In very general terms, fat is a storage system. It allows animals to lock away energy for later use, much as plants lock energy away in the form of starch. Of course plants make some fat too, though it’s usually in liquid form — oil. Animals make more fat because it’s a denser storage medium, and that’s better if you happen to have to move around (chase prey or, conversely, run away from predators). You can store 4 calories (kcals for you Europeans) in a gram of starch, bur more than twice as many (9 calories) in a gram of fat.
Chemically speaking, fats and oils are triglycerides, sub-members of the extremely broad lipid family of molecules. Triglycerides, as the name implies, are made up of a trio of fatty acids, all connected to a “backbone” of glycerol. They are all more or less “E”-shaped, though since the type and configurations of the fatty acid molecules they contain are highly variable, different triglycerides can behave in very different ways. One factor that greatly influences the behavior of a triglyceride is its degree of saturation (for a discussion of that, go here).
At this juncture it’s important to note that the things we know specifically as fats or oils can contain many different types of triglyceride molecules and scores of different fatty acids, some of them saturated and some of them mono- and polyunsaturated . Thus a specific fat — like lard, for instance — can’t fairly be characterized as “saturated” or “unsaturated” in and of itself. Rather it is “high” in saturated fatty acids. Olive oil, by comparison, is high in monounsaturated fatty acids. Make sense?
The utility of fats in the kitchen hardly needs explaining, but because I’m by nature long-winded, I’ll explain anyway. Firstly, fats are outstanding cooking mediums. Being thick and viscous they cling to the outsides of foods, and because they have very high boiling points, they’re great for transferring heat and creating browning reactions that make foods extra-delicious.
Next, they’re outstanding carriers of flavor. I mentioned before that fats are members of the very large lipid family of molecules. That’s handy because it just so happens that lipid molecules (from pigments to vitamins, gums, waxes and a great many flavor-containing substances) abound in the foods we eat. Since similar molecules tend to dissolve in one another, fats allow these lipid molecules to disperse, either in the pan or in our mouths. So instead of tumbling across our tongues in clumps where they hardly register on our senses, they spread out, sliding languorously across our taste buds, lighting up our flavor receptors like fireflies on a summer evening. They result is, by and large, quite pleasurable.
For bakers and pastry makers, fats are especially valuable. They tenderize breads, cakes and crusts by interfering with the development of tough and chewy gluten networks. In a similar way they help stave off the effects of staling, thus extending the shelf life of various breads and sweets. Indeed it’s safe to say that without fat the world of baking would be a rather dull place. Indeed I don’t think I’m going too far to say that without fat, pastry itself would be impossible.