Definition of Milk
Milk may be defined as the whole, fresh, clean lacteal secretion obtained by the complete milking of one or more healthy milchy animals, excluding that obtained within 15 days before or 5 days after calving or such periods as may be necessary to render the milk practically colostrum free, and containing the minimum prescribed percentage of milk fat and milk-solids- not fat. In India, the term ‘milk’ when unqualified, refers to cow or buffalo milk or a combination of the two.
The term market milk refers to fluid whole milk that is sold to individuals usually for direct consumption. It excludes milk consumed on the farm and that used for the manufacture of dairy products.
Water :
Water constitutes the medium in which the other milk constituents are either dissolved or suspended. Most of it is free and only a very small portion is in the bound form, being firmly bounded by milk proteins, phospholipids etc.
Total Solids :
Total Solids constituents lipids (Fat) and solid not fat.
Milk Fat (Lipids) :
The bulk of the fat in the milk exists in the form of small globules, which average approximately 2 to 5 microns in size. This is an oil-in-water type emulsion. The surface of these fat globules is coated with an adsorbed layer of material commonly known as the fat globule membrane. This membrane contains phospholipids, and proteins in the form of a complex and stabilizes the fat emulsion. In other words, the membrane prevents the fat globules from coalescing and separating from one another. The emulsion may, however, be broken by agitation (at low temperature), heating, freezing etc.
Chemically, milk fat is composed of a number glyceride-esters of fatty acids Milk fat on hydrolysis gives a mixture of fatty acids and glycerol. (The milk fat is a mixture of true fats in established from the fact that it has no sharp melting point). The fatty acids are saturated or unsaturated fatty acids. Saturated fatty acids are relatively stable.
The fat associated substances are phospholipids, cholesterol, carotene and fat soluble vitamins (A, D, E, K).
Phospholipids :
Three types of phospholipids, ie. Lecithin, Cephalin and Sphingomylin. Lecithin, which forms an important constituent of the fat globule membrane, contributes to the richness of flavour of milk and other dairy products. It is highly sensitive to oxidative changes, giving rise to oxidized / metallic flavours. Phospholipids are excellent emulsifying agents, and no doubt serve to stabilize the milk fat emulsion. Cholesterol : This appears to be present in true solution in the fat, as a part of fat globule membrane complex and in complex formation with protein in the non-fat portion of milk.
Fat Soluble Pigments :
Carotene in fat soluble and responsible for the yellow colour of milk, cream, butter, ghee and other fat rich products. Carotene acts as antioxidant and also as a precursor of Vitamin A. One molecule of B – carotene gives two molecules of Vitamin A, where as α - carotene give one.
Fat Soluble Vitamins :
Milk is rich in Fat soluble vitamins ie. A, D, E and
Solid-not-fat content contains lactose, proteins and mineral contents.
Milk Sugar or Lactose :
This exists in milk only. It is in true solution in the milk serum. On crystallization from water, it forms hard gritty crystals. It is one-sixth as sweet as sucrose. Lactose is responsible for the defect known as sandiness in ice-cream or condensed milk. It is fermented by bacteria to yield lactic acid and other organic acids and is important both in the production of cultured milk products and in the spoilage of milk and milk products by souring.
Milk Proteins :
The proteins in milk consists mainly of casein, lactaglobulin, lactalbumin, milk serum albumin, immuno globulins etc. Casein forms more than 80% of the total proteins of the milk. Casein exists only in milk and is found in the form of calcium caseinate phosphate complex. It is present in colloidal state. It may be precipitated by acid, rennet, alcohol, heat and concentration. Casein compose of α,β and gamma fraction. α - casein constitutes 75% of casein which is responsible for stabilization of casein micelle in milk.β and gamma forms constitutes 22 and 3 percent respectively. α - casein constitute two fractions. αs is calcium sensitive which is coagulated by calcium ions and another form is k-casein which is called calcium insensitive casein fraction, not precipitated by calcium Ion. K-casein is the richest repository of carbohydrates as against other casein fractions. It is the site for rennin action.
Lactalbumin and lactaglobulin are known as ‘Whey or serum proteins’. They are also present in colloidal state and are easily coagulated by heat. Milk serum albumin is same as blood serum albumin of the blood. Immunoglobulins are present only in colostrum and gives immunity to the calves.
Non protein nitrogenous Compounds : Eg: Ammonia, aminiacids, proteose-peptones, urea, uric acid etc.
Mineral Matter or Ash :
The mineral matter or salts of milk although present in small quantities, exert considerable influence on the physico- chemical properties and nutritive value of milk. The major salt constituents i.e. those present in appreciable quantities, includes potassium, sodium, magnesium, calcium, phosphate, citrate, chloride, sulphate and bicarbonate.
The trace elements include all other minerals and salt compounds. The mineral salts of milk are usually determined after ashing.
Although milk is acidic, ash is distinctly basic. Part of the mineral salts occur in true solution, while a part are in colloidal state.
Other Constituents :
Pigments :
Water soluble pigments are Riboflevin and xanthophyll. Riboflavin besides being a vitamin, is a greenish yellow pigment which gives characteristic colour to whey. Earlier it is known as lactoflavin or lactochrome.
Dissolved Gases :
Milk contains gases like O2, Co2, N2 etc.
Vitamins : Water soluble vitamins B complex and vitamin ‘C’
Enzymes : These are biological catalysts. Milk contains Amylase, Lipase, Phosphatase, protease, peroxidase and catalase enzymes.
Detailed Composition of Milk
Constituents or Group of Constituent Approx. Conc. Wt per Litre of milk
Water 860 – 880 g
Lipids in emulsion phase
Milk fat 30 – 50 g.
Phospholipids 0.30 g.
Sterols 0.10 g.
Vitamin A D E K -
Proteins in Colloidal Dispersion
Casein (α, β, γ) 25 g.
Lactalbumin 3 g.
Lactaglobulin 0.7 g.
Albumin,
pscudoglobulin etc
Dissolved Materials
Lactose 45 – 50 g.
Inorganic and organic ions and salts
Calcium 1.25 g.
Phosphate 2.1 g.
Citrates 2.0 g.
Chlorides 1.0 g.
Trace Minerals
Cu, Fe, I Traces
Milk may be defined as the whole, fresh, clean lacteal secretion obtained by the complete milking of one or more healthy milchy animals, excluding that obtained within 15 days before or 5 days after calving or such periods as may be necessary to render the milk practically colostrum free, and containing the minimum prescribed percentage of milk fat and milk-solids- not fat. In India, the term ‘milk’ when unqualified, refers to cow or buffalo milk or a combination of the two.
The term market milk refers to fluid whole milk that is sold to individuals usually for direct consumption. It excludes milk consumed on the farm and that used for the manufacture of dairy products.
Water :
Water constitutes the medium in which the other milk constituents are either dissolved or suspended. Most of it is free and only a very small portion is in the bound form, being firmly bounded by milk proteins, phospholipids etc.
Total Solids :
Total Solids constituents lipids (Fat) and solid not fat.
Milk Fat (Lipids) :
The bulk of the fat in the milk exists in the form of small globules, which average approximately 2 to 5 microns in size. This is an oil-in-water type emulsion. The surface of these fat globules is coated with an adsorbed layer of material commonly known as the fat globule membrane. This membrane contains phospholipids, and proteins in the form of a complex and stabilizes the fat emulsion. In other words, the membrane prevents the fat globules from coalescing and separating from one another. The emulsion may, however, be broken by agitation (at low temperature), heating, freezing etc.
Chemically, milk fat is composed of a number glyceride-esters of fatty acids Milk fat on hydrolysis gives a mixture of fatty acids and glycerol. (The milk fat is a mixture of true fats in established from the fact that it has no sharp melting point). The fatty acids are saturated or unsaturated fatty acids. Saturated fatty acids are relatively stable.
The fat associated substances are phospholipids, cholesterol, carotene and fat soluble vitamins (A, D, E, K).
Phospholipids :
Three types of phospholipids, ie. Lecithin, Cephalin and Sphingomylin. Lecithin, which forms an important constituent of the fat globule membrane, contributes to the richness of flavour of milk and other dairy products. It is highly sensitive to oxidative changes, giving rise to oxidized / metallic flavours. Phospholipids are excellent emulsifying agents, and no doubt serve to stabilize the milk fat emulsion. Cholesterol : This appears to be present in true solution in the fat, as a part of fat globule membrane complex and in complex formation with protein in the non-fat portion of milk.
Fat Soluble Pigments :
Carotene in fat soluble and responsible for the yellow colour of milk, cream, butter, ghee and other fat rich products. Carotene acts as antioxidant and also as a precursor of Vitamin A. One molecule of B – carotene gives two molecules of Vitamin A, where as α - carotene give one.
Fat Soluble Vitamins :
Milk is rich in Fat soluble vitamins ie. A, D, E and
Solid-not-fat content contains lactose, proteins and mineral contents.
Milk Sugar or Lactose :
This exists in milk only. It is in true solution in the milk serum. On crystallization from water, it forms hard gritty crystals. It is one-sixth as sweet as sucrose. Lactose is responsible for the defect known as sandiness in ice-cream or condensed milk. It is fermented by bacteria to yield lactic acid and other organic acids and is important both in the production of cultured milk products and in the spoilage of milk and milk products by souring.
Milk Proteins :
The proteins in milk consists mainly of casein, lactaglobulin, lactalbumin, milk serum albumin, immuno globulins etc. Casein forms more than 80% of the total proteins of the milk. Casein exists only in milk and is found in the form of calcium caseinate phosphate complex. It is present in colloidal state. It may be precipitated by acid, rennet, alcohol, heat and concentration. Casein compose of α,β and gamma fraction. α - casein constitutes 75% of casein which is responsible for stabilization of casein micelle in milk.β and gamma forms constitutes 22 and 3 percent respectively. α - casein constitute two fractions. αs is calcium sensitive which is coagulated by calcium ions and another form is k-casein which is called calcium insensitive casein fraction, not precipitated by calcium Ion. K-casein is the richest repository of carbohydrates as against other casein fractions. It is the site for rennin action.
Lactalbumin and lactaglobulin are known as ‘Whey or serum proteins’. They are also present in colloidal state and are easily coagulated by heat. Milk serum albumin is same as blood serum albumin of the blood. Immunoglobulins are present only in colostrum and gives immunity to the calves.
Non protein nitrogenous Compounds : Eg: Ammonia, aminiacids, proteose-peptones, urea, uric acid etc.
Mineral Matter or Ash :
The mineral matter or salts of milk although present in small quantities, exert considerable influence on the physico- chemical properties and nutritive value of milk. The major salt constituents i.e. those present in appreciable quantities, includes potassium, sodium, magnesium, calcium, phosphate, citrate, chloride, sulphate and bicarbonate.
The trace elements include all other minerals and salt compounds. The mineral salts of milk are usually determined after ashing.
Although milk is acidic, ash is distinctly basic. Part of the mineral salts occur in true solution, while a part are in colloidal state.
Other Constituents :
Pigments :
Water soluble pigments are Riboflevin and xanthophyll. Riboflavin besides being a vitamin, is a greenish yellow pigment which gives characteristic colour to whey. Earlier it is known as lactoflavin or lactochrome.
Dissolved Gases :
Milk contains gases like O2, Co2, N2 etc.
Vitamins : Water soluble vitamins B complex and vitamin ‘C’
Enzymes : These are biological catalysts. Milk contains Amylase, Lipase, Phosphatase, protease, peroxidase and catalase enzymes.
Detailed Composition of Milk
Constituents or Group of Constituent Approx. Conc. Wt per Litre of milk
Water 860 – 880 g
Lipids in emulsion phase
Milk fat 30 – 50 g.
Phospholipids 0.30 g.
Sterols 0.10 g.
Vitamin A D E K -
Proteins in Colloidal Dispersion
Casein (α, β, γ) 25 g.
Lactalbumin 3 g.
Lactaglobulin 0.7 g.
Albumin,
pscudoglobulin etc
Dissolved Materials
Lactose 45 – 50 g.
Inorganic and organic ions and salts
Calcium 1.25 g.
Phosphate 2.1 g.
Citrates 2.0 g.
Chlorides 1.0 g.
Trace Minerals
Cu, Fe, I Traces
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