96 Principles of Food Science severe chemical burns. When examining bases that are safe to touch, you will find they have a slippery feel. As you may have guessed, soaps and cleansers are bases. Organic Dyes Historically, the second method of identifying acids and bases is with organic dyes. Organic dyes are naturally occurring color pigments that change color when exposed to acids or bases. The most widely used organic dye in science classes is litmus. Litmus is extracted from a plant and added to paper strips. These strips are convenient and economical to use in science laboratories. When litmus paper is dipped in acids, it will turn red. When litmus paper is dipped in bases, it will turn blue. Another common indicator used in science labs is pHydrion paper (pH paper). It turns shades of yellow to orange-red in acids and yellow-green to blue-green in bases. See 6-4. Acids and bases affect the colors of fruits and vegetables that contain organic dyes. These fruits and vegetables include plums, blueberries, cherries, red onions, and red cabbage. For instance, red cabbage retains its red color in cooking liquid that is acidic. If the cooking liquid is alkaline, however, the cabbage will turn bluish purple. Red and purple fruits and vegetables are not the only foods affected by acids and bases. Green vege- tables will be brighter green if you add baking soda while cooking. However, this is not a recommended practice. Remember that baking soda is a base. There- fore, you will neutralize the vitamin C (ascorbic acid) content of vegetables when you add baking soda. It is easier to observe the color changes of organic dyes if they are dissolved in water. You can simmer fruit or vegetable skins or pieces in distilled water. When the dye leaves the food and colors the water, you will have an organic dye solution. Measuring Acids and Bases Stomach acid is described as a “strong” acid and tea as a “weak” acid. For a scientist, such descriptions are not very accurate. One person’s definition of weak may be different from another’s. Scientists need a way to accurately define and measure the acidity of a substance. This can be critical in the food industry. Jodi Songer Riedel 6-4 This pH paper turned orange in an acidic solution of lemon juice and green in a basic solution of baking soda. dinazh/iStock/Getty Images Plus By Es75/Shutterstock.com 6-3 Citrus fruits and cranberry juice taste sour because they contain acids. Copyright Goodheart-Willcox Co., Inc.