What makes a can bulge?

For as long as I can remember, I have heard or read the admonition to NEVER eat food from a bulging can.

I always assumed this really meant “Never buy a bulging can at the grocery store, because the food was contaminated with bacteria during processing.”

But then I discovered a bulging can hiding in the back of my pantry! It was a can of non-fat sweetened condensed milk. I couldn’t begin to tell you how long it had been there. More than a couple of years, for sure. It’s the kind of thing I buy only if I have a specific purpose for it.

Anyhoo, both ends were bulging, noticeably. It frightened me. I thought about the “Cooler of Doom.”

What was going on in that can that would happen a considerable time after it was purchased? Granted, I don’t know how long ago the can began to bulge, but I am fairly certain it happened after it had been sitting there for quite some time. (Before it hid itself in the back of the pantry, I would come across it periodically and think, “What the hell did I buy this for anyway?”)

And what would have happened if it hadn’t been discovered? Would it have eventually exploded?

Bacterial growth is the usual reason a can bulges. The food will contain toxins,so don’t eat it. Cans will sometimes leak from the seam. I don’t think you have to worry about shrapnel flying around.

I understand bacterial growth as a reason for a can bulging. But why, if that’s the reason, did bacteria begin growing in a something that had been sitting around a long time? Is there a particular kind of bacterium that can be dormant in canned food for a long time? (And if so, what causes it to “wake up”?) Is there some kind of microscopic deterioration of the can’s seams that would allow bacteria enter?

As for why cans bulge, the reason is usually that bacteria within the can are multiplying.

Waste processes from anaerobic bacteria metabolism may include several gases - but is primarily CO2 I believe.

The buildup of gas pressure in the can causes it to bulge.

CO2 is of course not dangerous - heck, it’s released into the atmosphere quickly when the can is opened.

But what’s dangerous is that the bulging can indicates lots of bacterial activity inside, and these bacteria can harm or kill you (botulism is the classic example)

The big issue with botulism is that it creates toxins that persist even after the food is heated enough to kill the bacteria.

And from a cursory Google search, cans do explode. Carefully remove it from the can and dispose of it now.

Sweetened condensed milk is VERY high in sugar, though; which generally makes things bacteria-resistant.

I still wouldn’t eat it.

High acid foods can also do this, in Florida a few years ago quite a number of sudents suffered from metallic poisoning when were served canned pineapple - the cans had come from South Africa and had not been properly lacquered.

Blown cans can be caused any number of bacteria when part of the process has gone wrong, there are several ways this can happen, but as already mentioned, the biggest concern is that Bacillus Botulinum has grown and is deadly.

Given that the contents of the can are contaminated, please ensure that you are confident that when you dispose of it that no one else is likely to find it and try out the contents, incineration would be best as you are less likely to contaminate anything else, if you open it to dispose the contents down the toilet then make sure you clean up with powerful disinfectants afterwards.

Any time I’ve found a bulging can, it was because of this. A little patch of corrosion is all it takes to break the seal.

If the seal weren’t broken, the food would be fine indefinitely. I found that out last fall, on a lazy evening when I thought I’d “borrow” (with the intention of replacing) a can of soup for dinner from my roommate’s side of the cupboard. I thought the label design looked a little old, but heated it up and ate the soup with no problems. It was only later when I rinsed out the can for recycling that I noticed the expiration date was 1998. So the soup had to be even older than that. :eek:

But if the seal’s broken, why is there a buildup of pressure causing the can to bulge?

A plug forms at the puncture site by dehydration of the can contents- like the plug that forms in the nozzle of a tube of caulk or bottle of glue.

That has me puzzled as well.

Sweetened condensed milk (unlike evaporated milk or other low acid canned foods) is not heated to the point that botulism spores are killed during the canning process because the water activity in sweetened condensed milk is so low that botulism cannot grow in it.

The only thing I can think of is that something happened inside the can to change the water activity. If some of the sugar came out of solution and crystallized, the water activity could rise to a point where botulism or something else could grow. This is just a guess, I don’t know of a scenario where this could happen. But it would explain the apparent long period of dormancy between non-bulged and bulged can.

A friend got given some old tins of food by a neighbour about ten years ago. I pointed out that several of them were bulged and at least one was priced in £.s.d. [pre 1971 currency]

Dr. Lao, can you help me understand the concept of water activity? I just tried reading the Wikipedia article about it. I was lost after the first sentence. Actually, I was lost DURING the first sentence.

Basically, water activity is defined as the partial pressure of water vapor over your material divided by the pressure of vapor over pure water. It’s a measure of how much water is not only present, but available for use - not bound to sugars or whatnot. So, for instance, honey has a lot of water in it, but it also has a ton of sugar, which binds the water up and makes it unavailable for metabolism.

My wife used to be the quality control director at a vegetable cannery. The way the cans were prepared were that they were filled with beans or okra or whatever, then sealed, and then cooked in a retort - basically hot water.

They then stored the cooked cans in the warehouse just in case. One time, during a shift change, a retort full of cans never got cooked. About a week or so later, they started exploding all over the warehouse.

As for the seal issue, all it takes is one bacterium to get into a hole too small for the pressure to be relieved. The solder joints and seals for the lids are good but not perfect. It is also possible that one could be just out of alignment enough to produce a tiny hole with thermal expansion. Her lab tested the lids and the cans to make sure they were in spec, using sampling. Too many out of spec, and a whole railroad car full of cans went back to the manufacturer. She’s never had trouble returning things since.