Hydrogen sulfide is formed from the meat of baby clams during canning. The amounts of hydrogen sulfide are usually below 50 mg%, which may be the minimum amount necessary to cause blackening. When hydrogen sulfide is produced in amounts more than 50 mg% (e.g. when the raw material is not fresh), blackening may be accelerated, or the canned baby clams may decompose and hydrogen sulfide concentration increase slightly during storage. The blackening is caused by the formation of copper sulfide and iron sulfide. Iron transferred to the intestine from the mud where the clams live, is involved in blackening. Also implicated is copper freed from hemocyanin in baby clam by boiling. Some phosphate compounds (0.270) are effective in decreasing the blackening.