METHOD OF PURIFYING CYSTEINE

PURPOSE:To purify cysteine useful as an additional matter for drugs and foods in high purity, by adding a chelating agent having a chelate-forming ability with iron to an aqueous solution of iron-containing cysteine, and treating the solution with active carbon at a specific pH value to remove iron. CONSTITUTION:A chelating agent (e.g., o-phenanthroline) having chelate-forming ability with iron is added to an aqueous solution of iron-containing cysteine, and the solution is treated with active carbon in 1-8.5pH range, to remove iron. Iron admixed during cysteine production process deteriorates quality of product by iron coloring, and accelerates oxidation of cysteine to cystine by catalytic action of iron ion. Iron contained in cystein is removed by the treatment in high removal ratio. An amount of the chelating agent is >=an equimolor amount, preferably 2-8mol based on 1mol iron. Usually an alkali metal hydroxide, etc. can be used as an alkali for pH adjustment, and hydrochloric acid or sulfuric acid as an acid.