Detection of whey in margarine by analysis of sialic acid and κ-casein glycomacropeptide

Whey is a by-product of cheese making often used as an emulsifier in margarine. This study was undertaken to determine whether whey can be detected in commercial samples of whey added margarine by analyzing sialic acid, a carbohydrate moiety of kappa-casein glycomacropeptide (GMP) present in whey. Margarine samples were mixed with chloroform-methanol (2 : 1), and the insoluble residue obtained from each sample was dialyzed in water. After dialysis, the sialic acid content in the non-dialyzable fraction was determined by the thiobarbituric acid reaction. The content of sialic acid, expressed as mu g/g margarine +/- standard deviation, was 20.9 +/- 4.8 (n = 3). A major sialic acid containing fraction isolated by chromatography on Sephacryl S-200 was further examined using chemical composition analysis, gel electrophoresis and reverse phase HPLC. The results indicated that the sialic acid found in margarine is the constituent sugar of bovine kappa-casein GMP, a compound specific to whey. It was concluded that the analysis of sialic acid and GMP is a useful method for testing commercial samples of whey added margarine.