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Ask a Master Gardener: Follow tested recipes for safe canning

It’s fun to share recipes with friends and family, but unless you know that a canning recipe has been scientifically tested, you should not use it.

Canning process of tomato in mason jar
Canning tomatoes in mason jars.
Bychykhin_Olexandr / Getty Images / iStockphoto
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Q: I am going to get a tomato share from my CSA and I would like to learn how to can them. Any tips?

A: So glad you asked. Canning is a great way to preserve garden bounty. There’s something so satisfying about seeing the colorful jars lined up in the cupboard.

But there is a lot of advice about canning floating around the internet and on social media sites, and some of it is not just wrong — it’s dangerous. It’s fun to share recipes with friends and family, but unless you know that a canning recipe has been scientifically tested, you should not use it.

The problem is botulism. It’s caused by a bacterium that is everywhere. The bacterium isn’t dangerous, but sealed in where there isn’t oxygen — exactly the environment in a home-canned jar — it can produce a neurotoxin. People have died from eating home-canned food. So you need to make sure that what you’re canning is acidic enough to kill the bacteria or that the temperature gets high enough to kill it.

Some foods must be canned in a pressure canner, where the temperature gets hotter than the temperature in a water-bath canner. Some foods, such as pumpkin butter, should not be canned at home.

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Preserved vegetables in glass jars
Many older recipes will tell you that you have to boil the jar lids or hold them in hot water until you put them on the jars. This is no longer true of some popular brands of lids.
Olesia Shadrina / Getty Images / iStockphoto

Your tomatoes can be canned in a water-bath canner, but you must add an acid. Tomatoes are acidic, but they aren’t consistently acidic enough to prevent botulism. Whole or crushed tomatoes, tomato juice and tomato salsas require the addition of vinegar, bottled lemon juice, or citric acid. How much? You can find tested recipes on university Extension and USDA sites. There are lots of recipes and tips for safe canning on the site of the National Center for Home Food Preservation.

It is crucial to follow these recipes exactly. Do not substitute ingredients unless the recipe specifically says that you can do so. Do not add thickeners such as flour or cornstarch.

One last tip: Many older recipes will tell you that you have to boil the jar lids or hold them in hot water until you put them on the jars. This is no longer true of some popular brands of lids. Ball and Pur, for example, now recommend just washing the lids in hot, soapy water and then setting them aside until you’re ready to use them. Read the directions on the box of whatever brand you buy. Lids should only be used once.

Written by U of M Extension Master Gardeners in St. Louis County. Send questions to  features@duluthnews.com .

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