Wheat Gluten the Muscle in Baked Goods
The typical consumer probably sees wheat flour as a whitish, dusty powder, largely inert and not particularly interesting. In fact, it's a remarkably complex substance. Its composition and properties vary depending upon the type of wheat from which it was milled and upon the conditions under which that wheat was grown. For the serious home baker, it's important to have a basic knowledge of wheat and flour types, and to understand why particular flours perform best for particular baked goods, whether you're making bread, a pie crust, or a layer cake.
Aside from taste and nutrition, baked goods differ from one another in terms of structure. A light, airy wholewheat bread and a chewy, toothy pasta owe their unique textures largely to gluten. Gluten is the stuff in bread dough that holds everything together and imparts texture. By the same token, a crisp, flaky pie crust and a tender cake owe their textures, in part, to a lack of gluten. The baker can control the amount of gluten by selecting differ-ent types of flour and different methods of preparing dough.
Many thousands of wheat varieties have been catalogued. Major classes include hard red winter, hard red spring, soft red winter, white (both hard and soft) , and durum wheats. Within each class there are hundreds of varieties, each with its own personality, physical and chemical characteristics, and baking performance. (These differences can be likened to the distinctions among grape varieties and their functions in producing various wines.) A good flour miller has the knowledge and experience to draw upon appropriate wheat classes and varieties to mill flours suited to different purposes.
Protein and gluten-Largely due to their higher protein contents ( 11 to 13percent), hard-wheat flours are preferred for breads. Soft wheat flours, low in protein (8 to 9 percent) , are better for cakes, cookies, pastries, and similar baked goods. All-purpose flours are typically made from a combination of hard and soft wheats. With a protein content of9 to 1 1 percent, all-purpose flour can be used successfully to make many bakery products, although you usually can make much better products with flours milled for specific purposes. Durum, the hardest of wheats, has a protein content of 12 to 15 percent. When milled, it produces a coarse, yellowish flour called semolina, which is used to make dried pastas . Semolina produces a very strong dough, which holds up well to being stretched and cut, and is less liable to break when dried.
All of this talk about protein percentages is important because a flour's ability to produce gluten is directly related to its protein content. Strictly speaking, only the building blocks for gluten exist in flour. Gluten itself is formed only when the proteins, along with a small quantity of other components, link together to form long strands or molecular chains called polymers. This linking happens when the flour is hydrated and then worked by mixing or kneading. Gluten is a rubbery material that is viscoelastic (that is, it is both stretchable and elastic) . These properties come from gluten's two major components, glutenin and gliadin. Glutenin imparts elasticity, while gliadin contributes extensibility.
The balance of these two properties accounts for the truly unique properties of gluten, shared by no other cereal protein. How gluten works-Gluten is critical to the structure of doughs. In a wheatflour dough, the gluten provides a continuous network that envelopes all the starch granules, which are the major flour components. This three-dimensional network forms a lattice work for the dough. In a yeast dough, carbon-dioxide gas is generated as the yeast ferments. The expanding gases, trapped in thousands of small pockets, cause the gluten to stretch considerably (by four- or fivefold) . During baking, the expanded, porous dough structure is rendered stable because the heat causes the gluten to denature irreversibly into a gel-like or a rigid structure, depending on the final moisture content of the product. Similar things happen when you cook an egg or perm your hair.
Proper mixing or kneading is critical to the s uccess of breadmaking. Sufficient energy must be imparted into a dough to adequately develop the gluten into its most favorable structure. The more protein in the flour, the more mixing it will requ ire . Not only do high-protein flours develop more gluten, but their gluten is also stronger than that produced by lowprotein flours. (High- and low-protein flours are typically referred to as "strong" or "weak," respectively, a reflection of how much muscle it takes to develop the gluten.) Some bread flours used by commercial bakers really require mixing by heavy-duty machines. Such flours make dough that's difficult to knead by hand to the point where the gluten is optimally developed, and so are to be avoided by home bakers without mixing equipment.
It is possible to overmix dough. In any flour, there's an optimum point of mixing, which coincides with optimum gluten development. If worked past that point, the gluten polymers begin to break down. An overworked dough becomes sticky and results in a loaf of bread with less volume and a denser, coarser crumb than is desirable. If you're kneading bread by hand, it's difficult to overwork it; if you're using a machine, there's a greater danger of doing so, especially with a dough made with all-purpose flour.
How can you de termine when a dough is adequately kneaded? Short of employing special instruments used by baking technologists, the experienced baker relies on certain signals, such as the onset of a characteristic satin sheen on the dough surface, or the balanced extensible -elastic feel when a piece of dough is stretched between the fingers. Gluten in other baked goods Gluten is also important for some pastry items. Dough for danish pastry, which is rolled, typically contains some bread flour. Rolled cookie doughs and even puff pastry require some gluten for stability and strength; but the amount of gluten formed by soft or all-purpose flours is always adequate.
If gluten were allowed to develop in cake batter to the same extent it is in breads, you'd have to eat cake with a knife and fork. In cakes, gluten formation is minimized because the batters are alkaline (made so by the addition ofbaking soda) , and therefore they slightly dissolve the flour's proteins. Bread doughs, in contrast, are slightly acidic. In the United States and some other countries, high-quality cake flours (which are low in protein and therefore low in potential gluten to begin with) are usually subjected to very small quantities of gaseous chlorine. The purpose of chlorinating cake flour is to modify the starch so that it sets up more rapidly in baking, but the chlorine also impairs the flour's glutenforming ability.
Vital glu ten-Gluten also is available as an ingredient, called vital gluten. Vital gluten is produced by wet-extracting gluten from flour, then carefully drying and pulverizing the gluten to retain most of its original vitality. Added at the rate of about one tablespoon per loaf, vital gluten can help strengthen doughs made from weaker flours, as well as those containing appreciable quantities of inertingredients such as raisins, nuts, or nongluten-forming cereals like oats, rice, and soy flour. Vital gluten is quite absorptive, rapidly taking up about twice its weight in water, so extra water must be added to the dough. Gluten is also used in other foods: it's blended with meats as a binder or an extension, it's added to breading or coating batters, and it's used to strengthen pasta doughs made from relatively weak flours. In Japanese cuisine, cooked gluten, called seitan, is added to dishes in much the same way as firm tofu is used.
One ingredient that complements and strengthens gluten is salt. In solution, salt ionizes and interacts with gluten proteins to produce a stronger and tougher gluten. Salt is thus a very important dough ingredient. In addition, salt helps to control and moderate yeast fermentation as well as to enhance bread flavor. -Joseph G. Ponte, Jr., is a professor in the Department of Grain Science and Industry at Kansas S tate University in Manhattan, Kansas
