Determination of α-amino nitrogen in pea protein hydrolysates: a comparison of three analytical methods

Three spectrophotometric methods, using 2,4,6-trinitrobenzenesulphonic acid (TNBS), o-phthaldialdehyde (OPA) or ninhydrin, for the determination of α-amino nitrogen in pea protein isolates and hydrolysates were compared. The determined amounts of α-amino nitrogen differed greatly, depending on the method used. The TNBS and OPA methods produced comparable results, whereas the data obtained with the ninhydrin method were only half of the TNBS or OPA values. Colour stability, recovery of the standard from the protein matrix and reproducibility of the results were determined. The methods showed good accuracy (SE 1–3%) with recovery values of the standard (l-leucine) from the protein matrix of 91, 111 and 75% for the OPA, ninhydrin and TNBS method, respectively.