Nutrient Components of Dairy
As one of the core food groups, dairy plays a key role in a balanced diet. Dairy products are convenient, cost effective and tasty and naturally contain more than ten essential nutrients, including protein, carbohydrates, vitamin A, riboflavin, vitamin B12, calcium, potassium, phosphorus, magnesium and zinc. As a result of new technologies in genetics, molecular biology and analytical chemistry, a number of milk constituents with physiological benefits beyond milk’s traditional package of nutrients are being recognised. Milk is the primary ingredient of dairy products and the nutrients found in milk are therefore discussed first. Fresh, full-cream cow’s milk is a liquid food (87.5% water) and it contains an average of 12.5% total solids of which 9% are nonfat solids. Refer to Figure 1 for a schematic diagram of the major components of milk.
The energy (kilojoule) content of milk and other dairy products varies widely and depends mostly on the fat content of the product. However, the addition of nonfat milk solids, sweeteners and other energy yielding components to dairy products contributes to the total energy provided by the selected food. Milk is considered to be a nutrient rich food, providing a meaningful amount of nutrients relative to its energy content.
Cow’s milk is recognised as an excellent source of high quality protein. Cow’s milk contains about 3.25% protein, of which 82% is casein and 18% is whey protein. Each of these proteins have unique characteristics and biological functions. Whey protein concentrates and isolates are used as ingredients in a number of formulated food products.
Nutritionally, cow’s milk protein is considered to be a ‘complete protein’, because it contains, in varying amounts, all nine essential amino acids required for growth and which cannot be synthesised by the body.
Lactose, the principal carbohydrate in milk, is synthesised in a cow’s the mammary glands. Lactose accounts for approximately 54% of the total nonfat solids of milk and contributes about 30% of the energy of fullcream milk. Cow’s milk contains about 4.63% lactose, which translates to 12 g lactose per 250 ml.
Minor quantities of glucose, galactose and oligosaccharides are also present in milk. Glucose and galactose are the products of lactose hydrolysis by the enzyme lactase.
Milk fat contributes unique characteristics to the appearance, texture, flavour, and stability of dairy products. It is a source of energy, essential fatty acids, fat-soluble vitamins, and several other potential health promoting components.
Milk fat, the most complex of dietary fats, exists in microscopic globules in an oil-in-water emulsion in milk. These fats are unique amongst animal fats, because it contains a relatively high proportion of short-chain and medium-chain saturated fatty acids.
The composition of milk fat varies somewhat according to the breed of the cow, stage of lactation, season, geographical location and feed composition.
In full-cream milk, approximately 56% of the fat is in the form of saturated fatty acids, 25% is monounsaturated fatty acids, and 3% is polyunsaturated fatty acids.
Although saturated fatty acids generally contribute to an increase in blood cholesterol levels, individual saturated fatty acids differ in their blood cholesterol raising effects. Long-chain saturated fatty acids, such as lauric, myristic and palmitic acids, raise total and low-density lipoproteins (LDL) blood levels, whereas stearic acids and short-chain saturated fatty acids, such as butyric, caproic, caprylic and capric acids have either a neutral effect or may lower blood cholesterol levels. Oleic acid is the main monounsaturated fatty acid in milk fat. Polyunsaturated fatty acids, such as arachidonic acid, are present in trace amounts.
Omega3 linolenic acid and its products, eicosapentaenoic acid (EPA) and docosahexaaenoic acid (DHA), are also present in small but significant amounts. Current dietary recommendations advise moderation in total fat intake (20–35% of total kilojoules), with less than 10% of kilojoules from saturated fatty acids, and keeping intake of trans fatty acids as low as possible.
Almost all of the vitamins known to be essential to humans are found in milk. Vitamin A, D, E and K are associated with the fat component of milk. Vitamin A plays an important role in vision, gene expression, cellular differentiation, embryonic development, growth, reproduction and immune competence.
Both vitamin A and its precursors – principally beta-carotene – are present in variable amounts in milk fat.
Milk and milk products are an important dietary source of vitamin A. Three servings of full-cream dairy consisting of 250 ml milk, 200 ml yoghurt and 40 g cheese will provide 29% and 36% of the RDA for vitamin A for adult males and females, respectively. Cow’s milk is a good source not only of vitamin A, but also of beta lacto-globulin, the major protein component of bovine milk whey, which may enhance vitamin A absorption.
Vitamin D, a fat-soluble vitamin that enhances the intestinal absorption of calcium and phosphorus, is essential for the maintenance of a healthy skeleton throughout life. Vitamin D is present in low concentrations in South African milk as it is unfortified, but exposure to sunlight should provide adequate vitamin D for most people living in South Africa.
Vitamin E is an antioxidant, protecting cell membranes and lipoproteins from oxidative damage by free radicals. This vitamin helps to maintain cell membrane integrity and stimulate the immune response. Vitamin E is present in low concentrations in full-cream milk (109 μg per 100 g).
In addition to the essential fat-soluble vitamins, milk and other dairy products contain all of the water-soluble vitamins in varying amounts required by humans. Thiamine (vitamin B1), which acts as a coenzyme for many reactions in carbohydrate metabolism, are found in milk. Three servings of dairy provide about 8% and 10% of the thiamine recommended for adult males and females, respectively. Milk is also an excellent source of riboflavin, or vitamin B2. This vitamin functions as a precursor for certain essential coenzymes important in the oxidation of glucose, fatty acids, amino acids and purines. The average riboflavin content of full-cream milk is about 0.158 mg/100 g.
Niacin functions as part of a coenzyme in fat synthesis, tissue respiration and utilisation of carbohydrates. This vitamin promotes healthy skin, nerves and digestive tract, and aids digestion and the fostering of a normal appetite. The average amount of niacin in milk is 0.107 mg/100 g.
Milk is also a good source of pantothenic acid, a component of the coenzyme A involved in fatty acid metabolism. Folate, found in milk, is a growth factor and functions as a coenzyme in the transfer of nucleotides necessary for DNA synthesis. Cow’s milk contains a high-affinity folate-binding protein (FBP), a minor whey protein that promotes retention and increases the bio-availabilty of folate by slowing the rate of absorption.
Adequate folate nutrition is especially important for women of childbearing age and can reduce the risk of neural tube defects in infants. Three servings of dairy would supply approximately 9% of the 400μg folate recommended per day for adults.
Dairy is very high in vitamin B12, which is necessary for growth, maintenance of nerve tissue and normal blood formation. Milk provides 0.44 μg vitamin B12 per 100 g.
Milk and other dairy products are a good source of readily bio-available calcium, providing approximately 300 mg per serving.
It is also an important source of other minerals, such as phosphorus, magnesium, potassium, and trace minerals such as zinc.
Phosphorus is an essential mineral that plays a central role in metabolism and is a component of lipids, proteins, and carbohydrate. Magnesium, a required cofactor for more than 300 enzyme systems in the body, is related to calcium and phosphorus function. This mineral activates many of the body’s enzymes, participates in the synthesis of protein from amino acids, and plays a role in the metabolism of carbohydrates and fat. Because magnesium is widely distributed in foods, particularly those of vegetable origin, a deficiency of this nutrient is rare.
Potassium contributes to the transmission of nerve impulses and helps to control skeletal muscle contraction. Accumulating scientific evidence supports the beneficial role of potassium in blood pressure control as well as the prevention of hypertension. Milk contains about 157 mg potassium per 100 g and is ranked as the top food source of potassium.
Milk and other dairy products contain many trace elements or nutrients needed by the body at levels of only a few milligrams per day, such as zinc, selenium and iodine. Trace elements in cow’s milk are highly variable and depend on the stage of lactation, season, milk yield, the amount of trace elements in each cow’s diet, post-pasteurisation handling of milk and storage conditions.
Zinc is essential for growth and development, wound healing, immunity, and other physiological processes. Zinc is also a regulator of gene expression and helps maintain the integrity of cell membranes. Dairy products such as milk, cheese, yoghurt and maas are a source of zinc.
Iodine, which is naturally present in milk, is an essential component of the thyroid hormones, which regulate growth and metabolism. The iodine content of cow’s milk varies widely, depending on the geographical area and iodine intake of the cow.