A yeast extract containing gamma -glutamylcysteine in an amount of 1 % by weight or more based on the solid matters； a process for producing a food material (for example, yeast cells) being rich in cysteine which is characterized by maintaining the food material at 50 to 120 DEG C in the absence of sugar and in the presence of water at pH 1 to 7, or treating the food material with gamma -glutamylpeptide hydrolase at pH 3 to 9 at 15 to 70 DEG C； and a process for producing a flavor-enhancing agent for foods capable of enhancing the flavor (beef flavor, dried bonito flavor, ctc.) of foods or drinks which comprises treating food materials with gamma -glutamylpeptide hydrolase and then adding a reducing sugar to thereby solve the problems of, for example, causing a strong scorching taste.
