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 is small enough to go where the taste molecules never will. 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.