk.

Unfortunately, milk consumption among those 25 years and older is said to be too low to ensure adequate dietary calcium.

XIX. Answer the questions and discuss your opinion with your classmates:

1) Do you think that yogurt is really healthy for all people and milk is unhealthy for older people?

2) Are milk products the main foodstuffs in the life of any man?

Answer the questions:

1) What kinds of animals were domesticated for dairy purposes?

2) Why is it important for man to consume cow milk daily?

3) What factors influence milk composition?

4) How does milk composition vary with mammals?

5) Is milk as rich in proteins as meat?

6) What are the main two proteins in milk and why are they important?

7) What is known about the structure of lactose, the principle carbohydrate in milk?

8) Why is lactose so highly valued?

9) What important vitamins are provided by milk products?

10) What minerals is milk rich in?

11) Why is it necessary to increase the consumption of milk for older people?