As bakers, when we think of starch, we often think of its function as a thickener in custards and sauces. Although starch is indispensable for pie fillings and pastry cream, it also plays a role in the pie crust and choux that hold them. Starch is as important as gluten for structure and texture in baked goods. It feeds yeast and interacts with proteins, sugar, and fats. We use it to form thin, crisp crusts on bread, chewy crusts on bagels, and tall shells of choux pastry. So let’s dive into the science of starch, starting with a description of what it is and an understanding of how it interacts with heat and water.
Starch is a carbohydrate.
We briefly mentioned starch in our discussion about sugar. Since most of the chemical properties of sugars also apply to starches, it might help to review the introduction to sugar. Both starches and sugars are carbohydrates. Carbohydrates are chains of individual links called saccharides, which have a specific ratio of carbon, hydrogen, and oxygen atoms. While sugars only have one or two saccharides, starches contain thousands to millions of saccharides linked together. Starches are specifically made of one saccharide: glucose. (Glucose is also found in granulated sugar, corn syrup, and lactose in milk, either on its own or linked to another saccharide.) These glucose units can be arranged linearly to form the starch amylose, or in branched chains to form the starch amylopectin.
Starch granules store energy.
Glucose is an important source of energy. Yeast in bread dough metabolize glucose to survive and reproduce. So do most living organisms, including us humans, which is why carbohydrates are such an important part of our diet. To keep a reliable source of glucose ready, plants and animals link up glucose units when they photosynthesize or eat and store them away. When their cells need energy, they grab individual glucose molecules off the larger chain. In animals, glucose is stored as glycogen. In plants, it’s stored as starch.
Plants pack starch into granules, which are compact grains of both the linear amylose and the branched amylopectin starch molecules. The exact ratio of amylose to amylopectin and the size and shape of the granule vary depending on the plant. But as plants create more starch, they add it around the outside of the granule, building layer upon layer of energy reserves.
Some of the starches we use in the kitchen come from the tubers or roots of plants, where they supplement plants with energy when conditions are unfavorable for photosynthesis. Starches from these sources, like potato, tapioca, and arrowroot starch, are considered root starches, and they’re typically lower in amylose and higher in amylopectin. Plants also store starch in their seeds, where it feeds the seed until it sprouts and starts to photosynthesize. Starches that come from seeds, such as cornstarch and flour, are considered cereal starches. These starches typically have more amylose and less amylopectin. Although different starches have unique properties that make them more suitable for certain applications, all starches absorb water and thicken liquids when heated.
Starch granules are semicrystalline.
In the post about sugar, we described how sugar’s molecular composition makes it suitable for bonding to itself and to water. Essentially, the oxygen and hydrogen atoms in the –OH’s (or hydroxyl groups) have slight electrical charges. And much as opposite magnetic poles attract, opposite electrical charges attract. The oxygens and hydrogens look for other atoms with the opposite electrical charge. When they find one, often on the hydroxyl group of another sugar molecule or on water, they form a hydrogen bond.
The same holds true for the hydroxyl groups on the glucose units in starch—in fact, starch molecules can form an organized structure because individual glucose units from different starch molecules bond to each other. This forms a compact, crystalline structure. Since starch molecules are so large, however, they cannot form these bonds along their entire lengths. Thus, only some portions of starch granules are crystalline, and the granule as a whole is semi-crystalline. In a granule, the starch molecules form alternating layers of crystalline and non-crystalline (or amorphous) structure.
Uncooked (or undercooked) starch is hard and crunchy because of these crystals. Think about uncooked rice. All those semi-crystalline starch granules are hard enough to break a tooth! Similarly, if the starches we bake with aren’t cooked through, they have a sandy mouthfeel.
Starches thicken with heat and water.
At room temperature, the crystalline portions of a starch granule are compact. Each glucose holds on tightly to its hydrogen bonded partners, so there’s no room for water to squeeze in and form hydrogen bonds. This is why starches don’t dissolve at room temperature. Think about soaking rice. With time, the rice will absorb some water, but it will never fully soften because the starch crystals make much of the granule impenetrable. The water that is absorbed forms hydrogen bonds with glucose units in the amorphous (or non-crystalline) portions of the starch.
If we heat the starch and water, however, the hydrogen bonds in the starch granule start to loosen. (I discussed heat’s effect on chemical bonds in depth here.) The oxygen and hydrogen atoms in the glucose’s hydroxyl groups separate more and more frequently before bonding again with their partner. When glucose units separate, water can squeeze in and form its own hydrogen bonds to them. As the water gets hotter, it replaces the hydrogen bonds in more and more starch molecules and moves deep into the granule’s layers. And as more and more water enters a starch granule, the granule swells. The liquid begins to thicken as the water is absorbed and the granules press against each other.
If we continue to heat the mixture to the specific gelation temperature for the given starch, the granules swell so much they burst and leak starch molecules into the liquid. This is starch gelatinization. The network of released starch molecules, broken granules, and trapped water forms the thick liquid we expect from starch. At this point, the starch is perfectly cooked. If we think about rice again, cooked rice is soft because the water and heat have fully gelatinized the starch molecules and broken down their crystalline structure.
If we continue to cook the rice, however, the texture becomes mushy. This occurs because the starch granules completely rupture and release all of their starch molecules, and the empty granules do not have a bite. For root starches especially, which are high in amylopectin, deflated granules actually result in a thinner mixture despite the starch molecules now released into the liquid. Without the large, swollen granules pressing together, these starches lose some of their thickening power.
Starches recrystallize when cooled.
When we cool gelatinized starch, we remove the energy that enabled the starch molecules to loosen their hydrogen bonds, and all the molecules in the mixture move slower. If there’s enough starch, the mixture might become so thick that it gels into a solid we can slice. This is especially true of cereal starches, which typically contain more amylose. This is how many custard pie fillings are made.
But with so much less energy available, the starch molecules would really rather return to their organized, crystalline structure. Since there are now a ton of water molecules scattered throughout the starch, this is impossible, but the starch molecules are able to recrystallize to an extent in a process called retrogradation. Amylose is especially prone to retrogradation because its long, straight chains are easy to align. Over time, amylose molecules migrate closer together and squeeze out the water molecules that hold them apart in a process called syneresis. This is easier to see in custards, where the water beads on the surface, but retrogradation occurs in baked goods as well. The water is just absorbed by other ingredients.
The amylose crystals, much like the original semi-crystalline starch granules, are hard. Retrogradation is one of the processes that contributes to staling—as starches recrystallize and squeeze out water over time, baked goods lose their softness. We also see retrogradation in rice when it hardens in the refrigerator. If we reheat it, the rice regains its original texture because the heat separates the starch crystals again. Similarly, stale baked goods can be refreshed with a little heat.
The starches we use in the kitchen come from different plants, but they all absorb water and gelatinize with sufficient water and heat. These properties are the basis of starch’s roles in baking. In the next post, we’ll explore how starch from flour behaves in cookies, cakes, and breads. And we’ll highlight some of starch’s special roles when it comes to steam-injected ovens, bagels, and choux pastry.
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