Let's clear up some confusion about the lineage of the tomato. Botanically, it's a fruit. But in 1893, vegetables imported into the United States were taxed with a 10 percent tariff...

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Let’s clear up some confusion about the lineage of the tomato. Botanically, it’s a fruit. But in 1893, vegetables imported into the United States were taxed with a 10 percent tariff, while fruit was duty-free. An eager New York customs collector, looking to up the ante on importers, declared the tomato a vegetable. The importers sued but lost their case in the Supreme Court, the tariff was paid, and presto-chango: A fruit became a vegetable.

Mother Nature wasn’t fooled, though. The tomato is what it is. But when it comes to canning this particular fruit-of-the-vine, there is a catch. The pH level changes rapidly in tomatoes as they mature, and processing them is trickier than other high-acid fruits, said Val Hillers, an extension food specialist at Washington State University.

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She said it is the imperative to select tomatoes that are firm and very fresh, and use USDA-approved recipes and processing information to avoid problems. Hillers and other food safety professionals are concerned about people trying to use overripe or poor-quality fruit and old canning recipes because it could lead to botulism, which can be deadly.

Canning salsa and sauces

There are some basic differences between varieties of tomatoes. Paste tomatoes such as roma or plum tend to be denser with fewer seeds, and make thicker sauces. Slicing tomatoes — beefsteak and Early Girl — are more watery, but the addition of tomato paste gives sauces and salsas body. Yellow and heirloom varieties may fall into either category, but “all are treated with the same guidelines for acidification,” said Hillers.

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Lemon juice or vinegar is added to the tomatoes to raise the acidity level. Be sure to use bottled lemon juice, which has a standardized acid level suitable for canning.

A water-bath canning process is most often used for salsa. Generous amounts of bottled lemon juice or vinegar are added to raise the acidity level of the other ingredients so they’re safe to can.

Here again, there are special considerations. Bottled lemon juice has a standardized acid level suitable for canning, while the acidity of fresh lemons fluctuates and makes them unreliable for use in canning.

The acidity levels for vinegars aren’t consistent either. Many apple cider vinegars have a 5 percent acid level, the minimum accepted level of acidity for canning with vinegar. A red or white wine vinegar may be 6 or 7 percent, which makes it OK for canning. Rice vinegar has an acidity of about 4.2 percent, which is not adequate.

Always check on the bottle’s label for its acid percentage before using. The USDA and WSU-approved recipes have been designed assuming a 5 percent acidity for the vinegar, noted Hillers.

It is important not to tinker with any ingredient proportions or canning instructions because USDA and WSU-approved salsa and tomato recipes have been tested for proper acidity using the prescribed measurements and methods.

If a salsa recipe requires bottled lemon juice, don’t make a substitution with vinegar. Lemon is more acidic, and the amount of low-acid vegetables in a particular salsa may need the additional boost that lemon juice provides. If a recipe calls for using vinegar, however, it can be replaced with lemon juice.

Sauces that combine tomatoes with some vegetable proportions must be canned in a pressure cooker. But that doesn’t mean that a cherished recipe for tomato sauce passed down from your Italian grandmother is fit for canning. For a sauce to meet guidelines, a relatively small amount of vegetables are used. Consider this: A USDA-approved spaghetti sauce uses 30 pounds of tomatoes, 1 cup chopped onion and ½ cup each chopped celery and green pepper. Grandma’s sauce most likely has proportions that are very different, and are not OK for canning, given our present-day scientific knowledge of what constitutes safe canning. Grandma’s recipe may be more suitable for freezing. (See recipe on this page.)

You can be creative with the amounts of dried herbs and spices, but they’re the only ingredients that are safe to alter in these recipes. Don’t be tempted to substitute fresh herbs for dried, which may throw off the precise balance of the other ingredients. If you want fresh herb flavors, Hillers suggests adding the fresh herbs before serving. You’ll get fresher flavors and a more aromatic sauce.

Equipment

As for all types of canning, standard pint and quart mason-type canning jars are necessary. (Mayonnaise or condiment jars may crack when heated in boiling water.) Jar caps can be bought separately and come in two pieces: self-sealing lids and metal ring bands. Jars and bands can be reused if they’re still in good shape. Check jars for cracks, and make sure bands aren’t dented. Because the rubber seal on the lids are designed for a single use, you’ll need to purchase new ones every time you can.

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For a water-bath canner, you’ll need a pot that is deep enough for boiling water to cover jars by about 1 to 2 inches, with another 1½ inches of pot height for water expansion. Because of the longer processing times for tomatoes, you may need to periodically add boiling water to the canner to ensure a safe water level.

If you don’t have a rack that fits the bottom of the canner, put a clean, folded kitchen towel in the bottom to protect jars from breakage. Cover the pot during the canning process for consistent heat conduction.

When canning with a pressure cooker, follow the manufacturer’s directions with these guidelines in mind: There should be 2 to 3 inches of boiling water in the canner, and jars should sit on a rack so that steam can flow freely. Once the canner is closed, watch for steam that may escape around the rim. Steam should be released steadily through the petcock for 10 minutes before it’s closed. Even if your canner’s instructions tell you it’s a “self-exhausting” model, this step is necessary because any air that’s trapped in the canner may cause uneven heating of the jars.

Sterilizing jars and caps

As an added safety precaution for both water-bath canning process and pressure canning, jars can be sterilized in boiling water for 10 minutes; the water level should cover jars by about an inch. Hold the sterilized equipment in hot water and remove individually when it is time to fill each jar. Prepare lids and bands according to manufacturer’s directions.

Packing the tomatoes

There are two ways to pack whole or sliced tomatoes into jars, with specific advantages and disadvantages for each method. For a raw pack, tomatoes are put into canning jars and covered with boiling water or tomato juice. The processed tomatoes will have a firmer texture, but will float to the top of the jars.

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With the hot pack process, the tomatoes are brought to a boil in liquid. Then they’re packed into the jars while still hot and covered with the boiling liquid. Because heated tomatoes are softer, more can fit into the jars, and there will be less floating.

Acidifying the tomatoes is necessary to prevent spoilage. Add 1 tablespoon bottled lemon juice per pint or 2 tablespoons to each quart. Or add ¼ teaspoon citric acid per pint and ½ teaspoon per quart. A teaspoon sugar can be added to each pint, or 2 teaspoons per quart, to balance the tartness.

Salt can be omitted, but if you prefer to add it for flavor, ½ teaspoon per pint and 1 teaspoon for each quart are starting points. (You can also add salt to taste when serving the tomatoes.)

Leave ½-inch headspace when packing jars, which is measured from the top of the liquid to the bottom of the flat lid. The tomatoes and their juices will expand when heated, and if the jars are filled too full, the seal may be broken as the food bubbles. Too much headspace may also keep jars from sealing. Once jars are filled, run a clean plastic spatula around the inside edge to release air bubbles.

Wipe the rim and threads of jars with a clean, damp paper towel. Immediately place a hot lid on top and tighten band. As each jar is filled and capped, use the jar lifter to place it in the boiling water-bath. (A jar lifter is a tool well worth its $5 price tag.) When the canner is full, cover and bring back to a steady boil. Process according to a USDA-approved canning time. (See related story about processing tomatoes.)

Storage

When finished processing, use the jar lifter to transfer them from the canner to a rack or folded kitchen towels. Don’t tighten the bands or move jars for at least 12 hours. Then check the seal of each jar by pressing the center of the lid — it should not flex. If it does, the tomatoes should be frozen immediately or refrigerated and eaten within 10 days.

After checking seals, remove the bands and wipe jars with a clean paper towel. Date jars and store in a cool, dark place.

For maximum quality and flavor, use the tomatoes within a year. As long as the seal of the jars remains intact, they can be stored up to 3 years, but the quality will begin to decline, affecting color, flavor and nutrition.

Before using canned tomatoes, always check for bulging lids or leaks. If either of these situations are present, throw the product out without tasting it. Once opened, spurting liquid, foam, mold or an off odor are clear trouble signs, and again, the food should not be tasted and should instead be discarded immediately.

Sources: Pacific Northwest Extension publications “Canning Tomatoes and Tomato Products: PNW300” and “Salsa Recipes for Canning: PNW395.” Val Hillers, extension food specialist at Washington State University; Olga Fusté, with the Washington State University’s Extension Service in Pierce County. Food preservation booklets are available online at www.pierce.wsu.edu, or by calling 253-798-7180.