Determination of residual lactose in lactose-free cow milk by hydrophilic interaction liquid chromatography (HILIC) coupled to tandem mass spectrometry

Lactose is the major carbohydrate found in milk and dairy products. Lactose intolerance means the body cannot digest foods with this natural sugar in them. In this context, the lactose-free market has experienced a steep increase in recent years. A new method for the determination of residual lactose in lactose-free dairy products using liquid chromatography coupled to tandem mass spectrometry triple quadrupole (HPLC-MS/MS) has been developed. Hydrophilic interaction chromatography (HILIC) has been used for this purpose. An amide chromatographic column with an alkaline mobile phase were selected as optimal. In addition, a fast, cost-effective and reliable sample treatment has been developed for routine analytical laboratory use. The method has been validated by using matrix-matched calibration standards and a recovery assay on a lactose-free milk sample obtained by lactase hydrolysis of regular milk. The limit of quantification (LOQ) was 15 mg L-1, while the recovery was close to 100% with relative standard deviation lower than 90/0 in all cases. The method was applied to several lactose-free products and the results showed that lactose values in these products are not always below the recommended maximum value of 100 mg L-1.