Physical and chemical properties of milk

Physical and chemical properties of milk

Milk is the fluid secreted by mammals for the nourishment of their young while colostrum is a much more concentrated liquid secreted immediately after parturition. A number of animals produce milk, but cow is most commercially utilized.  Milk is a white or yellow-white, opaque liquid.

Physical Properties of Milk

Colour and optical properties: Milk is turbid and opaque due to light scattering by fat globules and protein micelles. Skim milk appears slightly blue due to the higher protein or casein content. Beta-carotene content is responsible for the creamy colour of cow milk. Riboflavin is responsible for the greenish tinge in whey.  Refractive index of milk ranges from 1.3440 to 1.3485 at 20ºC.

Flavour of milk: Naturally milk have a sweet flavour. Off-flavours quickly develop in milk due to several factors such as feed consumed by animals, bacterial growth, enzyme activities, oxidative reactions, etc.   Bacterial growth causes fruity, barny, malty or acid flavours, enzyme activities cause rancidity, oxidative reactions cause a cardboard flavour and processing of milk cause cooked flavours.

Specific gravity and density: Milk is heavier than water and its specific gravity varies from 1.018 to 1.036.  The density of milk varies within the range of 1.027 to 1.033 kg/cm3 at 20°C. The density of milk indicates the solid content.

Viscosity: Viscosity of milk depends on the temperature and the amount and state of dispersion of the solid constituents such as casein and fat. Viscosity of the whole milk at 25°C is about 2.0 cP. Cooler temperatures increase viscosity due to the increased voluminosity of casein micelles and temperatures above 65°C increase viscosity due to the denaturation of whey proteins. An increase or decrease in the milk pH also increase viscosity due to increase in casein micelle voluminosity. The effect of agitation on milk viscosity is not uniform.

Surface tension: Milk has a surface tension of 50 dyne/cm at 20°C. It is dependent upon proteins, fat, phospholipids and fresh fatty acids present. Homogenization and heating increase the surface tension of milk.

Freezing point and boiling point of milk: The freezing points of milk vary from -0.512 to -0.572ºC and the boiling point of milk is 100.17ºC.

Acidity and pH: Freshly drawn milk has a pH ranging from 6.5 to 6.7.  Milk contains 0.14 to 0.18% titratable acid calculated as lactic acid.

Heat stability of milk: Heat stability is defined as the length of time required to induce coagulation at a given temperature or the temperature required to induce coagulation in a given time. Heat stability depends on caseins and salt balance of milk. Added citrates, phosphates and calcium also influence the heat stability.

Chemical Properties of Milk - Composition of Milk

There are several factors that influence the chemical composition of milk.  They are the age and health of the animal, breed of animal, seasonal changes, weather, nature and quality of feed, stage of lactation, the quarter of the udder of the animal from which milk is drawn, etc.

The major components of milk are water, fat, protein and lactose.  It also contains minerals, vitamins and enzymes. Milk contains Lipids 37 g/L, Proteins 34 g/L, Non-protein Nitrogen 1.9 g/L, Lactose 48 g/L, Citric acid 1.75 g/L, Ash 7.0 g/L, Calcium 1.25 g/L, Phosphorus 0.96 g/L.  Fresh milk from a healthy cow is generally free from bacteria.

Water is the principal constituent in milk, 79% to 90%.  All other constituents of milk are either dissolved or suspended in water.   The water activity in milk is relatively high, it is 0.993.  Removal of water increases shelf life of milk, powdered milk have more shelf life. 

The fat or lipid content of milk is the most variable feature. It is comprised mainly of C14 (myristic), C16 (palmitic), C18 (stearic), and C18:1 (oleic) fatty acids.  They are present in fresh milk in the form of fat globules surrounded by a phospholipid rich layer and this is known as the milk fat globule membrane. These globules have a diameter of about 5 mm and the milk contains about 10¹² fat globules per litre. Upon standing, the fat in fresh milk rises to the surface of the milk and produce a distinct cream line.  Homogenization or the passing of milk through a small orifice under pressure reduces this tendency.

About 80–85% of the protein in milk is present as caseins. Casein is milk-specific protein and are precipitated from milk by decreasing the pH to 4.6.  There are five main classes of caseins.  They aggregate together in association with calcium phosphate in milk to form colloidal particles known as micelles.   They have an average diameter of around 0.2 mm. Milk contains around 10¹⁵casein micelles per liter. The micelle is stabilized and maintained by k-casein near or on the surface of the particle. When k-casein is cleaved by chymosin it leads to the sticking together of micelles to form a coagulum, for example during cheese production.

Whey proteins are the rest of protein in milk. These comprise the compact globular proteins β - lactoglobulin and α-lactalbumin.  Milk also contains serum albumin and immunoglobulins.  Immunoglobulins are present in higher concentration in colostrum.

Raw whole milk contains 29.36% fat, 26.98% protein (22.22% casein, 4.76% whey proteins), 38.1% lactose, and 5.56% minerals on a dry basis. Milk contains calcium, phosphates and riboflavin. Fresh milk contains 84–87% water in which all other constituents of milk are dissolved and dispersed as two different systems. Fat globules are enclosed as an oil-in-water emulsion, and protein and insoluble salts are in a colloidal suspension.

Several carbohydrates such as Lactose, Glucose, Galactose, Glycoconjugates (oligosaccharides, glycoproteins, and glycoaminoglycans), etc are present in milk. Main carbohydrate is lactose.  Lactose exists in three forms, α - lactose monohydrate, β – lactose and anhydrous α - lactose.

Milk contains almost all minerals essential for human nutrition including potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), chloride (Cl), and phosphate esters.

Approximately 20 different enzymes have been characterized in milk such as Lipoprotein lipase, Alkaline phosphatase, Sulfhydryl oxidase, Lactoperoxidase, Catalase, Xanthine oxidase, Superoxide dismutase, γ- Glutamyltransferase, etc.

References

De, S. 1980. Outlines of Dairy Technology. Oxford University Press, Delhi

Robert Jenness, R., Noble P. Wong, N.P. and Elmer H. Marth, E.H. 1999. Fundamentals of Dairy Chemistry. Aspen Publishers, USA.

Food Microbiology, Third Edition,  Martin R. Adams and Maurice O. Moss University of Surrey, Surrey, Guildford, UK.