The main reason people use mustard on pork butt is so that the dry rub sticks better to the meat. The idea being that the mustard will encourage a markedly better bark.
The mustard layer is also called a "binder, slather, or schmear." The overarching purpose of the mustard is to bind the dry ingredients to the pork butt. When cooked, the mustard imparts no flavor that's capable of being tasted.
The topic of binders in the Barbecue World has existed for a long time.
While we know the main reason they're used is to bind dry rub to meat like pork butt, ribs, and brisket, there isn't a clear explanation as to why mustard is used.
In my opinion, the reason mustard is popular is because it's vinegar based, it's cheap, and it's found in most folk's refrigerators.
After smoking tons of pork butts at this point, I find a binder - whether it be mustard or an alternative below - to be useful.
The main reason being, pork butt is rather irregular in shape.
In comparison, a cut like brisket or a rack of ribs is rectangular and lays flat. There also isn't a ton of thickness to the meat. I find that these cuts don't need a binder because of the way they're oriented.
Pork butt is like a miniature boulder of muscle and applying dry rub to an odd shaped cut can be troublesome. Having a wet/tacky surface simply makes applying the dry rub easier.
Since the meat is being pulled apart, having uniform bark is less of a concern. Where-as with brisket you're often creating pencil thin slices with strips of bark across the top.
With brisket, I've had several instances where I'll completely lose bark when I've used mustard as a binder; Almost like old paint chipping off a car.
Almost all yellow mustard is made from a combination of the same ingredients: water, vinegar, and mustard seed.
For example, one of the most popular brands of Yellow mustard is French's. Their ingredients list:
Another example is Heinz:
Above I've bolded the liquid ingredients found in both brands of mustards - which make up most of the mustard in the bottle.
When these ingredients are "cooked" they vaporize. The boiling point of water is 212F and the boiling point of vinegar is 213F.
Meaning, we're left behind with soluble ingredients like mustard seed, salt, turmeric, paprika, etc.
Almost all of these ingredients are found in barbecue dry rubs. Meaning, when you apply a dry rub, the mustard is indistinguishable from the rub.
With all that said, if your goal is to impart a "mustard" flavor on the meat, the mustard seed in the yellow mustard isn't enough to be tasted.
Mustard is a condiment that most people have in their refrigerators. It's used for a variety of dishes and it's readily available - it's also very cheap.
If you're someone who likes using a binder, it's a lot more affordable than something like Olive oil.
To illustrate, we can look at the cost comparison between Yellow Mustard and Olive oil:
In my house we usually will opt for a better brand of olive oil.
Even if you were to buy a cheap brand of olive oil, you'd still pay 2-3x less for mustard.
While mustard is the most popular binder for barbecue, there are a number of different binders that people use.
Another really common one with pork is Hot sauce - which is also very cheap.
Note: Before we explore the different alternatives, I wanted to make it abundantly clear that almost none of these impart a flavor that's capable of actually being tasted.
Simply examine the ingredients list - the liquid ingredients will vaporize and the soluble ingredients will remain, albeit in small quantities.
For various cuts of meat I prefer not to use a binder.
Remember, a dry rub is just that, a dehydrated dry rub. At a minimum, water will re-hydrate the spices and offer the dry rub something to stick to.
Technically, you could simply dry brine the meat too. Pork butt is a large muscle and enough surface area where using water isn't super necessary.
Remember too, not all folks have access to clean drinking water. Mustard is a great option in this circumstance.
If I don't have mustard or hot sauce, I'll usually forgo using a binder.
Hot sauce is a common binder for cuts of pork - I also regularly use it for pork butt.
Most hot sauces are also super cheap.
I'm from New Hampshire and Frank's Red Hot is really popular here. A 12 fl.oz. bottle of Frank's Red Hot (Original - not the wing sauce) is $2.98.
The ingredients in Frank's Red Hot:
In this case the distilled vinegar and water are vaporized and you're left with spicy notes from the aged cayenne peppers.
Keep in mind: If your goal is to have the Frank's Red Hot function as the spicy component, there isn't enough aged cayenne to be tasted.
Using Worcestershire sauce for pork cuts isn't very common. Most folks will use Worcestershire sauce for cuts of beef.
The idea being that the Worcestershire sauce brings out the "beefy" flavors - similar to why people inject with beef broth/stock (both could also function as binders).
The ingredients in Worcestershire Sauce are as follows:
Again, water and distilled white vinegar are vaporized and we're left with sweet and spicy notes - both of which aren't enough to be tasted.
In my house we usually have Lea and Perrin's - a 10 oz. bottle costs $4.28 - which is double the price of mustard.
Cooking oils are really common with various cuts of meat - whether it's beef brisket, pork butt, ribs - however, it's most popular with chicken.
Most homes have olive oil on hand just for all-purpose cooking. Whether it's Olive oil, vegetable oil, canola oil, etc. In my house we'll usually buy a better brand like Saratoga's, Meyer Lemon Olive Oil - which is $19.95 for 12.6 oz.
There is almost no chance I'll waste good olive oil on an inexpensive meat - especially when it doesn't do much for flavor.
Apple juice and apple cider vinegar are both really common with pork cuts. People will use apple juice/ACV in their liquid wraps to beat the stall.
Lots of people also like to use a 50/50 mixture of ACV/Apple juice and water to spritz with. Spritz is applied during the smoking process as apposed to at the start of the smoking process.
Either way though, if you're someone who uses Apple juice or ACV in a wrap or for a spritz, you can also use it as a binder.
I've never used mayonnaise as a binder for any cut of meat but I do know people use it; It's also more common with beef cuts.
A popular brand of Mayonnaise is Hellmann's - the ingredients list is fairly neutral:
Aside from basic mayonnaise, there are also other variations that can include a number of ingredients - like chipotle for example.
To this day, I've yet to find a binder that does much of anything to affect the taste of my pork butt/pulled pork - It plays such a small role in a cook that takes 12+ hours to do.
A lot of the taste components can be achieved simply through the rub.
For instance, if you're trying to get heat from the hot sauce binder, you're better off incorporating cayenne in your rub - or adding a heat component to a finishing sauce.