Why brine meat proteins? To slightly season, to increase water retention during cooking and to plump dry meats.
In high doses, salt draws moisture out of meat and is perhaps the oldest means of preservation.
However, at low doses, salt can do the opposite and increase the water-holding capacity of meat.
Salt is a mineral (halite), not a rock as implied by the term ‘rock salt’, and not a spice either. Pedantically, when salt is dissolved in water it is no longer salt, but a solution of sodium ions (positively-charged) and chloride ions (negatively-charged). The sodium ions underlie our perception of ‘salty’ because they activate sodium channels in salt-taste receptors that trigger a signal to the brain.
At the start of brining, salt ions move into the meat (lower salinity region). This is an equilibrium-driven process. The inward movement of salt occurs by diffusion, much the same way as heat diffuses into a roast in the oven from the outer layers to the core. Ion diffusion is very much slower though, which is why you can roast in a few hours but may need to brine for a few days.
As salt ions enter the meat, they position themselves on the surface of muscle filaments in such a way that the surfaces acquire a net charge. This creates a weak electrostatic repulsion between adjacent filaments that drives them slightly apart and allows water molecules to enter the gaps.
As well, the presence of an increased salinity can denature muscle proteins and cause them to unravel or partially break down. This has two effects: the unraveled proteins can trap water much as a gel does, holding on to the water better; and the broken-down proteins will not tighten as strongly when heated and so will not squeeze out as much moisture when cooked.
The net result of low-salinity brining is that water has been drawn into the meat, water will be held there better during cooking, it won’t be so easily squeezed out and the meat has been lightly seasoned.
Brining is most effective in modern lean meats such as chicken breast, low-fat pork and seafood. Although there are always exceptions — pork belly is one.
A salinity to aim for is just a few percent (0.5% is about right for seafood, around 1–1.5% for white meats). Over 2% and the meat will start to taste over-seasoned. From about 5% onwards, a different process (osmosis) takes over, and moisture is drawn out of the meat (as in bacon, hams etc).
There are two main brining strategies for the home — gradient brining and equilibrium brining. They can both be accelerated by injection brining.
This method uses a strong brine (5–10%) but only leaves the meat in it a little while and then it is rinsed and rested to allow diffusion of the salt ions throughout the meat. It’s like roasting followed by resting — oven temperatures are much higher than the target meat temperature and resting equilibrates heat. There is a lot of guesswork involved. Meat thickness matters. It is best to follow a reliable recipe.
Sometimes, a surface-brine is all that is wanted. This is so for seafood, which benefits from a brief 10% brine for 5 minutes. The seafood needn’t be rinsed first. It is quick, it seasons the surface slightly, inhibits surface microbes and the high salt concentration denatures surface proteins and stops them coagulating as white ooze when cooked.
Dry, salted rubs are a kind of gradient brining. Although they will not draw water into the meat, they will help the meat retain its natural water when cooked. Some argue that rubs are advantageous because they don’t dilute the meat flavour with additional water. Others argue that the additional juiciness from wet brines increases the perception of flavour.
This is more like sous vide. The meat is weighed, and an equal weight of water is measured out. The salinity of this water is set to be twice the final desired meat salinity. When the meat is brined in this solution, and equilibrium is reached, the water and the meat will have the same salinity, which will be half the starting salinity of the brine.
You can use any quantity of water, but making water weight = meat weight makes the salt calculation (doubling) easier. The advantage of this method is that the meat will never get over salted (just like food cooked sous vide can never get hotter than the surrounding water) — once equilibrium is reached, salt diffusion is stabilised.
Salt can be added from other sources, for example soy sauce. If soy is 20% by weight salt, then adding 5% of soy to water is equivalent to adding 1% of salt.
Flavours can be added to a brine, but be aware that larger flavour molecules will not be able to diffuse into the meat. The gaps provided by electrostatic repulsion are very small. Nothing much bigger than a water molecule will get in.
Diffusion is slow, especially for thick cuts or a whole chicken (brining times go up as the square of meat thickness). One way to speed it up is to inject the brine into the meat. A meat injector will do the job, or a large surgical syringe. The meat is then placed in its brine for diffusion to continue.
Be aware that meat may have already been brined or tenderised in some way before sale. The benefits of brining mean that consumers are more satisfied with the product, but it adds water to the meat which the consumer pays for at meat prices. Brining can be achieved fairly quickly by banks of fine needle injectors. Tenderising may be achieved without brining, such as with hydrodynamic shockwaves (underwater sonic-booms that travel through the meat disrupting fibres). Mechanical tenderising is also achieved by blade tenderising (jaccarding). Enzymatic tenderising occurs with ageing, or by injection of microbial or fruit derived proteolytic enzymes into the bloodstream after slaughter (the problem is to control when they stop breaking down proteins). Any such pre-processing is usually (except perhaps for ageing) invisible to the consumer at point of sale.
I’ve been discussing brining of meat, but of course vegetables can be brined. They can also be marinated. The terms are not usually separately applied to vegetables, and go by the composite term — pickling.