ABSTRACT: Scallop (Patinopecten yessoensis) adductor muscles were heated using superheated steam (150 and 200 °C), boiling (98 °C), and normal steaming (95 °C). The amounts of amino acids, water-soluble peptides, and nucleotides, expressed as extractive nitrogen in scallop products, are very important elements of quality and taste. After 15-min heating of scallop muscles with normal steaming and boiling, respective losses of 50% and 64% of the extractive nitrogen were observed. However, most extractive nitrogen (> 86%) remained in the scallop muscles treated with superheated steam at 150 and 200 °C. Protective effects of superheated steam against elution loss of nitrogen compounds were also observed in amino acid analyses of the heated products. The scallop-muscle surface temperature during treatment with superheated steam increased more quickly than that with normal steaming and boiling. The rapid water loss and the surface protein denaturation engendered formation of a 30-μm-thick film covering the surface, which prevented extractive nitrogen loss from internal tissues. Superheated steam treatment at 200 °C caused browning, surface shrinkage, and 47% weight loss. In marked contrast, the appearance and the weight loss of sample treated at 150 °C were almost the same as those of normal steaming and boiling-treated samples. These results suggested that superheated steaming at 150 °C is an optimal heat treatment of scallop adductor muscles.