Since 1811, the year Gottlieb Kirchhoff combined sulfuric acid with potato starch and created syrup, scientists have been aware that starches are rich sources of sugar. The problem for the next hundred years or so was how to produce it on a mass scale. In time it was discovered that plant enzymes (non-living organic molecules that perform specific tasks for living organisms) could do the job just as effectively, breaking down (hydrolyzing) long-chain starches into their sweet-tasting pieces.
Corn syrup is made by steeping corn kernels in water to loosen their husks and hydrate their starch. Once the kernels are soft the water is drained off and they’re ground into a wet paste. The whole soup is then passed through a series of filters and centrifuges that separate out the husks and germ to yield the pure starch. Lastly, enzymes are let loose in the slurry to start breaking the big corn starch molecules down.
Today corn syrup is made via a more sophisticated process than it was in the mid-20th century, but it tastes and performs about the same. It’s made of something like 15% glucose, 10% maltose (two glucose molecules bonded together) and 55% longer sugars which both blunt its sweetness and give it thickness by reducing flow (the remaining 20% of the volume is water). Overall corn syrup has a sweetness level on par with molasses which is was created to replace. Which is to say it’s about 85% as sweet as table sugar.
You might be surprise to learn, given all the controversy surrounding corn syrup, that it meets the criteria of a “natural” product and is sold in many health food stores.