My later references to processes in this discussion possibly related to what is done by Arby's® in making their roast beef are all my own thoughts and guesses, not firm knowledge of what Arby's® actually does to make their beef roasts.
Think about delicatessen cold meat products like ham, or consider how Arby's® creates that obviously combined processed beef so that it slices very thinly and doesn't fall apart. It is rather obvious that the delicatessen ham products are not cut from one integral piece of cured, smoked pork ... instead, they are a loaf formed of pieces of ham chemically joined by a product such as transglutaminase to behave as if they were simply one piece of meat (in a very convenient loaf shape). It is also obvious that Arby's® roast beef is a composite, highly processed product that retains the taste of beef, because it is mostly beef, but certainly not a single integral roast of beef.
The point is obvious. A "Meat Glue" product like transglutaminase is essential in making many of the products we enjoy from delicatessens and fast food restaurants. You have consumed much of it and it is very safe and recognized as safe by the FDA. That having been said, you may well be wondering why a home chef would have any interest at all in a commercial product like transglutaminase. Now the discussion becomes quite interesting, as you will read below.
I have a recipe for gyro meat in Food Nirvana. It is obvious when reading the recipe that a great deal of effort goes into making a final product that will behave as a loaf of combined meats that can be sliced, grilled, and hold it's general shape until it is consumed within a pita bread wrap. Now consider how many steps might be eliminated from that recipe if transglutaminase (TG) were used during raw meat grinding and mixing with herbs and seasonings, then formed into a loaf that, after refrigeration, seems to magically become integrated. Yes, if you can easily bond ground beef and ground lamb to form an easily sliceable loaf, then you have a perfect method for making gyro meat. Actually, that method is far superior to what has been done historically to create the same general type of product, sometimes called souvlaki.
Now, for the fun of it, think about being able to do what Arby's® does with beef ... use tenderizer and salt and the enzyme and join the meat pieces by vacuum sealing until the enzyme has bonded the pieces of meat, then bag and seal the composite meat in a brine in a heat proof polymer bag and bake it for a few hours to get the flavors of the brine into the meat, producing a delightful, thinly sliceable "roast" of beef for hot sandwiches.
Chalk another success up on the board for applied science! And yes, we owe Ajinomoto®, a Japanese company, the applause for doing all of the research to create, biologically, a perfect form of transglutaminase for commercial or home use.
I will now describe the use of their product named Activa® RM. It is one of a family of transglutaminase products used for different applications. It is sold as a powder that is sprinkled on raw flesh or used in a slurry to coat pieces of raw flesh. Ajinomoto produces transglutaminase and sells it in vacuum sealed foil pouches. I have modified the content of an Internet transglutaminase information source that I read in the usage paragraphs that follow, to limit and direct discussion to what the home chef is most likely to need.
Unopened, the pouches of RM are good for 18 months if stored below 21°C (70°F). Once open, however, the enzymes become vulnerable to moisture. Opened pouches should be tightly wrapped and stored in the freezer. The freezing temperatures will not hurt the enzyme and the low humidity in the freezer will prevent degradation. A vacuum sealer is especially useful for breaking a batch of RM into small packages for sealed storage in the freezer.
Never leave large quantities of RM out on the counter, especially in open containers, for they will quickly become useless. Remove only the amount needed using clean, dry utensils and return the rest to the freezer. Keep the working container/pouch/plastic bag closed when not in use.
Salt has an interesting effect on meats when combined with RM. Salting meat makes more of its proteins soluble and allows the meat to bind together naturally, as in a traditional sausage: Meat and salt are mixed together, the salt forces some of the meat’s protein into solution, and this protein forms a gel that binds the sausage together.
In charcuterie this is called the primary bind. Because these soluble proteins are an excellent bonding substrate for RM, salting meat can dramatically increase bond strength. In some recipes, however, the combination of RM and salt can make food rubbery. You can avoid this problem by limiting the amounts of salt and/or RM. I can easily imagine salt and RM being used by Arby's® together to both season the meat and perfectly bond the pieces together.
When bonding a mixture like ground beef and ground lamb, add RM by weight. Typically, RM is used at .75% to 1% by weight of the mixture being bonded. 1 kilogram of mixture will require 10 grams of RM. One pound thus requires no more than 4.5 grams of RM. An exception is pure chicken breast, which is more difficult to bond and sometimes requires up to 2%, though leg meat will work at 1%. Adding water or water-based flavors (like wine) to mixtures in small quantities does not affect bond strength, but adding fat-based flavors (like cream) will weaken the bond.
After adding RM, remove any air pockets between the bonding surfaces. If you can, vacuum pack food items to assure a tight seal. For shapes like rolls and sausages, simply roll the products in an ample length of plastic wrap and twist the ends. The plastic wrap technique is great. It is simple, fast and cheap; foods can be cooked in water directly in the plastic wrap if it is sealed or tied off. Putting a weight on top of pieces to be glued is also a good and different technique from vacuum sealing or rolling in plastic wrap for having all inner surfaces touch.
Try to get all gluing and forming done within 20-30 minutes and then allow the product to remain undisturbed in the refrigerator for 4 hours or more, unless you are using the heat-set method described below. RM will bond pieces of flesh within 3 hours but bond strength will be substantially higher after 4 or 5 hours at refrigeration temperatures.
Highly acidic environments (below pH 4) cannot be glued effectively. For example, to make a RM ceviche, glue the seafood together before marinating in acidic lime juice. Most other marinades, however, won’t lower the pH of the proteins enough to inhibit bonding, allowing for post-marinade gluing. Thus, it is easy to imagine a flavorful marinade and a meat tenderizer being used for products like Arby's® roast beef prior to gluing/vacuum sealing and later baking in brine in a sealed bag.
RM, like other enzyme products, increases its activity as the temperature is raised ... to a point. The same bonding reaction that takes 4 hours in the refrigerator takes place in just 5 to 20 minutes at 50-58°C (122- 136°F). This technique is called heat setting. Typically you place items to be heat set vacuum sealed in a plastic bag in a 55°C water bath. 5 minutes after the core has reached temperature the meat is bonded.
The heat denaturing of an enzyme is time and temperature dependant. Enzymes are typically stable up to a given temperature, which for RM is around 40°C (104°F), above which the enzyme is slowly inactivated. As temperature is increased, inactivation becomes faster and faster. Although is takes many hours to inactivate RM at 45°C (113°F), 10 minutes at 60°C (140°F) will destroy 94% of RM.
So, what are you to conclude from this primer? In the spirit of Food Nirvana you should delight in finding ways to create at home what appears to be magic in commercial applications. Even better, this stuff is easy and inexpensive, and overall adds considerably to your perceptions of what you are capable of doing in your advanced kitchen.
Go for it! Experiment. Share what you learn. You now have all the essential information to use RM in making gyro meat, so try it. Then think about actually making nice beef roasts or even steaks from pieces of raw beef. Use your imagination.