Why 10.75ml milk is taken for fat estimation by Gerber method in India?
The Gerber Method is a primary and historic chemical test to determine the fat content of substances, most commonly milk. The Gerber Method was developed and patented by Dr. Niklaus Gerber of Switzerland in 1891. In this method milk fat is separated from proteins by adding sulfuric acid (1.820-1.825 sp. gr. at 60°F). The separation is facilitated by using amyl alcohol and centrifugation. The fat content is read directly via a special calibrated butyrometer.
The theory behind using 10.75 ml milk in the pipette is as follows:
Gerber butyrometer is graduated on 0-10 scale and calibrated in such a way that each 1% division represents 0.125 ml of fat.
The weight of the fat in the area is equal to Volume × Density = Mass; 0.125 x 0.9 = 0.1125 (because density of fat = 0.9 g/lit)
If 1 % represents 0.1125 then 100 % will be represented 11.25 g.
As per this, we should be pipetting 11.25 g of milk, but there are certain impurities due to iso-amyl alcohol, which affects the fat reading. These impurities are estimated at 2.5-3% (average (2.5+3)/2 = 2.667%).
So the fat is 1.125-(1.125×2.667/100) = 1.095 g.
According to this we should be pipetting 10.95 g of milk.
This is equivalent to 10.65 ml of milk (10.95/1.02547, the denominator being the density of milk).
Since 0.1 ml residual milk remains in the glass pipette sticking to the walls, we take 10.75 ml of milk.
Prepared By: Vishal Trivedi