The present invention relates to a method for producing a cysteine-rich food material by maintaining a food material containing .gamma.-glutamylcysteine at a ratio of at least 1 wt % to the solid content thereof at a temperature ranging from 50 to 120.degree. C. and at a pH ranging from 1 to 7. This process is conducted in the absence of a sugar and in the presence of water. The present invention also relates to a method for producing a cysteine-rich food material by reacting the food material with a .gamma.-glutamyl peptide hydrolase at a pH ranging from 3 to 9 and at a temperature ranging from 15 to 70.degree. C. in the present of water. Further, the present invention relates to a method for producing a flavor-enhancing material for use in food, food products obtained by these processes, and yeast cells or extracts for use in food products.