In the last post, we discussed the molecular details of starch: what it is, where it comes from, and how it changes with water and heat. Today, we’ll apply those concepts to baked goods with a focus on the starch in wheat flour. Although flour is often noted for its gluten, it actually contains 68–75% starch. So when we consider the chemistry of any baked good that contains flour, be it cake, bread, or cookies, starch always plays a role. And in foods cooked in steam or boiling water, starch helps create textures as diverse as soft skins on steamed buns, chewy crusts on bagels, and crisp shells of choux pastry.
Starch absorbs water.
When we hydrate flour, close to half the liquid is absorbed by starch. The exact amount depends on the temperature of the water. As we discussed in the last post, the hotter the water, the more the starch absorbs. This is one reason temperature control is important in baking. Chinese cooks, for example, differentiate between 燙麵 (tàngmiàn, hot water dough), which is made with boiling water, and 冷水面 (lěngshuǐmiàn, cold water dough), made with water at room temperature. The hotter the water, the more the starch absorbs, the more starch gelatinizes, the softer the dough. The colder the water, the larger the role of gluten in the dough structure. By changing only the temperature of the water and, by extension, the amount of water in the dough, we can tailor the dough for specific cooking methods, tastes, colors, and textures and create wheat products as diverse as dumpling wrappers, noodles, scallion pancakes, and chive pocket wrappers.
Type of flour
Properties of the flour itself also affect how much water it absorbs. As we discussed in the last post, the starch in flour comes from the starch granules in wheat grains. When wheat is milled into flour, a lot of these granules are crushed. Because these granules are split open, they’re easier for water to penetrate, and they can absorb three to four times more water than whole granules at room temperature. Flours made from harder wheat kernels, such as bread flour, contain more broken granules because they require more force to grind. These damaged starch granules are one reason bread flour absorbs more water than all-purpose or cake flour. (Bread flour also contains more gluten, which also absorbs water.)
Chlorination, a chemical treatment used on some cake flours, also increases starch’s ability to absorb water by changing the molecular composition of the glucose units and breaking down large starch chains. The more liquid we can add into our cake batters, the more sugar we can dissolve in it and the more tender the cake. The swollen starch also provides more thickening power and structural support for cakes as they bake. Some chefs increase the water absorption capacity of their cake batters by using a combination of flour and pure starch.
Starch adds structure in high-moisture environments.
Starch provides structure for cakes and other high-moisture baked goods such as bread. In these recipes, which contain a lot of liquid, the starch starts to gelatinize around 170°F (75°C). This process, along with protein coagulation, thickens the batter and sets it into its final shape and crumb. As with the other structure molecules like gluten and egg proteins, gelatinized starch adds texture. We taste it in the softness of bread and cake.
The exact temperature at which starch gelatinizes depends on the amount of water available to it. Ingredients like sugar, which holds onto water, and fat, which prevents water molecules from reaching the starch, increase the gelation temperature. In most recipes, starch gelatinization continues until around 200°F (95°C)
Starch tenderizes in low-moisture environments.
However, in baked goods with minimal liquid, such as cookies and pie dough, there isn’t enough water to fully gelatinize the starch. Starch granules still absorb water and swell, but they do not thicken. It is solely the proteins from flour and egg that set the structure of these baked goods. The large, swollen granules of starch actually act as tenderizers in these recipes. They present a physical obstacle to the formation of protein networks.
Starch contributes to staling.
As we discussed in the last post, starches, especially amylose, recrystallize as they cool in a process called retrogradation. As they do so, they squeeze water out from between them. The water is usually absorbed by other ingredients, such as sugar, but this migration of water is one of the processes that contributes to staling. Flour, as a cereal starch, is relatively high in amylose and thus prone to retrogradation. Since temperatures in the refrigerator accelerate this process, most bakers recommend you store baked goods at room temperature or in the freezer. Even so, many baked goods taste better when they’re reheated. Heat breaks up the crystals and softens the texture, refreshing the baked good.
Starch feeds yeast in bread.
As we mentioned in previous discussions, starch also provides food for yeast in bread doughs. An enzyme called amylase separates individual glucose links from the larger starch chains, and the yeast metabolize the glucose and produce carbon dioxide and ethanol. (Amylase is also at work in living organisms, including in our saliva!) Damaged starch granules are especially accessible to amylase, which is another reason bread flour made from hard wheat kernels is suitable for yeast breads. However, white flour itself typically contains very little amylase, so most commercial millers supplement their flours with malted barley flour. You can find it on the ingredients list on a bag of flour. Malted flours are ground from barely sprouted seeds, so they contain many active enzymes, including amylase, that the sprouts need to grow.
Certain cooking methods encourage starch gelatinization.
Baking with steam
In addition to feeding yeast and adding structure and texture, starch has another important application for bread in the texture of the crust. Some bread recipes, such as baguette recipes, place a pan of water on the oven floor as the bread bakes. Other breads, like no-knead breads and sourdough, are baked in covered Dutch ovens to produce crusty exteriors. Both these methods mimic steam-injected ovens in commercial bakeries. Why is steam so important?
Steam is made of water molecules in the gas phase. When we surround bread dough with water molecules, the dough doesn’t lose water as quickly. This keeps the surface of the bread cooler for longer, so the crust sets later and the loaf rises more. The steam also provides enough water to gelatinize the starch on the crust. Since starch is stretchier than gluten, the bread can expand even more. The expanding bread dough thins out the layer of gelatinized starch on its surface, ultimately creating a thinner crust.
Toward the end of the baking time, the water on the floor of the oven boils off or the Dutch oven is uncovered. In both cases, we expose the bread, baked this long with plenty of water vapor around, to dry heat. The crust dries and crisps. Without water molecules around, the crust also reaches higher temperatures, which promotes browning. And finally, since gelatinized starch forms a film, breads baked with steam have a glossy crust. (You know that dried papery stuff at the bottom of your empty rice or pasta pot? That’s a film of gelatinized starch!)
In some cultures, dough is steamed until it’s cooked through. For example, Chinese buns with (包子, bāozi) or without (饅頭, mántóu) fillings are cooked in steamers that continuously provide wet heat. The crusts stay soft because they never have a chance to dry out, and the buns stay pale because temperatures don’t rise above the boiling point of water. However, steamed buns do have a shiny skin on the outside, formed from the starch that gelatinized in the wet heat of the steamer. Check out a comparison of steamed and baked buns here!
Boiling before baking
Other bread products such as bagels and pretzels are also shiny, but not from steaming. Instead, they’re briefly boiled in water before baking. The hot water gelatinizes starch on the surface of the bagel or pretzel, once again creating a shiny crust. Since the crust is effectively cooked when it’s boiled, it can’t expand once it enters the oven. As a result, the bagel rises less, which gives it its characteristic dense texture. The longer the bagels are boiled, the more the crust sets, the lesser the rise, and the denser the texture.
For other recipes, the flour is cooked with water before it’s baked. In choux pastry, for example, the flour is dumped into boiling butter and water (or milk), then cooked until the mixture is dry. This process combines flour with plenty of hot water—the perfect conditions for gelatinizing starch.
Why is this so important? The starch now holds plenty of liquid, so we can add more eggs to the the paste. Because choux pastry is leavened by steam, the more water we can add to the dough with the eggs, the more leavening power we have. Gelatinizing the starch ensures that we have plenty of water in the dough for a beautiful puff.
A similar technique can be applied to bread. Asian cultures use the tangzhong (湯種, tāngzhǒng, literally “soup seed”) method to produce soft and fluffy yeast breads. (Tangzhong is derived from the Japanese yukone or yudane method, and it’s also called kappi in Malayalam.) Small portions of the flour and water (or milk) in a recipe are cooked together to form a paste. This paste is then added to the rest of the ingredients. As with choux pastry, cooking the flour with water gelatinizes the starch, which makes it absorb more water. As a result, the dough is drier and easier to knead. The gelatinized starch is also more accessible to amylase, which breaks it down into smaller sugar molecules. After baking, the extra moisture held by the gelatinized starch increases the moistness of the bread, and the sugars created by amylase extend its shelf life.
Starch, as the main component of flour, is at play in most of our baked goods, whether as a tenderizer or as a toughener. Its roles are even more diverse in breads, where it can change the texture of the crust or create a bigger loaf that holds more moisture. Starch’s ability to hold water makes it indispensable for baked goods, and it also makes it an ideal thickener and stabilizer. In the next post, we’ll explore these roles of starch in foods such as custards, meringues, and buttercream.
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