• Purple staining on the inner surface of the cans in which sulphur- containing foods are packed may occur with all fish and meat products, especially, liver, kidneys and tongue.
  • It is due to the breaking down of sulphur containing proteins at a high processing temperature with a result that, hydrogen sulphide is liberated and a thin layer of tin sulphide is formed on the inside of the can.
  • This discoloration, which does not involves foodstuffs itself and varies from a light pink to a dark purple but a blackening of both inside of the can and the surface of the foodstuffs may accompany it.
  • These latter changes are due to attack of hydrogen sulphide on the steel base with a formation of iron sulphide.
  • It is of more serious impart than the deposition of tin sulphide as it may lead to pitting of the steel and disfigurement of the surface of the meat pack.

Prevention or Treatment of the Can

  • The basis of which is, copal gum, dissolved in a suitable solvent to which an added substance capable of uniting with volatile “sulphur” gases released while the food is being processed.


  • Purple staining of the inside of the can is of no significance, while blackening of surface of the foodstuffs may be removed by trimming.
Last modified: Tuesday, 7 September 2010, 4:28 AM