In general, when working with a recipe for 6 to 12 people, “I’d think you should be able to substitute [the sugars] directly,” says Pichet Ong, author of The Sweet Spot: Asian Inspired Desserts. “The most noticeable difference is that brown sugar makes [baked goods] more moist,” he says. This means you may have to adjust some of the other proportions in the recipe, like slightly decreasing the wet ingredients or upping the dry ones. Brown sugar will also add a hint of rich caramel flavor and affect the color.
Ong says he decides which to use based on what texture he wants. For something like a banana or zucchini bread, where he’s looking for a moist texture, brown sugar is good. But he likes layer cakes to be more dry and aerated, so he sticks to white. He says you can also use a combo like many cookie recipes do.
Ong estimates that brown sugar adds a net of about 1 percent more moisture overall, because most brown sugar in the United States is made by adding 5 percent molasses by volume to granulated white sugar. Substituting turbinado or Demerara (the “natural” brown sugars usually sold as “raw sugar”) doesn’t work so well, says Ong. They won’t melt down the way granulated white or conventional brown sugar would in a recipe because of the crystal size. He likes to take advantage of the texture of these bigger crystals by using them to crust cakes or cookies. Muscovado, another “natural” brown sugar, is wetter and stickier than conventional brown sugar, with a more intense molasses taste. It can be subbed for conventional brown sugar (again, you’d need to reduce the wet ingredients a bit), but it’s probably too strong in flavor to use in most recipes that call for white sugar.
In her book BakeWise, Shirley Corriher notes that white sugar used in high proportions “makes a very crisp cookie that stays crisp,” while brown sugar is more “hygroscopic,” meaning it draws in water more easily from the air. Therefore cookies made with brown sugar “will absorb moisture from the atmosphere and soften on standing.”