Science and tips
The difference between brining and marinating is that one uses salt and the other acidity for its active ingredient. They are part of a spectrum though, and it is not unusual to combine these actions. Seawater (brine) is aqua marina, and the word marinade derives from marina, further clouding the distinction They can also both contain flavouring, but this is more common for marinades.
In my view, marinating isn’t worth the trouble, at least not for flavouring the interior of meat — most flavour molecules don’t penetrate much more than a few millimetres from the surface, even with long marination.
So, if the marinade flavours don’t penetrate, but stay on the surface, why not just make a nicely constructed sauce to serve with the meat instead?
Having expressed my marinade-skepticism, explanations and observations on marinating follow. The acidity of marinades still serve a tenderising purpose.
Acids are characterised by easily giving up a hydrogen ion (positively charged), and acidity is measured on a scale called pH (partial Hydrogen-ion concentration) that runs from 0 to 14. Distilled water sits in the middle of the scale (7, neutral), anything acidic is <7, and anything alkaline >7. Meat has a pH of around 5.5, and so it is acidic. Living muscle has a neutral pH of 7. The process of turning muscle into meat leads to a decline in pH. Meat Standards Australia assigns a high importance to pH for its grading system. Too high a pH can indicate that the animal was stressed before slaughter.
The meat pH of 5.5 is a kind of set-point that maintains the condition of meat proteins. The aim of a marinade is to lower or raise this pH value. The change in the positively-charged hydrogen ion concentration establishes weak electrostatic forces on the surface of meat filaments that pushes them slightly apart and allows water to enter. As with brines, acids will also unravel (denature) or break up/dissolve proteins. Thus while the mechanism is different, the effect is similar to a brine.
If taken to an extreme, acidity can break up and denature sufficient protein to give a cooked-like effect. This is the aim of ceviche. Typically fish responds best to this approach because of its more open texture.
If left too long in an acidic marinade, the protein begins to dissolve and the meat goes mushy. Thus a long marinade is not necessarily a good marinade.
Acidic dairy marinades (yoghurt or buttermilk) seem to have additional benefits because of calcium availability. The mechanisms are not certain, but calcium can activate natural enzymes in the meat that will help break down proteins.
Wine/beer marinades are also acidic, and have their effect in that way. The alcohol is not beneficial (in a marinade at least) and will actually dry-out the meat (it’s a preservative). If using, it is best to simmer the marinade to reduce alcohol concentration.
Other acidic ingredients (lemon juice, vinegar, fruit juices etc) are also employed to lower pH. But raising pH (making the marinade more alkaline) also works. Baking soda is used in some Asian marinades for this purpose. It has the advantage that the maillard (browning) reactions proceed faster under alkaline conditions, so flavour may be improved when the meat is cooked.
After deciding on the active ingredient, the next step is usually to add sugar, oil and flavourings. Sugar is too large to penetrate the meat, but any absorbed on the surface will also promote the maillard reaction and help browning when cooking. Oil will not penetrate either (remember that meat is mostly water, and oil and water don’t mix). It can accumulate in surface fissures where it might be helpful in browning. Finally, flavour molecules are too large and unwieldy to get between meat filaments and will remain on the surface.
Some fruit juices (pineapple, kiwi and papaya) and root ginger contain enzymes that split proteins apart. This can be used as an active ingredient in place of, or with, acidity but if not done with care it can turn the surface mushy too. The enzyme in fresh pineapple (bromelain) can actually break down proteins inside our mouth and on our tongue, contributing to some of the mouthfeel of this fruit. Canned or bottled juice will have no effect because it is pasteurised and enzymes are easily destroyed by heat.
Marinades can be injected into the meat, or the meat gashed, increasing their distribution. For small pieces of meat (e.g. cubed chicken) marinade penetration can be increased under pressure — place the meat in a whipping siphon with its marinade and charge with nitrous oxide or carbon dioxide. Cooking meat in a pressure cooker in the presence of flavouring has a similar effect, briefly marinating under pressure while being cooked.