The Three Parts of a Marinade: Acid, Fat, Aromatics
A good marinade balances three things: an acid (lemon juice, vinegar, yoghurt), a fat (olive oil), and aromatics (garlic, spices, fresh herbs). The acid loosens the proteins on the meat's surface and helps flavour work its way in; the fat carries the aromatic compounds and slows the meat from drying out as it cooks.
Salt is also an important part of a marinade, but it is usually added last; added too early, it can pull moisture out of the meat ahead of time, which can leave it tougher.
Practical Tip
Shaking the marinade in a sealed bag instead of whisking it in a bowl means less washing up, and makes it easier for every surface of the meat to touch the marinade.
Don't Overdo the Acid
If a marinade is too acidic — say, just lemon juice and vinegar — the meat's surface turns a whitish, seemingly cooked texture; this is a chemical cooking effect and toughens the texture in an unwanted way. Acid should not make up more than about a quarter of the total marinade liquid.
Recipes where raw fish is "cooked" by lemon juice, like ceviche, are the exception; that chemical effect is the whole point there, but the technique needs specific know-how and very fresh, trustworthy fish.
A marinade is not a shortcut, it's a waiting period. Too much acid does not speed things up — it only ruins the texture.
Marinating Time by Ingredient
Fish and seafood are delicate, so they should not sit in a marinade long — 15-30 minutes is enough; leave them longer in an acidic mixture and the texture starts to fall apart. Chicken breast can take 2-4 hours, and red meat, being firmer, can go 4-12 hours, or even a whole roast marinated overnight.
Vegetables (especially zucchini, aubergine and peppers meant for the grill) do not need long — 30 minutes to an hour is enough, since a longer soak draws out their liquid and softens them too much.
Don't Forget Safety While Marinating
Raw meat or chicken should marinate in the fridge in a covered container; a marinade left at room temperature for more than 2 hours creates conditions bacteria can grow in.
The liquid raw meat or chicken marinated in should never be poured straight over cooked food as a sauce; if you want to use it, boil it first and keep it at a rolling boil for at least a few minutes.
Make the Most of Leftover Marinade
If you set some of the marinade aside before it touches the raw meat, you can brush it on during cooking for extra flavour — but that portion must never have come into contact with the raw meat. Marinade that has touched raw meat can only be used after boiling, as described above.