Chemistry of Cooking

The Structure of Flour

Flour is made up of small starch granules.  Each granule is composed of a mixture of starch molecules. These starch granules contain some proteins which vary based on the source of the starch granules.  There are two starch molecules – amylose (a “linear” molecule) and amylopectin (a heavily branched molecule).  Both molecules are polysaccharides (long string molecules made up from lots of sugar molecules all strung together).  The arrangement of the different molecules and their proportions determine the “type” of flour and how the flour will behave when used in baking.

For bread making, the most important aspect of the flour is the formation of gluten sheets.  Gluten consists mainly of protein and includes what may be the largest protein molecules to be found in the natural world.  Proteins are long, chain-like molecules built up from smaller molecules called amino acids.  Gluten is a complex mixture of certain wheat proteins that cannot dissolve in water, but do form associations with water molecules and each other.  When water is added to flour, the proteins on the outside of the starch molecules quickly absorb the moisture and become “sticky.” This process is technically called hydration.  The hydrated protein molecules begin to stick together and bind the granules together.  As the granules are stretched, the proteins change their shape.  From this process, gluten is formed. 

Gluten is not a protein itself nor does it occur in nature.  It is formed when two different protein molecules (gliadin and glutenin) are made to interact with each other by the kneading of the wet dough.  Gluten is a highly elastic and plastic material. It will change its shape under pressure yet resists the pressure and moves back toward its original shape when the pressure is removed. Because of this combination of properties, wheat dough can expand to incorporate the carbon dioxide gas produced by the yeast.  Its gluten forms sheets that behave like rubber balloons, which fill with the carbon dioxide as the bread leavens and thus causes the bread to rise.  To make good bread, you need these gluten sheets to be robust enough not to break as the carbon dioxide is formed and plentiful enough to capture the gas in very small bubbles.  Large bubbles would create holes in the final loaf.⁵

How Wheat Becomes Flour

Several kinds of wheat are grown today, each with its own characteristics and uses.  Most are species of bread wheat, Triticum aestivuum.  The most important distinguishing characteristic is the content and quality of gluten proteins.  Hard wheat grains constitute about 75% of the American crop.  Soft wheats, which make up 20% of the crop have a lower amount of somewhat weaker gluten proteins.  North American wheats are named by their growth habit and kernel color.  Spring wheats are sown in the spring and harvested in the fall.  Winter wheats are sown in late fall, live through the winter as a seedling and are harvested in the summer. 

The baking qualities of any flour are determined by the wheat from which it’s made and how the wheat is turned into flour.  Flour has been made since prehistoric times. The earliest methods used for producing flour all involved grinding grain between stones. These methods included the mortar and pestle (a stone club striking grain held in a stone bowl), the saddlestone (a cylindrical stone rolling against grain held in a stone bowl), and the quern (a horizontal, disk-shaped stone spinning on top of grain held on another horizontal stone). These devices were all operated by hand.

The millstone, a later development, consisted of one vertical, disk-shaped stone rolling on grain sitting on a horizontal, disk-shaped stone. Millstones were first operated by human or animal power. The ancient Romans used waterwheels to power millstones. Windmills were also used to power millstones in Europe by the twelfth century.

The first mills in the North American colonies were powered by either wind or water. During the nineteenth century, numerous improvements were made in mill technology. In 1865, Edmund La Croix introduced the first middlings purifier, a device that included a vibrating screen through which air was blown to remove bran from ground wheat. The resulting product, known as middlings or farina, could be further ground into high-quality flour. This new type of mill used metal rollers, rather than millstones, to grind wheat. Roller mills were less expensive, more efficient, more uniform, and cleaner than millstones. Modern versions of middlings purifiers and roller mills are still used to make flour today.⁶

Current day milling continues as the process of breaking the wheat kernel down into small particles and sifting the particles to make a flour of desired consistency. Most flours are refined, meaning they have been sieved to remove the germ and bran layers from the particles of protein and starch-rich endosperm.  Bran and germ are rich in nutrients and flavor, but go rancid quickly and interfere with the formation of strong gluten. In conventional milling, grooved metal roller shears open the grain, squeeze out the germ and scrape the endosperm away to be ground, sieved and reground until the particles reach the desired size.  Stone grinding, which is not as common, crushes the whole grain more thoroughly before sieving so that some of the germ and bran ends up in the refined flours.  Breads made with stoned-ground flours tend to have more flavor, but also have a shorter shelf-life.⁷

Types of Wheat Flour

There are many different types of flour available.  A general difference between the flours is the type of grain they are milled from.  All flours start life as cereal grains; these are then ground down to a fine powder.  As we’ve learned, in some flours the outer skin is removed while in others (brown and whole-grain), it is not.  Common grains used for flour include wheat, rice, corn (or maize), barley and several types of beans.  Although these flours can be used in a variety of recipes, with adjustments, it is wheat flour that is, by far the most common.

There are many strains of wheat and even a single strain will produce different proportions of starch and protein in the grains.  These variations occur as a result of the climate and soil conditions where the wheat is grown.  Flours are often blended to provide a more consistent product.

Most flours that we buy in the supermarket are labeled based on their intended use. Recipes developed with a particular flour often turn out differently when made with another.  For general baking, you need a moderate protein content of around 7–10% by weight.  It is generally better to have a flour with a protein content at the lower end of this range for pastry (where the gluten needs to be minimized).  For breads, where the generation of gluten is essential, a higher protein content is necessary.  In choosing the best flour for the recipe, it helps to know the basics about the wheat from which flour is milled.  Soft wheat thrives in temperate, moist climates, like the mid-Atlantic region, while hard wheat flourishes in the Midwest.  Soft wheat is milled into pastry flour, while hard wheat becomes bread flour.  All-purpose flour is a combination of the two.⁸  

All-purpose flour: If a recipe calls simply for "flour," it's calling for all-purpose flour. Milled from a mixture of soft and hard wheat, with a moderate protein content in the 10 to 12 percent range, all-purpose flour is a staple among staples. While not necessarily good for all purposes, it is the most versatile of flours.

Cake flour: The flour with the lowest protein content at 5-8%. The relative lack of gluten-forming proteins makes cake flour useful for cakes (of course), but also biscuits, muffins and scones. Cake flour is generally chlorinated, a bleaching process that further weakens the gluten proteins and, just as important, alters the flour's starch to increase its capacity to absorb more liquid and sugar, and creates a moist product.

Bread flour: With a protein content of 12 to 14 percent, bread flour is the strongest of all flours, providing the most structural support. This is especially important in yeasted breads, where a strong gluten network is required to contain the CO₂ gases produced during fermentation. The extra protein doesn't just make for better volume and a chewier consistency; it also results in more browning in the crust. Bread flour can be found in white or whole wheat, bleached or unbleached forms. Unbleached all-purpose flour can generally be substituted for bread flour with good results.

Self-rising flour: As the name suggests, self-rising flours contain baking powder (1½ teaspoons baking powder per cup flour) and, therefore, don’t require added leavening for the making of quickbreads, pancakes, or other chemically raised foods.

Whole wheat flour:  During milling, the wheat kernel is separated into its three components: the endosperm, the germ (the embryo) and the bran (the outer coating). In whole-wheat flours, varying amounts of the germ and bran are added back into the flour. Whole-wheat flour tends to be high in protein, but its gluten-forming ability is compromised by the bran and germ and tends to produce heavier, denser baked goods.  In most recipes, whole-wheat flour can be substituted for up to half of the all-purpose flour but because wheat germ is high in oils, whole-wheat flour is far more perishable than white.⁹