Many tomato varieties grown today are not as acidic as varieties grown in years past. The United States Department of Agriculture recommends acidifying tomatoes before canning to avoid illness.

Angela Treadaway, an Alabama Extension food safety and quality regional agent, said acidifying tomatoes before canning will help prevent the possibility of botulism poisoning and other bacterial concerns.

“The bacteria that causes botulism poisoning can grow and produce toxins in sealed jars of moist food at room temperature if the pH — or measure of acidity — is above 4.6,” she said. “Vegetables, meat and fish have pH measures naturally higher than 4.6, so pressure processes were developed for those to kill the heat-resistant spores of C. botulinum that may likely contaminate them.”

The pH scale ranges from 0 (highly acidic) to 14 (highly alkaline). Traditionally, tomatoes are canned in a boiling-water-bath canner at 212 degrees. However, because the tomatoes may not be sufficiently acidic, one should either can tomatoes in the pressure canner to reach 240 degrees or add some type of acid to ensure the proper pH. Find more complete instructions below.

Acidify tomatoes

Tomatoes can have a natural pH above 4.6, and up to 4.8. Treadaway said a pH of 4.6 and below is needed to prevent favorable growing conditions for botulism. Instead of developing a pressure-only process to assume all tomatoes fall below 4.6, the USDA recommends adding a small amount of acid. “This allows treatment of tomatoes as a food with a pH of less than 4.6 for home canning,” she said. “Therefore, they are suitable for boiling-water canning with the addition of acid.”

Acidify with lemon juice or citric acid

• Quarts: Add 2 tablespoons of bottled lemon juice or ½ teaspoon of citric acid

• Pints: Add 1 tablespoon of bottled lemon juice or ¼ teaspoon of citric acid

Acidify with vinegar

Quarts: Add 4 tablespoons

Pints: Add 2 tablespoons

Note: Vinegar may change the taste of tomatoes.

Recommendations

The USDA offers recommendations for both boiling water and pressure canning. The pressure process requires a shorter amount of time to preserve because it cooks at a higher temperature.

“Whole tomatoes in a pressure canner take only 10 minutes to process at 10 pounds of pressure,” Treadaway said. “Pints require 40 minutes in a water bath with the addition of acid.”

There are, however, some tomato products that require pressure-canner processing.

These include:

• Tomatoes with okra or zucchini;

• Spaghetti sauce;

• Mexican tomato sauce with little to no addition of acid; and

• Tomato-based vegetable soup.

“If a pressure process is the only listed option, then it is the required processing method and there is not another available water-process option,” she said.

Storing, ripening fresh tomatoes

Treadaway said it is important to avoid storing fresh tomatoes in the refrigerator.

“Cold temperatures make the flesh of a tomato pulpy and destroy the flavor,” she said.

To ripen, place green or unripened tomatoes in a brown paper bag. Place the bag in a dark place for three to four days, depending on the degree of greenness. Do not put tomatoes in the sun to ripen, as this often softens the fruit.

— For information on topics related to the home and garden, contact any office of the Alabama Cooperative Extension System. The Limestone County Office is located at 1109 W. Market St. in Athens. Office hours are 8 a.m. to 4:30 p.m. Mondays through Fridays. For more information, call 256-232-5510 or visit www.aces.edu.