Why do I have to add acid when canning tomatoes in the pressure canner?
Tomatoes that are acidified for canning are done so to prevent botulism poisoning and other bacterial concerns by a combination of acid and heat; the control in vegetables, meat and other naturally low-acid foods is by heat alone.
The bacteria that cause botulism poisoning can grow and produce toxin in sealed jars of moist food at room temperature if the pH (measure of acidity) is above 4.6.
Vegetables, meat, fish, etc. are naturally fairly high above pH 4.6 (close to 6.0) and so pressure processes were developed for those to kill the heat-resistant spores of C. botulinum bacteria that are likely to be contaminating them.
Tomatoes also can have a natural pH above 4.6 (at least up to 4.8). But rather than develop a pressure-only process as if they were all low-acid, since they are so close to 4.6, USDA decided instead to recommend a small amount of acid be added so they can be treated as a food with a pH less than 4.6 for home canning. Therefore they are suitable for boiling water canning when the acid is added. (The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls.)
When you see the tomato product recommendations in USDA canning directions that offer both boiling water and pressure canning options, those pressure processes are still only the same amount of heat treatment as the boiling water option. (Higher temperature=shorter process time.) Those pressure processes are not the amount of heat and time that would be required for canning a low-acid food to control for botulism. There has not been a properly researched process for pressure canning of low-acid tomatoes without added acid, so the available process times still require the addition of acid as if they are being processed in boiling water.
There are some tomato products in the USDA canning procedures that only have a pressure process listed (for example, tomatoes with okra or zucchini, spaghetti sauces, Mexican tomato sauce, etc.). If a pressure process is the only listed option, then it is the required processing method and we do not have a boiling water process option available. These products made according to the stated recipes and procedures are low-acid food mixtures.
Storing and Ripening Tomatoes
Never Refrigerate Fresh Tomatoes!! Cold temperatures make the flesh of a tomato pulpy and destroys the flavor.
To ripen, place green or unripened tomatoes in a brown paper bag and place in a dark spot for three or four days, depending on the degree of greenness. Do not put tomatoes in the sun to ripen - this softens them.
Tomato Equivalents
- 2½ pounds fresh tomatoes equals 3 cups chopped and drained fresh tomatoes
- A 16-oz. can equals 2 cups drained tomatoes or 1 cup undrained tomatoes
- A 28-oz can equals 3 cups undrained or 2 to 2½ cups drained tomatoes
- 25 to 30 cherry tomatoes equals 2 cups chopped tomatoes
- A 6-oz. can tomato paste equals ¾ cup
- An average bushel of tomatoes weighs about 53 pounds
Measures
1 cup = 16 tablespoons 1 pound = 16 ounces
3/4 cup = 12 tablespoons 1/2 pound = 8ounces
1/2 cup = 8 tablespoons 1/4 pound = 4 ounces
1/4 cup = 4 tablespoons 1 cup = 8 fluid ounces
1/8 cup = 2 tablespoons 2 cups = 1 pint
1/3 cup = 5 tablespoons plus 1 teaspoon 4 cups = 1 quart
2/3 cup = 10 tablespoons plus 2 teaspoons 4 quarts = 1 gallon
1 tablespoon = 3 teaspoons 1 peck = 8 quarts
1/2 tablespoon = 1 1/2 teaspoons 1 bushel = 4 pecks
1/3 tablespoon = 1 teaspoon
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