The Scientific Reason Some Mustards Are Spicier Than Others
Compared to other spicy foods, it can be challenging to talk about spice when it comes to things like mustard, ginger, or black pepper. Chile peppers have a defined Scoville scale that tells you exactly how much eating that scotch bonnet is going to hurt (via Nature's Path). Other foods don't fit into this measurement system as easily, though.
One of the reasons for this is that the sensation of heat or spice comes from a different source. Foods like mustard, wasabi, and horseradish get their kick from an oil called Allyl Isothiocyanate(AITC) (via Scott Roberts). At the same time, chile peppers get their heat from a compound called capsaicin. This is the reason why hot peppers have a fiery bite, and these other foods are known more for clearing sinuses without the same accompanying burning sensation. There's a spice there, it's just affecting your body differently.
Because mustard uses allyl AITC as its source of the spice, there are special methods to determine how spicy a mustard sauce will be.
Not all mustards are created equal
One of the main influences on how spicy mustard will be is the type of mustard seeds used. There are yellow, black, and brown mustard seeds that all have different properties. According to Serious Eats, the yellow mustard seeds are the mildest. The type used in the mild French's yellow mustard is the standard go-to for barbecuing ribs and briskets (via Spruce Eats). Brown and black seeds are much more pungent and are typically reserved for sauces with a serious kick.
All mustard sauces are made with the same basic method. The chosen seeds are ground and then mixed with a liquid (via Serious Eats). The acidity of the chosen juice will play a huge role in mustard's spice level. This is because it will determine how quickly the enzymes that give the mustard its spicy flavor will break down. Mustards made with vinegar tend to have a better preserved but milder flavor. Mustards made with water, however, will have more flavor but will dull more quickly. This is why it's especially important to check expiration dates on mustards made with water instead of vinegar. Of course, with a projected mustard shortage on our hands, you may not be able to be as choosey.
Now you've got all the tools and science you need to grab the right mustard when you're gearing up for your next hot dog and hamburger-loaded cookout.