Brown sugar, really, is just sugar that hasn’t been refined as much. Basically, sugar cane (a lot of sugar — and even a little bit of “molasses”– is produced from sugar beets, but we won’t complicate things here) is mashed or pulverized to release “cane juice” which is then boiled and processed repeatedly to force the sugar to crystalize. The viscous liquid stuff left behind is molasses. So, basically, brown sugar is just sugar from which the molasses has not been removed.
Except it’s actually not, oddly enough. The brown sugar you buy in the grocery is often created by adding molasses back in with refined sugar crystals (somehow this presents LESS manufacturing cost — I don’t get it either), so that there’s more control and consistency (light brown sugar has about 3.5% molasses by volume, and dark brown sugar about 6.5%.)
“Raw sugar” is just sugar from the first crystalization of the sugar cane mash, and its characteristic light brown color indicates the presence of some molasses. There are actually three ‘boilings’ of the mash recognized during the refining process, resulting in “first molasses,” “second molasses” and “blackstrap molasses” (that last actually marketed as a nutritional supplement because it has such significant amounts of various minerals) with a decreasing sugar content, and thus a decreasing “sweetness.”
Brown sugar, as you might imagine, has slightly less caloric value than refined white sugar (which is white not only because the molasses has been removed, but also because it has been BLEACHED– as unnecessary with sugar, in my opinion, as with refined flour). because of its slightly higher moisture content. (as would also stand to reason, brown sugar is not quite as “sweet” as refined white sugar).
If you’re preparing a recipe that calls for brown sugar (like a lot of oatmeal cookie recipes, for example) and you don’t have any on hand, you can easily make your own by adding about 1 Tbsp. molasses (1:16 by volume) to a cup of sugar. Molasses has a relatively high specific gravitiy, while sugar’s specific gravity is only about 0.85– 849 kg/m^3 (if the air between the crystals werre all trapped when it was immersed in water, it’d float!) You can do this math, or just take my word for it … the result is 6.25% molasses by volume (or almost twice that: over 11% by weight, which is interesting, I suppose, but not important), so a relatively “dark” brown sugar. The more molasses, the darker brown the sugar and the more distinctive the taste: molasses is often added to baked goods to impart a “caramel-like” flavor.
Really, how much molasses you add to the sugar is pretty much a matter of personal preference, although the moisture content in theory could be a factor in baking, although not practically, because it’s so very little. For cooking, you usually needn’t blend the molasses with the granulated sugar– you can really just add them to the recipe separately; for serving at table (like with oatmeal, perhaps?) you can mix the molasses thoroughly in with the sugar in a blender first.
Back in the late 19th century, when “refined sugar” was the “newest, best thing,” the refined sugar folks launched a smear campaign against the “natural” sugar folks, displaying in their advertisements images of harmless, yet fairly hideous-looking, microbes commonly found in brown sugar. They claimed that brown sugar was “infested” with “tiny insects.” Of course, we now know that those microbes are completely harmless (and probably killed by the sulfuring of a lot of sugar — sulfur is used as a preservative, and is required to make sugar out of young, not-yet-matured cane), but the refined sugar folks were successful in creating a perception of brown sugar’s inferiority. The racist implications of bleached, “white” sugar being better, of course, we’ll leave alone, although I’m sure there’s something poetic to be said about the exploited (and in some cases, even enslaved) labor that worked all those sugar plantations.
To summarize, molasses has increasingly higher mineral content as more and more sugar is removed from it, and slightly (about 8%) fewer calories. Quite the opposite of the claims of the “food manufacturing” industry 100 years ago, brown sugar is actually better for you than white sugar. “Raw” sugar is truly less refined than white sugar (“brown sugar” is often “remanufactured” by adding the molasses back into already-refined sugar, which may be made from cane or, increasingly commonly, from beets) and it’s important to make sure that you’re getting “unsulfured” molasses, prepared from cane that has been allowed to ripen– not that ingesting a small amount of sulfur is really a big deal for most folks.