Determination of Activity (Titrable Acidity) of Milk
Milk is amphoteric in reaction which turns blue litmus to Red and Red litmus to blue. The acidity of milk is of two kinds.
i. Apparent or natural acidity which is due to citrates and phosphates present in the milk and dissolved CO2 during the process of milking and thereafter.
ii. Real acidity or developed acidity which is due to lactic acid produced by the action of bacteria on lactose in milk.
Generally the acidity of milk means the total acidity (Natural + developed) or tirratable acidity. It is determine by titrating a know volume of milk with standard alkali to the point of an indicator line phenolphthalein.
Acidity is expressed as percent lactic acid. Since 1 ml of 0.1 N lactic acid contains 0.009 grams of lactic acid, the number of ml. of 0.1 N NaOH required to neutralize the lactic acid in the sample, multiplied by 0.009 will give the amount of lactic acid (grams) in the sample, when the result is divided by weight of milk sample and multiplied by 100 the percent lactic acid will be obtained.
Use fresh N/10 NaOH solution.
1. Milk sample.
2. N/10 NaOH solution.
3. Phenolphthalein indicator.
a) 10 ml capacity pipette.
b) 100 ml conical flask or porcelain dish.
c) 50 ml capacity burette with stand.
d) Porcelain tile.
e) Glass rod.
1. Fill the burette with N/10 NaOH solution.
2. Mix the milk sample thoroughly by avoiding incorporation of air.
3. Transfer 10 ml milk with the pipette in porcelain dish/conical flask.
5. Add equal quantity of glass distilled water.
6. Add 3-4 drops of phenolphthalein indicator and stir with glass rod.
7. Take the initially reading of the alkali in the burette at the lowest point of meniscus.
8. Rapidly titrate the contents with N/10 NaOH solution continue to add alkali drop by the drop and stirring the content with glass rod till first definite change to pink colour which remains constant for 10 to 15 seconds.
9. Complete the titration within 20 seconds.
10. Note down the final burette reading.
No of ml. of 0.1 N NaOH solutions
required for neutralization x 0.009
% Lactic acid = ———————————————————————- x 100
Weight of sample
(Weight of sample = Volume of milk x specific gravity)