Application of heat, after the filling and sealing of the can, is relied on for complete destruction of all micro-organisms. Occasionally, organisms enter the can after heat processing, or bacteria spores survive the heating process, and spoilage may occur. Where organisms gain entrance after processing, gas production will normally cause swelling or doming of a can. Almost any bacterium or fungus may be the cause. Doming of cans due to microbial growth should not be confused with physical distortion of the can due to mechanical damage.

If, due to a manufacturing defect, spore-forming bacteria such as Clostridium spp. and Bacillus spp. survive the heating process, they may grow and cause putrefaction, fermentation or rancidity. Many of these bacteria are adapted to high temperatures and develop only where the storage temperature is high. Some do not produce gas; accordingly the can does not become swollen.

Damaged cans should be destroyed because of the risk of contamination with Clostridium botulinum, which produces a heat-stable botulinum toxin that is potentially fatal. Samples should be sent for examination in all instances where claims or retort actions are contemplated against the donor, shipper or manufacturer as a result of poor or defective quality of cans.