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Home canning the safe way

A marmalade project with local cranberries and bitter oranges.
Elspeth Hay
A marmalade project with local cranberries and bitter oranges.

When Kim Concra first started working with the Barnstable County Extension Service as a food safety and nutrition specialist in 2005, she says she didn’t know much about canning. But very quickly, it became clear that she would need to learn.

"Somebody came in with a jar of canned tomatoes and it looked like there were little worms in it. And it was frightening," she explained. "

And we had like five of us around looking to figure out what was going on. Somebody to look at it under a microscope. And we were trying to figure it out. What it was is it wasn't properly canned. It wasn't canned properly long enough to disable basically the seeds from sprouting."

Kim signed up for a Master Food Preservation Workshop at Cornell.

"We did pressure canning, we did boil water canning, we did homemade sauerkraut, we did pickles, we did all kinds of things. It was a really, really intense three days, but it was absolutely awesome."

While she was there, Kim gained a new respect for canning — both as an incredible way to preserve food in a shelf stable form and as a practice that demands precision. For instance, she hadn’t realized before just how important acidity is in home canning.

"When you talk about food preservation and you're talking about canning foods, there's low acid foods and there's higher acid foods."

Higher acid foods can generally be canned at home without any fancy equipment.

"So for instance, with a higher acid food like we'll say tomatoes, right? We can boil water can those," she said.

Boil water canning is also called water bath processing, and it involves filling sterile jars with food — usually hot food — sealing them with a lid, and then boiling the contents submerged in water for a prescribed amount of time. But with lower acid foods, this isn’t always safe.

"Basically, we're looking for less than 4.6 on a pH scale. So for people that haven't taken chemistry in a long time, that pH scale is 0 to 14. So zero you think of acid, 14, you think of as base. So it seems kind of backwards, but a lower number is a higher acid. So we're trying to shift that food pH to a lower number, higher acid."

This is easy to achieve for many foods by adding a little vinegar or lemon juice — this is why we make pickles for instance, and why recipes often call for a little bit of lemon juice for foods on the cusp, like applesauce. The problem with low acid foods in canning is botulism — a disease caused by bacteria that are common on all sorts of fruits and vegetables. We actually ingest these bacteria all the time without harm, but when they reproduce in low-oxygen, low acid, low sugar, or low salt environments, they produce a toxin called botulin and it’s so poisonous one millionth of a gram can kill a person.

"Foods like cucumbers, potatoes, green beans, those are low acid so they’re more apt to have an issue with botulism, so what we do is we put them under pressure, for low acid foods like cucumbers, potatoes, green beans, those are low acid. So they're more apt to have an issue with botulism," Kim explained.

She added, "So what we do is we put them under pressure. And why we put them under pressure is because in a boil water canner, you'll only going to get up to 212 degrees. That is not enough to kill botulism spores. So under pressure, we can achieve about 240 degrees that will kill the spores."

This explains why foods that are both acidic and high in sugar — I’m thinking about marmalade, don’t even need a hot water bath for canning, they can just be sealed in sterile jars — but things like meat and fish are dangerous to can this way and need to be pressure canned. You simply need the right equipment — and an understanding of the science behind preserving these foods safely. It’s a dance. Kim Concra says now that she has this understanding, she’s tried all sorts of new recipes.

"Most recently I did a salsa verde, which was wonderful with roasted green tomatoes, because I had a lot of leftover green tomatoes; that was amazing."

The recipe also called for roasted jalapenos and lime juice for acid and salt, which I now understand were key to canning it safely.

Here's a link to the Barnstable County Food Safety website—you can use it to get in touch with Kim Concra, find out how to get your pressure canner tested, and explore safe home canning recipes.

An avid locavore, Elspeth lives in Wellfleet and writes a blog about food. Elspeth is constantly exploring the Cape, Islands, and South Coast and all our farmer's markets to find out what's good, what's growing and what to do with it. Her Local Food Report airs Thursdays at 8:30 on Morning Edition and 5:45pm on All Things Considered, as well as Saturday mornings at 9:30.