Here's a little primer on how corn syrup gets made.
First, corn cobs are squeezed under extreme mechanical pressure to extract all their starchy, sugary juices. The juice is cooked with calcium hydroxide (lime) to kill any enzymes, after which it's cooked further in big pots until most of its water evaporates and it concentrates down to a thick syrup. The syrup gets so saturated that sugar crystals fall out of the solution. The crystals are then drained, centrifuged, dried, and bleached of impurities with sulphur or carbon dioxide.
Oh, wait, I got my notes mixed up. That's the process for refining sugar from sugar cane.
After years of making ice cream and sorbet, I have no bigger gripe than hearing complaints about using corn syrup in recipes. "I don't like using unnatural or processed foods," people tell me. "I just don't think corn syrup is good for me."
You're right. Corn syrup isn't good for you. But neither is refined sugar, or ice cream for that matter. If using highly processed ingredients is a problem for you, you may want to rethink making desserts from scratch. And to be clear, we're not talking about high fructose corn syrup. The Karo syrup you buy at the grocery store is an entirely different product.
When recipe writers call for corn syrup, they aren't doing it for kicks. So why do I use it in some of my sorbets and sherbets? Because corn syrup behaves similar to invert sugar, and invert sugar makes sorbets smoother and less icy. The pros use invert sugar in some of their recipes, and you should too.
The Scoop on Invert Sugars
Invert sugar specifically refers to a syrup of table sugar (sucrose) broken down into its component molecules, glucose and fructose. The glossy syrup, also called Trimoline, is sweeter than plain sugar and has greater body than simple syrup.
Invert sugar isn't the only alternative sweetener on the market. Other sugars, like corn syrup and glucose, behave similarly in ice cream recipes. They have two main advantages:
They're more viscous than sugar syrup. Many sorbet recipes call for making a syrup of sugar and water, then adding that syrup to a fruit purée. If you've done this, you've probably noticed that the sugar syrup you make isn't very thick. Corn syrup, on the other hand, is quite viscous, and it adds a rich, full-bodied texture to a sorbet base. The thicker a sorbet base, the creamier it'll be.
Many alternate sugars are less sweet than table sugar. Nothing impacts the texture of a sorbet more than how much sugar is in it. Sorbet needs plenty of sugar to stay soft and scoopable, and sometimes the amount of sugar you need for a smooth texture makes a sorbet that tastes candy sweet. But if an alternative sugar is less sweet than table sugar, you can use more of it without killing the sorbet with sweetness.
Invert sugar also resists crystallization, which isn't too vital for ice cream but is a big help for candy makers.
The Pros of Using Corn Syrup
Now let's look at corn syrup, which is very thick and gooey, impervious to crystallization, and only 33% as sweet as table sugar (by weight).* It behaves a lot like invert sugar, adding smoothness and creaminess to ice cream and sorbet, but is far less sweet, and it's the only neutral-flavored liquid sugar you'll find in every supermarket. Honey, agave nectar, maple syrup, and molasses also have properties to invert sugar, but their strong flavors limit their flexibility.
You can see a whole table of the relative sweetness of different sugars here. Karo corn syrup doesn't list its exact chemical composition, but we conducted some tests to pinpoint its sweetness. If you want to try this at home, mix 10 grams of sugar into 50 grams of water. Then mix 30 grams of corn syrup into 30 grams of water and taste them blind. The syrups won't taste exactly the same, but they'll be similarly sweet. On Twitter, Catherine Oddenino shared some independent lab results suggesting that Karo is a 47% dextrose equivalent syrup, which puts it right on target for a 33% relative sweetness to sugar with our own taste test.
To see how corn syrup affected a sorbet's texture, I made four batches of lemon sorbet. One was my standard recipe, made with all corn syrup, which I like for its balance of sweet and tart flavors as well as its lush, ice cream-like texture. I also made three other sorbets, equal in volume and sweetness, with different amounts of sugar: one that derived two thirds of its sweetness from corn syrup and one third from sugar, one that had only one third of its sweetness from corn syrup and two thirds from sugar, and a third made with plain sugar, which had only one third of the total sugar of my corn syrup base. The Serious Eats team tasted all four blind and shared their comments.
The results were clear: even a small amount of corn syrup drastically improved a sorbet's texture. While the sugar-based sorbets won out on flavor, their texture suffered. The all-sugar sorbet sucked eggs: dry, icy, clumpy, impossible to scoop. A small amount of corn syrup added substantial creaminess, and greater amounts improved texture even more.
How much corn syrup to add is a question of personal taste. I like the super-dense, super-smooth, ice cream-like texture of my all corn syrup sorbet, though it takes a good 12 hours in the freezer to harden and it melts quickly. Others preferred the mostly corn syrup version, which wasn't quite as smooth, but close. There's no right or wrong answer here, except to dismiss corn syrup out of hand.
The Cons of Using Corn Syrup
Ice cream is a balancing act, and each ingredient has its cost. Corn syrup is a great texture enhancer, but it's not without its flaws. To wit:
Table sugar tastes better. Karo corn syrup doesn't taste bad, but it has a slight metallic flavor compared to cleaner-tasting table sugar. Our taste test bore this difference out, and while I think a small difference in flavor is worth the immense gains in texture, a discerning palate can spot the difference side by side. If you can find them, alternative sugars like glucose (which also comes powdered) and of course Trimoline (inverted cane sugar syrup) taste better, though both are sweeter than corn syrup.
It can dilute flavors. Since corn syrup is a liquid, it adds volume to a sorbet while diluting its flavor. With strong citrus sorbets like lemon and orange, this isn't a problem since you're diluting the juice with water anyway, and some of that water can be substituted out for the water in corn syrup. But in my root beer sherbet for example, it's important to use a boldly flavored root beer so its flavor stands up to the blandness of the corn syrup. The more subtle your sorbet's flavor is, the higher a ratio of sugar to corn syrup you might want to use, as that'll dilute your other ingredients less.
Not all sorbets need corn syrup. Thick ones like strawberry, cherry, or peach, for instance, are viscous enough that table sugar works just fine. (My general rule is that if you can make jam out of the fruit, it doesn't need corn syrup to improve its texture.)
Trust Your Own Taste, Not a Label
When it comes to dessert, ice cream is forgiving stuff. It's easy to modify to your tastes, and if you don't like the end result, you can always melt it down, add some ingredients, and churn it again. I'm not saying you have to use a pint of corn syrup to get good sorbet. But it's an ingredient worth exploring for its versatility and handy chemical properties. Just remember that there's more than one kind of sugar out there, and you don't reach next-level ice cream Jedi status until you've tried them all.