Acidify Tomatoes before Canning
Posted: August 10, 2012
A common misconception is that tomatoes are a high acid food. With a pH of 4.6 tomatoes are right on the border between being high or low acid. The tomato variety and growing conditions (blight and diseased plants) can easily tip the acidity level to the low acid side of the scale.
Other misconceptions are that yellow tomatoes are lower in acid than red tomatoes and that heirloom tomatoes are high in acid. Both are still borderline. Yellow tomatoes taste less acid because they contain more natural sugars.
Why acidify tomatoes before canning them?
The bacterial spores that cause botulism cannot produce their deadly toxin in a high acid environment. Before filling a quart jar with tomatoes or tomato product, place 2 tablespoons bottled lemon juice, ½ teaspoon citric acid crystals, or 4 tablespoons (1/4 cup) vinegar in the jar. For a pint jars, use 1 tablespoon bottled lemon juice, ¼ teaspoon citric acid crystals, or 2 tablespoons vinegar.
Why not fresh lemon juice?
Bottled lemon juice has a controlled amount of acid. It is becoming easier to find small jars or cans of citric acid where canning supplies are sold. Vinegar is seldom used because it may cause undesirable flavor changes. Many people like the complimentary flavor of lemon and tomatoes. If you feel it is too tart, sugar can be added to sweeten the tomatoes.
Most combination tomato products also need to be acidified. Be especially cautious when canning tomato sauces that have low acid vegetables such as onions, celery, pepper, or garlic added for flavor. If too many of these flavor enhancers are used, the lemon juice or citric acid may not be adequate to safely process the sauce in a boiling water bath.
Never process tomato sauces with meat or mushrooms in a boiling water bath. Use a pressure canner. Never home can sauces containing cheese.
Processing times are also important to the safety of canning tomatoes and tomato products. Acidified tomatoes that are packed raw into pint or quart jars with no added liquid or in tomato juice need to be processed in a boiling water bath for 85 minutes or a pressure canner for 25 minutes (Use 10 pounds pressure in a weighted gauge canner and 11 pounds pressure in a dial gauge canner at altitudes below 1000 feet). Hot packed crushed tomatoes are processed in a boiling water bath for 35 minutes for pints, 45 minutes for quarts or either size jar in a pressure canner for 15 minutes. Process hot packed tomato juice in a boiling water bath for 35 minutes for pints and 40 minutes for quarts or pressure can either size jar for 15 minutes. Tomato juice and tomatoes should not be canned in half-gallon jars. These processing times may be longer than those found in old canning books.
The safest practice is to use canning recipes from the USDA and cooperative extension or from a Ball Blue Book® (both with a copyright date of 1994 or later); these have been scientifically tested to control bacterial growth.