AUBURN UNIVERSITY, Ala.—Many tomato varieties grown today are not as acidic as varieties grown in years past. The USDA recommends acidifying tomatoes before canning to avoid health concerns.
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,” Treadaway 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°F. However, because they may not be sufficiently acidic, it is recommended to can in the pressure canner to reach 240°F or add some type of acid to ensure the proper pH. Find more complete instructions below.
Tomatoes can have a natural pH above 4.6, and up to 4.8. Treadaway said a pH of 4.6 and below are 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 tbsp bottled lemon juice or ½ tsp of citric acid
Pints: Add 1 tbsp bottled lemon juice or ¼ tsp of citric acid
Acidify with Vinegar
Quarts: Add 4 tbsp
Pints: Add 2 tbsp
**Vinegar may change the taste of tomatoes.
The USDA offers tomato product recommendations for both boiling water and pressure canning. However, the pressure processes require a shorter amount of time to preserve because it is 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 still require pressure canner processing. These include:
- Tomatoes with okra or zucchini
- Spaghetti sauces
- Mexican tomato sauce where there is little to no addition of acid
- Tomato based vegetable soups
“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 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 un-ripened 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.