As a method, equilibrium curing differs from the more traditional methods of packing the meat in salt (the Salt Box Method) or submerging it in a concentrated saline solution (Brining). While traditional methods have the virtue of diffusing a lot of salt (more accurately, sodium ions) into the meat quickly (and thus accelerating curing and preventing spoiling), equilibrium curing has the virtue of eliminating the risk of the cured meat becoming too salty. Thus the equilibrium curing method is less wasteful of curing salts can be particularly useful for beginners.
This method is quite straightforward, as long as you know the weight of the meat in the item you're curing and the desired concentration of salt in the final product. When calculating the weight of the meat, you will need to subtract the weight of any bones, which will not absorb salt. (In the case of whole poultry, this can be significant, with bones accounting for up to 40% of total weight of the bird.) To the weight of the meat, add that of the water used (if brining), and then calculate the amount of salt you need. The metric system makes this particularly easy. For example, say you want a final salt concentration in the cured meat of 1.75%. Your meat weighs 0.5kg and you're using 0.5kg (500ml) of water, for 1kg total weight. 1.75% of 1000g is simply 17.5g of salt you will need to dissolve in the water. To be strictly accurate, you would also need to take the weight of the salt into account and add an extra 0.31g to compensate, but in practical terms this is insignificant, as, in the example given, it only lowers the final sodium level by 1.77% for a final concentration in the meat of 1.72% instead of the target of 1.75%. Perhaps a simpler way to do this is to reduce the amount of water in the brine, so that the weight of the water PLUS the cure equals the target weight for the brine.
At first, all this sodium will be in the water and not in the meat, but it will gradually diffuse through the meat until the salt concentration in the meat and the water are the same or in "equilibrium." No matter how long the meat sits in the brine, it cannot become saltier than it, and so there is no risk of over-salting.
Equilibrium brining can be done in a container or in a vacuum-sealed bag. In either case, it's desirable to agitate the water occasionally, in order to keep the sodium evenly dispersed. If vacuum-sealing, you can even eliminate the added water. The desired amount of salt is simply applied to all surfaces of the meat (for 500g of meat, 8.75g of salt will yield a concentration of 1.75%) and vacuum-sealed with it. The salt will extract moisture from the meat and create a concentrated brine around it.
Modified Equilibrium Curing
The rate at which sodium ions are absorbed is not linear, but follows a curve, in which the cure enters the meat quite quickly initially and becomes slower over time, taking awhile to reach true equilibrium. A limited test of this method by individuals on the sausagemaking forum found that 50% of the desired amount of sodium had been taken up by the meat in just a few days while full equilibrium was unlikely to be reached in less than about 19 days per kg of meat. As a result of these findings, they recommended a modified equilibrium curing method that significantly reduces the curing time.
Since their test indicated that 85% of equilibrium was reached in about 10 days, they recommend increasing the initial amount of the cure by 17.5% in order to reach the desired sodium concentration in the meat in 10 days or half the time it would take for a true equilibrium cure. Using this modified method, it's important to note that the sodium will not be evenly distributed throughout the meat (as it would if it had reached true equilibrium), and so the meat will still need a resting period after curing to allow the sodium to equalize.
Notes and Limitations
If a whole muscle is going to be dried after curing, then the moisture loss during drying has to be taken into account when calculating the salt. For example, if a final salt concentration of 3% is desired, and the meat weighs 1kg, but will eventually be hung to dry until it loses 40% of its weight, ie achieves a final weight of 600g, then the amount of salt needed is 3 X 6 or 18g (assuming no added water).
It is also important to note that vacuum-packing meat with salt is not necessarily the same as equilibrium curing. Unless the meat stays in the cure long enough to reach equilibrium with the brine, this is simply a modified version of the Salt Box Method.
Finally, it's important to understand that the equilibrium is between the meat and the surrounding brine and does not refer to the diffusion of sodium within the meat, although in practice it comes to the same thing. By the time the meat reaches equilibrium with the brine, sodium will be relatively evenly dispersed throughout the meat. In contrast, with traditional cures, the meat will come out of the cure sooner, but then it will still need a resting period to allow the salt levels to equalize throughout.
Nathan Myhrvold et al., Modernist Cuisine