Rapid determination of lactulose in milk by microdialysis and biosensors

A simple and rapid flow system for the determination of lactulose in milk samples was developed. It is based on the hydrolysis of lactulose to galactose and fructose by the enzyme β-galactosidase immobilised in a reactor. The amount of fructose produced was measured with an electrochemical biosensor based on the fructose dehydrogenase enzyme, K3[Fe(CN)6] as mediator and a platinum based electrochemical transducer. Parameters such as the enzyme immobilisation in the reactor and under the electrode surface, the lifetime of the β-galactosidase reactor and of the dehydrogenase biosensor and the flow parameters were studied and optimised. Fructose was determined in the range 1 × 10–6–5 × 10–3 mol l–1 with an RSD of about 2% and a detection limit of 5 × 10–7 mol l–1. The use of a microdialysis probe as the sampling system permitted the direct measurement of lactulose in milk samples without pre-treatment in the range 1 × 10–5–5 × 10–3 mol l–1. The sensitivity of the procedure allowed pasteurised, UHT and in-container sterilised milk to be distinguished.