Obviously, the concern about dented cans is that the food they contain may not be safe to eat. To address this question we have to know a little about the canning process. The preservation of food historically has been a problem. Microbes such as bacteria and molds like the same kinds of food that we do and are ready to dive in if given the chance. And if we then eat the bacteria or molds or the toxins they produce, we can get sick. Furthermore, food itself tends to degrade with time as its own enzymes begin to break down the proteins and carbohydrates and fats. The canning process is an effective way around this problem.
In canning, the heat destroys harmful organisms and deactivates enzymes and the tight seal protects the food from outside contamination. Nicholas Appert invented canning in 1809 in response to a prize offered in 1795 by the French government. He heated food in glass flasks sealed with corks but had no theoretical explanation for why this worked. It was left to Louis Pasteur to eventually explain what had happened.
The idea of the can was conceived by the Englishman Peter Durand, but the early cans were crude and heavy. The idea is that the can is sealed and heated to a high enough temperature to inactivate the enzymes and kill the microorganisms. The most acidic foods need the least severe treatment, about 30 minutes in a bath of boiling water because low pH inhibits most microbes. Vegetables with a pH of 5-6 are much more hospitable and usually need 30-90 minutes at 116C. The greatest worry is the bacterium Clostridium botulinum because it is anaerobic and produces botulin, a nerve toxin. The toxin is destroyed by boiling but the spores can survive boiling for 5 hours. You actually need higher than boiling temperatures to destroy them. The answer is a pressure cooker which can reach temperatures of 116C.
The canning process is very effective and cans which were produced over a hundred years ago have been opened and eaten. Problems arise if there is a defect in the can or if the heating isn’t done properly. One giveaway is a bulging can. Bacterial activity produces gas and any can that bulges should be thrown away. Dented cans are a different story. The concern here is that denting puts strain on the metal and microscopic cracks may develop. The sterility is then lost and microbes and molds can enter. The risk is very small because usually dents do not produce holes. Dented cans do not necessarily have to be thrown out but their contents should be boiled to kill any microbes and destroy any toxin that could have been produced by the Clostridium botulinum bacteria.
It is noteworthy that the can opener was not developed until about 50 years after the invention of the can. Canned foods were mostly used as military rations and soldiers would open them with their bayonets or at least in one case, with a rifle bullet. Some cans actually carried instructions to cut around the top with a hammer and chisel. The can opener as we know it today was invented in 1870 by William Lyman and used the same principle as today’s openers.
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