As a renewable resource, starch has become an important raw material for industry. China’s current starch production has reached 1.5 million tons, starch and its deep-processed products are widely used in food, textiles, paper, medicine, feed, oil drilling, casting, construction and other industries.

Due to many inherent properties of the original starch (cold water insolubility, paste in acid, heat, shear, etc.) constraints on starch applications in industry, in order to improve starch performance, expanding its scope of application, can be processed through the physical, chemical, enzyme and other methods to prepare modified starch, to change the natural properties of starch, to increase its functionality or the introduction of new features to meet the needs of industrial development.

Pregelatinized starch is a kind of physically modified starch, also known as α-starch; it is a kind of porous, non-apparent crystalline starch granule obtained by rapid dehydration and drying after starch paste is dispersed and the starch molecules are too late for rearrangement. Since the pregelatinized starch has a porous, hydrogen-bond-breaking structure, and has the characteristics of soluble in cold water, good cold water stability and strong water retention, it is used in many industrial sectors.

Raw material and preparation method of pregelatinized starch

Pregelatinized starch is the original starch heated in the presence of a certain amount of water or hydrophilic solvent. The use of water or hydrophilic solvent to make its intermolecular hydrogen bond breakage, destroy its regular arrangement of micellar structure, completely paste, and then quickly dry at high temperature to get hydrogen bond breakage, porous, no obvious crystallization of starch particles.

The process of pasting can be divided into three stages:

(1) Reversible water absorption stage.

Moisture enters the amorphous part of the starch granule, the volume is slightly expanded, at this time cooling and drying, the granule can be restored, and the birefringence phenomenon is unchanged;

(2) Irreversible water absorption stage.

With the rise in temperature, water into the starch microcrystalline interstitial, irreversible water absorption, birefringence phenomenon gradually fuzzy to disappear, also known as crystallization “dissolution”, starch granules swollen to the original volume of 50 to 100 times;

(3) The final disintegration of starch granules, starch molecules all into solution.

After the paste starch is also known as alpha-chemical starch.

Raw materials for the production of pregelatinized starch include raw starch, such as potato starch, sweet potato starch, mung bean starch, wheat starch, tapioca starch, corn starch, waxy corn starch, oat starch, etc., as well as seed crop extracted vegetable starch and chemically modified starch, such as crosslinked starch, modified glutinous corn starch, etc.

Compared with other pregelatinized starches, potato pregelatinized starch has strong adhesion and good film-forming properties. Tapioca starch is also widely used in the production of pregelatinized starch, for example, in 2002, Sheng et al. introduced the modified pregelatinized tapioca starch prepared by chemical modification and then pregelatinized to obtain modified pregelatinized tapioca starch performance, such as viscosity, drawing length, viscous resistance, etc. more than the Thailand Thai Flower brand pregelatinized tapioca starch, but also far better than the domestic pregelatinized tapioca starch.

The traditional pre-gelatinized starch production is mainly to heat the original starch to make the starch granules paste, and then fast drying, grinding, sieving and packaging. Specifically using the process route: starch and water, a starch emulsion, a pasteurization, a drying, a crushing, a packaging. The most commonly used production methods for the drum drying method and extrusion method, of which the drum method is the most common. The method was invented in 1908 by the European Wulkank Supf, 1920 formally applied to industrialized production, while the extrusion method is generally applicable to the laboratory four.

1, drum drying method

Drum drying method is also known as hot drum method. According to the drum structure is different, and is divided into single-roll and double-roll two kinds. Double-roller drum turning the opposite, the steam into the drum heating, so that the surface temperature of the drum up to 150 ℃ ~ 180 ℃, the concentration of 40% or so starch emulsion distribution on the surface of the drum, in the drum rotation to form a uniform thin layer. Under the heating, the starch began to paste, drying, until the moisture is reduced to 5%, you can use a scraper to peel off the starch layer, after crushing, sieving, you can get pre-pasteurized starch.

The operation process is the key to prepare qualified pre-gelatinized starch, such as starch coating thickness, drum speed, drum surface temperature, product final moisture and other parameters will affect the quality of pre-gelatinized starch. In order to improve the temperature difference between inside and outside of the starch coating film and speed up the heat transfer, the starch emulsion can also be preheated with jet or heat exchanger first, and then introduced into the drum surface.

2、Screw extrusion method

With the promotion and popularization of screw extrusion technology in food processing, pre-gelatinized starch can also be produced by extrusion method. The starch will be wetted first, so that its moisture content of about 20%, introduced into the extruder, the extruder heating part of the temperature up to 120 ℃ ~ 200 ℃, the pressure can be up to 3-10MPa.

The starch is pushed forward by the screw in the extruder, and released through the several millimeter die hole of the extruder. Due to the high-pressure difference between the inside of the extruder and the outside atmosphere, the starch is instantly swollen and dried to achieve the effect of pre-gelatinization. The process of screw extrusion method is characterized by production continuity, low energy consumption, low input, and simple equipment; however, the viscosity of the final product is lower than that of the roller drying method, which is due to the breakage of glycosidic bond and other part of the bond within the molecule caused by mechanical shear in the extrusion process.

Properties of pregelatinized starch

Natural starch has a microcrystalline structure, does not dissolve or swell in cold water, and is insensitive to amylase. Heating natural starch with a certain amount of water can make the regular arrangement of colloidal structure damage, intermolecular hydrogen bond breakage. Water molecules into its internal, crystalline structure disappears, lose birefringence phenomenon, and susceptible to the action of the enzyme, this process is the starch paste. Completely pasted starch is rapidly dried and dehydrated at high temperature, and the starch granules with broken hydrogen bonds, porous shape and no obvious crystallization phenomenon will be obtained, i.e. pre-pasteurized starch.

The study of the structure found that there are a large number of submicrocrystalline structures in the pregelatinized starch, and these submicrocrystalline structures are formed gradually with the decrease of the average distance between the starch molecular chains during the drying process of the starch paste. Due to the formation of submicrocrystals, the X-ray diffraction curve of the pregelatinized starch shows a combination of a diffuse crystalline diffraction peak and a diffuse amorphous diffraction peak.

The change of starch paste during the drying process of preparing pregelatinized starch, manifested in the x-ray diffraction curve, is a double-peak formation and separation process. Among them, the diffuse crystalline diffraction peak is from none to some, from small to large, and the peak position moves to the direction of decreasing diffraction angle according to the law of crystalline diffraction; while the diffuse amorphous diffraction peak is from large to small, and finally tends to stabilize, and the peak position moves to the direction of increasing diffraction angle according to the law of amorphous diffraction.

As the pregelatinized starch has a porous, hydrogen-bond breaking structure, compared with the original starch, it has the characteristics of high dispersibility, high oil absorption, high hydration rate, high viscosity and high swelling, which can be applied to foodstuffs. The performance of pregelatinized starch can be measured by viscosity, particle size, pasting degree, whiteness and pH value, gel strength and elasticity. The main performance indexes of pregelatinized starch used for food are as follows:

1、Paste degree (α-degree)

It refers to the proportion of pregelatinized starch in a certain number of products, and the value of α-degree directly affects the product quality. α-degree is determined by birefringence method and enzyme analysis method.

2、Particle size

The particle size of pregelatinized starch directly affects the viscosity, rehydration and surface finish of the product. Generally speaking, fine particle size, the product dissolved in water to form a paste with a high viscosity of cold paste, low viscosity of hot paste, the surface gloss is also good: but rehydration is too fast, the organization can not be contacted with the water, resulting in uneven dispersion, prone to agglomeration phenomenon. Coarse particle size, the product dissolves in cold water slower, there is a water transfer time is too long, so the final product should be properly controlled particle size. Particle size is usually measured by sieving method.

3、Viscosity

Different raw material varieties of starch paste due to its different molecular chain structure, and has a different viscosity value; the same raw material with different methods of production of pre-gelatinized starch, its viscosity value is not exactly the same. Under the same conditions, potato starch as raw material to produce pre-gelatinized starch viscosity is the largest, and good stability. Usually, the viscosity is measured by Brabender viscometer and En-type viscometer.

Application of pregelatinized starch in food industry

1, in the pre-gelatinization process, the water molecules destroy the starch molecules hydrogen bonding, thereby destroying the crystalline structure of starch particles, so that the run-up dissolved in water, and therefore easy to be amylase action, conducive to human digestion and absorption. This property of pre-gelatinized starch can be used in the production of food for the elderly and infants.

2, pre-gelatinized starch in traditional food has certain application advantages.

Some scholars have found that potato pre-gelatinized starch has good viscoelasticity, can replace the gluten in flour, to flour weight 8% added to wheat starch can be produced better steamed bread. According to the U.S. patent reports, will contain straight chain amylose pregelatinized starch added to the dough, can improve its morphological properties. In addition, add the right amount of pre-gelatinized starch in the noodles, can reduce the noodle head, and can be cooked quickly, especially the best effect of cassava phosphate crosslinked starch, the amount of 10%.

3、Pre-gelatinized starch has strong water retention and can be used to improve the quality of baked goods.

When making cake, adding a certain amount of pregelatinized starch, it is easy to form dough when mixing powder, and because pregelatinized starch increases water absorption and improves gas production capacity, it makes the cake have good volume and increases the freshness and structural homogeneity of the finished product, which makes the product fluffy and has good texture.

4、Pre-gelatinized starch has good freezing stability and can be used to stabilize the internal structure of frozen food.

Adding appropriate amount of pregelatinized starch in frozen food can avoid the product cracking in the process of freezing and improve the yield, thus reducing the production cost.

In addition, pregelatinized starch has good viscoelasticity and shape-keeping property, which can enhance the elasticity of dumplings, keep the shape of dumplings and help avoid the collapse of dumplings.

5、Pre-gelatinized starch is a good raw material for leisure food such as rice crackers and thin crackers, which is better than common starch.

The reason is made of pre-gelatinized starch mixture blanks when part of the starch has absorbed water, baking, the water from the starch particles to escape, resulting in expansion. Moreover, sometimes, in order to achieve better results, will also use the denaturation over the pre-gelatinized starch.

6, pre-gelatinized starch can be dissolved in cold water, the formation of a certain viscosity paste dispersion is good, and has a thickening and stabilizing effect, easy to use, so it is widely used in a variety of convenience foods. In the consumption of cooking and heating, and also has the effect of thickening, improve the taste; and in a variety of nutritious pastes, instant soups, jams and apple pie filler are applied.

For example, the use of pre-gelatinized starch formulated instant cereal, almond paste, chicken essence, etc., with warm water can be taken to eat, generally in the jam and sweet pie filling dosage of 5% to 10%. Pregelatinized starch gel has a certain strength, instead of pudding gel, in Europe and the United States, pregelatinized starch as the base material, add a certain amount of starch sugar, nutrient enhancers, flavoring agents, etc. made of instant pudding powder is a very best-selling convenience food.

7, pre-gelatinized starch is highly absorbent, paste and viscoelasticity are higher, can also be used for surimi series products, ham, sausage and other food products to enhance the elasticity, improve molding properties, and prevent water loss. If you add a certain percentage of pre-gelatinized starch when making fishballs, it can be used as adhesive and prevent adhesion between fishballs, and the structure of fishballs, color, taste and texture are greatly improved, so that the product is full and smooth: but the amount of additive is appropriate for 10% to 40%, and it is easy to aging if it is too much.

Pregelatinized starch with cold water solubility, high viscosity and high expansion characteristics, so that its use is extremely wide, foreign countries have been widely used in food, medicine, chemical industry, feed, casting, oil drilling, textile, paper and other industries. China’s pre-gelatinized starch industry is developing rapidly, the current annual production capacity has reached about 200,000 tons, but due to the production of raw materials, equipment and technology, the quality of China’s production of pre-gelatinized starch has been lagging behind the international advanced level, both in terms of product quality or production, are unable to meet the demand of China’s market.

Pregelatinized starch raw material selection, pasting methods and conditions, other ingredients in the food will affect the starch pasting effect, and different foods have different requirements on the nature of the pregelatinized starch, such as viscosity, transmittance, freezing and thawing, etc.; excellent pregelatinized starch should be combined with the specific requirements for the preparation. How to improve the production technology of pregelatinized starch, improve the quality of pregelatinized starch and further promote its application in the food industry is of great significance to the development of China’s starch industry.