This article is intended as an overview of the two main categories of cuts of meat (muscles) and the common properties these groups share. A lot of this may be boring science-nerd stuff (which I am all here for), but the reason I think it’s interesting is because these shared properties shape the way we cook them. For example, we would alter our cooking approach if we were cooking chicken breast vs chicken thighs, and likewise, there is no way you would want to cook a fillet steak the same way you cook a brisket. This is because they are in different categories and have different properties, likewise, meats in the same categories can be cooked in basically the same way. Cooked short rib before but never ox-cheek? Don’t worry, just think of them like different shaped short ribs and you’ll be fine.
1. Muscle types
1.1 Fast twitch muscles
Fast twitch muscles are the muscles designed for short, sharp bursts of activity, such as sprinting. From an evolutionary point of view these are the quick burst muscles for escaping danger. Generally speaking they don’t get much use day-to-day.
1.2 Slow twitch muscles
Slow twitch muscles, inversely, are the slower, hard working muscle groups. The muscles that get lots of use on a regular basis. These kinds of muscles are used for slower movement (carrying, chewing). If someone describes a muscle as hard-working they are most likely talking about a slow twitch muscle.
3. What difference does it make?
So, why does this matter? As mentioned in the intro, there are specific attributes that come with the muscle’s role in the body. Hard working muscles need a greater supply of energy, and that comes in the form of oxygen and fat.
The increased oxygen demand doesn’t make much difference to our cooking, however as a fun(?) side note, hard working muscles may appear darker in colour (in white meat at least), because they require a greater supply of oxygen so will have a greater presence of myoglobin (oxygenated myoglobin is red, and it the reason your meat might sometimes bleed e.g. your meat isn’t bleeding, that red liquid is mostly water and myoglobin). This is why chicken legs are usually a slightly darker meat - chickens spend large periods walking or standing so those muscles are slow twitch, compared to say the breast or wing muscles on a chicken that get a lot less use, other than an occasional flapping of its wings (you may have already worked this out, but this is also why duck is a darker meat than chicken, as a flying bird its breast and wings get a lot more use than on a flightless chicken).
More interesting than levels of myoglobin in the meat is the increased levels of fat in the muscle. The fat we are interested in here is intramuscular fat, that is not the fat that might encase a muscle (you might hear it referred to as a fat cap), but rather those specks and lines within the muscle often referred to as marbling. This fat helps provide energy to the muscle and is a crucial part in providing delicious flavour as well as moisture to meat as it’s cooked. Fat caps (a big slab of fat that you might finding sitting on top of your pork shoulder, for example) does not flavour the meat in the same way, any fat that renders off doesn’t somehow penetrate the meat to add moisture, it just renders off in to the pan/fire (and whilst its running off the surface of the meat, there’s also a chance it washes off some of the seasoning or rub that you have applied in the process). That’s not to say it’s no good, you just need to consider the end product - the fat drippings from a fat cap can be used to make delicious gravies or jus, and lots of people enjoy eating pieces of fat, but in other cooks it can get in the way of a good smokey bark forming - so depending on what you are cooking you need to make sure you trim it judiciously.
3. Connective tissue
The third important feature of hard working muscles, is perhaps the most wonderful feature. The connective tissue.
Connective tissues are proteins in the body that help keep muscles in place, and connected. There are a couple different types of connective tissues, with different purposes. One purpose is to keep the muscles encased and connected to bones, so when muscles contract, the body is able to move. As you might imagine, if the tissue is keeping your muscles connected and in the right place, then any hard working muscles are going to need plenty of connective tissue to endure the more strenuous work they are subjected to. Take a brisket, for example, one of the most connective-tissue rich cuts you might cook with, this muscle may have been responsible for permanently supporting up to 60% of the cow’s standing weight, so as you can imagine requires a wealth of connective tissue to take that workload. Likewise, all the harder working muscles, such as the shoulder, will contain more connective tissues.
When not properly cooked, connective tissue is really chewy (to the point of not really being edible), however, when cooked properly, connective tissue can transform into gelatin. This is a silky smooth liquid that lubricates the meat and is the magic that makes people happy. It’s what transforms tough, chewy (traditionally cheaper) cuts of meat into the kind of results that we all BBQ for.
This is the reason we slow cook pieces like brisket, short rib, pork shoulder to a higher internal temperature - it’s to break down the connective tissue into gelatin. In a fast-twitch muscle, without connective tissue, such as chicken breast or fillet steak, if we cooked it to those higher internal temperatures the meat proteins would contract, forcing the moisture out of the meat and making for a tough, dry piece of meat. In the slow-twitch muscles, with increased intra-muscular fat and lots of connective tissue, at those higher temperatures is where the magic happens!
3.1 Sidenote: Elastin vs Collagen
Boring science stuff alert: The two main types of connective tissue are Elastin and Collagen - Collagen is our favourite connective tissue, the BBQ-ers best friend. Collagen breaks down, when subject to the right conditions, into gelatin, enhancing our meat with huge rewards for our patience. Elastin, on the other hand, won’t break down for love nor money and will always remain chewy. In recipes or instructional videos this is often referred to as “silverskin” - this will be a slightly translucent, silvery skin that will often be on the surface of pieces of meat you get from a butcher, and unfortunately these just need to be trimmed off and removed as best you can.
The reason it can be useful to be able to recognise these two groups of muscles, and understand their particular traits, is that it means you can easily understand how to cook it or how it might fit into other recipes or cooking techniques you have in your arsenal.
For example, let’s say you have never cooked beef cheeks before but you have cooked short ribs, and you know they are both hard-working slow twitch muscles, then we can use similar recipes and techniques to cook the two.
Inversely, imagine you get your first hanger steak, this is a very tender, fast-twitch muscle (as the name suggests, it just hangs on the cow, basically doing no work) - knowing this we know that it will be a fairly tender cut and will need to be cooked accordingly and definitely not to the temperatures of a brisket or short rib.
This knowledge helps understand what pieces of meat in recipes can be interchanged with minimal other changes to the recipe. For example, in my chilli recipe I mention in the ingredients that you can basically choose the meat, can be pork shoulder, short rib, ox cheek, brisket etc, and nothing else really needs to change in the recipe.