Types of wheat flour
When a recipe calls for flour it usually means wheat flour, unless otherwise stated. (Common types of non-wheat flour include buckwheat flour, cornflour, potato flour, rice flour, spelt, chickpea flour - indeed, almost anything with a high starch content can be made into flour.)
Under the umbrella of wheat flour there are a variety of products which have different properties, each resulting in a different outcome when cooking.
Classified by the amount of the whole grain used in the final product
Sub-groups of wheat flour
Classified by the amount of protein
present in the flour.
The 2 main proteins are gliadin and glutenin and these become gluten once water is added to the flour in a recipe. Most commercial flours contain a mixture of different varieties of wheat, blended to produce the required characteristics.
Gluten provides a strengthening network within the basic structure of the finished product, and the more the flour mixture is handled, the more gluten is released. Too much gluten results in stiff, heavy cakes and pastry, which is why you need to use a low protein type flour for these, and why handling should be kept to a minimum during mixing.
A strong gluten network is essential for well risen bread, therefore strong flour is a must; heavy kneading draws out more gluten, and the process may be further enhanced by the addition of ascorbic acid and salt.
Flour as a thickening agent
As well as being the primary ingredient in bread, cakes, cookies, pasta etc, flour is often added to liquids as a thickening agent. It is the starch which is responsible for thickening. When the starch granules are heated in a liquid, they swell and eventually burst, releasing starch. The starch forms a molecular network which traps and absorbs water, thereby thickening the mixture.
Tip: never, ever lick the spoon and put it back into the sauce! As well as being unhygienic, saliva contains an enzyme (amylase) which breaks down the starch network - your thickened sauce will rapidly become runny again and there's nothing you can do to save it.
Flour provides 310 - 340 kcal/100g (1320 - 1450 KJ/100g) depending on the type. It provides protein, a range of vitamins, particularly the B vitamins thiamin and niacin, and a range of minerals. It is low in fat.