C. botulinum is an anaerobic organism. This means that it does not grow in the presence of oxygen. However, I caution the reader not to think that a food has to be in a tightly sealed glass or metal container to be anaerobic. Foods that are resealed into plastic bags can be sufficiently anaerobic to support the growth of C. botulinum . During the canning process, foods are heated and filled hot into a container or are heated in the container.
Heating drives oxygen from the food and leaves the food anaerobic.
Thus canned foods and packaged cooked foods are at risk of becoming toxic due to the growth of C. botulinum . Even fresh produce can be a risk. There is at least one outbreak of botulism due to shredded cabbage packaged in a sealed plastic bag. Apparently, the cabbage was left unrefrigerated and oxygen in the bag was consumed by the metabolic activity of the cabbage. Once the oxygen was consumed, C. botulinum grew and produced toxin.