You may have heard that it’s important to add lemon juice, citric acid or another acid to tomatoes before canning, but maybe you are not sure why. It’s all about pH.

It is the measurement of the acidity or alkalinity of a solution. Lemon juice is acidic, which means it has a low pH. Soap is very alkaline, or basic, which means it has a high pH. The figure below illustrates pH.

Microorganisms like Clostridium botulinum, the type of bacteria that causes botulism, can survive or grow in some foods at certain pH levels. The Food and Drug Administration and U.S. Department of Agriculture recommend that naturally acidic foods (like fruit, jams/jellies) and acidified foods (like pickles and salsa) have safe pH levels before canning. A pH of 4.6 or lower is required for safe canning without the use of pressure processing.

Foods like pickles or salsa need to have an acid added if they are to reach a pH level of 4.6 or lower to prevent microorganism survival and/or growth.

For safety, "low acid" foods like vegetables, meat and many mixtures of foods require pressure canning using current time/pressure recommendations. Acidic foods like most fruits, jellies/jams and properly acidified foods like tomatoes and pickles can be processed in a water bath canner.

To ensure safe acidity in whole, crushed, or juiced tomatoes, add two tablespoons of bottled lemon juice or ½ teaspoon of citric acid per quart of tomatoes. For pints, use one tablespoon bottled lemon juice or ¼ teaspoon citric acid. Acid can be added directly to the jars before filling with product. Add sugar to offset acid taste, if desired. One to two teaspoons of sugar may be enough to enhance the flavor. For more Information see "Canning and Freezing Tomatoes" available on-line at: http://www.ext.nodak.edu/extpubs/yf/foods/he175w.htm#Canning

The salsa recipe used in the experiment and listed below appears in the NDSU Extension Service publication "From the Garden to the Table: Salsa!" available on-line at:

7 qt. paste tomatoes, peeled, cored and chopped
4 c. long green chili peppers, seeded and chopped (about 12 chilies)
5 c. onion, chopped (about 5 medium)
½ c. jalapeno peppers, seeded and finely chopped (about 2 peppers)
6 cloves garlic, minced
2 c. bottled lemon or lime juice
2 Tbsp. salt
½ Tbsp. red pepper
2 Tbsp. ground cumin 3 Tbsp. oregano leaves
2 Tbsp. fresh cilantro (optional)

Selecting Tomatoes: Select only disease-free, preferably vine ripened, firm fruit for canning. Caution: Do not can tomatoes from dead or frost-killed vines because the acidity of the fruit may be altered.

Procedure: Wash tomatoes. Dip in boiling water for 30 to 60 seconds or until skins split; then dip in cold water.

Slip off skins, remove cores and dice.

Wash and prepare peppers. Wear rubber gloves while handling chili peppers.

Combine all ingredients except cumin, oregano and cilantro in a large pot and bring to a boil, stirring frequently. Reduce heat and boil 10 minutes. Add herbs and spices and simmer for another 20 minutes, stirring occasionally.

Wash jars. Prepare lids according to manufacturer's instructions. Ladle hot into hot pint jars, leaving ½-inch headspace. Remove air bubbles by inserting a flat plastic spatula between the food and the jar. Wipe sealing edge of jars with a clean, damp paper towel. Cap with properly pretreated lids. Adjust lids and process in a boiling water canner for 20 minutes.

*This recipe is best suited for paste tomatoes. Slicing tomatoes require a much longer cooking time to achieve a desirable consistency. You may decrease the amounts of spices, but increasing the amount is not advised. You may make adjustments to suit your tastes after opening the jars.