Gelatinization of starch: The function of starch particles expanding, splitting and forming a uniform paste solution in water at a proper temperature (the required temperature of starch from various sources is different, generally 60~80℃) is called gelatinization. The essence of gelatinization is that the hydrogen bond between ordered and disordered (crystalline and amorphous) starch molecules in starch particles is broken and dispersed in water to become colloidal solution.
The gelatinization process can be divided into three stages: (1) reversible water absorption stage, in which water enters the amorphous part of starch granules and its volume expands slightly. At this time, the particles can be recovered after cooling and drying, and the birefringence phenomenon remains unchanged; (2) In the irreversible water absorption stage, with the increase of temperature, water enters the starch microcrystal gap, irreversibly absorbs a large amount of water, and the birefringence gradually blurs or even disappears, also called "crystallization", and the starch granules expand to 50~ 100 times of the original volume; (3) The starch particles finally disintegrate and all starch molecules enter the solution.
Gelatinized starch is also called α starch. The freshly prepared gelatinized starch slurry is dehydrated and dried to obtain amorphous powder which is easy to disperse and cold water, namely "soluble α-starch".
2. Determination methods of starch gelatinization: optical microscope, electron microscope, light propagation method, viscosity determination method, swelling degree and solubility determination method, enzyme analysis method, nuclear magnetic resonance method, laser scattering method, etc. Viscosity determination, swelling and solubility determination are commonly used methods in industry.
Second, acid modified starch
Below the gelatinization temperature, starch is treated with inorganic acid to change its properties, and the product is called acid-modified starch.
Reaction mechanism: In the process of treating starch with acid, acid acts on glycosidic bonds to hydrolyze starch molecules, making starch molecules smaller. Starch granules are composed of amylose and amylopectin. The former has α- 1, 4 bonds, while the latter has a few α- 1, 6 bonds besides α- 1, 4 bonds. The acid hydrolysis difficulty of these two glycosidic bonds is different. Due to the influence of the crystalline structure of starch granules, amylose molecules are combined into crystalline structure through hydrogen bonds, so acid penetration is difficult, and its α- 1, 4 bond is not easily hydrolyzed by acid. However, the α- 1 4 bond and α- 1 6 bond of amorphous region amylopectin molecules in the granules are easy to be infiltrated and hydrolyzed by acid.
Technology and principle: Under normal circumstances, acid-modified starch is made from concentrated starch slurry with solid content of about 36%~40%, heated to below gelatinization temperature (generally 40~60℃), added with inorganic acid, and stirred 1 hour or several hours. When the required acidity or conversion rate is reached, thirdly, the starch is oxidized.
Many reagents can oxidize starch, but alkaline hypochlorite is the most commonly used in industrial production. Starch oxidized with hypochlorite is called "chlorinated starch" (although chlorine is not introduced into starch molecules during treatment).
The hypochlorite oxidation of starch emulsion is carried out in alkaline sodium hypochlorite solution. At this time, it is necessary to control pH, temperature and the concentrations of hypochlorite, alkali and starch. Adjust the pH value to 8~ 10 with about 3% sodium hydroxide solution, and add 5~ 10% hypochlorite solution of available chlorine within the specified time. The pH value is controlled by adding dilute sodium hydroxide solution to neutralize the acidic substances produced in the reaction. Many different products can be produced by changing time, temperature, pH value, starch type, hypochlorite concentration and hypochlorite addition speed. When the oxidation reaction reaches the required level, the pH value is reduced to 5~7, and sodium bisulfite solution or sulfur dioxide gas is added to remove excess chlorine to terminate the reaction.
Four, the classification of modified starch
At present, there are more than two thousand varieties and specifications of modified starch, and the classification of modified starch is generally carried out according to treatment methods.
(1) Physical denaturation: pre-gelatinized (α-gelatinized) starch, γ-ray, ultra-high frequency radiation treated starch, mechanical grinding treated starch, moist heat treated starch, etc.
(2) Chemical denaturation: modified starch obtained by treating with various chemical reagents. There are two types: one is to reduce the molecular weight of starch, such as acid hydrolysis starch, oxidized starch, baking dextrin and so on. The other is to increase the molecular weight of starch, such as cross-linked starch, esterified starch, etherified starch, grafted starch and so on.
(3) Enzymatic denaturation (biological modification): Starch is treated by various enzymes. Such as α, β, γ-cyclodextrin, maltodextrin, amylose, etc.
(4) Compound denaturation: modified starch obtained by two or more treatment methods. Such as oxidized cross-linked starch, cross-linked esterified starch and the like. The modified starch obtained by compound modification has the advantages of both modified starches.
In addition, modified starch can also be classified according to the production process route, including dry method (such as phosphate starch, acid hydrolysis starch, cationic starch, carboxymethyl starch, etc. ), wet method, organic solvent method (for example, ethanol is generally used as the solvent for preparing carboxyl starch), extrusion method and roller drying method (for example, pregelatinized starch is produced from natural starch or modified starch).
Properties of verb (abbreviation of verb) modified starch
The availability of natural starch depends on the structure of starch particles and the content of amylose and amylopectin. Different sources of starch have different properties, so the availability of starch from different sources is also different.
The application of natural starch in modern industry is limited, especially in the case of new technology, new technology and new equipment. Most natural starches do not have good properties. According to the needs, combined with the structural rationalization characteristics of starch, starch modification technology is developed to produce modified starch with better performance, which is convenient for application, meets the operation requirements of new technology, opens up new uses and expands market space.
The main function of denaturation is to change the gelatinization and cooking characteristics, mainly changing the following properties:
(1) gelatinization temperature: depolymerization reduces gelatinization temperature (gt); In the non-depolymerization process, the gelation temperature increases and decreases. Generally, introducing hydrophilic groups into starch molecules can enhance the interaction between starch molecules and water and reduce GT. Crosslinking plays a blocking role, which is not conducive to the entry of water molecules and increases GT. High amylose starch is closely combined with high lattice energy, and it is not easy to gelatinize.
(2) Thermal stability of starch paste: Generally, the thermal stability of cereal starch is greater than that of potato; By grafting or derivatizing some groups, the thermal stability of starch paste can be increased by changing the size or bridging of groups.
(3) Cold stability of starch paste: hydrophilic groups are introduced into the starch structure, resulting in space barriers and molecules are not easy to rearrange. In addition, the introduction of hydrophilic groups enhances hydrophilicity, strengthens the binding force with water and reduces the dehydration of starch.
(4) Acid stability: change the starch structure into a network structure as much as possible to make the starch resistant to pH 3-3. The acidity of 5.
(5) Shear resistance: Generally, acid-resistant starch is also shear resistant.
(6) Composite modification: it has multiple functions.
The properties of modified starch depend on the following factors. Sources of starch (corn, potatoes, wheat, rice, etc.). ), and the ratio or content of treatment (acidolysis or dextrinization, etc.). ), amylose and amylopectin, molecular weight distribution range (viscosity or fluidity), derivative types (esterification, etherification, etc. ), the nature of substituents (acetyl, hydroxypropyl, etc. ), degree of substitution (DS) or molar degree of substitution, and physical properties. That is to say, modified starch products with different properties can be obtained by different methods and degrees of denaturation of starch from different sources. The properties of modified starch are mainly investigated from the following aspects: transparency, solubility, swelling, freeze-thaw stability, viscosity and stability, acid resistance, shear resistance, adhesion, aging and emulsification.