Reaction SMILES: C(=O)([O-])O.[Na+].N[C@H:7](C(O)=O)[CH2:8]S.[NH2:13][C@H:14]([C:19]([OH:21])=[O:20])[CH2:15][CH2:16][S:17][CH3:18].S(=O)(=O)(O)O>O.C(=O)([O-])[O-].[Na+].[Na+].C(O)C>[CH2:7]([O:20][C:19](=[O:21])[C@H:14]([CH2:15][CH2:16][S:17][CH3:18])[NH2:13])[CH3:8]
Reactant: CSCC[C@H](N)C(=O)O
Reactant: N[C@@H](CS)C(=O)O
Agent: O=C([O-])O
Agent: O=C([O-])[O-]
Agent: O=S(=O)(O)O
Solvent: O
Solvent: CCO
Product: CCOC(=O)[C@@H](N)CCSC
Temperature: 90 °C
Yield: 92.3 %

Procedure:
In 2.5 liters of water containing 85 g of sodium carbonate and 26 g of sodium hydrogen carbonate, were dissolved 33 g of papain and 3.3 g of L cysteine as a papain activator. With stirring, 1 kg of a commercial soybean protein (manufactured and sold by Fuji Seiyu K.K. under trade name "Fujipro R") in limited amounts was added to the solution and thoroughly mixed and agitated. Separately, a suspension of 50 g of L-methionine in 320 ml of ethyl alcohol was gradually added to 35 ml of conc. sulfuric acid to completely dissolve the methionine. The solution was heated under reflux at 90° C. for 5 hours, obtaining a reaction solution of L-methionine ethyl ester in an yield of 92.3%. The reaction solution was diluted with ethyl alcohol to a volume of 500 ml. A 370-ml portion of the diluted reaction solution was added to the separately prepared protein system. After thorough mixing and agitation, the solution was kept at 37° C. for 24 hours to effect plastein reaction. The yield of L methionine incorporated in the thus produced protein-like material was 88.1% based on the amount of L-methionine ethyl ester added at the start of the reaction. The reaction solution was then diluted with water to a volume of 20 liters and adjusted to pH 6-7 with hydrochloric acid before it was subjected to repeated concentration operations for desalting and purification using a reverse osmosis membrane (tubular type, manufactured by Daicell Chemical K.K. under tradename DRS-10) under an operating pressure of 15 kg/cm2. The concentrate resulting from the reverse osmosis treatment was again diluted with water to a volume of 20 liters and adjusted to pH 3 with phosphoric acid. Then, 20 g of orientase (manufactured by Hankyu Kyoei Bussan K.K.) and 20 g of molsin (manufactured by Fujisawa Chemicals K.K.) were added to the solution, which was kept at 40° C. for 40 hours to effect hydrolysis. During the reaction, the solution was controlled to pH 3 with a 6N phosphoric acid. At the end of the reaction, the solution was heated to 90° C. for 10 minutes to deactivate the proteases. The solids were removed by filtration, and the filtrate was spray dried to yield 685 g of a dry powder, which was a low-molecular weight peptide mixture having L-methionine replenished in the form of an acid residue. The low-molecular weight peptide mixture was found to have an average molecular weight of 320. A sample of the powder was subjected to gel filtration using Cephadex G-10 to find that 87.3% of the sample had a molecular weight of 700 or lower. It was also found that the content of free amino acids was 9.8%. Table 1 shows the amino acid composition of the starting soybean protein and low-molecular weight peptide mixture produced therefrom.