Milkotester Master Pro Operating Instructions Manual page 21

5.1.2.1.3. Determination of total proteins
А/ Kjeldahl method
Heating with concentrated sulphuric acid in the presence of catalyst
mineralizes a definite volume of the milk sample. The liberated ammonium
combines with the sulphuric acid and forms ammonium sulphate. After
adding surplus soda caustic ammonium is liberated. When distilled it
combines with the boronic acid.
The quantity of the combined ammonium is determined by titration
with acid with determined titer. Combined with the ammonium acid the
initial nitrogen content is determined, and also the proteins in the milk.
B/ Titration with formalin
Formalin, added to the milk, combines with the amino group in the
protein's molecule and forms methyl groups, which have no alkaline
reaction. Milk acidity increases by the liberated carboxylic groups, which
are titrated with soda caustic solution. The used volume soda caustic is
proportional to the protein content in the milk.
5.1.2.1.4. Determination of casein content in the milk
А/ Kjeldahl method
The total nitrogen content in the milk is determined. Casein is
precipitated with acetic acid (acetate buffer) and is filtrated. The content of
nitrogen in the filtrate is determined. Casein content is the difference
between the two results for nitrogen using the Kjeldahl's method.
B/ Titration with formalin
More details for this method – see Appendix Methods.
5.1.2.1.5. Determination of salts in milk.
For the salts in milk and its derivatives is judged by its ashes
content. Milk dries, becomes carbonized and turns to ashes till constant
mass. The ashes received are calculated in percentage.
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Operating Instructions Master Pro
Milkotester Ltd
Ultrasonic milk analyzers