Planning to can tomatoes?

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Home canning continues to make a comeback, and tomatoes are among the most popular commodities to can says Dr. Janie Burney, a professor and food preservation specialist with the University of Tennessee Extension Department of Family and Consumer Sciences.

As the canning season begins, Burney answers questions that beginners and those experienced in food preservation often ask about canning tomatoes.

Question: Why do I need to add lemon juice to tomatoes and salsa before canning?

Burney: It's all about pH. A food's pH is a measure of its acidity or alkalinity. You may think of tomatoes as an acidic food, but they may not be acidic enough to make your canned tomatoes or salsa safe to eat.

When you can foods so that they can be stored at room temperature, you have to be sure to destroy harmful bacteria called Clostridium botulinum, the type of bacteria that cause botulism. These bacteria produce a dangerous neurotoxin that can be fatal. When a food is acidic enough, these bacteria cannot grow and produce the toxin.

Question: Do all tomatoes have the same amount of acid?

Burney: No, they don't and that is part of the problem. The amount of acid in a tomato can depend on many factors including variety of tomato and growing and harvesting conditions. Studies have demonstrated that when a tomato stays on the vine it loses some of its acidity over time. The amount of direct sunlight a tomato receives as it ripens on the vine, or whether it is ripened on the vine, also can affect pH. A tomato harvested from dead vines, or a damaged or decayed tomato, may be less acidic. The North Dakota Extension Service tested several varieties and found their pH varied considerably.

Question: What are heirloom tomatoes, and are they more acidic than other tomatoes?

Burney: Tomatoes referred to as "heirloom" or "heritage" are open-pollinated varieties that are valued for their unique colors, shapes, flavors and legacies. Open-pollinated means they are pollinated by insects, birds, wind or other natural means. The name generally refers to varieties that existed before the 1940s, before industrial farming spread dramatically in the U.S. Some people have assumed that heirloom tomatoes are more acidic than others, but one study by Utah State University Extension actually demonstrated that some may be less acidic that hybrid varieties. The bottom line is that most home canners don't know the pH of their tomatoes, so adding an acid is the safe way to go.

Question: How do I acidify my tomatoes before canning?

Burney: Add two tablespoons of bottled lemon juice (not fresh) or one-half teaspoon of citric acid per quart of tomatoes. For pints, use one tablespoon bottled lemon juice or one-quarter teaspoon of citric acid. Acid can be added directly to the jars before filling.

Question: Do I need to acidify if I use the pressure canner to process my tomatoes?

Burney: Yes, you do. Currently, USDA does not have a time established for processing tomatoes that have not been acidified. A time established using a scientific method is necessary to ensure that Clostridium botulinum bacteria are destroyed. However, processing in a pressure canner can save you time and will result in a more nutritious product of higher quality than processing in a water-bath canner.

UT Extension provides a gateway to the University of Tennessee as the outreach unit of the Institute of Agriculture. With an office in every Tennessee county, UT Extension delivers educational programs and research-based information to citizens throughout the state. In cooperation with Tennessee State University, UT Extension works with farmers, families, youth and communities to improve lives by addressing problems and issues at the local, state and national levels.

Carla Y. Bush, MVTE

UT Extension, Cannon County

Family and Consumer Sciences

614 Lehman Street

Woodbury, TN 37190


615-563-1285 - FAX

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