Q: What is the substitution of vanilla extract for beans?
There isn't an easy answer to this question. According to Nielsen-Massey Vanillas, Inc., the substitution of whole vanilla beans for vanilla extract is one bean equals approximately one tablespoon of extract. However, The Vanilla.COMpany says ½ vanilla bean is the equivalent of 1 teaspoon of vanilla extract, so the "experts" differ by 50%.
I find that for custards and ice cream, one teaspoon of extract and about 2 ½ inches of vanilla bean give the same strength of flavor. That said, the bean has a rounder, more subtle flavor than extract, especially in dishes where it is the star player, like classic french vanilla ice cream.
To substitute extract for beans, you generally don't need to worry about the added volume of liquid, except perhaps in candy making. If you are substituting the other way, split the bean lengthwise, scrape the tiny seeds from inside the bean using the edge of a knife, and then steep the bean and seeds in the liquid being used in the recipe for about 10 minutes. Remove the bean before proceeding.