What can I use as a substitute for corn syrup in a recipe for boiled icing?
When heating sugar up in boiled icing or in making candy, the problem is sugar crystallization. This happens because the solution becomes supersaturated and any movement can cause it to shift back into a crystal state.
The corn syrup is there to prevent this from happening by providing glucose to 'get in the way'. You can get just 'glucose' at the drugstore to replace the corn syrup. Any other sugary thing that's high in fructose or glucose should work.
Another option is an acid. Some acid (like lemon juice) should make some of the sucrose break apart into simpler sugars and prevent crystallization.
Boiled icing is essentially an Italian meringue which I never use corn syrup in. But anyway, corn syrup is basically fructose syrup which is used in the recipe to preven the sugar crystallizing, there are other substances (all fairly similar in chemical structure) that can prevent this: glucose syrup, golden syrup if you live in the UK and even honey will do this although this will of course give you a honey flavored icing.
If all else fails and you can't find any of the ingredients listed above you could always substitute it for a simple syrup (use equal amounts of sugar and water in place of the amount of corn syrup) and just be very carefull not to let it crystalise.
You can stop this by making sure the sugar has dissolved before you boil it, brush away any sugar grains with a pastry brush before boiling and most importantly don't stir it, just give it a swirl or it will crystalise.
Hope this helps!
To improve on the suggestions for using simple syrup: you can make invert sugar syrup at home, the sugar composition of which should be pretty similar to corn syrup.
The recipe I found involves two parts sugar, one part water, 1/500 part citric acid or cream of tartar. Bring to a simmer, let cook for about 20 minutes, brushing the sides of the pot down with water to avoid caramelized deposits. It's done when the temperature is above 236F/114C. If you don't have a thermometer, for me that was the point where the syrup started having color, not much but distinctly so. I.e. you want to go a bit beyond the "first hint of colour" stage, but before amber. (Cooking it too much won't hurt anything, but might affect the final flavour.)
Because of the water driven off during the cooking, I can't tell you off the top of my head how to substitute this. If you have a scale, you can use that to dilute the final product to the concentration your corn syrup has. E.g. Karo Light is 30% carbohydrates by weight, so if you start with 500g of sugar, you weigh out the cooked invert syrup, then add water until the total weight is 500/30%=1667g, maybe briefly bring back to a simmer again to homogenize the mix, then cool down again.
If you are simply wanting to replace corn syrup, and sugar (white cane granulated) will do fine, you can make a "Simple Syrup" (1 cup sugar to 1 cup boiling water, stir till all is dissolved... pretty 'simple'...). You might want to experiment with the quantity but this will give you both the sweet and liquid elements of corn syrup.
If you are looking eschew 'sugars' altogether then you can substitute Splenda (or any competitive sucralose product) for the sugar and make a sugar free simple syrup.