High fructose corn syrup (HFCS) is, as the name implies, corn syrup whose glucose has been partially changed into a different sugar, fructose. To make HFCS, you start with corn, then mill it to produce starch -corn starch. Starch, the most important carbohydrate in the human diet, consists of long chains of glucose. To make corn syrup, you mix the corn starch with water and then add an enzyme, produced by a bacterium, that breaks the starch down into shorter chains of glucose. Then you add another enzyme, produced by a fungus, that breaks the short chains down into glucose molecules. At that point, you have regular corn syrup.
To make the corn syrup into high fructose corn syrup, you turn some of its glucose molecules into fructose molecules by exposing the syrup to yet another enzyme, again produced by bacteria. This enzyme converts the glucose to a mixture of about 42 percent fructose and 53 percent glucose, with some other sugars as well. This syrup, called HFCS 42, is about as sweet as natural sugar (sucrose) and is used in foods and bakery items. HFCS 55, which contains approximately 55 percent fructose and 42 percent glucose, is sweeter than sucrose and is used mostly in soft drinks