Corn syrup is a surprisingly useful and versatile sweetener. It helps candy makers turn out soft caramels and marshmallows, and is included in most pecan pie recipes; it even makes frozen desserts smoother and creamier. But that means it's all the more difficult to replace if you have a corn allergy or object to the use of genetically-modified corn. Other liquid sweeteners are sometimes acceptable substitutes, but the simplest option is often a sugar syrup.
Real corn syrup is made by breaking down cornstarch with enzymes -- a bit like the natural process that occurs when corn sprouts -- and then refining it into a liquid sweetener. That's beyond the scope of a home kitchen, but anyone can make the sugar-based alternative.
To do this properly, you'll need a heavy-bottomed saucepan and a reliable candy or deep-fry thermometer. The proportion of ingredients is flexible because it's the thermometer that tells you when your syrup is done. Some recipes measure the sugar and acid by volume and others measure by weight, but the process is the same in either case.
Measure your sugar into a heavy-bottomed saucepan. For every pound of sugar -- approximately 2 cups -- add 1/2 teaspoon cream of tartar, a dry acid found in the baking section of any supermarket.
Add enough water to dissolve the sugar, and clip your candy thermometer to the side of the pot.
Place the saucepan over a medium-high burner. Bring the sugar mixture to a boil, and then dip a pastry brush in water and brush down any crystals left clinging to the sides of the saucepan.
Boil the sugar, without stirring it or swirling the pot, until the mixture reaches a temperature of 236 to 240 degrees Fahrenheit.
Remove the saucepan immediately to a cooling rack or unheated burner, and let it cool to room temperature. Pour the cooled syrup into sterile jars, and store it in the refrigerator until needed.
You can use the same quantity of powdered citric acid, sold at many pharmacies and bulk food stores, in place of the cream of tartar.
Corn syrup is what's called an "invert" sugar, meaning it remains liquid rather than forming crystals. If it's mixed with other sugars, it limits their ability to crystallize as well. That's why corn syrup is used so widely by home candy makers to control the crystallization of their sweets. Ordinary sugars can be converted to invert sugars by treating them with heat and acidity, which is what you've done here with the cream of tartar.
The finished sugar syrup will be thicker than corn syrup and harder to pour. Heat it gently in a pan of hot water, or in your microwave, to soften it and make it more pourable. Over time this can cause the sugar to crystallize, so it's best to store the syrup in several small jars rather than one big one, and to only heat as much as you need.
One of corn syrup's virtues is that it's not as sweet as regular sugar. You can add enough to achieve the right texture, without your treats being overpoweringly sweet. When you substitute your homemade syrup for regular corn syrup, the end result will be sweeter than the original recipe. Sometimes you can compensate by reducing the recipe's other sweeteners or adding a tart flavoring, but not always.
Other liquid sweeteners can be used in its place, but they vary widely in sweetness and consistency and often have strong flavors of their own. Thick sugar syrup is neutral in both color and flavor, so it's often the best available substitute.