In the last few posts, we discussed starch’s different roles in baked goods and other desserts. But some starches are better than others for certain applications. Cornstarch, for example, is useful for gelling custard pie fillings, but if we refrigerate a cornstarch fruit filling, it gets cloudy. Tapioca starch, on the other hand, won’t gel into a solid you can slice, but it remains clear if refrigerated. In this post, we’ll explore the molecular differences between different starches and their consequent effects in our desserts.
Starches differ in several ways.
As we alluded to in the introduction to starch, starch granules vary in size and shape depending on the plant they’re from. They’re also processed differently. Ultimately, starches differ in factors such as the temperature at which their granules swell and their thickening power, which affects how we cook the starch and the amount we use. Root starches, for example, tend to swell at a lower temperature than cereal starches. Although the list of differences between starches seems daunting, a lot of them can be predicted by the proportion of amylose to amylopectin in the starch.
Starches can be broadly classified based on their composition
Remember from the introduction that starches are long chains of glucose sugar links, and the chains can be straight or branched. The straight chains are called amylose, and the branched chains are amylopectin. Starch granules can contain 1 to 28% amylose (with the remaining starch as amylopectin). Most cereal starches (flour, cornstarch) are high in amylose, while root starches (potato starch, tapioca starch, arrowroot starch) contain less amylose and more amylopectin.
Texture in rice
To illustrate the differences in texture between these two types of starch, we can compare varieties of rice. Long-grain jasmine or basmati rice, which you get at an Indian restaurant, has a high percentage of amylose. The cooked grains are fairly loose and do not stick. In contrast, short-grain glutinous (or sticky) rice, which you find in a mango sticky rice dessert at a Thai restaurant or in a savory lo mai gai at a Cantonese dim sum restaurant, is low in amylose and high in amylopectin. When you unwrap lo mai gai from its leaf, the rice stays in a cake exactly in the shape it was wrapped. True to their name, the cooked grains stick—to each other, to the table, to your hands—because of the amylopectin. The rice you might get with Chinese takeout has a medium level of amylopectin. As a result, its rice grains hold together, but they are not as sticky as sticky rice.
In the kitchen, we can choose starches for different applications based on the starch’s behavior. Mochi, for example, is traditionally made from cooked glutinous rice pounded into a smooth dough. But today, many recipes simply cook glutinous rice flour in water to obtain a similar consistency. Other types of mochi use tapioca, sweet potato, or arrowroot starch to create different flavors and textures. Because all of these recipes use starches with more amylopectin, they create chewy, stretchy mochi. If we were to try the same process with a starch low in amylopectin, such as cornstarch, the texture would not be the same. It would probably gel into a solid that we can slice.
Cereal starches (high amylose)
This difference in texture arises from the different structures of amylose and amylopectin. As we discussed in the introduction, cooked starch molecules want to regain an organized structure as they cool. Although starch gelatinization is irreversible, the starch molecules can realign to an extent. Because amylose is a straight chain, it’s more likely to form a tight, orderly structure, both in the starch granule and after it gelatinizes. This leads to several differences in high-amylose mixtures.
First, raw high-amylose starches need more heat and water to gelatinize. And not only do high-amylose starches thicken, but at high enough concentrations, they gel into a solid. Cream pie fillings, for example, often contain cornstarch. The compact amylose molecules also block light, creating cloudy fillings you might notice in a homestyle apple pie filling made with flour. This isn’t a problem in custards and pudding, which are already opaque from ingredients like milk and cream, but it might be undesirable for a strawberry pie where we want the color of the fruit to shine through. Furthermore, the colder the mixture, the more the amylose molecules draw together. This is why custard pie fillings thicken as they cool to room temperature. They also weep and tighten in the refrigerator or freezer because the amylose molecules squeeze water out as they draw together. Cereal starches like cornstarch and flour also add a flavor of their own and contribute a heavy body.
Root starches (medium amylose)
Root starches, on the other hand, do not contribute much flavor. Because they contain less amylose, they cannot gel, but low levels of amylose can also be advantageous. Root starches remain clear, which is often preferable for fruit pie fillings. They are also less prone to weeping, making them more stable through refrigeration or freezing. However, some root starches, such as tapioca and arrowroot starch, can become stringy, especially if they’re overcooked. While this is desirable for mochi, it’s unappealing in fruit pies. Root starches also thin if they’re reheated, so if we use a root starch to thicken a fruit pie filling, we should not serve it hot unless the filling also contains a high-amylose starch.
All starches gelatinize and thicken, but their individual characteristics vary depending on the size and shape of their granules, their processing, and their proportion of amylose to amylopectin. Recipe developers use the combination of starches they need to achieve the texture and appearance they want, but with a better understanding of how starches behave, we can also modify recipes to suit our own needs and pantries.
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