Shortening is pure vegetable oil, hydrogenated to give it a firm texture. It’s different from margarine (another hydrogenated fat) in that it was not created to be a substitute for butter, but rather as a substitute for animal fat, specifically lard. How does it stack up? Rather well. Like lard it’s all fat with no water in it. That means it’s great for things like biscuits and pie doughs, which lose some of their crispiness and flake with butter because of the water butter contains.
Unlike lard, however, it’s completely neutral in flavor. It’s also quite a bit less expensive. It also melts at a much higher temperature, around 118 degrees Fahrenheit, which is a good thing for things like cookies and (American) biscuits, since you get a lot less spread. And did I mention it keeps indefinitely at room temperature? No wonder that the emergence of shortening in the very early 1900’s coincided with a steep drop-off in the popularity of lard.
Like margarine, the chief argument against it lately has been that it contains trans fats. In response to that criticism the food industry has developed a version without trans fats. This has been no easy feat, for as you may recall from the post on margarine, the process of hydrogenation only has two gears: full hydrogenation (which yields a fat that’s as rigid as ice) and partial hydrogenation (which creates at least some trans fats). The industry’s solution: to blend fully hydrogenated oil with liquid vegetable oil. It’s not a terribly easy thing to do, and produces a not terribly convincing fat, at least from a baking perspective. My own fiddlings with trans-free shortenings have been only mildly satisfactory. The biscuits don’t rise as high, aren’t as crispy and seem to me to have a different taste. Since I’m not worried about trans fats at all, I’ll skip it and go with the regular stuff, me.
In breads and other baked goods, shortening, like all fats, undermines gluten networks and hence the rise of the breads in question. Breads with a high proportion of fat are therefore shorter than their fat-free counterparts. Hence the word “shortening.”