Apart from anatomical location on the hog, the main thing that separates the pork loin and the pork butt (pork shoulder) is fat content.
Pork butt contains far more intramuscular fat than pork loin - which typically contains minimal fat.
As a result of the fat differences, these cuts of pork are also better served when cooked to completely different finishing temperatures; Pork loin is best served at around 145F and pork butt will "finish" in the range of 195 -205F for pulling.
With that said, pork loin can work for pulled pork, provided you use the blade end (discussed below).
When most people hear pork "butt" they often think that the meat is sourced from the hindquarters. However, pork butt or shoulder actually comes from the front of the pig.
Note: The "ham" is from the hindquarters.
Pork butt - also colloquially referred to as Boston butt - is a cut of pork sourced from the pig's upper shoulder, behind the neck/head.
The name "Boston butt" originates from Colonial New England before refrigerators were invented. The Butchers would pack inexpensive cuts of meat into large barrels called "butts" for transportation purposes.
The name stuck and we're left with the "Boston Butt."
People also often think of the pork butt as a singular muscle. It's actually comprised of several muscles (roughly 12) that all converge at the shoulder.
These muscles are then attached to the shoulder blade via connective tissue. This is the reason why when you buy a boneless pork butt, it will look like a relaxed mass of meat. Where-as a bone-in pork butt looks like a boulder of muscle.
Unlike the picnic (also commonly called the shoulder), the pork butt isn't heavily exercised; Meaning, it's less tough and sinewy. Instead, pork butt is prized for it's marbled intramuscular fat.
Pork loin is a cut of pork from the back of the pig. More specifically, it's found between the back fat and the ribs.
Meaning, if you've ever had baby back ribs, you've had pork loin meat.
Aside from pork tenderloin, pork loin is the leanest cut on the entire pig. The loin isn't exercised much and it isn't used for movement.
From the pork loin you can pull a number of different cuts - ranging from boneless pork chops (with a variety of different thicknesses), pork roasts, stew meat, pork ribeye steaks, etc.
Something these two cuts of pork have in common is that they're super cheap.
Both pork loin and pork butt are very affordable on a per pound basis and usually cost around $0.99 - $2.29/lb.
For instance, the meat used for this article was:
These cuts can also be cut up in a variety of ways. For instance, pork butt can be sold bone-in and boneless and is used to make things like pork steaks or pulled pork.
Pork loin can be sold whole (the entire pork loin muscle), or divided into two halves: a blade end and a sirloin end.
Pork butt and pork loin are usually cooked much differently.
Pork butt is commonly smoked until 195 - 205F internal. At this temperature the muscle can be quite literally pulled apart.
Conversely, pork loin is only cooked until 145F and can be prepped into any number of cuts.
The previous recommendation for cuts like pork roasts, chops, steaks, ham was 160F internal. However, in 2011 the United States Department of Agriculture changed their recommendations to 145F.
This change was made due to improvements in food safety practices and a decrease in the prevalence of trichinosis.
An added benefit of this change is that you can markedly improve moisture content without drying out the meat or risk of trichinosis infection. Pork loin already doesn't have a ton of intramuscular fat so the above improves the eating experience greatly.
A better way to think about this concept: The above is the same reason people cook beef steaks to medium-rare as apposed to well done - you're maximizing free moisture content (juiciness).
While not as common, Pork butt can also be used to make pork steaks. If you're making pork steaks at home, I'd strongly suggest buying bone-less pork butts as you'll need a bone saw to get through the blade bone of bone-in pork butts.
While it's possible to use pork loin for pulled pork, I think it's important that people are educated on the muscle and how certain parts of the loin work better than others for pulled pork.
Any recipe that calls for pork loin for pulled pork is doing a disservice to readers. Mainly because they're not qualifying what they mean by "pork loin."
A whole pork loin is a huge muscle, often weighing around the same as a pork butt. On a whole pork loin there are two ends:
When a pork loin is cut up, you can get any number of different cuts; Different parts of the loin work better than others for different methods of cooking.
For instance, the center rib/loin works great for roasts; The blade-end works great for thick cut pork ribeye steaks; The sirloin end works great for thin cut chops and stew meat, etc.
In a grocery store you can also find pork loins sold halved. Meaning, you get one half with the blade end and a portion of the center loin/rib. There's also the sirloin end sold with a portion of the center-rib.
Of these two halves, the blade-end is better suited for pulled pork. It shares similar qualities to the pork butt (which is where the blade bone exists) and contains intramuscular fat that works well to keep the meat moist.
The sirloin end is more so suited for cubing up into stew meat or slicing into thin chops.
If your goal is to simply use pork loin for pulled pork, do yourself a favor and look for the blade-halved loins. The blade end is a darker hue of pink and looks more like a relaxed muscle; The sirloin end is pinker and looks "contracted."