The free amino acids are usually analysed without hydrolysing
the sample, but acid hydrolysis (usually 100–120 C, 6 N
hydrochloric acid and 22–24 h) is necessary when the total amino
acids in protein are determined and Trp is unstable under these
conditions (Caballero et al., 2003; The JP Request, 2002). Stability
and recovery of Trp is poor in conditions where acid and oxygen
are present as well as acid hydrolysis due to the indole group of
Trp. Sodium hydroxide is the most effective alkaline used for protein hydrolysis with or without a carbohydrate or thiodiglycol as
an antioxidant. Trp has often being ignored in the assessment of
overall amino acid profile of foods because, Trp exhibits a strong
native fluorescence, and fluorometric method has been widely
used to determine Trp content without derivatization