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Milk and milk products supply almost one-third of the world's intake of animal protein.
COMPOSITION AND NUTRITIVE VALUE OF MILK
The average composition of milk can vary quite considerably between different species of mammals. The factors responsible for this variation include the species of the animal, breed of the animal, stage of lactation, age of the animal, season of the year, feed, time of milking, period of time between milking, and the physiological condition of the animal. These factors also affect the quality of the milk.
Milk composition may also be affected by adulteration with water or other materials, the action of bacteria, lack of agitation during sampling and similar factors.
Nutritionally, milk is an excellent source of good quality protein and calcium which are necessary for growth and sound bone and teeth development of children.
Milk fat is cherished both as cream and as the main component of butter.
It is palatable, highly digestible and assimilable.
Milk is the only food in which lactose is found naturally.
It is used as a constituent in infant foods and medicinal products.
Milk is rich in various minerals and vitamins including vitamins A and D, thiamin, riboflavin, pyridoxin, biotin, niacin, pantothenic acid, etc.
Fresh milk contains about 87.1% water in which are dispersed milk sugar, lipoprotein, calcium salts and other minor components.
Small amounts of ascorbic acid are also present in raw milk.
PHYSICAL PROPERTIES OF MILK
Physically, milk is both a dilute emulsion, a coloidal dispersion and a true solution. Its physical properties are essentially those of water, modified somewhat by the concentration and state of dispersion of the solid constituents.
Milk has a buffering capacity that maintain its pH at about 6.5 to 6.7 and its titratable acidity at 0.14 to 0.18 percent.
The density which varies between 1.0260 and 1.0320 depends on the fat composition and to a lesser extent on dissolved gases such as Carbon IV oxide and Nitrogen which are present in freshly drawn milk to the extent of 4-5%.
Casein is the major component of milk affecting viscosity. However, fat contributes considerably to viscosity depending on the amount and size of the globules. Lactose, proteins and salt make small contributions to viscosity.
The refractive index, freezing point and boiling point are affected by dissolved substances in the milk. These increase the refractive index and boiling point but depress the freezing point of milk.
Surface tension and interfacial tension are affected by protein, phospholipids, fat, free fatty acids and treatment that change the concentration of these substances.
CHEMICAL CONSTITUENTS OF MILK
The principal constituents of milk are protein(mainly casein), fat, milk sugar(lactose), minerals and other miscellaneous factors. These vary in amounts for different animal species.
PROTEIN:
The main protein in milk is casein, representing about 80% of the total milk protein in coloidal suspension.
It also contains small quantities of lactalbumin (0.5%) and lactoglobulin (0.06%).
In fresh milk, casein exists as calcium caseinate - a complex calcium: calcium phosphate suspension of casein miscelles.
Casein may be precipitated out in form of calcium paracaseinate from milk by treating with dilute acid or by heating under pressure.
Casein is used to improve the whipping property of cream topping from vegetable fat to improve the body of yogurt and sour cream.
LIPIDS:
The lipid fraction of milk is composed primarily of fat although there are also small amounts of sterols, phospholipids, the fat soluble vitamins A and D, carotenes and xanthophyll.
The main sterol found in milk is cholestrol which occurs to the extent of about 0.015%.
Lipids are significant in milk system because:
(i) they are active emulsifying agents and (ii) they are rich in unsaturated fatty acids and easily oxidized giving rise to the "oxidized" flavour of milk and the "richness" flavour of milk products.
MILK SUGAR(LACTOSE):
Lactose is the principal carbohydrate in milk. It has a solubility of only about 20% at room temperature.
Lactose is readily assimilated as food, being hydrolysed into glucose by the enzyme lactase.
In souring of milk(fermentation), lactic acid bacteria convert lactose to lactic acid, thus giving the soured milk its characteristics flavour.
Some individuals lose the ability to digest lactose and become lactose-intolerant due to a deficiency of the enzyme lactase in the intestinal mucosa as they grow older.
The phenomenon is more common among adult Africans, American Indians and Asians.
MINERALS:
Milk is noted for its abundant supply of minerals. The major minerals contained in milk are calcium, phosphorus, potassium, sodium, magnesium, chlorine and sulphur.
Traces of other minerals such as aluminium, boron, zinc, manganese and silicon are also present.
It is however deficient in copper and iron as the presence of these minerals would catalyse oxidation thereby producing a metallic or oxidized flavour.
Calcium is the chief mineral in milk. As calcium phosphate, it forms a part of the casein particle and influences its behaviour towards precipitation with rennin, heat and acids.
VITAMINS AND ENZYMES:
Milk is a fairly good source of the water soluble vitamins B and C, the fat soluble vitamins A, D, E, K.
Also present in milk are the enzymes: phosphatase, lipase, catalase, peroxidase, protease, diastase, amylase and lactase.
Phosphatase and peroxidase are useful as indices of heat of treatment.
REFERENCES
Campbell, J. R. and Marshall, R. T. (1975). The Science of Providing Milk for Man. McGraw-Hill Book Co. NY.
Egbekun, M. K. (1997). Food Biotechnology in Sustenable Food Production and Food Security in Nigeria. Proc 28th Annual Conf. Nutri. Soc. Nigeria.
Fox, B. A. and Cameron, A. G. (1989). Food Science, Nutrition and Health. 5th ed. Edward Amold; A division of Hodder & Stoughton, London.
Passmore R. and Eastwood M. A. (1987). Human Nutrition and Dietetics. 8th Ed. Churchill Livingstone, London.
Pyke M. (1981). Food Science and Technology. 4th Ed. John Murray, London.
