BY-PRODUCTS TECHNOLOGY

Module 2. Skim milk and its by-products

Lesson 4
PHYSICO-CHEMICAL CHARACTERISTICS OF SKIM MILK

4.1 Introduction

Skim milk is a by-product obtained during the manufacture of cream. It is rich in solid not fat content and has high nutritional value. This milk is also referred as non-fat, fat free or defatted milk.

4.2 Physico-Chemical Characteristics of Skim Milk

4.2.1 Chemical composition

Skim milk and cream are the products of whole milk separation through a cream separator. As per FSSAI (2011) skimmed milk means the product prepared from milk from which almost all the milk fat has been removed mechanically and it should have not more than 0.5% fat and minimum 8.7% milk SNF. A fat content in skim milk below 0.1% is desirable. Buffalo skim milk contains higher total solids i.e. about 10.7% including higher lactose and protein mainly casein content as compared to cow skim milk having about 9.3% total solids. Typical chemical composition of skim milk is shown in

4.2.2 Density/ Specific gravity

The density of milk is a resultant of the densities of its components complicated by variations in the ratio of solid to liquid fat and in the degree of hydration of the proteins. Thus the density of a given specimen of milk is determined by its percentage composition, by its temperature and processing treatments. Specific gravity of skim milk at 15.5°C is 1.036.

4.2.3 Viscosity

The viscosity of the milk depends on the temperature and on the amount and state of dispersion of the solid components. Representative values at 20°C for skim milk is 1.5 cP. It is evident that the caseinate micelles are the most important contributors to the viscosity. There is decline of viscosity from 5 to 30°C, reflecting a decrease in voluminosity of the caseinate micelles. Above 30°C, the decrease is less marked until about 65°C, where the whey proteins begin to be denatured.

4.2.4 Surface and interfacial tension

The area of contact between two phases is called “interface”, or especially if one of the phases is gaseous, the “surface”. Properties of interfaces and surfaces are determined by the number, kind and orientation of molecules located in them. Surface tension of skim milk at 20°C is 51 dynes cm^-1.

4.2.5 Electrical conductivity

Electrical conductivity of milk is mainly due to Na, K, and Cl etc. The fat globules of milk reduce the conductivity by occupying volume and by impeding the mobility of ions. Thus the conductivity of the whole milk is less than that of skim milk by about 10%. The production of acid by bacterial action, of course, increases the conductivity of milk. Temperature control is important in the measurement, since the conductivity increases about 0.0001 ohm^-1 cm^-1 per °C rise in temperature.

4.2.6 Refracting index

Refracting index of a substance is defined as the ratio of the speed of light in a vacuum to its speed in that substance. One consequence of refraction is to change the direction of a light ray as it enters or leave the substance. Measurement of this bending gives a direct measure of refractive index. Refractive index of skim milk falls in the range of 1.3440-1.3485.

4.2.7 Solubility

Between pH 3.5 and 5.5, caseins are very insoluble which restricts its use in low pH foods.

4.2.8 Heat stability

Skim milk normally withstands heating at 140°C for 20 min, while concentrated skim milk (20% TS) is usually stable at 120°C for 20 min. Heat induced interaction between β-Lg and k-casein plays a major role in determining the heat stability and rennet clotting behavior of milk.

4.2.9 Heat capacity and thermal conductivity

The heat capacity of a substance is the quantity of heat required to raise the unit temperature of a unit mass. It is usually expressed in terms of cal g^-1 °C^-1. Thermal conductivity is the rate of heat transfer by conduction through unit thickness across unit area of substance for a unit difference of temperature.

Skim milk exhibits a small but definite linear increase in heat capacity between 0 and 50°C from about 0.933 to 0.954 cal g^-1 °C^-1.There is a marked decrease in heat capacity as the total solids contents of the sample is increased. Dried skim milk products have heat capacities of 0.28 to 0.32 cal g^-1 °C^-1 in 18 to 30°C temperature range.

Selected reference

Nutting, G. C. 1970. The by-products of milk. In: By-products from milk, second edition, ed. Earle O. Whittier and B. H. Webb. The AVI Publishing Company, Inc.: 17.