Methodological parameters for determination of GMP and CMP in milk by spectrophotometric (ANSM) and by chromatographic method (HPLC)

Abstract

Sialic acids are terminal components of many glycoproteins and its position contribute to the structural properties, being an important regulator of molecular interactions. Glycomacropeptide is a glycosilated fraction of bovine kappa-casein that remains soluble when milk is clotted by rennin. Determinations of milk sialic acid content are useful because its concentration reflects the amount of free GMP of milk. In normal milk these amounts are very low, 12 to 16 times lower than in sweet whey. Therefore, its determination may be applied to verify possible frauds with whey addictions, since it works as a fingerprint. With the description of a new spectrophotometric method for determination of free GMP (ANSM) occurred a simplification of procedures, being faster than others (HPLC method), without loss of accuracy. However, due to variations of glycosilation in kappa-casein between animals, during the lactation period, due to mastitis and yet due to proteolysis on milk, it was necessary to know these variations to interpret correctly the analytical results. It was analyzed 1,703 samples of producer’s raw milk and 1,189 samples of processed milk (HTST and UHT). The results showed that normal milk from herd (producer’s milk) have only small amounts of free GMP, with A_470nm = 0.232±0.088 or 3.89±1.25mg of sialic acid/L. The upper limit of this distribution was A = 0.496; thus every bigger value may represent a problem, being outside of normal distribution.