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.