Sorting out our standard strains of sucrose
If you were looking to sweeten your coffee, would you consider adding brown sugar to it? Would you only use white sugar in a pumpkin pie? At their core, they’re the same sugar crystals, made from carbon, hydrogen and oxygen, and yet there are some important differences. The trick is that it’s not the sugar that’s different between these two baking ingredients, but what’s coming along with the sugar.
Brown sugar is basically sugar with some extra molasses left on the crystals. This syrupy coating carries some mineral content that can (slightly) add to brown sugar’s nutritional profile thanks to the extra calcium, iron or magnesium. Thanks to these extra ingredients, there’s less room in a single cup of brown sugar for the actual sucrose crystals, meaning it’s a little less sweet and calorically intense than white sugar as well. This molasses coating can be removed, of course, with an intense regimen of boiling, rising and if possible, centrifuging.
Why does my brown sugar become a brick?
You probably wouldn’t worry about converting brown into white sugar for taste or nutritional reasons. That extra spec of magnesium isn’t going to be a big deal in your banana bread. The more practical differences are tied more to the sugar before you eat it, since that brown molasses also carries and absorbs water, changing the properties of brown sugar when it’s cooked or left in the cabinet for a few weeks. In the latter scenario, brown sugar can turn into an annoying, impossible-to-measure-out block, and this is thanks to the water evaporating out of the syrup leaving you with a brick of minerals and sucrose.
Hydration is an issue when baking with brown sugar as well. Baking a cookie with only white sugar should give you thinner, dryer and more dispersed cookies. Brown sugar will have some of the water cooked out, and just as in the back of your cupboard, clump up into thicker cookies. The brown sugar will even absorb liquid out of the surrounding dough, further preventing its dispersal on the cookie sheet.
My first-grader asked: What happens when you mix the sugars when you cook? Well, you end up somewhere in the middle. When baking, you’ll have a mixture of the two ingredients, not a new compound. The two sugars will mingle but not react to each other. No new “tan” sugar hybrids, at least on a chemical level.
Source: Kitchen Science: What's the Difference Between Brown and White Sugar? by Esther Inglis-Arkell, io9