In barbecuing, a rub is a mix of seasoning and flavoring ingredients that are combined and applied to the outside of meat or poultry before cooking.
Rubs Are for When You Barbecue, Not Grill
Any discussion of rubs ought, to begin with clarifying the difference between grilling and barbecuing.
Grilling is a fast, high-temperature method suitable for cooking, for example, burgers or steaks.
Barbecuing is a slow, low-temperature (typically 225 F) method you'd use to cook, for example, a whole pork shoulder or beef brisket.
This is a huge difference, and its misunderstanding leads to the misconception that rubs should be used for grilling. No! Rubs, either wet or dry, are for when you barbecue, not grill.
This is mainly because rubs will burn in the scorching heat of a grill, leaving you with a blackened, smoky mess. Sugar is one of the primary components of a rub and starts to burn at 265 F. Consider that steaks are grilled at 450 to 550 F, and even chicken is grilled at 350 to 450 F, and you'll see why rubs and grilling don't mix.
So, use rubs for low-heat barbecuing and smoking, not grilling. For high-heat grilling, stick to a simple seasoning of Kosher salt and freshly ground black pepper.
Flavoring Ingredients in Rubs
Beyond salt and sugar, other rub ingredients typically include garlic and onion powders, cumin, oregano, paprika, and chili powder. These last two contribute color as well as flavor. Color is important because, at 225 F, meat is not going to turn brown via the Maillard reaction, which happens at temperatures of 310 F or higher.
Since there is no set formula for the relationship between a piece of meat's weight to its surface area, there is no formula for how much rub you'll need per pound. You simply want enough to cover the entire surface. Any excess simply won't stick and will fall off. Fortunately, dry rub keeps for a few months in a cool, dry place, so make extra.
We can, however, talk in terms of ratios. In general, a good rub recipe will combine equal parts (by weight) of salt, black pepper, sugar, chili powder (including paprika and chipotle powder) and aromatics (such as garlic powder, onion powder, cumin, oregano, mustard powder, and so on).
Wet vs. Dry Rubs
In a sense, the choice of wet vs dry is mainly a choice relating to the flavor. Which is to say, there is no way to add the flavor of Worcestershire sauce without using Worcestershire sauce. And since Worcestershire sauce is wet, you're using a wet rub. The same goes for citrus juice or vinegar.
Beyond that, liquid applied to the surface of a piece of meat is going to evaporate very quickly when exposed to heat. But though the liquid may evaporate, the flavor compounds it contained still remain. Thus the liquid is merely the medium for applying the flavor.
Oil (another liquid) doesn't evaporate—but other ingredients don't dissolve in it, either. Therefore, an oil-based rub (dry ingredients moistened with oil and formed into a paste) is using oil as a glue to adhere the dry ingredients to the surface of the meat.
And remember, the flavors of the rub aren't going any deeper than the outer millimeter or two of meat. That's why spice rubs need to be bold. You're trying to apply enough flavor to the surface of the meat to season the entire meat.
Note too that when discussing the difference between wet and dry rubs, that is not the same as the difference between wet and dry barbecue. The former relates to the form of rub that is used, while the latter has to do with the use of sauce—either during the cooking, at the table, or both.
Sweetness Is the Key to Barbecue Rubs
Speaking of wet vs dry, molasses is a great ingredient for making wet rubs. As the byproduct of refining raw sugar into granulated white sugar, molasses functions both as a glue and as a medium for sweetness. And remember, because barbecue is a slow, low-temperature affair, you don't have to worry about the sugar burning.
Brown sugar (which is what you get if you mix white sugar with molasses) is a standard foundation for dry rubs. Because it is slightly moist, it forms a good glue between the meat and other ingredients in the rubs. Maple sugar and turbinado sugar are also good choices.