Shortening
Solid fat that is essentially all fat — no water, a high melting point, and behaviour no butter can quite reproduce.
Also called vegetable shortening (US); white fat (UK); Trex (UK) — A common UK brand name used generically.
Shortening is hydrogenated or otherwise solidified vegetable fat, and its defining property is what it lacks: water. Butter is roughly a sixth water; shortening is essentially none.
That difference explains most of what it does. No water means no steam contribution, so pastry made with it is short and tender rather than flaky. It also means no water available to hydrate flour proteins, so gluten development is restrained more effectively than butter restrains it — the shortening effect the ingredient is named for.
Its higher melting point is the second property. It stays solid at temperatures where butter has already softened, so cookies made with it hold their shape and spread less, and pastry made with it is more forgiving in a warm kitchen. What it does not do is taste of anything, which is why many bakers use a mixture.
Composition and specification
Water content, against butter
Butter contains a meaningful proportion of water; shortening contains essentially none. Substituting one for the other by weight therefore changes both the fat content and the liquid content of a formulation at the same time.
RecordedHarold McGee, Scribner
What changes if it is left out
Replacing shortening with butter adds water and flavour and lowers the melting point: expect more spread, more browning, a less tender crumb and a better taste.
Follows from the composition difference rather than from a measured comparison.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Gluten inhibition
Named for exactly this. With essentially no water, it restrains gluten more effectively than butter does.
Can stand in for
Adds no water and melts higher: less spread, less browning, more tenderness and no flavour. Many bakers use a mixture for exactly that reason.
The science behind it
Fat plasticity and crystal structure
Engineered for a wide plastic range, which is the property it is bought for and the one a recipe is relying on when it specifies shortening rather than butter.
Can be replaced by
The closest pair in the catalogue: both are plastic, both are essentially water-free, and both shorten strongly. Lard brings a savoury flavour that shortening does not and is not vegetarian, which is the reason the substitution usually runs in the other direction.
Compared in
Named for the effect it has rather than for what it is made of, which is the clue to when it is the right choice.
One side of the comparison.