Food sealed in metal cans that you buy at your supermarket is hermetically sealed. Hermetic is scientific jargon for airtight. This means that microorganisms cannot mix with your food and make you sick. Dented cans may have compromised seals. Here's how cans are sealed and why you might want to pass on those dented cans:
How Cans Are Sealed
Cans are sealed with machines called can seamers. These large industrial machines turn the edges of your can's top cover (the lid) under so that it can be fitted and sealed to the top of the can. This is called the cover curl. The top of the can is flanged or flared out. Between the cover curl and the flanged top of the can is a very thin layer of sealant. This compound fills in the gaps so no air can get into the canned food. The can seamer performs two very fast operations that seal your can.
The first seamer operation rolls the can in an upright position using pressure to bond the the cover curl and the flange together. They must meet perfectly. This is called a pre-seaming. The second operation flattens the seam so that the can is airtight. This seams the can so that the food cannot be contaminated. Or can they? What if there are dents in the can?
Why Dents May Mean Compromised Food
If airtight cans are dented during processing, they will not pass quality inspection. However, cans that are dented while being transported or dented by humans that drop them don't have quality inspections. This is how dented cans can end up on store shelves. Obvious can breaches are thrown out. However, many dented cans are simply reduced in price to sell quickly. These cans do not have open breaches where the food within gets mixed with microorganisms in the air or on surfaces. So is the food still compromised? Well, it depends.
According to the USDA, cans with small dents not on the seam of the can should be safe to buy and eat. If the dents are located on the seam—and if you can lay your finger in the dent—then don't buy the can no matter how reduced the price. Dents in the seams of metal cans can mean that the hermetic seal has been compromised. Bacteria can easily get in and cause your food to be unsafe to eat.
If it's not airtight, it's not all right. Don't buy any cans of food with large dents and don't buy any cans that have large (or small) dents along on the seam.