Gluten, also known as active gluten flour, is a natural grain protein extracted from wheat by deep processing. As early as 1728, Italian scientist Beccari washed gluten from wheat flour, and established the existence of wheat gluten, but people did not pay attention to it. Until 1907, Osborne divided it into four kinds of protein, namely albumin, globulin, gliadin and glutenin, according to the solubility characteristics of protein in wheat grains. Gluten protein mainly contains gliadin and glutenin, which are collectively called storage protein (account for about 70%-80% of wheat gluten dry basis).
Gliadin in gluten is a monomer protein , it is a small molecular weight of about 35000u, which is insoluble in water and anhydrous ethanol, but it is soluble in 70%-80% ethanol. The composition is characterized by more proline and amide, more nonpolar side chains than polar side chains, no subunit structure exists in the molecule and no disulfide bond between peptide chains, and single peptide chains are connected by hydrogen bonds, hydrophobic bonds and intramolecular disulfide bonds to form a compact three-dimensional structure, which is spherical. Because gliadin is mostly composed of nonpolar amino acids, it has viscosity and expansibility, which mainly provides extensibility for dough.
Glutenin is a heterogeneous macromolecular polymer with a molecular weight of 40000-300000u, some molecular weight of polymer can reach several billion u. It is insoluble in water, alcohol and neutral salt solution, but it is soluble in dilute acid or dilute alkali. Glutenin is generally composed of 17-20 different polypeptide subunits, which are connected by intramolecular and intermolecular disulfide bonds and it is fibrous. Its amino acid composition is mostly polar amino acids, which is prone to aggregation. Disulfide bonds between peptide chains and polar amino acids are the main factors that determine the strength of dough, which endows dough with elasticity.
The unique amino acid composition of gliadin and glutenin endows wheat protein with the characteristics of viscoelastic network structure, which is unmatched by other protein. When water molecules interact with hydrophilic groups of protein, it will form hydrate-wet gluten. Hydration is gradually carried out from the surface to the inside, and the volume increases and the water absorption is less in the surface acting stage. When the water absorption swells further, water molecules further diffuse into protein molecules, and protein colloidal particles are like a permeable bag, which greatly increases the amount of water absorption. Wet gluten after water absorption maintains its original natural activity and natural physical state, and it has viscoelasticity, extensibility, film formability and liposuction emulsification.
Physical characteristics of gluten proteins are as follows:
1. Viscoelasticity
Gliadin molecules in gluten protein are spherical, with small molecular weight, it is blessed with extensibility, but it is low elasticity; Glutenin molecules are fibrous with large molecular weight and elasticity, but they are blessed with small extensibility. The combination of the above two characteristics makes gluten have unique viscoelasticity that other plant proteins do not have.
2. Extensibility
Extensibility refers to the performance of pulling gluten blocks to certain length without breaking, which can be expressed by the maximum length of gluten blocks when they are pulled to break. The extensibility of gluten is divided into three levels: gluten with poor extensibility, gluten with medium extensibility and gluten with good extensibility.
3. Film formability
The film formability of gluten is the direct expression of its viscoelasticity. Because gluten is elastic, CO2 or water vapour is surrounded by continuous protein phase, and the it is filled with gas inside, which makes gluten to show the appearance of sponge or fibrous structure and forms film gluten.
4. Hydroscopicity
High-quality gluten can absorb twice as much water as gluten. This hydroscopicity of gluten can increase the product yield and prolong the shelf life of food.
5. Thermal coagulability
When water-soluble protein is heated to a critical temperature, it will denature, and after denaturation, it will not be easily soluble in water, which is the thermal coagulability. Gluten protein is different from other protein, gluten is not sensitive to heat. If it is not heated to about 80℃, it will not gelatinization. This shows that most of the molecules in gluten are S-S cross-linked, that is, gluten protein is composed of solid tertiary structure or quaternary structure. Therefore, if the reducing agent is used to cut off the S-S cross-linking of gluten, its thermal sensitivity will be significantly improved.
6. Isoelectric point
The pH of isoelectric point for gluten protein is 6-8, and its solubility is the smallest in this pH range.
7. Taste
The gluten which is properly processed and reasonably stored has the "light and mellow taste" or a slightly "cereal taste", which is the taste that people like. When gluten is mixed with other food ingredients, even if it is added in large quantities, it will not produce odor.
Gluten has extensive applications in food, feed, chemical industry and paper industry. As food or ingredient, wheat protein must have appropriate functional properties for food application and consumer acceptance. These properties affect the composition and conformation of protein, and their internal reactions with other food ingredients , and they are influenced by processing conditions and processing environment.
The functional characteristics of gluten protein interact with each other, they play the performance of synergistic effect in the food system, and the main controlling factors of each functional property are as follows:
1. Solubility
Due to the unique properties of gliadin and glutenin, gluten has low solubility, so the most important factors to control solubility are charge rate and hydrophobicity.
2. Water holding property
The interaction between wheat protein and water can be divided into water absorption capability and water holding property, the former is "chemical combination" and the latter is "physical interception". The water holding property is mainly determined by pH value rather than concentration.
3. Emulsifiability
The production of emulsifying phenomenon depends on the rapid absorption of substance, which expands and resets internally; The emulsion stability depends on the reduction of internal free energy and the rheological properties of the membrane. The formation of emulsification is directly related to pH value.
4. Foamability
Foamability requires that protein molecules can reach the inner surface and expand rapidly. The foaming ability of gluten protein is affected by viscosity, hydrophobicity and solubility in descending order.
5. Gelatinity
The influence factors of gelation are closely related to the external conditions of gel formation, such as temperature, PH value ,salt concentration,etc.
6. Oil absorbency
It is the reaction between protein conformation and protein that affects the oil absorbency of protein. Non-covalent bond is the main force involved in the reaction between protein and oil, followed by hydrogen bond.
7. Viscosity
Gluten protein solution is the pseudoplastic liquid belonging to non-Newtonian fluid, and its viscosity increases with the increase of concentration.