When President Johnson threw a barbecue he called upon is favorite Pitmaster Walter Jetton to cook up a meal that often feed hundreds of people. This meal would be cooked on an open-air fire pit that measured about 40 square feet. Walter would cover every square inch of this surface in ribs, roasts, and meats of every variety. To keep the meat moist he mopped it, with a real mop. Hence the barbecue term, "mop".
Today you can buy a miniature tool that looks like a kitchen mop to mop your meat. The cotton fibers hold the thin mop sauce and make it easy to dash large amounts on at once. But a mop isn't just another kind of barbecue sauce. It is a thin, watery solution that drips over meat adding moisture to combat the drying of an open fire. Think of it this way; a sauce is applied with a brush, like a paint brush. A mop, sometimes called a sop, is applied with, well a mop. Sauces are thicker than mops. Mops should have a consistency close to water.
While traditional, low and slow, barbecue cooks will claim that a mop is purely a smoking sauce; the truth is that mops started out in grilling. Many cultures and traditions use mops and they are widely varied. Of course, the ingredients you use in your mops needs to reflect not just the meat but the cooking method. For instance, if you cooking temperature is above 265 degrees F/130 degrees C, the burning temperature of sugar, don';t use sugar.
Basically, if you are smoking at low temperatures sugar is okay but if you are grilling you should avoid it.
Mops can be based a number of primary ingredients. If you just want a thin, simple mop, base it with water. If you want to increase the tenderness of meat then you want to use vinegar. And always you want to enhance the flavor so you can start with liquids like, beer, wine, Worcestershire sauce, or fruit juices, or a combination of these.
From here you want to add seasonings to the mix. I always recommend that you stick with a single flavor strategy. If you are using a rub, marinade, or barbecue sauce you want to use the same flavorings from these in your mop. Remember that every flavor you add to something should work together.
Mops can be easily made from other things. Add a couple of tablespoons of your favorite rub to a cup of cider vinegar or beer. Boil the marinade you used already, add some water to keep it thin and use that. Add water, beer or vinegar to barbecue sauces and mop that on. Just remember what I said about the sugar. Now that you have your mop, apply it about three to four times during the cooking at regular intervals. So if you are grilling something for an hour, mop every 15 to 20 minutes. Smoking for 20 hours, then mop every 4 to 5 hours. This will add the moisture and flavor that makes for great barbecue and grilling.