Potatoes aren’t sweet at all. So how in the world did a potato like the sweet potato come to exist? Well, sweet potatoes aren’t really potatoes. Potatoes are members of the Nightshade family, while sweet potatoes are members of the Morning Glory family. So, they’re allowed to be sweet without owing any apologies to potatoes. What makes them sweet?
Well, like a potato, they contain most of their carbohydrates in the form of starch. Starch is a way for the plant to store sugar, by connecting saccharides together in long, branched complexes. Starches, unlike simple sugars, which have only one or two saccharides (sucrose or table sugar has two), do not taste sweet, but deliver a huge carbohydrate punch because our bodies can use digestive enzymes to break them down into simple sugars. Sweet potatoes themselves have a lot of enzyme systems that are responsible for various functions, including degradative ones, within the root.
One of these enzymes is the same kind of enzyme that we use to digest starches: Amylase. In fact, sweet potatoes have so much amylase, in the form of alpha- and beta-amylase, that they are used to produce commercial preparations of the enzyme. All of the commercial production of beta-amylase in the U.S. comes from sweet potatoes. These amylase enzymes break down a type of starch called amylose. Makes sense, right? The enzyme that breaks down amylose is called amylase.
You will notice that different sweet potatoes have different degrees of sweetness when raw. Some of them are hardly sweet or not sweet at all, while others can be fairly and pleasantly sweet even before cooking. This has to do with the cultivar (variety), and the levels and activities of the amylase enzymes during storage. The way the sweet potato is stored, the temperature, etc., can also influence this. There are other enzymes, as well, that can influence the taste by affecting mouth-feel.
Now, all this enzyme activity, being degradative, means that while the potatoes might get a bit sweeter during storage, other not so desirable things can happen as well. Once they start going bad, they go bad very quickly and you really can’t save a sweet potato once it seems like it has begun to rot. Just because you cut off the bad part, the rest of the flesh will be wonky and texture and taste will be bad. So don’t bother. Store them in a dry place at about 50 to 60°F, by the way. And, like actual potatoes, never wash them until you are about to use them.
But of course, once you cook a good sweet potato, even if it’s not so sweet to begin with, it gets much sweeter. This is because the amylase is more active at higher temperatures, so when the potato starts cooking. the amylose starch really starts breaking down much faster into a simple sugar called maltose.
You can influence the sweetness of the final result with the way you cook the sweet potato. The faster you cook it, the less sweet it will be. The faster the cooking time the less time the enzymes have to break down the starches. For the sweetest taste, you want to bake them in the oven.
Although you’d have to taste it to know how sweet a particular sweet potato is before you cook it, there are a couple rules of thumb. There are two basic types. One has a pale yellow skin and flesh, and the other has a dark orange or red skin with a deep orange flesh. The darker orange ones tend to be a little sweeter than the pale ones. Keep in mind that within these varieties the colors can vary from an almost white to a deep orange or red. There are even some with a purplish skin. Still, all sweet potatoes are sweet once you cook them.