Tomato plants are beginning to yield, and with the new crop comes a new activity: home canning. But whether canning whole tomatoes, homemade ketchup, pasta sauce or anything in between, adding acid to canned tomato products is a must, according to University of Wisconsin-Extension food safety specialist Barbara Ingham.

“Tomatoes can be preserved by canning, drying, freezing, or pickling,” says Ingham. “And when foods are home canned, the safety depends primarily on the amount of acid in the product.” Though tomatoes are usually considered a high-acid food, food safety researchers now know that the pH (acid) levels of tomatoes and other fruits can vary greatly because of many factors, including climate, soil, cultivar variety and ripeness. Because of this variation in acid levels, the United States Department of Agriculture recommends adding acid to all home-canned tomato products.

Ingham warns that improperly canned foods are dangerous to consume. “Foods canned with too little acid may allow the bacteria that cause botulism to grow in the jars, producing a deadly neurotoxin,” Ingham says.

Adding acid to home-canned tomatoes is one way to help prevent the incidence of botulism. “The rule is ½ teaspoon of citric acid or two tablespoons of bottled lemon juice for every quart of tomatoes,” she says. “The acid can be mixed in to the tomatoes or added to the jar directly before filling the jars with product.” Using vinegar is also an option (five percent acetic acid at four tablespoons per quart), but because vinegar will affect the flavor, it may not be the best choice for things like plain canned tomatoes or tomato juice. And be sure to use bottled lemon juice, not fresh-squeezed, for the assurance that your home-canned tomatoes will be safe and tasty, Ingham says.

There are a few other important safety tips Ingham recommends keeping in mind when home-canning tomatoes and other fruits and vegetables. “When choosing tomatoes to can, do not use tomatoes that are overripe or have bruises, cracks or insect damage,” she says. “Tomatoes growing on dead or frost-killed vines are also unsafe, because these fruits will have lower acidity.” It is also unsafe to add thickening agents like flour and starch to tomato products before canning; Ingham instead recommends thickening things like tomato sauce and soup immediately before serving.

Ingham also recommends using current, research-tested recipes for all home canning. “Just because a recipe is in print, doesn’t mean it’s safe for you and your family,” she says. “Canning recommendations have changed dramatically over the last 15 years, so if you are using recipes that date before 1994, then it’s a good idea to set those aside and find an up-to-date recipe that has been tested for safety.” It is also important to make sure all canning equipment, such as boiling water or pressure canners, are in good working order.