PHYSICAL CHEMISTRY OF MILK

Module 1. Milk composition

Lesson 1
COMPOSITION OF COW MILK

1.1 Introduction

Milk derived from cattle species is an important food. It has many nutrients and the precise nutrient composition of raw milk vary by species and by a number of other factors. Cow milk is available for marketing and utilization in the preparation of various products world over in a large proportion. Globally milk from other species like ovine, caprine, is utilized in a very insignificant proportion. For millennia, cow's milk has been processed into dairy products such as cream, butter, fermented milk products like dahi & yogurt, ice cream, cheese etc.

1.2 Gross Composition of Cow Milk

Majority of the people utilize cow milk for their day to day needs. As such it is necessary to study the gross composition of cow’s milk. It could be observed from the Table 1.1 that the major constituent of milk is water which ranges from 85.5 % to 88.7%. Lactose, the milk sugar, is high in its concentration ranging from 3.8 % to 5.3%. There is considerable variation in fat content and it ranges from 2.4 % to 5.5%. The building block of the living cells, the proteins, range between 2.3 % and 4.4%. The concentration of the major protein of milk, casein ranges from 1.7% to 3.5%. The remaining constituents although less in their concentration play an important role in maintaining the subtle balance among the various constituents in milk. Organic acids and other miscellaneous substance such as vitamins, citrate, etc. play a vital role both in the nutrition of the young calves and in humans.

Table 1.1 Gross composition of cow milk

Component	Average Content percentage (w/w)	Range Percentage (w/w)	Average % of Dry matter
Water	87.3	85.5 -88.7
Solids not fat	8.8	7.9-10.0	69
Lactose	4.6	3.8-5.3	36
Fat	3.9	2.4-5.5	31
Protein	3.25	2.3-4.4	26
Casein	2.6	1.7-3.5	20
Mineral substances	0.65	0.53-0.80	5.1
Organic acids	0.18	--	1.4
Miscellaneous	0.14	--	1.1

(Source: Dairy Chemistry and Physics, Walstra and Jenness, 1984)

A considerable variation in the composition of milk from various mammals is observed as evident from the data presented in table 1.2. Buffalo milk and sheep milk has highest fat percentage while human milk has highest lactose content. The casein content is low in human milk when compared with other milks.

Table 1.2 Gross composition of milk from different species of mammals

Species	Scientific name	COMPOSITION (g/100g)
		Water	Fat	Casein	Whey proteins	Lactose	Ash
Human	Homo sapiens	87.1	4.5	0.4	0.5	7.1	0.2
Cow	Bos Taurus	87.3	3.9	2.6	0.6	4.6	0.7
Zebu	Bos indicus	86.5	4.7	2.6	0.6	4.7	0.7
Buffalo	Bubalus bubalis	82.8	7.4	3.2	0.6	4.8	0.8
Goat	Capr hircus	86.7	4.5	2.6	0.6	4.3	0.8
Sheep	Ovis aries	82.0	7.2	3.9	0.7	4.8	0.9
Horse	Equus caballus	88.8	1.9	1.3	1.2	6.2	0.5
Camel	Camelus dromedaries	86.5	4.0	2.7	0.9	5.0	0.8

(Source: Jenness and Sloan,1970, Dairy science abstracts,32,599-612)

1.3 Major Constituents of Milk

The major constituents of milk are, water, fat, proteins, lactose, minerals and miscellaneous compounds.

1.3.1 Water

It is the medium in which all the milk constituents are dispersed. It is a universal vehicle and plays an important role in the physical properties exhibited by the milk. It also assists in maintaining the much desirable equilibrium and keeps the constituents of the milk in their native state. Water acts as a solvent in keeping the constituents in soluble, colloidal or emulsion form and influences the collegative properties of milk.

1.3.2 Fat

Fat is the costliest component of milk. It is characterized by its presence as an emulsion. The unique feature of milk fat is its fatty acid composition. Milk fat is rich in saturated fatty acids and mono unsaturated fatty acids and also acts as a carrier for the fat soluble vitamins. The unique feature of the milk fat from ruminants is that they are having short chain saturated fatty acids in substantial quantities and imparts the unique flavour to the milk and milk products. Presence of long chain unsaturated fatty acids reduce the melting point of milk fat. In order to maintain the emulsion stability several surface active substance are also associated with the milk fat and present in higher proportion in fat globule membrane.

1.3.3 Proteins

Milk is rich in protein content and has a unique protein namely the casein. It is present in the form of colloidal dispersion in milk and is responsible for several physical properties of milk. It also supplies all the essential amino acids, hence a complete protein and is easily digestible. In addition to casein, milk also contains (other) proteins which are known as whey proteins. Milk is a biological secretion of living cells synthesized through several (enzyme mediated) biochemical reactions (which are often mediated through enzymes). It is a well known fact that these enzymes are basically protein in nature and act as biological catalysts. Milk is a good source for several enzymes which have both desirable and also undesirable effect on preservation and processing of milk.

1.3.4 Lactose

Lactose is a major soluble component present in milk. Milk contains approximately 4.6% carbohydrate that is predominately lactose with trace amounts of monosaccharides and oligosaccharides. Lactose is a disaccharide of glucose and galactose. It is first carbon source for the microbes gaining entry into milk. It is a reducing sugar and is present in soluble state. It is a good source of energy for the young ones. In addition to lactose, milk also contain several other carbohydrates in trace levels (negligible).

1.3.5 Minerals

Minerals in milk are mainly inorganic salts, partly ionized and partly present as complex salts. Some inorganic matter is bound covalently such as calcium phosphate groups in casein. They are responsible for the ionic balance of milk and helps in maintaining the equilibrium between the soluble and colloidal state.

1.3.6 Vitamins

Milk contains almost all the essential vitamins which include fat soluble and water soluble vitamins. It is a very good source for the most essential fat soluble vitamin namely the Vitamin A. It is also interesting to observe that this vitamin is existing in the cow milk in its precursor form i.e. carotene a provitamin A imparts golden yellow colour to milk. Milk is not a good source of Vitamin K and Vitamin C.

1.3.7 Miscellaneous compounds

Milk contains many components which are in low concentrations (less than 100 mg/liter) which do not fall into any of the above categories. These compounds may be considered as natural if they present in freshly drawn milk and have been detected in most samples tested for that component. The compounds considered in this category are gases, alcohols, carbonyl compounds, carboxylic acids, conjugated compounds, non protein nitrogenous compounds, phosphate esters, nucleotides, nucleic acids, sulfur containing compounds, hormones etc.