© Denzil Green
Shortening is a solid white fat product made from vegetable oils, using the technique known as hydrogenation discovered at the beginning of the 1900s.
The word is older than that, though. At the beginning of the 1800s, "shortening" was a generic term used to describe fats or oils added to baked goods to "shorten" or break up gluten masses, weakening the gluten structure to produce a more tender result. Thus, shortbread, made with flour which has gluten in it, has a mass of butter added to "shorten" the product and make tender shortbread cookies.
Shortening certainly acts like this in pie crusts. Its role in cake batters, however, is a bit different. Part of the reason for creaming butter with sugar is that the sugar crystals cut into the butter allowing air in and air cells to be created in the butter. Well, commercial Shortening is like creamed butter without the sugar, as it already has the air cells in it. It further tenderizes a cake by separating starch in the cake from coagulating protein -- not to mention that the added fat makes the cake taste moister and smoother.
Hydrogenation helps make shortening a solid, flavourless fat product that will keep its form regardless of room temperature, and not puddle or pool like butter or other animal fats do on hot days or in warm rooms. The disadvantage is a health one: the process creates trans-fatty acids and saturated fat which is bad, and destroys the normal polyunsaturate benefits of the vegetable oil that it started from.
Many people in the UK are confused by the term "Shortening" and don't think it is available in the UK. In fact, in the UK shortening is available under brand names such as Cookeen, Trex, White Flora, etc.
In France, the predominate brand, which is called "Végétaline", is made from hydrogenated coconut oil.
Unless a recipe says otherwise, you should use Shortening cold -- such as in crusts, biscuits, dumplings, etc.
Instead of Shortening, you can use any vegetable oil for times when a liquid fat is in order, such as frying, or use butter, margarine, lard or dripping, depending on what you are doing.
Crisco 1911 packaging
[Ed: some sources feel that Snowdrift Shortening (made from cotton-seed oil purified in the process discovered by David Wesson in 1899) was the first all-vegetable-oil shortening, but they do not give a year to support their claim.]
Many budget-conscious households came to use Shortening a great deal in their baking as it was more economical than butter. This was particularly true in areas where another alternative, margarine, was either banned or subject to prohibitive taxes.