Home Canning Tomatoes Safely - Acidity - Are Tomatoes Low Acid or High Acid?

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Are tomatoes low acid or high acid (acidic)?  In short, they borderline, and whether they are acidic or not, for the purposes of home canning, depends upon the variety. In practice, it is a moot point: as long as you add a small amount of lemon juice (it won't affect the taste) to each jar, it will always be acidic enough to be safe! Don't miss: Answers to common tomato canning problems


Tomato Acidity Explained

Tomatoes borderline acidic, and it depends on the variety.  Some are more or less acidic than others. 

 

The USDA  says: 

Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water canner.

 

The University of Minnesota has a page dedicated to home canning tomatoes and says:  

Researchers at USDA and at the University of Minnesota have found that most underripe to ripe, cooked tomatoes have a pH below 4.6. Unfortunately, a few varieties may have a pH above or close to 4.6. These include

  • Ace,
  • Ace 55VF,
  • Beefmaster Hybrid,
  • Big Early Hybrid,
  • Big Girl,
  • Big Set,
  • Burpee VF Hybrid,
  • Cal Ace,
  • Delicious,
  • Fireball,
  • Garden State,
  • Royal Chico,
  • San Marzano. and
  • there may be others!

Some of these are grown for commercial purposes and are not found in home gardens. However, safely canning these varieties requires additional acid for water bath processing or a pressure canning process similar to low acid vegetables.

USDA Acidification Methods

To ensure safe acidity in whole, crushed, or juiced tomatoes, add 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes. For pints, use 1 tablespoon bottled lemon juice or 1/4 teaspoon citric acid. Acid can be added directly to the jars before filling with the product. Add sugar to offset acid taste, if desired. Four tablespoons of a 5 percent acidity vinegar per quart may be used instead of lemon juice or citric acid. However, vinegar may cause undesirable flavor changes.

 

Background Information: Ensuring Safe Canned Foods

Growth of the bacterium Clostridium botulinum in canned food may cause botulism—a deadly form of food poisoning. These bacteria exist either as spores or as vegetative cells. The spores, which are comparable to plant seeds, can survive harmlessly in soil and water for many years. When ideal conditions exist for growth, the spores produce vegetative cells which multiply rapidly and may produce a deadly toxin within 3 to 4 days of growth in an environment consisting of:

* a moist, low-acid food
* a temperature between 40° and 120°F
* less than 2 percent oxygen

Botulinum spores are on most fresh food surfaces. Because they grow only in the absence of air, they are harmless on fresh foods.

Most bacteria, yeasts, and molds are difficult to remove from food surfaces. Washing fresh food reduces their numbers only slightly. Peeling root crops, underground stem crops, and tomatoes reduces their numbers greatly. Blanching also helps, but the vital controls are the method of canning and making sure the recommended research-based process times found in the USDA's Complete Guide to Home Canning are used.

The processing times in this book ensure destruction of the largest expected number of heat-resistant microorganisms in home-canned foods. Properly sanitized canned food will be free of spoilage if lids seal and jars are stored below 95°F. Storing jars at 50° to 70°F enhances retention of quality.

Food acidity and processing methods

Whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity of the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods are not acidic enough to prevent the growth of these bacteria. Acid foods contain enough acid to block their growth, or destroy them more rapidly when heated. The term "pH" is a measure of acidity; the lower its value, the more acid the food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.

Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6 unless their recipes include enough lemon juice, citric acid, or vinegar to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.

Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water canner.

Temperatures for Food Preservation

Botulinum spores are very hard to destroy at boiling-water temperatures; the higher the canner temperature, the more easily they are destroyed. Therefore, all low-acid foods should be sanitized at temperatures of 240° to 250°F, attainable with pressure canners operated at 10 to 15 PSIG. PSIG means pounds per square inch of pressure as measured by gauge. The more familiar "PSI" designation is used hereafter in this publication (the Complete Guide to Home Canning). At temperatures of 240° to 250°F, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes.

The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling-water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes. Keep in mind that each recipe is tested individually, so always follow the times and methods in the recipe.



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