When you drizzled a little lemon juice over some cut apples, you might have said to yourself, "thank heavens for citric acid and its wonderful ability to keep fruits and vegetables from turning brown. Vitamin C is a great thing!" Assuming your high-school chemistry student was raiding the refrigerator for the 11th time that day and that he was listening to you (a preposterous assumption), he would simply have rolled his eyes and come up with one more reason to mock you.
The fact is, there is citric acid and ascorbic acid in lemon juice, but they are not the same thing, and it is the ascorbic acid that keeps the fruits and vegetables from turning brown. Indeed, ascorbic acid is the more versatile and essential of the two acids.Ascorbic acid, or vitamin C, has a chemical make-up of C 6 H 8 O 6 . While it is found in citrus fruits, billygoat plums, rose hips, blackcurrants, guavas, kiwi fruits, broccoli, and Brussels sprouts are much better sources. Most animals can also produce it themselves, although most fish, some birds, all guinea pigs, and some primates - and man - cannot, and so must get it from other sources.
In cooking, besides keeping cut fruits and vegetables from turning brown, ascorbic acid promotes the growth of yeast, and so is also often added as an enhancer to bread dough. In commercial food processing it is used as an antioxidant preservative.
Citric acid has one more oxygen atom than ascorbic acid; its formula is C6H8O7. It occurs naturally in citrus fruits and some other fruits and vegetables. It can also be synthetically produced from sugar.
Citric acid is used commercially to enhance the tartness in fruit-flavored candy and in soft drinks. It is also added to some ice creams to keep fat globules from coagulating. Some bakers use it in sourdough bread to produce an especially assertive tang.