SOP: Determination of Acidity in Milk Powder

GIFSQ — Global Institute for Food Safety & Quality

Scope

This method is applicable to fluid milk, powder milk and other milk products.

1. Principle

The method is based on the titration of the sample with sodium hydroxide to a phenolphthalein end point, and by comparing the colour with the colour obtained by mixing rosaniline acetate or cobalt sulphate to a known volume of milk sample.

2. Apparatus

a. Porcelain dishes, 100 mL

b. Glass burette, 50 mL capacity with 0.1 mL graduation

c. 10 mL pipette

d. Glass rod for stirring

3. Reagents

a. 0.1 N sodium hydroxide solution

b. Phenolphthalein indicator

c. Rosaniline acetate solution: dissolve 0.12 g rosaniline acetate in 95% ethyl alcohol containing 0.5 mL of glacial acetic acid and dilute to 100 mL (store in dark). To prepare the working standard, dilute 1 mL to 500 mL with 95% ethyl alcohol and water in the ratio of 1:1.

d. Cobalt sulphate solution: dissolve 1.5 g of cobalt sulphate in water and dilute to 100 mL.

4. Reconstitution of milk powder

Weigh 10 g of skim milk or 13 g of whole milk powder, and make up to 100 mL with warm distilled water at 24°C. Stir exactly for 90 seconds. Allow the sample to stand until the foam settles. The period of standing after mixing should not exceed 15 minutes.

5. Procedure

Pipette 10 mL of reconstituted milk into each porcelain dish

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To one dish, add 1 mL of working solution of rosaniline acetate or cobalt sulphate solution and stir with a glass rod. This solution will be the external end point reference

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To the other porcelain dish, add 1 mL of phenolphthalein solution and titrate with 0.1 N sodium hydroxide, stirring to mix the sample

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Continue titration until the colour is comparable to the reference solution

6. Calculation

% Acidity as lactic acid = (mL NaOH ÷ 1000) × 0.1 × 90 × [100 ÷ Weight of milk powder taken]

(With the standard reconstitution above, the 10 mL portion titrated contains 1.0 g of skim milk powder or 1.3 g of whole milk powder.)

References

Manual of Methods of Analysis of Foods, Milk and Milk Products, FSSAI 2016, Method 10.4.