What it actually is
In muscle, actin and myosin are bound together as actomyosin and neither can do much. Salt raises ionic strength and phosphate dissociates the complex, which frees myosin to dissolve. Dissolved myosin is what forms the tacky exudate on the surface of tumbled meat, the film around a fat droplet, and the gel that sets on cooking.
Why it matters
Almost everything a meat processor measures depends on it. Bind, slice integrity, emulsion stability and yield are all downstream of how much protein was extracted. A product that will not hold together has usually failed here rather than later.
What changes it
Salt level, phosphate, pH, and mechanical action: tumbling, massaging or chopping. In whole muscle there is no chopping step, so the mechanical work has to come from the tumbler, which is why tumbling time is a formulation variable and not just a schedule.
