Quantification of lactose using ion-pair RP-HPLC during enzymatic lactose hydrolysis of skim milk

The correct labelling of dairy foods as "lactose-free" requires a suitably sensitive and valid analytical method for the quantification of lactose in complex food matrices. Thus, an ion-pair RP-HPLC method for the simultaneous determination of lactose, glucose and galactose in original skim milk was investigated. The samples derived from an enzymatic lactose hydrolysis approach (0.5L) using the commercial β-galactosidase Godo-YNL2. After derivatisation with p-aminobenzoic acid and sodium cyanoborohydride, the samples were injected on a RP-C(18) column. Tetrabutylammonium hydrogen sulphate was used as the ion-pair reagent in the eluent system. The sugars were quantified using photometric- (UV; 303 nm) and fluorescence-detection (λ(ex) 313 nm, λ(em) 358 nm). The overall run time was 27 min. The limits of detection (LOD) were estimated at 2 mgL(-1) (UV detection) and at 0.13 mgL(-1) (fluorescence detection). The limits of quantification were 6 mgL(-1) (UV detection) and 0.45 mgL(-1) (fluorescence detection). Thus, this analytical method is suitable for sensitive lactose quantification in milk systems of less than 10 mgL(-1).