Categorized by protein content, at present we usually divide flour into three categories:
1. High gluten flour (strong flour, high protein flour or bread flour): protein content of 12% to 15%, wet gluten weight >35%. High gluten flour is suitable for making bread, shortcrust pastry, puff pastry and muffins.

2. Low gluten flour (weak gluten flour, low protein flour or cookie flour): protein content of 7% to 9%. Wet weight <25%. Low gluten is suitable for making cakes, cookies, mixed pastry and so on.

3. Medium gluten flour (general-purpose flour, medium-protein flour): a type of flour between high-gluten flour and low-gluten flour. Protein content of 9% to 11%, wet gluten weight of 25% to 35%. Medium gluten flour is suitable for making fruit cake and can also be used to make bread.

4. Special Flour: It is a kind of flour which should be used as raw material for food, and it is suitable for the production of special food after special mixing.

5. Pre-mixed flour: It is the flour that is pre-mixed with flour, sugar, powdered fat, milk powder, improver, emulsifier, salt and so on according to the recipe of bakery products. Currently sold in the market sponge cake mixes, cookie mixes, muffin mixes are such.

6. Whole Wheat Flour: It is made from the whole wheat grain and contains the germ, most of the wheat husk and endosperm. The wheat husk and germ are rich in protein, fiber, vitamins and minerals, and have high nutritional value.

Process properties of flour

1. Properties of starch
The starch in flour is divided into straight-chain starch and branched-chain starch due to the different connection between glucose molecules. Straight-chain starch is easily soluble in hot water, and the resulting colloid is not very viscous and has the property of enhancing the plasticity of dough. Branched-chain starch needs to be heated and pressurized before dissolving in water, and the resulting colloid is very viscous, which has the property of enhancing gluten strength.

Starch is insoluble in water at room temperature, but when the water temperature is above 53℃, the physical properties of starch change significantly. The property of starch dissolving and splitting to form a uniform paste solution at high temperature is called starch dextrinization. The pasting of starch improves the plasticity of dough.

The change from raw to cooked pasta food is actually the change of β starch to α starch. However, when α starch is left at room temperature, it will gradually change into β starch, which is called aging of starch. When bakery products are fresh out of the oven, the starch is. state, but will age after being left for a period of time for this reason.

In fermented dough, the starch in the flour is converted into sugar under the action of amylase and saccharase, which can provide nutrients for yeast fermentation, thus improving the dough’s ability to ferment and produce gas.The ability of the starch in the flour to be converted into sugar is known as the saccharification power of the flour.

Under the same conditions, the stronger the saccharification power of the flour, the more nutrients it provides for the yeast, the more gas the dough produces, and the larger the volume of the bread produced. In the baking process, the role of starch is also very important, when the center temperature of the dough reaches 55 ℃, the yeast will make the amylase accelerate the activation of the flour saccharification force accelerated, the dough becomes softer, at this time the starch absorbs water and pasteurization, and the mesh gluten together with the formation of the organizational structure of the baked goods.

2. Protein properties
Protein in flour is mainly gliadin and wheat gluten, accounting for about 80% of the flour protein, is the main component of the formation of gluten. Gliadin and wheat gluten absorb water to form a soft gelatinous substance is gluten. Gluten has elasticity, extensibility, toughness, ductility and plasticity.

The process of water absorption by proteins and the properties of the gluten formed by them are of great importance in the baking process.
During dough preparation, the gluten formed by the water absorption of proteins gives the dough a soft texture with elasticity, toughness and extensibility. During dough fermentation, due to the mesh structure formed by the gluten, when the yeast exhales carbon dioxide gas, the extensibility of the mesh gluten forms a film containing gas bubbles, which resists the expansion of the gas and prevents the gas from escaping, and the yeast continues to produce gas, which gradually increases the size of the dough.

During the ripening process, due to the gluten mesh structure and starch filling, flour plays the role of “skeleton” in bakery products, which enables the dough embryo to form a stable organizational structure during the ripening process.

Protein water absorption to form gluten is related to the resting time of the dough, mixing intensity and dough temperature. Protein water absorption to form gluten needs to go through a period of time, therefore, the dough resting for a period of time so that the protein water absorption is sufficient, conducive to the formation of gluten, the general resting time of the dough to 20 minutes is appropriate.

Dough in the mixing process, can promote the water absorption rate of protein, but be careful not to mix too long, otherwise it will destroy the gluten has been formed, and reduce the generation of gluten. Temperature has a great influence on the formation of gluten. The optimal temperature is 30-40 ℃, when the protein water absorption rate of up to 150%, gluten generation is higher. The temperature is too low, the gluten swelling process is delayed, the gluten generation rate is low. Temperature is too high, such as the temperature at 60-70 ℃, protein heat denaturation, water absorption ability to decline, swelling reduced, the dough gradually solidified, gluten decreases, the elasticity and elongation of the dough weakened, plasticity increased.

3. Properties of other chemical components
In addition to starch and protein, flour also contains soluble sugar, cellulose, fat, enzymes and inorganic salts, vitamins and so on. These chemical components will also have a certain impact on the baking process.

(1) Soluble sugar: soluble sugar in flour includes sucrose, maltose and glucose. The content is not much, but in the dough fermentation process can be used as yeast nutrients, but also conducive to the formation of products color, aroma and taste.

(2) Cellulose: mainly found in the wheat husk. The presence of a certain amount of cellulose is conducive to gastrointestinal peristalsis, promoting the body’s digestion and absorption of food. Semi-cellulose has the function of enhancing the strength of dough and preventing the aging of products.

(3) Fat: The fat content in flour is only 1 -2%. Lipids in the endosperm are an important part of gluten formation. Among them, lecithin is a good emulsifier, which can make the product organization fine, soft and anti-aging effect.

(4) Enzymes: The enzymes in flour are mainly amylase, protease and lipase.

Enzyme is a kind of protein, and it is amylase and protease that have great influence on baking process. Amylase can convert starch into maltose and glucose in fermented dough, providing energy for yeast fermentation and greatly improving the quality of bread in baking. The catabolic action of proteases softens the flour and reduces the process properties of the flour.

During mixing and fermentation, it reduces gluten strength, helps gluten to expand completely and shortens the mixing time. The decomposition action of lipase in flour storage tends to cause rancidity of flour and reduces the quality of flour.

Application of flour

1. Bread:
Bread should be used protein content of 1 2 -1 5 %, wet gluten in about 35% of the bread special flour.

2. Mixed pastry and pastry dry points:
Protein content of 7 -9%, wet gluten <25% of the flour should be used.

3. Cake:
Protein content of 7 -9%, wet gluten <25% of the flour.

4. Crisp pastry:
Protein content of 10-12%, wet gluten content of about 30% of the flour is appropriate.