Of the different beef primals and sub-primal cuts, the flank steak and skirt steak are cuts that are most often confused for one another.
One of the main reasons for this is because a number of resources, retailers, and butchers will use "Flank steak" as a catch all term to refer to skirt steak, flank steak, and hanger steak.
Both flank steak and skirt steak are tight-grained, exercised muscles. Skirt steak has much more fat than the the flank steak - and is much more flavorful. Both take well to marinades and are cooked in similar ways (typically seared over high heat).
Skirt steak comes from the plate primal - also called the short plate. The name skirt describes it's shape as it resembles a sash, girdle, belt, or skirt.
Skirt steak used to be considered offal (organ meat) and wasn't as popular as it is today. Most folks are now very much aware of how tasty and delicious skirt steak is.
From a single steer there are only two inside skirt steaks and two outside skirt steaks.
Both are boneless, trimmed cuts of meat from the diaphragm muscle attached to ribs 6 - 12 on the underside of the short plate.
There are two types of skirt steak - either inside or outside. The inside skirt steak is from the transverse abdominal muscle. The outside skirt steak comes from the diaphragm (not the adjacent hanger steak).
The inside and outside skirt steaks are similar. Both have lts of connective tissues and a fibrous, loose grain structure.
The outside skirt steak is typically sold to restaurants where-as the inside skirt steak is found at retail. This is rather unfortunate as the inside skirt steak is smaller than the outside skirt steak.
There are a number of different ways to use Skirt steak but the most popular include:
I've also grilled skirt steak like I would any steak - using kosher salt, pepper, and Lawry's seasoned salt.
Regardless of how you choose to cook it, it's a wonderful cut of meat.
Flank steak comes from the beef flank primal. The name flank describes the anatomical location on the steer - the flank.
Flank steak is a thin, lean cut of meat from the abdominal wall of the steer. Flank steak is a well exercised muscle and as a result the meat fibers are tight-grained.
Aside from being called flank steak, there are a number of different names you might see.
To name a few:
Flank steak is very common in Asian cuisine where it's commonly sold as "stir-fry meat."
It's also very common for folks to mistakenly refer to flank steak as skirt steak. This is seen with dishes like arrachera where in fact arrachera is skirt steak, not flank steak.
Flank steak can substitute for a lot of the same dishes as skirt steak. However, there are a few dishes that exclusively use skirt steak like:
Both flank steak and skirt steak take well to marinades. The marinade will typically include fruit that can tenderize meat.
For example, pineapple contains an enzyme called bromelain that can degrade the collagen fibers that make flank steak tough.
Other fruits that contain digestive enzymes include papaya (papain), kiwifruit (actindin), and figs (ficin).
Common marinades for flank steak will also feature ingredients like oil, garlic, black pepper, chili flakes, lime juice, soy sauce, ginger, etc. A number of these ingredients I use in my Beef on a stick recipe.
Once marinated for 4-24 hours, the meat should be seared over high on either on a grill or in a hot pan/cast iron skillet. Skirt steak is typically seared for 2-3 minutes per side where-as flank steak can take 4-5 minutes per side.
Skirt steak and flank steak should not be cooked past medium-rare; Doing so will make them tough and dry.
Both Flank Steak and Skirt Steak should be sliced on a bias and against the grain.
The reason you slice against the grain and on a bias (knife angled roughly 45 degrees) is to minimize the muscle fiber lengths. Meaning, your teeth are able to bite through the meat easier which improves the eating experience.
As you can sort of see above, there aren't major differences between skirt steak and flank steak. This is another reason recipes that call for either steak can be substituted for one another quite easily.
With that said, the major difference between flank steak and skirt steak is the fat content. Skirt steak has more fat/marbling than flank steak, and as the saying goes, fat is flavor.
Truth be told, flank steak has very little fat, if any.
Another difference is in the size of the meat. Skirt steak is a long (20-24 inches), thin cut of meat (4 inches wide). Flank steak is a roughly 10-12 inches long and 5-6 inches wide. However both are only at most 1/2 to 1 inch thick.
While both cuts of meat consist of tough muscle fibers, flank steak has a tighter grain where-as skirt steak is loser yet more fibrous. Meaning, Skirt steak will take better to marinades than flank steak.
I personally like skirt steak more than flank steak. However, I almost never find skirt steak in my grocery store. Flank steak is also very hit and miss in terms of availability.
To reiterate, both cuts are fibrous and take well to marinades. They should both be grilled over or seared over high heat and can normally be substituted for one another in dishes that call for them.
Unfortunately, most folks know about flank steak and skirt steak these days. Meaning, the price of both cuts has continued to go up and in some cases they're borderline impossible to find.