Vitamin C (chemically C6H8O6) can be derived from several sources. It can be obtained directly from foods that contain naturally-high amounts of ascorbic acid. Although citrus fruits contain large amounts, certain other plants contain much higher levels (e.g., rosehips). One of the first large consumers of ascorbic acid was the Royal Navy, which used lime juice to ward off scurvy among the sailors during prolonged periods at sea. Lime juice was selected because it kept well using the storage methods available at the time on the ships of the day. Although orange and lemon juice contain more ascorbic acid than does lime, it was an effective treatement. This is also the source of the nickname "Limey" for the English. The difficulty with plant-based sources of vitamin C is that the amount of the vitamin declines with time. Most storage, cooking, and processing methods deplete vitamin C levels in plant products. It is best obtained by consuming produce when it is fresh. Although humans are unable to synthesize vitamin C, many animals can. Beef liver is as good a source of the vitamin as citrus juices. The two industrial methods used to produce vitamin C synthetically employ the sugar glucose. The Reichstein process was developed in tthe early 1930s and uses a short fermentation process, followed by chemical processing: Glucose > Sorbitol + fermentation > Sorbose > Diacetone-Sorbose > Keto-Gulonic acid > Keto-Gulonic acid methylester > Ascorbic acid. An improved method using a two-step fermentation process was developed in China in the 1960s: Glucose > Sorbitol + fermentation > Sorbose + fermentation > Keto-Gluconic acid > ascorbic acid. Most vitamin C is currently produced in China using this method; there are only a couple of manufacturers outside of China producing vitamin C. A new one-step fermentation method is being developed to compete against the 2-step process. [With information from the Nutrition Almanac, BASF, and Wikipedia.] Endnotes: I have observed that some people who answers questions on industrial production methods of foodstuffs frequently attempt to create artificial links between some of the chemicals used in manufacturing and the end product. For example, chemicals such as H2SO4 (sulfuric acid) and HCl (hydrochloric acid) are used in different steps in the Reichstein process. Some people state that since dangerous chemicals are used in the manufacturing process, the final product is therefore dangerous. This indicates a general lack of knowledge of chemistry. Different substances are used to provide different components (e.g., atoms) of new compounds. From the reasoning of these folks, you would expect they must claim that salt is a deadly poison. Salt is made from sodium (Na) and chlorine (Cl). Chlorine is a corrosive poisonous gas and sodium is a violently reactive toxic metal. However, they seem to combine without any of these characteristics into salt. Ho-hum. Take these uninformed statements of production methods creating toxic products with a grain of salt (pun intended).