Corn syrup, especially in the form of high-fructose corn syrup, is one of the most common additives to commercial fruit and vegetable juices manufactured in the United States. It is usually labeled as high-fructose corn syrup, HFCS, or corn syrup solids. It is not used as a sweetener in Europe and does not have an E number.
Corn syrup is made from corn starch. The starch is first dissolved in a mixture of hydrochloric acid and water. Then it is treated with an enzyme called alpha-amylase, which is extracted from Aspergillus mold. This produces a stringy mat of complex carbohydrates. The mash is then treated with gamma-amylase to release glucose sugar, and the glucose sugar can be pumped through a tower with a third enzyme, D-xylose isomerase, to transform glucose into fructose.
Fructose is intensely sweet. It also absorbs moisture, keeping baked good from drying out and keeping juices and soft drinks from becoming syrupy. And in the United States, it's a lot cheaper than cane or beet sugar. The problem with high-fructose corn syrup is that it tastes so good that people naturally want to eat or drink too much of it.
And that's the bottom line with high-fructose corn syrup. It's sweet. It's cheap. But it's empty of any nutritional value other than the calories it provides. And if you tend to get too many calories in your diet, that's enough reason to leave it alone.