A new enzymatic spectrophotometric assay for the determination of lactulose in milk

A new enzymatic spectrophotometric assay for the determination of lactulose in milk samples was developed. This method entailed the use of beta-galactosidase which hydrolyses lactulose giving fructose and galactose, and fructose dehydrogenase which reacts with fructose in presence of a tetrazolium salt (MTT), giving a coloured compound which can be detected spectrophotometrically at 570 nm. The assay showed a lactulose detection limit in milk of about 10 mg l(-1), a linear range of 20 divided by 800 mg l(-1) and a relative standard deviation of 5%. The correlation with the determination of lactulose in milk using reference procedures was good. Moreover this procedure was found suitable for the quantitation of lactulose in milk after the heat treatment process, and more convenient for the rapid and sensitive estimation of lactulose if compared with previous published enzymatic methods. (C) 2000 Elsevier Science B.V. All rights reserved.