Quantification of lactose and lactulose in hydrolysed-lactose UHT milk using capillary zone electrophoresis

Abstract

A new method for quantification of lactose and lactulose in hydrolysed-lactose UHT milk, using capillary zone electrophoresis with indirect detection in the ultraviolet region (CZE-UV), is proposed. A 33 Box–Behnken design for instrumental electrophoretic parameters optimisation was used, with the best conditions shown to be an injection pressure of 15 mbar 2 s, applied voltage of −20 kV, and cartridge temperature of 20 °C. Extraction procedures were investigated and protein precipitation using trichloroacetic acid solution was shown to give the best results. Limits of quantification for lactose and lactulose were 0.024 g 100 mL−1 and 0.0094 g 100 mL−1, respectively. Recovery levels ranging from 86% to 102% were obtained. The method was successful applied to several hydrolysed-lactose and regular UHT milk samples from six commercial leading brands. Results showed that all hydrolysed-lactose samples are in accordance with the legal recommendations to be classified as “lactose-free” products.