Like cooked food, canned food is subject to changes in appearance and nutritional content. Food is canned by heating what goes into the container and then sealing the container to keep out air and microbes. It is then reheated after the can/jar is sealed.
Heating food often changes its color and texture. It also destroys some vitamin C. But canning effectively destroys a variety of pathogens, and it deactivates enzymes that might otherwise cause continued deterioration of the food.
A modern variation on canning is the sealed plastic or aluminum bag known as the retort pouch. Food sealed in the pouch is heated but for a shorter period than that required for canning. As a result, the pouch method does a better job of preserving flavor, appearance, and heat-sensitive vitamin C.
The sealed can or pouch also protects food from deterioration caused by light or air, so the seal must remain intact. When the seal is broken, air seeps into the can or pouch, spoiling the food.
A more serious hazard associated with canned food is botulism, a potentially fatal form of food poisoning caused by the failure to heat the food to high-enough temperatures or for a long-enough time to kill all Clostridium botulinum (or C. botulinum) bacteria.
Canning is based on temperatures and times necessary to destroy C. bot spores. C. botulinum is an anaerobic (an = without; aerobic = air) organism that thrives in the absence of oxygen, a condition nicely fulfilled by a sealed can. Botulinum spores not destroyed by high heat during the canning process may produce a toxin that can kill by paralyzing your heart muscles and the muscles that enable you to breathe.
To avoid potentially hazardous canned food do not buy, store, or use any can that is
Swollen, which indicates that bacteria are growing inside and producing gas.
Damaged, rusted, or deeply dented along the seam, because a break in the can permits air to enter and may promote the growth of organisms (other than botulinum).
