*4.6. Analysis of Amino Acid Composition*

According to the previous method with a slight modification [81]. Approximately 30 mg of the sample and 10 mL of 6 mol/L HCl were added to a hydrolysis tube containing phenol. After the tube was vacuumed, the mixture was washed with nitrogen and hydrolyzed at 110 ◦C for 22 h. After cooling to room temperature, the filtrate is filtered and spun dry under reduced pressure in a centrifuge tube. The 0.02 mol/L HCl solution was added to a dried centrifuge tube and dissolved, and the solution was transferred to the upper sample bottle and determined using an amino acid analyzer (L-8900, Hitachi, Tokyo, Japan). Then, the contents of amino acids in the sample could be determined according to the peak area in comparison with the standard. The content of tryptophan in OP was determined after hydrolysis with 6 mol/L of NaOH instead of HCl.
