Growing up in Kentucky, I always took it for granted that everyone had a few jars of local molasses on the shelf. My dad taught me to drizzle a big spoonful over a pat of butter to mash up and slather on cornbread, and my mom taught me how to grease a measuring cup so I could get the sticky stuff into cookie dough without any fuss.
It wasn't until I moved to New York that I ever encountered blackstrap molasses at the supermarket, which was something of a shock. In my neck of the woods, blackstrap was strictly reserved for doctoring fertilizer or livestock feed—not something you'd want anywhere near a batch of gingerbread. Blackstrap has become an increasingly common sight recently, as its ultra-high mineral content makes it alluring to health food junkies.
For the uninitiated, molasses is produced from crushed sugarcane. Similar to olive oil, the first extraction has the lightest flavor and color, while each subsequent batch is darker and more robust. The process is pretty complex and can be approached in a few different ways, both traditional and modern, but broadly speaking, here's what's going on.
After the sugarcane is crushed, the juice is gently boiled to drive off some of the water, creating something thick, wonderfully sweet, and not bitter at all: cane syrup. Sugarcane juice can also be boiled quite vigorously, driving off enough water to significantly concentrate its flavor, mineral content, and acidity, until there's so little water that the sucrose in the syrup is forced to crystallize. These crystals are extracted as "raw" sugar, leaving an acidic, bittersweet, and mineral-rich syrup called molasses. (Its dark color and flavor come in part from the caramelization of fructose and glucose during the cooking process.) Molasses can be light or dark, depending on the specific degree to which it's been concentrated, but it's still molasses, with a sugar content of around 70%.
With further boiling, molasses can be concentrated even more, allowing thermal decomposition to carbonize its remaining sugars into a black, bitter, and downright salty sludge. This, my friends, is blackstrap. It has a sugar content of only 45%, and a whopping 1% of your daily sodium content in every spoonful. For those reasons, it should absolutely never be used in place of molasses unless a recipe specifically calls for blackstrap by name.
On the left, we have a batch of gingersnap dough made with true molasses. It's light in both color and texture, and moist to the touch; if you're the type who's inclined to eat raw cookie dough, you'll find it spicy but sweet. On the right is the same dough made with blackstrap, which is actually thick enough to prevent the butter and sugar from creaming up as light as they should (more on the importance of creaming here). It's dark, dense, and comparatively dry, thanks to the lower moisture content of blackstrap. It's also bitter and salty, so there's nothing to temper the heat of the ginger, making it all but inedible.
After baking, the gingersnaps made with true molasses (top left) spread nicely and developed big cracks on the surface, while their sugar coating dissolved enough to create a more delicate crust, with some reasonable browning along the bottom (bottom left). The gingersnaps made with blackstrap (top right) didn't spread much at all; rather, they puffed straight up, making the cookies cakey inside. The lower moisture content of the dough prevented the sugar coating from dissolving, giving the gingersnaps a heavy crust. The blackstrap version also browned more along the bottom (bottom right), so the cookies developed even more bitterness in the oven.
Compared to true molasses (left), blackstrap is also denser. In the photo above, each ramekin contains three ounces (85 grams); the molasses occupies a volume of about half a cup, while the blackstrap falls a few tablespoons shy. If you're baking by volume, that discrepancy will compound the disaster of using blackstrap instead of true molasses, adding insult to injury.
When shopping for molasses at the supermarket, always check the ingredient list to make sure what you buy contains "molasses" and nothing more—a few shady brands try to pawn off blackstrap as true molasses, but you'll discover the difference in the fine print. If you've got a jar of local molasses that's not clearly labeled, chances are it's the real deal, but do give it a taste to be sure. Molasses should always be pleasantly bittersweet, with a bright note of acidity. (Whether it's cane molasses or sorghum molasses is another question, but fortunately those styles can be used interchangeably, so this isn't a make-or-break detail.)
In closing, because it can't be said too many times: Blackstrap is never an acceptable substitute for true molasses, even in a pinch. Whether you're making Kenji's pulled pork or Daniel's Boston baked beans, or whipping up something sweet, when recipes call for "molasses," they never mean blackstrap. If blackstrap is to be used, the recipe will spell it out by name.