Use: test for screening, determine suitability for processing.
Advantages: more precise than alcohol and clot on boiling tests.
Disadvantages: variation exists between cattle breeds.
A dye, which changes colour at a specific pH, is added to the milk, and titrated with a base (added little by little) until the colour changes. By recording the volume of base required and the volume of the milk sample, the amount of lactic acid can be calculated.
Equipment and materials:
- White porcelain dish (hemispherical, 60 ml capacity).
- 10 ml pipette reading 1-10 ml and 1 ml pipette.
- Measuring cylinder (25 ml).
- Burette (0.1 ml graduations).
- Glass rod for stirring (flattened at one end).
- Phenolphthalein indicator solution (0.5% in 50% alcohol): Dissolve one gram of phenolphthalein in 100 ml of 95 percent ethyl alcohol. Add 0.1 N sodium hydroxide solution until one drop gives a faint pink colouration. Dilute the distilled water to 200 ml.
- 0.1 N sodium hydroxide solution: (i) prepare a concentrated stock solution of sodium hydroxide by dissolving equal parts of sodium hydroxide (sticks or pellets) in equal parts of water in a flask;(ii) close the flask with a rubber stopper and allow any insoluble sodium carbonate to settle out for 3 to 4 days; (iii) use the clear supernatant liquid for preparing the standard 0.1 N Solution. About 8 ml of stock solution is required per litre of distilled water.
1) Measure accurately 10 ml of milk in porcelain dish.
2) Add 1.0 ml of the phenolphthalein indicator using 1ml pipette.
3) Titrate the contents with N/10 sodium hydroxide (NaOH) solution stirring the contents.
4) Observe occurrence of pink color as end point for the titration.
5) Complete the titration within 20 seconds.
6) Note the titre value. Calculate percent acidity of the sample as lactic acid.
The more sodium hydroxide added, the more acid the milk. You can calculate the titratable acidity (as lactic acid per 100 ml of milk) as follows:
V1 = volume in ml of the standard sodium hydroxide required for titration;
N = normality of the standard sodium hydroxide solution, and
V2 = volume in ml of milk taken for the test
Normal milk acidity ranges from 0.10 to 0.20% lactic acid. Any value in excess of 0.20 % can safely be reckoned as developed lactic acid. Due to the opacity of milk, the end-point of titration is not sharp, so care has to be taken to adjust the conditions to reach the same end-point.