With both ascorbic and citric acids on so many food labels, it isn't hard to confuse the two similar, yet disparate, chemicals. The fact that you can find both in high quantities in all citrus fruits can further compound the confusion. Many think of citrus fruits as a primary source of vitamin C, so it would seem natural to associate citric acid with the essential vitamin, but the opposite is actually true: vitamin C and ascorbic acid are chemically the same.
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Both ascorbic acid (vitamin C) and citric acid are weak, water-soluble crystalline acids.
The chemical compositions of both acids are extremely similar, differing in only a single oxygen atom (ascorbic acid contains six carbon atoms, eight hydrogen atoms and six oxygen atoms; citric acid contains six carbon atoms, eight hydrogen atoms and seven oxygen atoms). However, the molecular structures of these acids differ significantly. Additionally, citric acid is a crucial intermediate chemical in the Krebs cycle (the second stage in cellular metabolism that converts glucose into carbon dioxide in virtually every cell on Earth), making it essential to practically all living things.
While both are present in citrus fruits (pineapples contain citric acid as well), ascorbic acid exists naturally in tomatoes, potatoes and leafy green vegetables, such as spinach. The best sources of vitamin C include Brussels sprouts, bell peppers, kiwi fruit and broccoli (dark green vegetables are a particularly good source of ascorbic acid).
Due to ease and the low cost of manufacturing, most commercial food processors use ascorbic and citric acids as an additive and as antioxidant preservatives (they use citric acid to add a mildly sour taste to soft drinks). During food processing, most fruits and vegetables lose naturally occurring vitamin C, usually through cooking; therefore, the manufacturers add artificially produced ascorbic acid to replace what is lost. Citric acid is also a main component of many metal polishes.
In addition to its critical role in cellular metabolism, citric acid also aids in the body's absorption of certain minerals, such as calcium (this is why calcium citrate---a combination of both calcium and citric acid---is often found in orange juice). In the past, vitamin C played a primary role in preventing scurvy; now its principal uses are as a natural antioxidant and in the prevention of cancer.
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