## Specific gravity:

**Specific gravity** is the ratio of the density of a substance to the density of a reference substance; relative density.

## Lactometer:

A **lactometer** is just like a hydrometer, consisting of a long narrow graduated (10 or 15 to 40; Quevenne scale) glass stem which is enlarged below into a hollow glass bulb weighted with mercury and lead shots.

## Principle:

When a lactometer floats in a milk sample it displaces an amount of milk equal in weight to the weight of the floating lactometer.

## Apparatus:

Lactometer, Glass cylinder, and thermometer.

## Procedure:

- The given sample of milk is thoroughly mixed and brought to a temperature between 50-70°F.
- It is then poured into a high glass cylinder by its sides and the lactometer is slowly lowered into the fluid until it floats.
- There it is allowed to remain in position for about half a minute.
- The lactometer must float freely without touching the walls of the cylinder or its bottom.
- When it becomes stationary records:
- Scale reading &
- Temperature

## Calculations:

If the temperature is above or below the standard, (60°F) the lactometer reading is corrected according to the following rule:

- Add 0.1 to the lactometer reading, or 0.0001 to the specific gravity for every one degree Fahrenheit above the standard temperature.
- Subtract 0.1 from the lactometer reading or 0.0001 from the specific gravity for every one degree Fahrenheit below the standard temperature.
- The specific gravity of the milk is then calculated by following the formula;

Specific Gravity of Milk=1+(CLR)/1000

CLR= Corrected lactometer reading

## EXAMPLE:

The lactometer reading for a milk sample of a cow is 31.9 at 63°F, calculating its specific gravity (1.029-1.032).

### Solution:

CLR = 63°F – 60°F =3

= 3 x 0.1 =0.3

CLR = 31.9+0.3 = **32.2**

### Formula:

Specific Gravity =1+(CLR)/1000

SG = 1+ 32.2 / 1000

= 1+ 0.0322

**=1.0****322.**

### Values for specific gravity of different milk:

For **Cow’s milk**: The value is between **1.026 – 1.032**.

For **Buffalo milk**: The value between **1.030 – 1.034**.

For **Skim milk**: The value is between **1.034 – 1.036.**