Milk lactose and lactulose determination by the differential pH technique

A differential pH technique for lactose and lactulose determination in milk samples (UHT, pasteurised, sterilised and fresh milk) is presented. With enzymatic reactions, causing a pH variation directly proportional to the lactose and lactulose contents in samples, in less than two and four minutes, respectively, the lactose and lactulose can be analysed without sample preparation procedures. The automatic methods are very simple, precise and accurate, with an excellent linearity for lactose (R2 = 0.9999) and lactulose (R2 = 0.9997) throughout the whole range of concentrations and good repeatability (the coefficient of variation was lower than 0.5%). With the optimised experimental conditions, a sensitivity of 0.1 mmol·L–1 lactulose was obtained.