Pay attention to food safety when canning tomatoes

Sevie Kenyon
UW Madison

Over the years, best management practices for canning tomatoes have changed. With newer varieties of tomatoes being grown in home gardens, home canners should be aware of the need to add extra acid to ensure that their summer produce will remain safe while on the pantry shelf.

A pot of tomatoes, cooked down, blended, and then canned, brings some summer-y flavor to the dead of winter.

According to Barb Ingham, Department of Food Science, University of Wisconsin Extension in the College of Agricultural and Life Sciences, said many home gardeners may not realize that in the late 1990’s some canning recommendations changed, including those for tomatoes.

"We now pretty much require that acid is added to tomato products when they are canned and this would include both those that are canned in the boiling water canner as well as those that are pressure canned," Ingham said in a recent interview.

Ingham said the changes became necessary as newer varieties of tomatoes hit the market, and were bred for less acidity or more sugar.

"We also know that there are some organisms that are somewhat newer on the scene, the E. coli 0157H7, it’s relatively acid tolerant," Ingham pointed out. "So if we take all of those things together we have to be a little more careful with tomatoes than we used to be."

Ingham says the difference in the newer varieties of tomatoes can add up to as much as as an entire PH unit.

"That would mean 10 times difference in acid between those that are high in acid and those varieties that are low in acid," she said.

Acid is key in the canning process because it actually works as a hurdle for microbial growth.

"So, acid works with us and allows us to process at lower temperatures so it does give us some flexibility. But we add the acid whether we’re pressure canning or boiling water canning. It doesn’t matter. We simply need it," she said. "It’s like an insurance policy."

Adding acid to home canned tomatoes is relatively simple.

Another tip in ensuring safely canned tomatoes is to inspect the tomatoes for mold, over-ripeness and injured skins.

"We’re actually adding just a tablespoon of bottled lemon juice per pint of tomatoes or two tablespoons per quart," she said. "Or we can use citric acid; it’s a powder crystalline, it looks a little bit like sugar or salt, that you might add and that you would add only a ¼ teaspoon per pint or a ½ teaspoon per quart."

Another tip in ensuring safely canned tomatoes is to inspect the tomatoes themselves, Ingham said.

"You want to process tomatoes that are of good quality, which means that we don’t want those that are practically so rotten they’re liquid," she said. "You don’t want those that are heavily damaged to the point that they are supporting a lot of mold or other bacterial growth because those tomatoes might be problems when we come to canning."

It's also important to follow a tested recipe and to observe processing times for tomatoes especially for those using a boiling water canner, which can be up to 85 minutes.

"You can check with your local county extension office to make sure you’re using an up to date recipe, and that you’re processing fruit (tomatoes are fruits) of really good quality and so you will have something to enjoy all year long," she said.