Rapid determination of lactulose in milk by microdialysis and biosensors

A simple and rapid flow system for the determination of lactulose in milli samples was developed. It is based on the hydrolysis of lactulose to galact ose and fructose by the enzyme beta-galactosidase immobilised in a reactor. The amount of fructose produced was measured with an electrochemical biose nsor based on the fructose dehydrogenase enzyme, K-3[Fe(CN)(6)] as mediator and a platinum based electrochemical transducer. Parameters such as the en zyme immobilisation in the reactor and under the electrode surface, the lif etime of the beta-galactosidase reactor and of the dehydrogenase biosensor and the flow parameters were studied and optimised. Fructose was determined in the range 1 x 10(-6)-5 x 10(-3) mol l(-1) with an RSD of about 2% and a detection limit of 5 x 10(-7) mol l(-1). The use of a microdialysis probe as the sampling system permitted the direct measurement of lactulose in mil li samples without pre-treatment in the range 1 x 10(-5)-5 x 10(-3) mol l(- 1). The sensitivity of the procedure allowed pasteurised, UHT and in-contai ner sterilised mill, to be distinguished.