As a follow-up to one of my earlier posts discussing the role of autolysis in bread making, I thought it would be worthwhile to go through the various components of flour (specifically, wheat flour), and identify how each impacts the functionality of this ingredient fundamental to the process of baking bread.
Like all (once) living things, at its most basic flour can be broken down into a mix of proteins, carbohydrates, fats, vitamins and minerals. Despite making up a relatively low percentage of the overall content—7 to 13 percent, depending on the growing season and manner of processing—the focus tends to be on the proteins, which give wheat flour that unique ability to form a tensile, elastic dough. However, since we've already gone through (albeit very briefly) how glutenin and gliandin interact to form these vital, cohesive structures I thought I'd focus on what all the other bits do instead.
WHEAT BRAN & WHEAT GERM
A wheat kernel is comprised of three major compartments: the protective outer covering (bran), the wheat plant embryo (germ), and the growing plants first food reservoir (endosperm). The bran (high in fibre) and germ (full of lipids)—which are rich in both flavour and nutrients and give flour that creamy, flecked appearance—are fully retained in wholewheat flours, and also present in small amounts in stone-ground flour (unless it's been sifted). For most flours, however, they are removed during the first stages of milling as the sharp edges of the bran flakes interfere with gluten development, while the fats contained within the germ can turn rancid quite quickly. The majority of what makes up flour, therefore, comes from the wheat kernel's endosperm.
CARBOHYDRATES: STARCH & SUGARS
Flour is approximately 70% by weight carbohydrate, the bulk of which is starch. A polysaccharide comprised of repeating glucose molecules present in the form of two polymers, amylose and amylopectin, starch is the primary energy store of most green plants and the most common carbohydrate in the human diet. When combined with water in the presence of heat, starch undergoes gelatinisation, a process by which the granules begin to take up water and become irreversibly dissolved. At the molecular level, as heat is applied this increase in temperature causes relaxation of the granule's crystalline structure and water is absorbed into the amorphous space of starch, making it swell and permitting water to penetrate even further into the more tightly bound regions of the granule. As the highly branched, soluble amylopectin molecules swell the structural organisation of the granule is dissolved, releasing the insoluble amylose molecules into the surrounding water. What results is a viscous gel that eventually forms the rigid bulk of the walls, which surround the trapped bubbles of carbon dioxide in a leavened dough, giving the crumb its structure. Starch also effects the structural development of the dough by interpenetrating the developing gluten network, which helps to tenderise it, while a further function of starch is in providing a food source for yeasts during the early stages of fermentation. Granules damaged during milling are easily broken down into simple sugars (see below), which not only feed the yeasts actively growing within the dough, but add flavour and help to produce a browner crust when the bread is baked.
Enzymes are a specific class of proteins essential to the metabolic processes that occur in all living cells, functioning to convert one molecule into another and thus provide the food and energy necessary to sustain life. They are grouped on their ability to break down proteins, carbohydrates or fats, and each enzyme has a specific target molecule—a substrate—with which it interacts. While there are a number of enzymes present in flour the most numerous, and therefore most important to bakers, is amylase. Amylase is the enzyme responsible for breaking down starch into maltose and glucose, which, in the natural world, would help feed the growing plant during germination of the wheat kernel. In breadmaking, this reaction instead provides food for yeasts, which utilise glucose during fermentation. Starch granules damaged during milling are the first to be targeted, but will be protected from degradation until the amylase becomes activated by the presence water in the dough. If this activity were ongoing and the amylase continued to digest the starch made accessible during baking (i.e. the molecules undergoing gelatinisation), too much of the starch would be converted into sugar and the structure of the loaf would be destroyed. Fortunately, the amylase in wheat is heat-sensitive, and therefore becomes denatured (inactivated) before the process of gelatinisation begins to occur. For flours such as rye that carry a more heat-stable version of the amylase enzyme, inactivation is achieved by creating a more acidic environment through the use of sour starters or buttermilk as the liquid component.
Vitamins and minerals are present as a very small percentage that is directly related to the bran content, around 1-2% of a flour's weight. The vitamins are primarily E, or B-complex vitamins, while the minerals are derived exclusively from the soil, their composition therefore being heavily influenced by soil quality, rainfall, and farming-related practices such as fertiliser use. The amount of minerals present is referred to as the 'ash content', as it represents the mineral residue left behind following a controlled burn of a flour sample. The ash content is significant in breadmaking as it can appreciably affect the vigorousness of natural fermentation, and also helps strengthen the gluten by causing the network being formed to become more tensile.
Like vitamins and minerals, lipids account for only 1% of a dough's weight, as much of it is removed along with the germ during milling. In many respects this is a good thing as they become rancid and can easily spoil a flour if not stored correctly, however, fats are also beneficial in softening the bread structure, and for slowing staling by clinging to the starch granules to assist in moisture retention. They can also increase plasticity by helping glutenin bind starch and gliandin, and preserve dough volume by enabling proteins to retain carbon dioxide produced during fermentation.
So there you have it, a very brief overview of the ingredient so integral to the practice of making bread. Flour is fundamental in providing strength and structure, imparting flavour and nutritional value, absorbing liquids and affecting a loaf's keeping qualities. The type of grain and methods of milling, blending and processing can all impact significantly on these various parameters, providing a baker with endless possibilities when it comes to manipulating their breads. Not bad for a seemingly simple ingredient, eh?
- Amendola J, Rees N. (2003) The Art and Science of Baking, 3rd edition. John Wiley & Sons Inc. Hoboken, New Jersey, USA. ISBN 13-978-0-471-40546-7.
- McGee H. (2004) On Food and Cooking: The Science and Lore of the Kitchen. Scribner. New York, New York, USA. ISBN 13-978-0-684-80001-1.
- Stevens, D. (2009) River Cottage Handbook No.3: Bread. Bloomsbury Publishing Plc. London, England, UK. ISBN 978-0-7475-9533-5.
- Wing D, Scott A. (1999) The Bread Builders: Hearth Loaves and Masonry Ovens. Chelsea Green Publishing Company. White River Junction, Vermont, USA. ISBN 1-890132-05-5.