Titratable acidity is defined as the number of milliliters of 0.1 mol/l sodium hydroxide solution required to neutralize a quantity of milk corresponding to 10 g of Solids Not Fat.
Natural acidity of milk is due to its constituents such as casein, albumin, citrates, phosphates and carbon dioxide. This acidity can be measured by titrating milk against standard alkali using an indicator like phenolphthalein and is expressed in terms of lactic acid.
- Transfer the whole content of the bottle to a beaker.
- Whisk it for a minute or two with a brush made of a fine wire.
- Scrap inside of the bottle with a wire brush.
- Shake the contents in the bottle by pouring a little from the beaker in to it.
- Transfer completely to the beaker and whisk properly.
- Proceed as in case of Pasteurized Milk.
(1) Standard Sodium Hydroxide Solution – 0.1 N
(2) Phenolphthalein Indicator – Dissolve 0.5 gm phenolphthalein in 100 ml of 50 % ethyl alcohol
- Take accurately 10 ml milk in a beaker.
- Add 3-4 drop of phenolphthalein indicator. Shake well and titrate against standard NaOH solution.
- Complete the titration in 20 seconds.
- Keep a blank by taking 10 ml of material in another beaker for comparison of color.
Titratable acidity as Lactic acid = 9AN/W
Where A = Volume of standard NaOH required for titration
N = Normality of Standard NaOH solution
W = weight of sample taken for test