Tryptophan degradation during heat treatments: Part 2—Degradation of protein-bound tryptophan

Abstract

The stability of free or protein-bound tryptophan was determined during acid (4 m methane sulfonic acid) or alkaline (4·2 m sodium hydroxide) hydrolysis at 110°C under nitrogen. The influence of time and of glucose, starch or amino acid addition was studied. It was found that hydrolysis with methane sulfonic acid caused tryptophan losses when glucose or starch was present. The thermal degradation of protein-bound tryptophan was studied using either glycyl- l-tryptophyl-glycine or casein. Analysis of tryptophan was carried out after acid or alkaline hydrolysis, through high performance liquid chromatography and UV detection. Heat treatment of tripeptide (9·4 m M, pH 8, 125°C, 3 to 48 h) in the presence of oxygen, air or nitrogen resulted in peptide hydrolysis, with the formation of the corresponding dipeptides and of free tryptophan. After acid or alkaline hydrolysis, tryptophan loss was seen to be higher when the initial heat treatment was performed in the presence of oxygen than in the presence of air. Thermal treatment of 4 or 5% w/v casein solutions (pH 7 or 8) was carried out at 125°C, under oxygen, air or nitrogen. A marked loss of tryptophan was found to occur after 24 h at 125°C when oxygen or air was present. Under nitrogen, protein-bound tryptophan was heat stable, even in the presence of glucose or starch. It is therefore unlikely that the indole ring of protein-bound tryptophan may react with reducing carbohydrates through Maillard-type reactions. Oxidative degradation of protein-bound tryptophan is also unlikely to reach any significant extent during the heat processing of foods, unless catalysts or strong oxidizing agents are present.