Spectrophotometric Assay Using o-Phthaldialdehyde for Determination of Proteolysis in Milk and Isolated Milk Proteins

A rapid, sensitive, and convenient spectrophotometric assay was developed and characterized for measurement of proteolysis of milk proteins in buffered solutions or in milk. a-Amino groups released by hydrolysis react with o-phthaldialdehyde and /3-mercaptoethanol to form an adduct that absorbs strongly at 340 nm. The absorptivity (e = 6000 M -1 cm -1) is similar for all oe-amino groups.