What Different Acids Are in a Tomato?

Tomatoes are actually citrus fruits, although they are cultivated and typically eaten as vegetables. As such, tomatoes contain relatively high levels of organic acids, such as citric acid, which strongly influence their flavors. The acid contents of tomatoes vary widely and depend on many factors, although certain acids are always more predominate than others.

Organic acids are natural acids that contribute to the sourness of fruit, vegetables and other foods. As a result, the more organic acids a food contains the lower its pH value, which is a measure of acidity. Foods with a pH value less than 7 are acidic, and tomatoes have a pH range of between 4.0 and 4.6. Many other nutrients in food that are called acids, such as amino acids and fatty acids, are not considered organic acids because they don’t contribute sourness and they don’t impact pH levels to any great extent. Overall, organic acids comprise about 15 percent of the dry content of fresh tomatoes, although their specific ratios vary according to species, ripeness, growing conditions, handling and storage.

Citric acid is the most abundant organic acid in fresh tomatoes and it comprises about 9 percent of their dry weight. Citric acid is a natural antimicrobial and preservative because it deters the growth of bacteria, fungi and other microbes. The longer a tomato ripens on the vine, the less acidic it is because citric acid slowly degrades with time. Interestingly, varieties of tomatoes with naturally higher citric acid levels do not usually taste more sour because they have more glucose sugar to balance the tangy flavor.

The next most prevalent organic acid in fresh tomatoes is malic acid and it comprises about 4 percent of their dry weight. Malic acid contributes tartness to tomatoes; it’s actually about 14 percent more sour than citric acid, but it has less influence on taste because of its much lower concentration.

Ascorbic acid, also called vitamin C, is the next most prevalent organic acid in fresh tomatoes, comprising about 0.5 percent of their dry weight. In more practical terms, a medium-size fresh tomato contains about 10 milligrams of ascorbic acid, according to the U.S. Department of Agriculture. The recommended daily requirement of ascorbic acid for adults ranges between 75 and 120 milligrams depending on gender and pregnancy, so tomatoes are a relatively good source, but not as rich as other citrus fruit such as lemons, grapefruits or oranges. Ascorbic acid boosts immune system function and is needed to make collagen -- the elastic-like fibers in skin and other connective tissues. Like most other organic acids, ascorbic acid also displays antimicrobial properties.