Determination of free amino acids in infant formulas.

A fast and reliable method for the determination of free amino acids in infant formulas was developed. The method emphasizes the determination of taurine, methionine, arginine and tyrosine, as they are often added to these formulas in order to adapt them to the nutritional requirements of infants. The method involves the precipitation of the protein fraction, its separation by centrifugation, and the HPLC analysis of the o-phthaldialdehyde (OPA) derivatives of the free amino acids. The method was applied to several commercial formulas, as their actual content of free amino acids may be reduced by the development of Maillard reaction during processing and storage. The detected amounts of free taurine and methionine were often lower than those listed by the manufacturers. In some samples signi"cant amounts of free arginine, tyrosine and methionine were detected, although they had not been listed among the added ingredients