What's the difference between light and dark brown sugar? If a recipe calls for one, can I substitute with the other?
To understand what brown sugar is, you have to look at how sugar—like the granulated white table sugar your grandmother stole packets of from the diner—is made.
The short version: Sugar is just sucrose (C12H22O11), a molecule which occurs naturally in a variety of plants, though most of the sugar we eat in this country comes from the stalks of sugarcane.
To turn sugarcane into refined table sugar, the stalks are cut then crushed with rollers until juice is extracted. Milk of lime and carbon dioxide are added to help clarify the juice, then it's sent through an evaporator to remove the water and concentrate it into a syrup. The sugar in the syrup is crystalized, and you end up with a big old vat of raw sugar crystals (which will be further refined into white sugar) covered in molasses. Yum.
While those molasses-coated raw sugar crystals are now technically brown sugar, most of what you buy in the store labeled "brown sugar" is sugar that has been refined to the white sugar stage, then re-mixed with molasses to produce a consistent shade and flavor.
So what's the difference between light and dark brown sugar?
The difference between light and dark brown sugar is simply the amount of molasses each contains. Light brown sugar has less molasses per total volume of sugar (about 3.5% according to Rose Levy Beranbaum) while dark brown sugar has more (6.5%). You can easily see the difference in their makeup using just your eyeballs: dark brown sugar is darker in color and looks more like molasses syrup. You can taste it too: dark brown sugar has a slightly more complex flavor, one which people often characterize as similar to caramel or toffee.
Can I use dark and light brown sugar interchangeably in cooking?
Given that you've probably Googled yourself into this article, panicking, hands full of flour and halfway through a recipe, the answer is: generally speaking, yes they are interchangeable. (I want to get that out there because no matter how many years go by, no matter how many cakes, cookies, and breads I make, I still manage to have the "oh s%&$" moment because I started a recipe without all the ingredients in the house.) In fact, many recipes don't even specify what shade of brown sugar to use, though it's best to assume that by "brown sugar" they mean light.
That doesn't mean they're identical. Because dark brown sugar contains more molasses, it weighs more, contains more moisture, and is more acidic.
Wait, that doesn't change things? Yes, but not really. The amount of moisture is so negligible that you'd never have to compensate by adding extra dry ingredients to your recipe. There are two ways in which you'll see a small difference: taste and texture. Taste is obvious: sweets made with dark brown sugar will have a slightly deeper flavor with those notes of caramel and toffee I mentioned. That's why I only use dark brown sugar when making gingerbread; but depending on the recipe, you may not even notice a difference. Regarding the extra acidity of dark brown sugar, acid activates baking soda, so if you use dark brown sugar to make, say, cookies, your cookies will rise higher, but only slightly.
To test this, I made two batches of our Chewy Brown Sugar Cookies, using the same recipe but changing the type of brown sugar used.
The most obvious difference is the color of the finished cookies: there's no way you'd miss which ones were made with dark brown sugar. The dark brown sugar cookies also tasted more of molasses, but that's because brown sugar is the main flavoring agent and it's not competing with other ingredients. Other than that, the cookies made with light brown sugar spread a bit more, and the ones made with dark brown sugar rose just every so slightly higher. But we're talking about the kind of changes you'd need a magnifying glass (or an obsessive food personality) to see.
What about white sugar? Can I replace it for brown?
Using white sugar when a recipe calls for brown (or vice versa) is much more problematic. Molasses is hygroscopic, meaning that it likes to hang on to moisture. As a result, baked goods made with brown sugar end up moister, heavier, and chewier, while those made with white sugar end up lighter and crisper (read up a bit more on the science in this article about the best chocolate chip cookies).
Cookies made with white sugar will also tend to spread more without the acidic boost of brown sugar to help them rise, like in these chocolate chip cookies:
If you've got a recipe that calls for brown sugar and all you've got is white sugar and molasses on hand, you can approximate light brown sugar by combining a cup of sugar and a tablespoon of molasses in the food processor and running it until it reaches a uniform color.
The difference between light and dark brown sugar mostly comes down to taste. While there will also be subtle differences in texture, feel free to swap them out without worry.