Viscosity of a fluid (liquid or gas) is a measure of its resistance towards flow. This resistance is due to internal friction within a liquid as they slide each other. The unit of expression of viscosity is poise (named after Poiseuille). A poise is the force of one dyne acting on area of one square centimeter between two parallel planes one centimeter apart, to produce a difference in flow rate between the planes of one centimeter per second.

In milk centipoise is commonly used to express viscosity, which is one hundreth of a poise.

Principle of Viscosity: Viscosity can be measured both in absolute or relative terms. Absolute viscosity is the viscosity in poise or centipoise. Relative viscosity is the rate of flow of liquid. It is either volume flow during a fixed period of time or time for a fixed volume under specified conditions. The absolute viscosity of water is 1.005 centipoise at 20 0 C. Thus centipoise is the viscosity exhibited by water at 20 0 C.

Viscosity can be measured by the following methods

i)Ostwald pipette which is based on the principle of time of flow under a fixed pressure

ii) MacMichael Viscometer-Measuring the force required to move two layers of liquid past each other.

iii) Falling Ball Viscometer-by measuring the fall of a ball through a column of liquid e.g. Hoeppler viscometer.

Viscosity of Milk: Viscosity of milk ranges between 1.5 to 2.0 centipoise at 20 0 C.Due to fat emulsion and colloidal particles milk is viscous than water. Any alternation in the physical nature of fat or protein hydrolysis, cooling or heating of milk affects proteins and fat and thus the viscosity. Clustering of fat globules affects viscosity e.g. cream where viscosity increases due to clustering of fat globules. Likewise homogenization of milk results in the state of sub-division of dispersed constituents e.g. fat. Thus homogenization of milk increases the viscosity. Viscosity increases also due to heating and concentration e.g. condensed milk due to increased total solids and changes in milk constituents.

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