As of late you will see many people asking questions about trussing their meat before aging, most specifically why and should they also do it to their next sub primal. In short, the answer is absolutely yes. Now if you’re interested in the why and perhaps even how you’ll want to continue reading.
To take a small step back for a second, most people are probably most familiar with chefs trussing their meat before they cook it, not dry age it. The principle behind it is essentially the same. When you truss before you cook you are trying to maintain shape of the meat because as you add heat, the meat will start to expand and can sometimes develop a less than desirable shape which can lead to poor presentation of your dish. Your meat can spread out and, in some cases, burst. To go one step further some cooks will truss to keep meat that they have stuffed together. Trussing your meat also promotes even cooking.
Now how does that relate to dry aging you ask? The exact same way, except the opposite. Its not that confusing. When you are dry aging you are removing surface moisture from the muscle that concentrates beef flavor while also promoting the natural enzymes in the muscle to break down connective tissue resulting in a more tender steak (or roast). Imagine a sponge that is soaked with water that you have set out on your counter to dry. As time passes that moisture will be released from the sponge into the air and that sponge will begin to shrink and become dry to the touch.
Your subprime piece of meat is going to behave in the same way. As moisture moves out of the subprimal the meat has a tendency to “flatten out.” Your ribeye roast can end up looking more like an oblong New York Strip, when you process your fillet the steaks may look more like chicken strips, and so on. Bone in pieces of meat are also no exception, however having the bones in tact will help against deformation.
Another benefit of trussing is that when the meat is compressed there is a lesser chance of air getting into the roast from where it was butchered. When the air is not allowed to penetrate there will not be interior oxidation and ultimately discoloration of the beef which often times is not that visually appealing.
Now for the “how.” Some people are content with just cutting individual strands of butcher’s twine and tying knots around the meat, and some people a fancier method of looping one long piece of twine the length of the subprimal, but either is OK. The end goal is to produce a tightly wrapped piece of meat (without damage) that has a similar thickness all the way through. Successfully doing this will yield more consistent cuts of meat when processing and ultimately cuts of meat that cook more evenly.