A rapid method for the determination of lactic acid in milk is described.

After precipitation with NaOH-ZnSO4 in the presence of BaCl2, the filtrate is treated with FeCl3-HCl reagent, and the colour is measured by one of the three common methods. Reference to standard curves prepared from similar observations on lactic-treated fresh mixed milk enables lactic acid to be determined with an accuracy which compares favourably with the p-hydroxydiphenyl method.

The approximate lactic content may be determined by adding to the filtrate a mixture of ferric chloride, potassium ferrocyanide and salicylic acid solutions.

Although the apparent lactic content of fresh mixed milk is 0.003%, advanced lactation may cause considerable increases in this value.