What does dry-ageing actually do to beef?
Three things simultaneously — enzymatic tenderising, moisture loss and flavour development — and only the first happens in wet-ageing, which is why the two produce genuinely different results.
What is happening:
Enzymatic breakdown. Naturally present enzymes, principally calpains and cathepsins, break down muscle proteins and connective tissue over weeks. This is the tenderising, and it happens in any ageing, wet or dry.
Moisture loss. In a controlled environment — cold, specific humidity, moving air — the surface dries and forms a hard crust, or pellicle, while the interior loses water. This concentrates flavour, because the same flavour compounds occupy less mass.
Enzymatic and microbial flavour development. Fats oxidise and proteins break into amino acids, producing the distinctive nutty, buttery, sometimes blue-cheese notes that dry-ageing is prized for. Specific moulds are encouraged on the surface and contribute.
Wet-ageing, by contrast, holds the meat vacuum-sealed in its own juices. Tenderising occurs; concentration and flavour development do not — there is nowhere for moisture to go and no oxygen for oxidation. It is cheaper, loses no weight, and produces a milder, sometimes slightly metallic result.
Why dry-ageing is expensive, and the reasons are all about loss:
Weight loss of a substantial proportion through evaporation.
Trim loss, since the dried crust and any mould must be cut away and discarded.
Time and space, occupying controlled refrigeration for weeks.
Risk, since the process can go wrong.
The timescales. Meaningful tenderising is largely complete within a few weeks; flavour development continues far longer, with 28 to 45 days common and much longer ageing producing intense, polarising results.
It requires the right cut. A fat cap and bone protect the meat, and the loss makes it viable only on primal cuts.
What it is not. Not spoilage, and not the same as meat simply left in a domestic fridge — the humidity and airflow control are what separate ageing from rotting, and attempting it at home without them is genuinely unsafe.