What Is a Marinade? Mostly Surface Treatment Pretending to Be Deep

Meat is mostly water held in a dense protein structure, and molecules entering it must move through that, which happens by diffusion and is slow. Salt is the exception, being a small ion that penetrates several millimetres over hours and, more importantly, alters the proteins so they hold water during cooking, which is the mechanism that produces most of the improvement people attribute to a marinade. Acids penetrate poorly and act on the surface, where they denature proteins into a mushy layer if left long enough. Oils do not penetrate at all, since meat is aqueous and oil and water do not mix, though oil does carry fat-soluble aroma compounds and helps them stay on the surface. Most flavour molecules from herbs, spices, garlic and wine are far too large to travel more than a millimetre or two. The practical consequence is that a marinade seasons the outside and affects the surface texture and browning, and a long soak mostly achieves what a short one does.

A typical recipe combines several ingredients and their roles are quite distinct:

• •Salt, which penetrates, retains moisture during cooking and seasons beyond the surface, and is the single most effective component
• •Acid from vinegar, citrus, wine or yoghurt, which flavours the surface, promotes browning and, beyond a certain time, turns the outer layer soft and chalky rather than tender
• •Enzymes from pineapple, papaya, kiwi, fig and ginger, which genuinely break down protein and do so aggressively and unevenly, producing a mushy exterior long before the interior changes
• •Sugar, which promotes browning and burning, and which is why sweet marinades need lower cooking temperatures
• •Oil, which carries fat-soluble aromatics, aids heat transfer and prevents sticking
• •Dairy and yoghurt, which are mildly acidic, contain calcium that may activate enzymes in the meat itself, and coat the surface in a way that browns well, which is why yoghurt marinades work better than their acidity alone suggests

The belief that marinating tenderises is the most persistent error, and the actual determinants of tenderness are largely fixed before the meat reaches a kitchen. The amount and type of connective tissue depends on which muscle the cut came from and how much work it did, which is why a shoulder needs long slow cooking to convert collagen into gelatine and a loin does not. Ageing, meaning controlled storage after slaughter, allows natural enzymes in the meat to break down structure and is responsible for much of the difference between cheap and expensive beef. Cooking method and internal temperature matter enormously, since overcooking contracts proteins and expels water. Mechanical tenderising by pounding or by needling cuts fibres physically and works immediately. Slicing against the grain shortens the fibres the eater has to chew through and is the single easiest improvement available. Against all of that, an acid soak contributes very little except at the surface.

The practical advice that follows from the chemistry is straightforward. Salt is worth applying well in advance, hours or overnight, since that is the component that travels and the one that changes cooking behaviour. Aromatics are better applied shortly before cooking or after it, as a sauce or a dressing, since they were never going to penetrate and are frequently destroyed by extended contact and by heat. Acidic marinades should be limited to a few hours for most meats and considerably less for fish, where the acid begins to cook the surface within minutes, which is the principle behind ceviche. Enzyme-containing fruit should be treated with real caution. The surface should be dried before cooking, since a wet surface prevents browning. And reusing a marinade that has held raw meat requires boiling it first. None of this argues against marinating, and it does argue for treating it as surface seasoning rather than as a transformation of the whole piece.

Most components travel only a millimetre or two, so the treatment seasons the surface rather than the interior. Salt is the exception and does the real work by altering proteins so they hold water during cooking. Acids and fruit enzymes act on the outside and produce mush rather than tenderness if left too long. Tenderness comes from the cut, from ageing, from cooking temperature and from slicing against the grain.