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Knowing the acidity level of the food you’re processing is important because the pH, the measure of acidity, determines which canning method you use: water-bath or pressure canning.

For canning purposes, food is divided into two categories based on the amount of acid the food registers:

 High-acid foods include fruits and pickled foods. (For detailed information on identifying and processing high-acid food, refer to Chapter 4.) Foods in this group have a pH of 4.6 or lower. Processing them in a water-bath canner destroys harmful microorganisms.Tomatoes are considered a low-high acid food (or in the low range of high-acid foods). With all of the new varieties of tomatoes, it is now recommended that the home canner add an acid to the canning process, to ensure that the proper acidity is reached every time.

 Low-acid foods, primarily vegetables, meat, poultry, and fish, contain little natural acid. Their pH level is higher than 4.6. (Check out Chapter 9 for detailed information on identifying and processing low-acid food.) Process these foods in a pressure canner, which superheats your food and destroys the more heat-resistant bacteria, like botulism.

If you want to feel like you’re back in science class all over again, you can buy litmus paper at teacher- or scientific-supply stores, or online, and test the acidity level of your food yourself. Also referred to as pH paper, litmus paper is an acid-sensitive paper that measures the acid in food. When you insert a strip of pH paper into your prepared food, the paper changes color. You then compare the wet strip to the pH chart of colors that accompanies the litmus paper.

The pH, or potential of hydrogen, is the measure of acidity or alkalinity in food. The values range from 1 to 14. Neutral is 7. Lower values are more acidic, while higher values are more alkaline. The lower the pH value in your food, the more acidic it is.