The formation of the comutagenic tryptophan derivate norharman in heat-processed food is a well known phenomenon. As a first step to investigate the possible formation of norharman during food storage particularly in regard to fortified food we developed a model system and studied the non-enzymatic formation of the comutagen at temperatures below 100 degrees C. With standard conditions (4 h, 80 degrees C) iron and copper (Fe2+/Cu2+) was required for the formation of norharman. Addition of glucose to the reaction mixture increased the norharman formation and a plateau was reached with 400 microM tryptophan. Decreasing the reaction temperature to 40 degrees C led to a significant formation of norharman after 72 hours of incubation indicating that the formation of norharman can take place at temperatures that are close to ranges occurring during food storage.