Mostly a surface job
What Does a Marinade Actually Do?
A marinade flavours the outside and softens it, but it barely gets in at all.
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Step 1 of 5
Flavour molecules are big and slow.
The molecules that carry taste are far too large to travel through dense muscle. In a whole night they move only a few millimetres.
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Step 2 of 5
Salt is the exception.
Salt dissolves into water, splits into tiny particles and does move through meat, changing its proteins so they hold moisture better.
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Step 3 of 5
Acid tightens the surface rather than tenderising it.
Vinegar or lemon juice unravels proteins near the surface. A little makes it tender; too long and the outside turns chalky and dry.
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Step 4 of 5
Oil carries flavours that water cannot.
Many aromatic molecules dissolve in oil but not in water. The oil in a marinade is what picks them up and holds them against the food.
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Step 5 of 5
And it helps most in the pan.
The sugars and proteins in a marinade brown quickly, which is why marinated food develops colour and crust faster than plain food does.
A dry rub of salt and spices does most of the same work with none of the mess.
The short version
Marinades work on the surface: salt travels in, acid tenderises then toughens, and oil carries the flavours that water cannot.
Try it yourself
Cut a marinated piece of meat in half after cooking. The coloured layer is only a few millimetres deep.
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