3 μm ion exchange resin

Finely ground and dried cotton kernels (0.2 g) were combined with 5 ml of HCl (6 mol/L), dried by blowing nitrogen gas, and reacted in an oven at 110 °C for 24 h. Afterward, the sample was diluted to 10 ml with water. A 1-ml aliquot was dried by blowing nitrogen, then diluted using 10 ml of HCl (0.02 mol/L) and filtered with a 0.22-μm filter. The filtered liquid (1–1.5 ml) was then placed in a sample bottle for protein content determination. The analysis conditions were as follows: ion exchange column specification 4.6 × 60 mm; Hitachi’s special 3 μm ion exchange resin; column temperature 135 °C; pump flow rate 0.00∼0. 999 ml/min; and pump pressure 0∼20 kpa. The analysis time was about 30 min, and the injection volume was 20 μL. Protein content was measured using a Hitachi L-8900 automatic amino acid analyzer and EZChrom Elite for HITACHI analysis systems.

After the powder samples were treated according to the reported method [10 ], the amino acid concentration was assayed using an automatic amino acid analyzer (Model L-8900; Hitachi, Tokyo, Japan) with postcolumn ninhydrin derivation and spectrophotometrical detection (wavelength 570 nm). An aliquot of 20 μL was injected into the amino acid analyzer equipped with an Hitachi custom column filled with 3-μm ion exchange resin (Hitachi). The amino acids were identified by comparing with the retention time of standard amino acids (Sigma-Aldrich Co, St. Louis, MO, USA) using norvaline as an internal standard. The content of the tested samples was quantified by comparing peak areas of samples with standard amino acid profiles. The amino acid content was expressed as g/100 g yam protein.