How to Safely Can Your Tomatoes

The produce available in August is fantastically varied and abundant, but truthfully, most of my farmers market basket is usually filled with tomatoes at this point in the summer. And while I do my best to pump out as many tomato toasts and tarts as I can, the delight of fresh tomato season also brings with it a chance to bottle up the best of summer's sweet, juicy tomatoes to enjoy during cold winters by canning them. 

Any type of tomato can be canned, and you can use home-canned tomatoes any way you'd use store-bought canned tomatoes, including in an Italian-American red sauce, shakshuka, and tomato soup. So by all means, can those beautiful Brandywines, Sungolds, and plum tomatoes bursting out of your garden or that you hauled home from the market, just be sure to do it properly—for the best canned tomatoes and for your safety.

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15-Minute Creamy Tomato Soup (Vegan)

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cook time: 15 mins

serves: 4 to 6

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Proper canning practices are key not only to preserving flavor, but to avoiding foodborne illness, including botulism, a serious type of food poisoning that’s caused by a type of bacteria that thrives in low oxygen environments like canned and vacuum-packed foods. The bacteria is called Clostridium botulinum, and it becomes poisonous when it’s introduced to low-oxygen, low-acid, or low-sugar environments. To ensure your canned goods are safe to eat and taste their best, there are two key steps you can take: raising acidity or using a pressure canner for high-heat canning. For the ultimate insurance policy, you can employ both techniques, but one or the other should put your canned tomatoes in the safe zone. 

Why You Should Raise the Acidity of Your Tomatoes Before Canning Them

In order to can foods safely, you need to understand the importance of pH, which is simply a measurement of acidity. The pH scale runs from 0 (most acidic) to 14 (most basic). A lower pH value means higher acidity, and a higher pH means lower acidity. For reference, lemon juice is a pH of about 2, while water is neutral with a pH of 7. Achieving the proper acidity (pH) level is key for food preservation. 

An ingredient’s pH level needs to be at or below 4.6 to resist microbes and be considered safe for preserving, and while most tomatoes are safely in that range, some types can clock in at slightly higher pH levels, meaning they are not acidic enough to can safely without added acid. Each variety of tomato will have a different pH level, and that pH level also varies depending on the ripeness of the fruit when it was picked, as well as how much it has ripened since. Various studies have shown that both commercially grown and heirloom varieties of tomatoes can vary anywhere from 3.7 to 5.2. Unless you plan on testing the pH level of every tomato you have before canning, this is where incorporating more acid can give you the extra safety you need. 

In order to properly adjust the pH range of tomatoes for canning, it’s best to add either lemon juice or citric acid. Bottled lemon juice is recommended over fresh since the acidity of fresh lemon juice varies from lemon to lemon, while bottled lemon juice has a consistent controlled pH. You’ll want to add two tablespoons of lemon juice or half a teaspoon of citric acid per one quart jar of tomatoes, according to multiple sources, including University of Georgia’s National Center for Home Food Preservation.

Boiling Water Canning vs. Pressure Canning

Both boiling-water canning and pressure canning drive out air from jars and create a vacuum that pulls the lid down more tightly, creating an airtight seal, but each comes with its own pros and cons. A boiling-water bath is better suited for foods that are already higher in acidity, such as blueberries and strawberries, since boiling-water canners tend to reach the atmospheric boiling point, which is about 212°F (100°C). A pressure canner, on the other hand, will drive the boiling point higher than the atmospheric point to reach temperatures upwards of 250°F (121°C), making the environment even better for killing off microbes. If you’re adding lemon juice or citric acid to your tomatoes to boost the acidity, a boiling-water bath is enough to ensure safety. If you’re opting not to, then pressure canning is a must in order to guarantee high enough temperatures to prevent bacterial growth. 

Another factor that can help you decide which route to go when canning your tomatoes is time. The processing time for tomatoes using  boiling-water canning is 85 minutes, which can sometimes create a mushy texture in tomatoes; the pressure canner, on the other hand, only requires 25 minutes of processing time, along with a venting time of 10 minutes. 

Lastly, if you want to add additional ingredients to your tomatoes like garlic, oregano, or basil leaves, you’ll need to use a pressure canner over a boiling-water canner, since these ingredients can lower the acidity of the tomatoes. 

More Tips for Successfully Canning Tomatoes

There are a few other factors to keep in mind when preserving your tomatoes to ensure the best results. Make sure to look for tomatoes that are ripe and unblemished, since fungus, mold, and other microbes can infect damaged tomatoes and affect acidity. You’ll also want to peel your tomatoes (which you can do by scoring them, blanching for one minute, and then shocking them in an ice bath before removing the skin) to eliminate the bitterness and toughness that the skins bring. Adding a teaspoon of sea salt or kosher salt per two pounds of tomatoes is optional, but will boost flavor. 

Senior culinary editor Leah Colins also emphasizes the importance of “minding the gap” when canning. “Tomatoes are watery, and as they heat, they will release a lot of liquid,” she says. “During processing and cooling periods, a harmless air gap can occur in the jars. Hot-packing the jars helps minimize this.” She advises leaving an inch to an inch and a half of space from the top of a quart-sized jar. “This ensures spaces for juices to move within the jar during the heating or pressure canning process. If a large gap remains after processing, but the lid is still fully sealed, the food inside is still safe.”

The Takeaway

Canning tomatoes is a great way to preserve their flavor, but due to the fruit’s varied levels of acidity, you need to take a couple of key steps to ensure safety. Adding acidity in the form of bottled lemon juice or citric acid is the best way to put the tomatoes’ pH level at the ideal range of 4.6 or below; if you’re employing this technique, then boiling-water canning is a safe route for preserving your tomatoes. Pressure canning raises the temperature inside the jar, further reducing the likelihood of microbial contamination and growth, so this is recommended if you prefer not to add acid to your tomatoes. 

With this in mind, you can get to canning those tomatoes with ease and little worry. And given that tomato season is more than halfway through in most of the United States, you’d better get to it!