Old recipe books and meal planning guides often speak of the "starch" in a general way, as a part of the meal along with the meat and two — or three — vegetables. Those starchy foods have a lot of similarities because they're all high in carbohydrates, but they don't behave the same way in a pan or on your plate. That's because there are actually two different forms of starch, and your foods cook differently when there's more of one or the other.
First, What Is Starch?
Starches are one of the basic molecules that make up most foods. Like every other carbohydrate, they're made up of smaller molecules of glucose, a simple sugar. Starches taste bland when raw rather than sweet because they don't break down into their basic sugars while you're chewing them. That doesn't happen until they're digested, when your gut breaks them back down into sugars your body can use for energy.
The Two Types of Starch
There are two types of starch that make up all starchy foods, in varying proportions. They're known to scientists and food science geeks as amylose and amylopectin. Like every other carbohydrate, they're made up of sugar molecules, bound together into big, complex molecules. The two starches have very different structures, which is why they behave differently in the kitchen. The explanation may seem a bit technical, especially if you found chemistry class difficult when you were at school, but it's actually pretty easy to visualize.
Understanding the Different Forms of Starch
If you just happened to have a scanning electron microscope gathering dust in your basement and you used it to look at amylose and amylopectin starch molecules, you'd see the differences between them in a heartbeat. Starch, by definition, is made up of sugar molecules, but how they're connected is crucial. In amylose molecules, the sugars are joined together in simple chains, while amylopectin molecules branch out into bushy tangles, with chains reaching in all directions.
The structure of the two molecules makes them behave differently. Amylose molecules pack neatly together, like sardines in a can, and form a relatively dense and compact structure. Amylopectin molecules are much larger and more helter-skelter, so foods full of those molecules have relatively lighter, fluffier textures and won't take as long to cook. Knowing which kind of starch molecule is most common in your food goes a long way in helping you understand how it should be handled.
Consider Your Pot of Pasta
For an illustration of the two different forms of starch in action, think about the last pot of pasta you boiled. Much of the starch was concentrated in the pasta dough, where it absorbed moisture and held its shape until it reached that stage we call al dente. At that point, it's tender enough to be pleasant, but firm enough to hold its shape.
That process is mostly the amylose molecules at work. The cooking water, in turn, has absorbed enough starch from the pasta to become milky and cloudy, and it's even used in some recipes as a thickener for the sauce. That's primarily caused by the amylopectin molecules.
Waxy vs. Floury Potatoes
For another practical example of the two types of starches, take a look at potatoes. Potatoes are one of the most versatile foods imaginable, from fluffy baked potatoes to hash browns, from potato salads to french fries, with thousands of stops in between. You've probably already noticed that some potatoes work better than others in any given recipe. When you want a fluffy baked potato, for example, you'll choose a russet or one of its close kin. For a potato that works well in salads or keeps its shape in soups, you might opt instead for a red potato or a fingerling.
These two broad types of potatoes are usually referred to as "waxy" and "floury" or "mealy." If you've guessed that it's their levels of the different kinds of starch that make them waxy or mealy, you're right. Waxy varieties tend to have more amylose, which gives them their dense, firm texture. Russets and other floury varieties have more starch overall and are richer in amylopectin, which helps them cook up light and dry when they're baked, steamed or boiled.
Two Types of Starch in Rices
Starches play a similar role in deciding which kinds of rice work best in a given dish. The long-grain rices used in most parts of India and China, for example, are rich in amylose and have relatively little amylopectin. They'll hold their shape nicely when cooked, and each forkful — if the rice is cooked properly — is light and fluffy, with distinct, separate grains.
The sticky or "glutinous" rice used in other cultures — it doesn't really contain any gluten — and the rice used in Italy's risotto and Spain's paella is higher in amylopectin. If you look at a grain of risotto rice, you'll see that it's pudgy and football-shaped. Much of that thickness comes from layers of amylopectin. When you cook the rice in plenty of broth, stirring as you go, the starch dissolves into the cooking liquid and thickens it to make a thick, creamy-tasting sauce.
Starches and Blood Sugars
There aren't many pure starches used in the kitchen, except for thickeners like cornstarch, so usually starch molecules come into your body along with a bunch of other food components, including fats, proteins, sugars and fiber. In general, your body turns sugars and starches into energy very quickly, while fats, proteins and fiber slow things down. That's a good thing, because fast-dissolving sugars and starches raise your blood sugar levels just as quickly, and this can lead to serious health risks including diabetes and metabolic syndrome.
Fiber is especially important for slowing down your absorption of starches, which is ironic because it's also a carbohydrate. Fiber molecules are even larger and more tangled than starch molecules, to the point that your body can't digest them at all. This slows the absorption of the parts you can digest.
It's like watching someone leisurely cleaning the last bits of meat from the bones of a rotisserie chicken: It's a lot slower than picking up a nice slice with your fork. When you get your starches in the form of high-fiber fruits, vegetables and whole grains, their impact on your blood glucose levels and your health is much less.
The Curious Character of Resistant Starches
One of the really interesting things about starches is what happens when they're cooked and cooled. When starches get to a certain heat, they expand and soften and absorb whatever moisture is available. That's what happens when you use flour or cornstarch to thicken a gravy, for example, and it works the same way in rice, pasta, potatoes and other starchy foods. The interesting thing is that when you cool those foods, the starch molecules re-crystallize with a different structure, which makes them slower to digest.
The cooked and cooled starches are called resistant starches, and they behave very differently in your system. They act just like soluble fiber, the really healthy kind that's found in oatmeal and beans. Resistant starches are sometimes referred to as "prebiotics," because they feed the beneficial bacteria that live in your gut. They're also known to slow the rise in your blood sugars and to help keep blood pressure in check. It's quite a party trick for the same starches that otherwise would behave very differently in your digestive system.
- Colorado State University: Dietary Polysaccharides
- International Journal of Research in Medical Sciences: Ratio of Amylose and Amylopectin as Indicators of Glycaemic Index and in Vitro Enzymatic Hydrolysisof Starches of Long, Medium and Short Grain Rice; Kavita Dipnaik, Praneeta Kokare
- BBC Tomorrow: The Surprisingly Complex Chemistry of the Humble Spud
- Johns Hopkins Patient Guide to Diabetes: Resistant Starches