Just like most aspects of barbecue, opinions on whether or not to use a mustard coat before a dry rub layer are mixed. Some people will state that it's an important part of the process, while others will say it has little to no purpose.
With that said, the sole reason people use mustard on ribs (as well as other cuts) is so that the dry rub adheres better to the meat.
People will often refer to the mustard as a "binder" as its functional purpose is to act as a glue for the dry rub and the meat. The mustard layer imparts no flavor that's capable of being perceived in the mouth.
The conversation about binders in barbecue has existed for a long time. While we know their purpose is to bind dry rubs to meat, there isn't a clear understanding as to why mustard is used.
I'd wager to say that most people use mustard as a binder because it's primarily vinegar based (depending on brand), it's readily accessible in most people's refrigerators, and it's cheap.
Most commercial mustard is made from a combination of the following ingredients: crushed mustard seeds, water, vinegar, and white wine.
One of the most popular is French's Yellow mustard. The ingredients include:
There is also Heinz Yellow Mustard. The ingredients include:
As you might expect, when mustard is cooked the liquid ingredients (distilled vinegar and water) will evaporate which leaves behind mustard powder, salt, turmeric, paprika, garlic powder, etc.
A number of those ingredients are used in most barbecue rubs. Meaning, when a dry rub is applied with a mustard layer, they're indistinguishable.
With that said, if your goal is to impart a mustard flavor, the amount of "mustard seed" or "ground mustard" that you're left with isn't enough to be tasted.
Mustard is a staple condiment in most households. It's commonly used in backyard cook-outs for food like hamburgers and hotdogs.
If you're someone who believes in using a binder, mustard is a cheap alternative to something like olive oil. To illustrate we'll use the example brands above:
My house typically buys California Olive Ranch brand olive oil. However, even if you were to purchase Walmart's Great Value, you'd still spend 2-3x more than mustard.
While mustard is likely the most popular binder used, several Pitmasters opt to use alternatives. While mustard imparts no flavor on the meat, something like hot sauce or Worcestershire sauce can.
Note: Before we discuss alternatives, you can look at a lot of these products and their ingredients list. Just like the mustard, when the liquids evaporate, the soluble ingredients remain.
It's important to remember that a dry rub is just that, dry/dehydrated spices. At a minimum, using water as a binder can help to both re-hydrate the rub and allow it to penetrate the meat.
Dry-brining can technically do the same thing, however not many people want to wait for 1-3 hours for this process to occur.
Dry-brining works through osmosis. When you add salt to the meat, the meat's juices are drawn out. Since the salt is soluble, it dissolves in the meat juices and creates it's own concentrated brine.
This process also allows muscle proteins to be broken down/unwind or denature. Salt is made of sodium and chloride ions that carry electrical charges. These ions attack the proteins. The loosened muscle fibers then allow this brine solution to be reabsorbed.
Note: While people might argue to use water instead of mustard, not everyone has access to clean tap water.
Using Worcestershire sauce on pork ribs isn't very common. You more so see people use Worcestershire sauce for beef cuts like Brisket, rib roasts, chuck roasts, beef ribs, etc.
It's similar to people injecting with beef broth/stock, soy sauce, etc. The idea being that the Worcestershire sauce enhances the beef flavor.
With that said, some Pitmasters do use Worcestershire as a binder for pork ribs. For example, "Big" Moe Cason uses Worcestershire as apposed to mustard.
Worcestershire Sauce also contains common ingredients found in Barbecue rubs:
Just like Mustard, the vinegar and water gets burned off. However, in this case you're left with spicy notes from the chili pepper extract and sweet from the sugar and molasses.
Molasses/sugar are commonly used with pork ribs either in rubs or as an ingredient in a liquid wrap.
Most households have some form of oil on-hand. My house uses olive oil a lot and we typically pay more for a better brand.
For this reason alone, I typically don't use Olive oil as a binder, simply because it's expensive.
Apple cider vinegar/apple juice are commonly used with pork cuts.
It's fairly common for people to use apple juice when they perform a Texas crutch because pork and apple juice compliment each other well.
Assuming you're going to liquid wrap your ribs with apple juice, you'll also have it on-hand to use as a binder.
It's also common for people to use a mixture of water and apple cider vinegar to spritz the meat throughout a cook in order to prevent bark from drying out.
Hot sauce is another common binder. Most brands are relatively inexpensive.
I'm from New England and Frank's Red Hot is popular here. The ingredients are as follows:
Again, distilled vinegar and water are burned off and you're left with spicy notes from the cayenne peppers.
I've personally never tested using mayonnaise a as a binder, however I know people use it. It's also a fairly neutral option based on ingredients.
There are also variations of mayonnaise that contain a variety of ingredients like chipotle.
Hellmann's is a popular mayonnaise brand. It's ingredients are as follows:
There are also people who opt to apply their mustard after they've applied dry rub. For instance, Pitmaster Melissa Cookston of Memphis BBQ Co. applies mustard after she's dry rubbed her ribs.
While I've never heard her say exactly why she does it after as apposed to before, her reasoning is the same, so the rub sticks to the meat better.
Melissa will dry rub her ribs with her rib rub, apply a mustard layer, and then a layer of turbinado sugar.
The overarching goal of using mustard on ribs is to act as a binder.
Apart from Mayonnaise, I've tried all the above. Personally, I opt to use mustard or no binder. When I'm not using a binder, I typically wake up early and allow the rub to stand on the meat for 1-2 hours before smoking and the end result is the same.
The best part about Barbecue as a hobby is that you get to experiment with everything you do. Try using a different binder and see what you like best.