Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
Celeste asks via the feedback form on my web site:
I learned that sugar is considered a wet ingredient in baking chemistry. Attempts to know why have been unsuccessful.
Your difficulties are perfectly understandable. The problem comes because sugar is considered a wet ingredient in certain circumstances, but not uniformly across all of cooking. Indeed, depending on the way you look at the recipe, it's different at different parts of the recipe. So I'm happy to try to clear things up.
So, to start with: sugar is clearly a solid. It's not damp, it doesn't magically turn into a liquid form at baking temperatures or anything weird like that.
When you're looking at baker's formulas for things like cakes and muffins, liquid and sugar and fat are all separate, because they each act differently. In Shirley Corriher's Bakewise, she even lists how to determine if a cake recipe is going to succeed based on the proportion of liquid to sugar, among other things. So in many instances, sugar is its own thing and should not be considered a liquid.
Where they are considered the same is in a couple of baking methods, especially the Dissolved Sugar Method for cakes and the Muffin Method which, as I'm sure you'd determined on your own, is for muffins.
Unlike breads, cakes and muffins want to limit gluten production. We talked some about gluten in Which Flour Is Best for Pasta? so I won't go into great detail here, but when water mixes with flour it creates gluten. Gluten adds strength and body to baked goods, which is great when it's holding together a sandwich and terrible if it's something delicate and perhaps decorated with frosting.
Still, you need the liquid in the recipe in order to make it easy for ingredients to mix and activate properly. Also, because a lot of the liquid is going to disappear at baking temperature, you'll have a dry and nasty muffin if you don't put enough in. On the downside, the more liquid you have, the easier it will be for glutenin and gliadin proteins to mix with it and form gluten, which will make the cake tough.
If only we had some way of incorporating lots of liquid but keeping it unavailable for gluten production. And we do! Better yet, it leads to the point: adding sugar to the liquid will tie up the water in the liquid and make it unavailable for gluten production.
I go into a bit more detail on The Mystery of the Moister Cake, but the general idea is that Sugar and Water are Best Friends Forever, and they swear that nothing will ever separate them. Pinky swear. Seriously. So when Glutenin and Gliadin come sweeping in and promise a life of mystery and drama for water, but I'm afraid they don't have room for Sugar in the car, Water says, "No, thank you," and Glutenin and Gliadin just hang out with everyone else rather than forming some gluten.
So, from a method perspective, it is very useful to add sugar to water rather than with the dry ingredients. It does make sense from a chemical perspective, but sugar isn't really a liquid from most perspectives. I hope this helps not only clear up the confusion, but lead to tastier and more tender muffins and cakes.