Foodstuffs, such as vegetables, treated with fertilizer, pesticides and fungicides, may acquire carcinogenic (and mutagenic) properties when cooked or digested, even though the residues in the uncooked food are below toxic levels. This is due to alteration in the properties of the residues during the preparation process or due to interaction between them. Burnt and browned materials formed by heating proteins during cooking are highly mutagenic. Salt-cured or pickled foods may be linked with increased risk of cancers of the stomach and oesophagus, and traditional smoking processes for ham, fish and sausage cause the absorption into the food of cancer-causing tars similar to those contained in tobacco smoke.
Certain methods of processing foods may lead to the formation of toxic contaminants. The smoking of foods has been shown to produce significant levels of a carcinogen and the incidence of neoplasms seems to be increased in areas where the consumption of smoked fish reaches a high level.