As with most topics in barbecue, opinions on whether or not to use a mustard schmear before applying a dry rub are by and large split. Some people will say that it's an important part of the process, while others will say it serves little to no purpose.
The reason people use mustard on brisket is so that the dry rub sticks better to the meat. The idea being that the mustard will encourage the development of a markedly better bark.
The mustard layer is often referred to as a binder, slather, or schmear; The functional purpose of the mustard is to act as a glue for the dry rub and the brisket. This mustard layer imparts no flavor that's capable of being tasted.
The topic of binders in barbecue has existed for a long time. While the main reason to use mustard is to bind dry rub to meat like brisket, there isn't explanation for why mustard is used.
In my opinion, the reason mustard is popular because it's vinegar based, it's cheap, and it's found in most people's refrigerators.
Almost all commercial yellow mustard is made from a combination of the same ingredients: water, vinegar, and crushed mustard seeds.
To illustrate, one of the most popular is French's Yellow Mustard. The ingredients include:
Another popular brand of mustard is Heinz; The ingredients include:
When these ingredients are cooked, the liquid ingredients like distilled vinegar will vaporize - the boiling point of water is 212F and the boiling point of vinegar is 213F - which leaves behind the other dry ingredients like mustard seed, salt, turmeric, paprika, etc.
A number of the remaining ingredients are used in barbecue rubs. Meaning, when the dry rub is applied over the mustard, they're indistinguishable.
Keep in mind, if your goal is to impart a mustard flavor on the meat, the amount of mustard seed or ground mustard in these products isn't enough to be tasted.
Mustard is a condiment found in most people's homes. It's often used in backyard cook-outs for food like hamburgers and hotdogs. Meaning, most folks already have it in their refrigerator.
If you're someone who uses a binder, mustard is more affordable than alternative options like olive oil.
To illustrate, we can also look at the cost of olive oil, which is also often used as a binder (more traditionally for poultry):
In my house we typically have a higher quality olive oil on-hand that I'd rather not waste using as a binder for meat.
However, even if I you were to use a brand like Walmart's Great Value, you'd still spend up to 2-3x more than mustard.
While mustard is likely the most popular binder that's used, there are also a number of alternatives that Pitmasters have come up with.
Note: Before exploring alternatives, you can simply review the ingredients lists of any of these products; Just like mustard, the liquid ingredients will vaporize and the soluble ingredients will remain.
Apart from Mustard, water and/or no binder is the best option.
It's important to remember that a dry rub is just that, dehydrated spices. At a minimum, using water will re-hydrate the spices and offer something for the rub to stick to.
Technically, you could simply dry-brine the meat too. Brisket has enough surface area and moisture where using water isn't super necessary.
While some people might argue to use water instead of mustard, not everyone has access to clean tap water.
Using Worcestershire sauce as a binder for brisket is fairly common. It's in a similar vein to injecting with beef broth/stock. The purpose is to enhance the beef flavor.
Again, Worcestershire sauce contains several ingredients found in barbecue rubs:
When Worcestershire sauce is used, the vinegar and water are vaporized and you're left with spicy and sweet notes.
In my experience though, I don't taste much of anything when using Worcestershire sauce. There isn't much spice from the chili pepper extract or sweetness from the molasses/sugar.
We usually have Lea and Perrin's Worcestershire sauce at my house. A 10 oz. bottle costs $4.28 which is double the price of any commercial mustard.
I've seen this concept of creating a slather of pickle juice and mustard. However, I've personally never tested it.
Most of the folks you see who do use a pickle juice and mustard slather own Barbecue Restaurants. At these places, not much goes to waste, including pickle juice.
On a typical barbecue plate, you're served with white bread, different types and cuts of meat, sides, as well as pickle slices. When all of the pickles are gone, you're left with pickle juice brine.
Not many people in the barbecue community will tell you their reasoning for doing much of anything.
However, in a video by Jonny White (Jirby BBQ) of Goldees BBQ, we can see him use a pickle juice and mustard slather for his brisket.
A popular brand of pickles is Mt. Olive. To get a good idea of what's in their brine, we can look at their "Pickle Juicers."
The ingredients are as follows:
So we have the same ingredients that are vaporized and we're left with sea salt, turmeric, and natural flavors.
In this situation, mustard and pickle juice are made of up nearly the same ingredients. When they're combined and cooked, you won't taste them.
Almost all households have some form of cooking oil on-hand - whether it be olive oil, vegetable oil, canola oil, etc.
However, olive oil is almost always more expensive than every other option - as detailed above.
Apple juice and apple cider vinegar are very common with pork cuts, however they can work in much the same way for beef brisket.
It's more-so common to use apple cider vinegar and water as part of a spritz. A spritz is usually applied during the smoking process to prevent the meat from drying out and to encourage smoke particles to stick to the meat.
Hot sauce is a pretty common binder for beef cuts. Most basic hot sauces are fairly inexpensive.
I'm from New England and Frank's Red Hot is quite popular here. A 12 fl.oz. bottle of Frank's Red Hot is $2.98.
The ingredients include:
In this scenario the distilled vinegar and water are vaporized and you're left with the spicy notes from the aged cayenne red peppers.
Similar to the Worcestershire sauce, I don't taste much of anything or a spice that's cable of being tasted.
I've never tested mayonnaise as a binder but I know it's commonly used for beef cuts. Overall, Mayonnaise is fairly neutral in terms of ingredients.
A popular brand of Mayonnaise is Hellmann's, the ingredients include:
There are also variations of mayonnaise that include a variety of ingredients like chipotle.
To this day, I've yet to find a binder that does much of anything to affect the taste of brisket. It plays such a small role in a cook that takes 12+ hours to do.
My personal preference is no binder as I don't really find them necessary for brisket, let alone ribs.
I've personally done brisket both with and without a binder and I'm on the fence of no binder - it makes less mess and the end result is the same.