PHYSICAL CHEMISTRY OF MILK

Module 5. Viscosity

Lesson 10
VISCOSITY OF MILK

10.1 Introduction

The viscosity of a substance refers to, its resistance of flow. It is a measure of the friction between molecules as they slide past one another. The viscosity of a heterogeneous substance such as milk at a given temperature depends upon its composition and the physica1 state of its colloidally dispersed substances, including milk fat. The viscosity of fluids is influenced by various factors and knowledge of which is of immense value in understanding the behavior of these fluids during processing.

10.2 Factors Influencing the Viscosity of Milk

Viscosity of milk and milk products is important in determining the flowing rate of cream, rates of mass and heat transfer, the flow conditions in dairy processes. Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior in which the viscosity is independent of the rate of shear. The viscosity of these products depends on the temperature and pH. An increase or decrease in pH of milk also causes an increase in casein micelle voluminosity.

Important factors that influence the viscosity of milk are as follows:

1. State and concentration of protein

2. State and concentration of fat

3. Temperature of milk

4. Age of the milk

10.2.1 State and concentration of the protein

The viscosity of colloidal systems depends upon the volume occupied by the colloidal particles. Changes in the caseinate micelles produced by either raising or lowering the pH results in increased viscosity. The viscosity is approximately doubled by the addition of 10 ml of 1.4 to 3.8 N ammonia to 90 ml milk. Addition of alkali (pH up to 11.7), urea (up to 4.8 M) and calcium complexing agents to concentrated (22.7% solids) skim milk causes a marked transient increase of several folds in viscosity followed by a sharp decline. This is due to the swelling of the micelles followed by their disintegration.

10.2.2 State and concentration of fat

Viscosity increases with increasing concentration of fat and solids-not -fat, but consistent general relationship could not be established

10.2.3 Temperature

The viscosity of milk and dairy products depends up on the temperature and on the amount and state of dispersion of the solid components. Cooling temperature increase viscosity due to the increased voluminosity of casein micelle and temperatures above 65°C increase viscosity due to the denaturation of whey proteins

Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependent on the shear rate. Agitation may cause partial coalescence of the fat globules (partial churning) which increases viscosity. Fat globules that have under gone cold agglutination may be dispersed due to agitation, causing a decrease in viscosity.

The viscosity of milk and other products at 20 ^oC is given in the Table 10.1

Table.10.1 Viscosity of whey and various types of milk

Type of Milk	Viscosity (centipoise)
Whey	1.2
Skim Milk	1.5
Whole milk	2.0

From these values it is evident that the caseinate micelles and the fat globules are the most important contributors to the viscosity. The effect of temperature on the viscosity of normal fluid milk and in comparison to the various other fluids is given in table 10.2.

Table.10.2 Viscosity of milk, skim milk and lactose at various temperatures

Temperature (°C)	Viscosity (cp)
	Whole milk	Skim milk	5% lactose	Water
5	3.254	3.96	1.76	1.519
10	2.809	2.47	1.50	1.308
15	2.463	2.10	..	1.140
20	2.127	1.79	1.15	1.005
25	1.857	1.54	1.03	0.894
30	1.640	1.33	0.91	0.894
35	--	1.17	. --	0.723
40	--	1.04	0.74	0.656

The viscosity of 5% lactose solution at a given temperature is not much greater than that of water but skim milk has an appreciable viscosity approaching to that of whole milk. The difference in the viscosity between 5% lactose and skim milk indicates the role of milk proteins on the viscosity of milk. Lactose, whey proteins and milk salts have relatively small contribution towards the viscosity of milk whereas the casein alone is contributing substantially towards this property. The tendency of milk to increase the viscosity upon heating as it will approaches to a point of coagulation of proteins. This tendency of milk is the basis for producing high viscosity super heated condensed milk. Conditions and treatments that affect the stability of casein are very significant in the viscosity of milk. Acidity, salt balance, heat treatment and the action of various enzymes and bacteria are some of the factors which affect the stability of caseins.

Heating of skim milk to such a degree that most of the whey proteins are denatured causes an increase in viscosity by about 10%. Homogenization has little effect volume fraction of fat increases (because of the larger surface covered with protein volume fraction of casein and whey decreases (by providing material for the surface layers)

10.3 Determination of Viscosity

The unit of viscosity is poise. It is defined as the force required to maintain a relative velocity of 1cm/sec between two parallel planes placed 1 cm apart. The unit commonly used for milk is centi poise (10^-2 poise)

A useful quantity in fluid flow calculations is the kinematics viscosity or viscosity/density.

In dealing with the solutions and colloidal dispersion the following quantities are often used

Relative viscosity: η_rel = η_{soln /} η_solv

Specific viscosity: η_sp = η_rel - 1

Reduced viscosity : η_red = η_spC where c is the concentration of the solute

Intrinsic Viscosity: [ η]= lim (η_sp/ C) as c goes to zero

10.3.1 Types of viscometers

Three types of viscometers (Fig. 10.1) that are used for determination of viscosities of dairy products are

• Coaxial cylinders: e.g. Mc Michaecouetter and Brookfield
• Falling sphere e.g. Hoeppler
• Capillary tubes e.g. Ostwalds, McKennell

Falling sphere and capillary tube viscometers are not suitable for measurements of non-Newtonian fluids because of the correction for non uniform shear rates. These methods are not only tedious but the results obtained are not accurate. A Mobil meter, which has some features of both the coaxial cylinder and falling sphere viscometers, has been used to measure viscosities of evaporated milk. A sealed micro viscometer of the falling sphere type in which the specimen can be sterilized has been proposed for studies of changes occurring in sterilization of concentrated milk products.