Canning and Fish Packaging Technology

Unit 5 - Changes in canned foods and spoilage

5.2.2.5.1. Sulphide blackening or iron sulphide blackening
It is most commonly observed in canned shrimp, lobster, crab less commonly in other products etc. Though the cans are coated with sulphur resistant lacquer, any imperfection in the lacquer coating during its manufacture or subsequent reforming or any scratches during handling expose the tin layer and trimethyl amine present in marine products dissolve tin layer exposing iron. Sulphur containing components of marine products chemically react with iron to form black iron-sulphide causing black discolouration on the inner can surface. This reaction is more under alkaline condition and when less fresh or spoilt materials are used.

Uniform lacquering of can, its careful handling and use of parchment paper while packing can minimise this problem.