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November 02, 2016

Infrequently Asked Questions: How does canning work?

Though it's never been quite as sexy (or tasty) to buy a can of carrots (as opposed to a frozen bag), the science behind room-temperature vegetables keeping for so long remains a true marvel of modern technology. Relatively modern.

Curious about how it all works, we sought answers — and a little history — from Jon Deutsch, a professor of culinary arts and food science at Drexel University's Center for Hospitality and Sports Management

How does canning actually work, scientifically? What’s going on in that can that's keeping fruits and vegetables and other things from going bad?

For the most part, it’s tough to generalize because different foods go through different processes, but basically, you're doing two things. You’re cooking the food and creating a vacuum — forcing all the air out of the can, and putting it in a vacuum. Creating an anaerobic environment which, for some foods, like high-acid foods like pickles and jam, that’s sufficient. For other foods, like beets and low-acid foods like vegetables that aren’t acidic, you have to further process using a pressure cooker or in a food manufacturing facility — get it above 212 degrees. A pressure cooker will bring the boiling point up to 240 degrees, which will kill just about everything.

So it’s not as simple as just sticking vegetables in a can and calling it a day.

Right. So if you do that you could kill yourself... There’s a difference between canning something and properly canning something, for sure.

What are the regulatory laws on the books to make sure it is being done correctly?

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To do it commercially, the big magic number is the pH of 4.6 [and some high-acid foods naturally can better]. And what that means is that it can be [canned] without further processing if it’s acidic. So, just the temperature of boiling water plus acid will do enough to keep people safe. And there have been instances of people getting sick from canning, and more often than getting sick or dying from canning, is the can will just blow up. People who home-can improperly, you get bacterial growth, the bacteria eat the food in the can, create gas and heat and, eventually, that gas and heat build up and the can explodes.

Well, that’s not good.

Definitely not. And that’s why you always see warnings that if you see a bulging can or a dented can or punctured can, you should discard it.

I’ve always wondered how true it is. It’s always a good rule of thumb to choose the not-dented can anyway, but still.

Dents are different than punctures. The reason people advise against dented cans is because if they crack or rust around the dent, you can allow oxygen and contaminants to get in. Which can then make the food go bad. A little dent probably isn’t a problem, though it is advisable to avoid them all. But a dent around the seam or top of the can where the seam is, it’s more likely to allow air into it so it should definitely be avoided. As is any visible sign that — a sticky can, sometimes you get, like, a sticky food caked on. It could be from another can or could be from one can. But it could be a sign that somewhere there’s food that should be on the inside that’s on the outside, and that should be avoided.

When did we start canning? Did someone invent this?

It’s a little controversial. It’s attributed to Nicholas Appert, who was doing this at the request of the Napoleonic army. Because they were looking for ways to transport food. There are some people who say he didn’t really invent anything; he just got some recognition for other people’s work. This often happens in food history. And it also kind of depends on how you define canning.

So, his significant contribution he’s given credit for is actually using glass jars that sort of resemble modern-day canning. If you look at it a little more expansively, canning goes back millennia, in terms of potting things. There are examples, in ancient times, of things preserved in honey, for example, that have lasted millennia. Because honey basically never goes bad. It can keep anything in it. If you think of that as a form of food preservation, like canning. And potted meats, meats covered in fat, sauerkraut — some of that goes back way before, hundreds of thousands of years back. But Appert is sort of attributed as the founder of modern-day canning.

That's certainly on-pattern, in a contemporary sense, for the military to be the root of an invention like that.

Yeah, a lot of people are really attributing military success — and success in the age of exploration, for that matter — to food tech and preservation. A lot of people, if you say military history, they go right to guns and weapons and toys. And they forget that to move — just think of if you're going for a daylong hike, what you have to do to provision yourself. And then think of that for thousands of people for multiple days. It’s really mind-boggling, all the logistics needed. Take that and go back 200, 500 years in technology, and it was just an immense challenge. Canning was very helpful in allowing foods to preserve and travel. Although they were very heavy. Things like hardtack, a dried cracker, still endured even though it wasn’t as sexy as the canned vegetable, which was really revolutionary for its time. The practicality of lugging around vegetables didn’t quite work for the common everyday soldier.

Has there been any kind of progress in how we preserve food in recent years, or are we really latched onto canning?

Canning is still super popular. I think it’s fallen out of fashion, especially for vegetables, because of the sodium and quality of canned vegetables. Although some vegetables like tomatoes, chefs still — except for when tomatoes are perfectly in season, canned tomatoes are sort of a staple for everyone at home and commercially. Foods like canned asparagus were all the rage in the ’40s and ’50s but are almost absent [today] — and probably should be, from a culinary perspective these days. It varies wildly. 

We’re seeing a lot of progress in other forms besides the cans. The pouches. Tuna, for example, has been canned a long time, but it’s coming in pouches, which is perceived to be a little bit fresher and less intensely heat-processed because you have a thinner layer of product with a larger surface area. You have technology like HPP, or high-pressure pasteurization, which allows things that have never been successfully preserved to be preserved. For example, guacamole. There’s really no good-quality canned guacamole because it’s totally disgusting, but now you can find refrigerated tubs of guacamole that last 30 days or 60 days. That’s HPP. Same thing for raw juices that have a 30- or 60-day shelf life. It's intense pressure to kill bacteria. That's been the biggest advance that’s caught on in the last decade or so, although the technology really predates it.