So ask readers Mari and Lisa. I live for this sort of stuff, so let’s get into it. First, let’s define a few terms. Corm syrup is something most of us are very familiar with. It’s a syrup made from corn endosperm (starch) that has been exposed to enzymes which break the long chain starch molecules down into short sugars that human tongues can taste. The result of all this molecule-breaking (hydrolysis) is a soup of roughly 15% glucose (one of the very simplest of sugars), 10% maltose (two glucose molecules bonded together) and 55% longer sugars. These longer sugars blunt the sweetness of corn syrup simply because we don’t have taste receptors for all of them. They also give it thickness. The last 20% is water.
Invert syrup is nothing more complicated than table sugar (sucrose) and water, heated with an acid added to it, typically cream of tartar, citric acid or even lemon juice. The addition of acid causes many of the sucrose molecules to split in two, into their component sugars: glucose and fructose. Invert syrups vary widely in how much water they contain. Some are as high as 50% water, but the most useful are the thicker ones which are boiled to the thread or soft ball stage, about 235-240 degrees Fahrenheit. At that point they’re between 15% and 20% water, about the same as corn syrup.
Can one be substituted for the other? The answer is yes, depending on the application. Corn syrup and thread-stage invert syrup have similar viscosities. They’re both smooth, so they’ll add sweetness to a recipe without the grit of crystal sugar. They’re also both good at inhibiting crystallization of sugar syrups, as their molecules — small or large — get between sucrose molecules, preventing them from stacking up on one another like LEGOs and forming crystals. Pastry chefs like this function a whole lot, which is why so many icing, candy, toffee and fudge recipes call for corn syrup. It helps keep these preparations from getting either grainy or so stiff that they shatter on a cake or in the mouth. A lot of ice cream makers also like a little syrup in the mix, since it performs the same function with ice crystals as it does with sucrose crystals, and thus keeps ice cream smooth.
Corn syrup and invert sugar syrup are roughly equivalent at crystal inhibition. Corn syrup is generally more effective at the job since there’s really nothing in it that will crystallize…the glucose, the maltose or the long chain sugars. Invert sugar isn’t as effective because even after the heat and acid treatment it’s still well over half sucrose. Still it’s close enough for jazz in a typical icing or batch of fudge.
The real problem with substituting invert sugar for corn syrup is the sweetness level. Corn syrup was originally marketed as a replacement for molasses, which is sweet, but not as sweet as a sucrose syrup. Interestingly, in the 60’s food scientists finally figured out how to create a corn syrup that was equivalent in sweetness to table sugar, which is now known as Fruit of Beelzebub or high-fructose corn syrup (we’ll say no more about that).
Invert syrup, being mostly sucrose, is every bit as sweet as table sugar. Which means that if you substitute it for corn syrup in any great quantity, you may end up creating a monster of sweetness.
Still, if you keep that in mind and compensate accordingly, you’ll find that you can indeed substitute invert sugar syrup for corn syrup in most pastry and candy applications.