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

Lesson 10

10.1 Introduction

Milk is a complex biological fluid consisting of seven main components: water, fat, protein, sugar (lactose), minerals, vitamins and enzymes. It is a white opaque fluid in which fat is present as an emulsion, protein and some mineral matters in colloidal suspension and lactose together with some minerals and soluble proteins in true solution. The opacity of milk is due to its content of suspended particles of fat, proteins and certain minerals. The colour varies from white to yellow depending on the carotene content of the fat. Milk has a pleasant, slightly sweet taste, and pleasant odour. It is an excellent source of calcium, phosphates and riboflavin.

10.2 Physical Properties of Milk

10.2.1 Colour and optical properties

Milk appears turbid and opaque owing to light scattering by fat globules and casein micelles. Optical properties are influenced by the manner of scattering of light by the molecules. Light scattering occurs when the wavelength of light matches the magnitude of the particle. Thus, smaller particles scatter light of shorter wavelengths and vice versa. Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red. Beta-carotene, the carotenoid precursor of vitamin A, is responsible for the creamy colour of cow milk. The greenish tinge in whey is due to the presence of riboflavin. Refractive index of milk is an optical property and ranges from 1.3440 to 1.3485 at 20ºC. The relation between solids content of milk and refractive index is linear, and the contributions of the several constituents is additive.

10.2.2 Flavour of milk

The natural sweet flavour of milk is due to the combined effect of its components. Off-flavours are very quickly developed in milk owing to several factors. The feed consumed by animals may lead to some undesirable flavours. Bacterial growth in milk causes fruity, barny, malty or acid flavours. Enzyme activities also may lead to unnatural flavours, rancidity due to lipase action being a classic example. Oxidative reactions may cause a cardboard flavour in milk. Processing of milk may produce cooked flavours.

10.2.3 Specific gravity and density

Milk is heavier than water. The specific gravity of cow milk varies from 1.018 to 1.036 and of buffalo milk from 1.018 to 1.038. Though specific gravity varies with temperature, (lower at higher temperature and vice versa), the rate of this variation is not uniform.

The density of milk varies within the range of 1.027 to 1.033 kg/cm3 at 20°C. The density of milk is used to estimate the solids content, to convert volume into mass and vice versa and to calculate other physical properties such as dynamic viscosity. It is dependant on temperature at the time of measurement, temperature history of the sample, composition of the sample (particularly fat content) and inclusion of air.

10.2.4 Viscosity

Viscosity of milk depends on the temperature and the amount and state of dispersion of the solid constituents, mainly 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 whereas temperatures above 65°C increase viscosity due to the denaturation of whey proteins. An increase or decrease in pH of milk also causes an increase in casein micelle voluminosity. The effect of agitation on viscosity is not uniform. Sometimes, agitation causes partial coalescence of the fat globules, hence increasing the viscosity and at other times, agitation may disperse fat globules that have undergone cold agglutination, leading to a decrease in viscosity.

10.2.5 Surface tension

The surface activity of milk is related to proteins, fat, phospholipids and fresh fatty acids present in it. Homogenization and heat sterilization increase the surface tension of milk. Milk has a surface tension of 50 dyne/cm at 20°C.

10.2.6 Freezing and boiling points of milk

The freezing points of cow and buffalo milk vary from -0.512 to -0.572ºC and from -0.521 to -0.575ºC respectively. Freezing point of milk is mainly used to determine added water. The boiling point of milk is 100.17ºC.

10.2.7 Acidity and pH

Freshly drawn milk has a pH value in the range of 6.5 to 6.7 and contains 0.14 to 0.18% titratable acid calculated as lactic acid. There is no developed acidity in freshly drawn milk, the slightly lower than the neutral pH being attributed to the presence of carbon dioxide, citrate, casein etc.

10.2.8 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. The stability of milk system at the high processing temperatures to which milk is exposed for the manufacture of certain products is very important. Caseins and salt balance of milk governs its heat stability. Added citrates, phosphates and calcium have a great impact on the heat stability.

10.3 Composition of Milk

Factors that influence the chemical composition of milk are individuality of the animal, breed variation, seasonal changes, weather, age and health of the animal, managerial practices including nature and quality of feed, stage of lactation, the quarter of the udder of the animal from which milk is drawn, different fractions of milking etc. Tables 10.1 and 10.2 summarize the composition of milk.

Table 10.1 Average composition of milk of different mammals (%)

Table 10.1
(Average compiled from various sources)

Table 10.2 Average composition of milk of selected breeds of cows (%)

Table 10.2
(Average compiled from various sources)

Fresh milk contains 84–87% water in which all other constituents of milk are dissolved and in which are dispersed two different systems, namely fat globules enclosed within their protective membrane as an oil-in-water emulsion, and protein, containing casein molecules and insoluble salts in a colloidal suspension.

Selected Readings

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.

Last modified: Monday, 29 October 2012, 5:03 AM