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 (0.5 L) using the commercial β-galactosidase Godo-YNL2. After derivatisation with p-aminobenzoic acid and sodium cyanoborohydride, the samples were injected on a RP-C₁₈ 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 mg L⁻¹ (UV detection) and at 0.13 mg L⁻¹ (fluorescence detection). The limits of quantification were 6 mg L⁻¹ (UV detection) and 0.45 mg L⁻¹ (fluorescence detection). Thus, this analytical method is suitable for sensitive lactose quantification in milk systems of less than 10 mg L⁻¹.