PHYSICAL CHEMISTRY OF MILK

Module 3. Density

Lesson 7
FACTOR AFFECTING DENSITY AND SPECIFIC GRAVITY OF MILK

7.1 Introduction

Milk is often subjected to various processes before being allowed for marketing. During processing of milk there will be change in the density or the specific gravity of milk. A number of factors influence density or specific gravity of milk such as addition or removal of water, cream or other components etc. and temperature and pressure used during the processing of milk.

7.2 Factor Affecting the Density/Specific Gravity of Milk

Density of milk is influenced by the combined effect of densities of its various components. As such it is dependent on the amount of dissolved or suspended matter, changes in the constituents and the physical state of components in milk. Milk density is further influenced by the various factors such as temperature history of samples, biological differences of micelles and processing conditions of milk. Among the various milk constituents, milk fat content is known to be the main cause for variation in the density of milk. The physical state of the milk fat was observed to have greater influence on density.

7.2.1 Recknagel phenomenon

Recknagel, observed that the density of specific gravity of milk as soon as milking is lower than the same milk held for long periods of time, especially of milk under cold storage. Such phenomenon is known as Reckngel phenomenon. He attributed the increase in the hydration of the protein at low temperature as the major cause for such a phenomenon rather than the escape of the air bubbles. Subsequent work carried out by other scientists attributed this phenomenon to the ratio between the liquid and solid.

7.2.2 Temperature

Density and specific gravity decreases with increase of temperature and decreased with increase in temperature. This is the reason for obtaining corrected lactometer reading while calculating the specific gravity of milk at designated temperature of the lactometer. Sp. gravity of milk is = 1 + CLR / 1000, Where CLR is Corrected lactometer reading at a particular temperature expression. For attaining the maximum density and to avoid the Recknagel phenomenon it is suggested that milk should be heated to a temperature of 40^°C and specific gravity is determined when cooled to 20^°C to ensure reproducible state of milk fat.

7.2.3 Type of the milk and breed

These two factors influence the density/specific gravity of milk since there is variation in the composition which would otherwise influence this property. As the milk fat is lighter constituent, the milk with higher fat content will have lower the specific gravity and vice versa. However, although buffalo milk contains more fat than cow milk, its specific gravity is higher than the latter. This is because buffalo milk contains more solids-not-fat as well, which ultimately results in a higher specific gravity.

The specific gravity of milk is decreased by:

• Addition of water
• Addition of cream (fat)
• Increased temperature.

The specific gravity of milk is increased by:

• Addition of separated milk
• Removal of fat
• Reduction of temperature.

7.2.4 Processing and other factors

There will be an increase in the milk density due to refrigerated storage. Slow crystallization of the fat and change in the hydration of the globule membrane is responsible for this increase in the density.

• Fat content and temperature have been related to the density of creams.
• Homogenization slightly increases the density of whole milk but not of skim milk. Sterilization decreases the density of both milks. These changes are very small and negligible due large variations observed from sample to sample.
• The specific gravity of cream decreases regularly with increase in the fat content. Skim milk has specific gravity of 1.0320 to 1.0365 at 15^°C /15^°C. The removal of water in the manufacture of concentrated milk products raises the specific gravity.

• Baume hydrometer is widely used for this purpose. The Baume’s reading is related to specific gravity as follows :

1. Sp. Gravity at 60°F/60°F = 145/145-Be Where Be = Baume scale reading at 60°F
2. The concentrated milk products will have high viscosity as such the Baume’s reading is often taken at 120°F instead of 60°F.

• The milk with higher SNF content will have higher density. Similarly the processes in which water is removed will have similar effect on the density of milk.
• Densities of the liquid dairy products such as whey, evaporated milk, sweetened condensed milk and freshly frozen ice cream vary in similar way as to milk.