Module 5. Physico-chemical, microbiological and nutritional properties of milk

Lesson 15

15.1 Introduction

The primary function of milk in nature is to nourish and provide immunological protection for young mammals. Milk provides excellent support towards human nutrition, as it contains balanced quantities of required nutrients such as protein, fat, carbohydrate, minerals, and vitamins. The energy value of milk of selected mammals is detailed in Table 15.1. Milk has a mildly sweet and pleasant flavour which is of wide appeal and is relished by most people and hence is innately palatable. The digestibility of milk is high, which makes it an ideal food for children, adults, invalids and convalescing patients. Milk is an economical source of energy and furnishes almost all essential dietary constituents required for normal physiological functions. It supplies body-building proteins, bone-forming minerals and health-giving vitamins and furnishes energy-giving lactose and milk fat. Besides supplying certain essential fatty acids, it contains the above nutrients in an easily digestible form. All these properties make milk an important food for pregnant and lactating women, growing children, adolescents, adults, invalids, convalescents and patients alike.

Table 15.1 Energy value of milk of selected breeds of animals (in 100 g)

Table 15.1
*Figures in parenthesis denote the value in kJoules
(Source: www.nutritiondata.com)
15.2 Milk Fat
Since milk fat stays in the stomach longer than do carbohydrates and proteins, they improve satiety value of diet. It is a carrier of the fat-soluble vitamins A, D, E and K. The short chain fatty acids are unique of milk lipids and are digested and absorbed quickly. The position of double bond linkages along the carbon chain in polyunsaturated fatty acids (PUFA) is of great nutritional importance. Milk lipids also contain phospholipids, which are vital part of brain and nervous tissues. They are extremely important as intermediary substance of fat metabolism. Conjugated linoleic acids (CLAs) are a group of naturally occurring isomers of linoleic acid and are of great interest with respect to their anticarcinogenic effect. CLAs also exhibit several biological activities such as immunomodulation, reduction in atherosclerosis, increase in bone mass and muscle mass.

15.3 Milk Sugar

Lactose stimulates the growth of lactobacilli in the large intestine; these organisms synthesize many of the B vitamins. Lactose improves the absorption of several important minerals like calcium, phosphorus and magnesium. It yields one molecule each of glucose and galactose upon hydrolysis, both of which are convertible into glycogen in the human body. Galactose is necessary for the synthesis of the galactosidases of brain and medullar sheaths of nerve tissues and myelin-formation of cerebrosides in infants. Lactose is digested and absorbed from the intestine very slowly. The low solubility of lactose also makes it less irritating to the stomach and intestinal musoca than the highly soluble sugars. It thus makes milk valuable in diets during treatment of ulcers of the stomach and duodenum.

15.4 Milk Proteins

Fifty percent of recommended daily allowance of protein for adults may be derived from one liter of milk. Casein has a protein efficiency ratio (PER) of 2.89–3.10. Milk proteins are of high biological value as they contain all the essential amino acids in amounts and proportions required to support growth and perform numerous functions within the body. A comparison of amino acid constituents of milk with those of common foods is given in Table 15.2. Proteins contribute to building and repair of body, participate in muscular contraction, act as antibodies in the body’s immunological defense mechanism, and supply energy.

Table 15.2 Essential amino acids content of some common foods (mg/g)

Table 15.2
(Source: Mahindru (1982); Gopalan et al. (1989)

Whey proteins function as binder and transporter of retinol and significantly enhance retinol uptake. It also is a carrier of antibodies, hence possessing immunological properties. à-lactalbumin is readily digestible and is anticarcinogenic. Antimicrobial components of milk such as immunoglobulins provide passive immunity against enteric and respiratory bacteria and viruses. Colostrum has a very high concentration of immunoglobulins, facilitating quick transfer of immunity to the young born calf.

15.5 Minerals

All the known essential minerals other than iron and magnesium are present in milk in varying quantities. Milk is an out standing source of calcium and phosphorus, the two major bone building elements. The absorption of calcium from milk is high and is promoted by the presence of lactose, the amino acids lysine and arginine, citric acid and vitamin A, all of which are present in milk in generous amounts. Fortification of milk with vitamin D is helpful in increasing calcium absorption. Although the quantity of iron in milk is less, it is present in readily soluble form and is, therefore, completely absorbed from the intestine than is iron from other iron-rich foods. The iodine content of milk is highly variable and its concentration in milk is dependent on the kind of feed. Because of the wide variety of minerals in milk, it can play an important role in making diets satisfactory in mineral content. A comparison of the mineral contents in milk and some common foods is made in Table 15.3.

Table 15.3 Mineral content of few common foods (mg/100g)

Table 15.3
(Source: Gopalan et al. (1989)
15.6 Vitamins

Milk supplies all vitamins essential for human nutrition and health. Vitamin A and its precursors are necessary for good vision, aid in keeping the skin clear and smooth and help keep mucous membranes healthy and resistant to infection. Bovine milk, particularly colostrum, has high level of vitamin A activity as retinol. Milk contains small amount of vitamin E, which functions as an antioxidant preventing the oxidation of carotenoids, vitamin A and PUFA and as an aid in preventing muscular dystrophy. Vitamin D, although is present in low concentration, is useful in the absorption of calcium and phosphorus. Milk also contains vitamin K that is required for synthesis of pro-thrombin and hence is useful in normal blood clotting. Milk is a fair supplier of vitamin B complex. One litre of milk supplies the daily recommended requirement of riboflavin, which aids in oxygen uptake and helps in adapting the eye to light. Milk is a good source of thiamine which is required for normal appetite and digestion. Milk also contains sufficient vitamin B12 and folic acid, both of which are necessary for synthesis of red blood corpuscles and nucleic acids. A comparison of the vitamin content in milk and some common food materials is listed in Table 15.4.

Table 15.4 Vitamin content of some common foods items (per 100g)

Table 15.4
(Source: Gopalan et al. (1989)

Selected Readings

Gopalan, C., Ramashastri, B.V. and Balasubramanian, S.C. 1989. Nutritive Value of Indian Food. National Institute of Nutrition. (ICMR). Hyderabad.

Mahindru, S.N. 1982. Milk Nutrients, National Book Trust, India
Last modified: Tuesday, 9 October 2012, 10:12 AM