Why do recipes say to salt aubergine or cucumber before cooking?
To draw out water by osmosis — and for aubergine, the traditional reason given is now largely obsolete, while the real benefits are different from the ones usually stated.
The mechanism. Salt on the surface creates a high concentration outside the cells. Water moves across cell membranes from low concentration to high, so moisture is drawn out and pools on the surface. The process also collapses some cell structure, which matters more than the water loss itself.
The traditional reason for aubergine: bitterness. Older varieties were genuinely bitter, and salting leached some of the bitter compounds out with the water. Modern cultivated aubergines have been bred to remove that bitterness almost entirely, so this reason no longer applies to what you buy — which is why many contemporary cooks skip the step and notice nothing.
The reason that still holds: oil absorption. Aubergine flesh is remarkably spongy, full of air pockets. Dropped into hot oil, it absorbs an enormous quantity almost instantly — far more than any other vegetable. Salting collapses those air pockets, so there is less space for oil to occupy, and the result is less greasy and browns better.
The same effect is achieved by microwaving briefly or by pre-roasting, both of which also collapse the structure without the salting time.
For cucumber, the reason is straightforward. Cucumber is about 96% water. In a salad with dressing or in a sandwich, that water leaches out and dilutes everything, producing a watery result within minutes. Salting and draining first removes much of it, concentrates the flavour, and improves the texture.
The same applies to courgette before making fritters, where excess water prevents binding and browning, and to cabbage for slaw.
How to do it: slice or cube, salt generously, leave in a colander for 30 minutes to an hour, then rinse — the salting is far heavier than seasoning — and pat thoroughly dry. Drying matters as much as the salting, since a wet surface steams rather than browns.
When to skip it: any preparation where the aubergine is grilled, roasted or braised rather than fried in a lot of oil.