G. O. Kohler

Determination of tryptophan in feeds

Much of the literature available on amino acid content of feeds lacks accurate tryptophan values owing to difficulties of tryptophan analysis. Three representative tryptophan methods commonly employed were found by Miller (1) to be unsatisfactory for the routine analysis of feeds. These included a microbiological method (2) and two colorimetric methods (3, 4). The modified method of Spies (5) is useful for the determination of tryptophan in isolated proteins but has not proved satisfactory for use with feeds in g,eneral (6) because of interference by unknown factors in calorimetric determinati.on of t’ryptophan. Miller (I) developed a method of Ba (OH), hydrolysis followed by :L p-dimethylamino benzaldehyde (D&IAB) determination of tryptophan in feeds. A key feature of his method was prec.ipitation of barium from an acid medium prior to nddit,ion of DMAB. His method gave only 80% recovery of added tryptophan for maiztl and 84% recovery for wheat. Slum,p and Schreuder (7) recently published a method eliminating Miller’s precipitation step. They used a low pH b,uffer (pH 3.25) tIo avoid precipitat’ion of barium while eluting hgdrolyzate from a Sephadex G-25 column. Although ~*ecovery of added tryptopharl was high (94 to IOO%), 80 min was required t’o resolve the trypt,ophan peak, limiting the number of samples which can bc run daily. The present paper reports improvements on the: ion exchange procedural for tryptophan analysis developed in our laboratory (8) prior to the appearance of the Slump and Schreuder report’ (7). A very recent8 report’ by Tkachuk and Irvine indicates that they have worked out a method similar to ours (9). They do not, however, report any comparat’ive results between the ion exchange and other tryptophan methods. In both procedures samples are hydrolyzed in Ba(OH)2 solut,ion in evacuat’ed, sealed tubes. Barium is precipitat’ed from an acid medium and tryptophan determined on t,he basic column of an amino acid analyzer. High recoveries