IndustryPreparation, Properties, and Applications of Pre-gelatinized Starch in the Food Industry
Starch, as a renewable resource, has become an important industrial raw material. Current starch production in China has reached 1.5 million tons. Starch and its deep-processed products are widely used in food, textiles, papermaking, pharmaceuticals, feed, petroleum drilling, foundry, construction, and other industries.
The application of starch in industry is constrained by many inherent properties of native starch, such as insolubility in cold water and instability of the paste under the influence of acid, heat, or shear. To improve starch performance and expand its application scope, native starch can be processed via physical, chemical, or enzymatic methods to produce modified starches, altering its natural properties, adding functionality, or introducing new characteristics to meet the needs of industrial development.
Pre-gelatinized starch is a physically modified starch, also known as alpha-starch (α-starch). It is obtained by rapidly dehydrating and drying a starch paste after it has been gelatinized into a dispersed state, preventing starch molecules from having time to realign, resulting in a porous starch granule with no distinct crystallinity. Due to its porous structure, broken hydrogen bonds, and characteristics such as cold-water solubility, good cold-water stability, and strong water retention, pre-gelatinized starch is used in many industrial sectors.
I. Raw Materials and Preparation Methods for Pre-gelatinized Starch
Pre-gelatinized starch is produced by heating native starch in the presence of a certain amount of water or hydrophilic solvents. The water or hydrophilic solvents break the intermolecular hydrogen bonds and disrupt the orderly arranged micellar structure. After complete gelatinization, rapid drying at high temperatures yields starch granules with broken hydrogen bonds, a porous structure, and no distinct crystallinity.
The gelatinization process can be divided into three stages:
(1) Reversible Water Absorption Stage: Water enters the amorphous regions of the starch granule, causing slight volume swelling. Cooling and drying at this stage allow the granule to revert to its original state, with birefringence unchanged.
(2) Irreversible Water Absorption Stage: As temperature increases, water enters the interstices of starch microcrystals, leading to irreversible and substantial water absorption. Birefringence gradually blurs and eventually disappears, also termed crystalline "dissolution." The starch granule swells to 50–100 times its original volume.
(3) Final Disintegration Stage: The starch granule ultimately disintegrates, and starch molecules fully enter the solution.
Gelatinized starch is also called α-starch. Raw materials for producing pre-gelatinized starch include native starches such as potato, sweet potato, mung bean, wheat, cassava, corn, waxy corn, oat starch, etc., as well as vegetable starches extracted from seed crops and chemically modified starches like cross-linked starch and modified waxy corn starch.
Compared to other pre-gelatinized starches, potato pre-gelatinized starch exhibits strong adhesive properties and good film-forming performance. Cassava starch is also widely used for producing pre-gelatinized starch. For instance, in 2002, Sheng Jianguo et al. reported that chemically modified and subsequently pre-gelatinized cassava starch exhibited properties (e.g., viscosity, filament length, viscous resistance) exceeding those of Thailand's "Taihua" brand pre-gelatinized cassava starch and far superior to domestic pre-gelatinized cassava starch.
Traditional pre-gelatinized starch production primarily involves heating native starch to gelatinize the granules, followed by rapid drying, grinding, sieving, and packaging. The specific process route is: starch + water → starch milk → gelatinization → drying → pulverization → packaging. The most commonly used production methods are drum drying and extrusion, with drum drying being the most prevalent. The drum drying method was invented in Europe by Wulkank Supf in 1908 and officially applied to industrial production in 1920, while the extrusion method is generally suitable for laboratory-scale production.
1. Drum Drying Method
Also known as the hot drum method. Depending on the drum structure, it can be single-drum or double-drum. In double-drum dryers, the drums rotate in opposite directions. Steam is introduced into the drums for heating, raising the drum surface temperature to 150–180°C. A starch milk slurry with a concentration of about 40% is distributed onto the drum surface, forming a uniform thin layer as the drum rotates. Under heating, the starch gelatinizes and dries. When the moisture content drops to about 5%, a scraper blade removes the dried starch film, which is then pulverized and sieved to obtain pre-gelatinized starch.
The operation process is crucial for producing qualified pre-gelatinized starch. Parameters such as coating thickness, drum speed, drum surface temperature, and final product moisture content all affect the quality. To improve the temperature difference inside and outside the starch coating and accelerate heat transfer, the starch milk can be preheated using a jet cooker or heat exchanger before being applied to the drum surface.
2. Screw Extrusion Method
With the promotion and popularization of screw extrusion technology in food processing, the extrusion method can also be used to produce pre-gelatinized starch. The starch is first moistened to a moisture content of about 20%, then fed into the extruder. The heating zone temperature in the extruder can reach 120–200°C, with pressure up to 3–10 MPa.
The starch is pushed forward by the screw inside the extruder and expelled through a die with an aperture of a few millimeters. The high-pressure difference between the extruder interior and the external atmosphere causes the starch to instantaneously expand and dry, achieving the pre-gelatinization effect. The characteristics of the screw extrusion process are continuous production, low energy consumption, low investment, and relatively simple equipment. However, the final product viscosity is lower than that from drum drying, due to the breakage of glycosidic bonds and other linkages within the molecules caused by mechanical shear forces during extrusion.
II. Properties of Pre-gelatinized Starch
Native starch has a microcrystalline structure; it is insoluble and does not swell in cold water and is insensitive to amylase. Heating native starch with a certain amount of water can disrupt the orderly colloidal structure, break intermolecular hydrogen bonds, allow water molecules to penetrate its interior, cause the crystalline structure to disappear, result in the loss of birefringence, and make it susceptible to enzyme action. This process is starch gelatinization. Rapidly drying and dehydrating fully gelatinized starch at high temperatures yields starch granules where the hydrogen bonds remain broken, with a porous structure and no distinct crystallinity, known as pre-gelatinized starch.
Studies on its structure have found that pre-gelatinized starch contains numerous sub-microcrystalline structures. These form gradually during the drying of the starch paste as the average distance between starch molecular chains decreases. Due to the formation of these sub-microcrystals, the X-ray diffraction curve of pre-gelatinized starch appears as a combination of a diffuse crystalline diffraction peak and a diffuse amorphous diffraction peak.
The changes occurring during the drying of a starch paste to prepare pre-gelatinized starch are manifested in the X-ray diffraction curve as the process of formation and separation of these two peaks. The diffuse crystalline diffraction peak develops from nothing to something, growing larger, and its peak position shifts toward smaller diffraction angles according to crystal diffraction principles. Conversely, the diffuse amorphous diffraction peak shrinks from large to small, eventually stabilizing, with its peak position shifting toward larger diffraction angles according to amorphous diffraction principles.
Due to its porous structure and broken hydrogen bonds, pre-gelatinized starch possesses characteristics such as high dispersibility, high oil absorption, rapid hydration rate, high viscosity, and high swelling power compared to native starch, making it applicable in food. The performance of pre-gelatinized starch can be measured by indicators such as viscosity, particle size, degree of gelatinization (α-degree), whiteness and pH value, gel strength, and elasticity. The main performance indicators for pre-gelatinized starch used in food are as follows:
1. Degree of Gelatinization (α-degree)
Refers to the proportion of pre-gelatinized starch in a given product quantity. The α-degree value directly affects product quality. Its determination mainly employs the birefringence method and enzymatic analysis.
2. Particle Size
The particle size of the finished pre-gelatinized starch directly influences product viscosity, rehydration properties, and surface smoothness of the resulting paste. Generally, finer particles yield a paste with higher cold paste viscosity, lower hot paste viscosity, and better surface gloss upon dissolution in water. However, overly rapid rehydration can prevent water from contacting the inner parts of the agglomerates, leading to uneven dispersion and lump formation. Coarser particles dissolve more slowly in cold water, posing issues with excessive water transmission time. Therefore, the final product particle size should be appropriately controlled. Particle size is typically measured by sieving.
3. Viscosity
Starch pastes from different raw material varieties have different viscosity values due to variations in molecular chain structures. Pre-gelatinized starch produced from the same raw material using different methods also exhibits different viscosity values. Under the same conditions, pre-gelatinized starch produced from potato starch has the highest viscosity and good stability. Viscosity is commonly measured using a Brabender viscometer or an Engler viscometer.
III. Applications of Pre-gelatinized Starch in the Food Industry
1. Easy Digestibility of Pre-gelatinized Starch
During pre-gelatinization, water molecules break the hydrogen bonds of starch molecules, thereby disrupting the crystalline structure of starch granules, causing them to swell and dissolve in water. This makes them more easily acted upon by amylases, facilitating human digestion and absorption. This property of pre-gelatinized starch can be utilized in the production of foods for the elderly and infants.
2. Application Advantages in Traditional Foods
Some research has found that potato pre-gelatinized starch has good viscoelasticity and can replace gluten in flour. Adding 8% (by flour weight) to wheat starch can produce satisfactory steamed buns. According to a U.S. patent, adding pre-gelatinized starch containing amylose to dough can improve its morphological characteristics. Additionally, adding an appropriate amount of pre-gelatinized starch to noodles can reduce breakage and enable rapid cooking, with cassava phosphate cross-linked starch being particularly effective at an addition level of 10%.
3. Strong Water Retention for Improving Baked Goods Quality
When making cakes, adding a certain amount of pre-gelatinized starch makes dough formation easier during mixing. Due to its increased water absorption and gas production capacity, pre-gelatinized starch contributes to good cake volume, enhances product freshness and structural uniformity, resulting in a soft texture and pleasant mouthfeel.
4. Good Freeze-Thaw Stability for Stabilizing Frozen Food Structure
Adding an appropriate amount of pre-gelatinized starch to quick-frozen foods can prevent product cracking during the freezing process, improve yield, and thus reduce production costs. Furthermore, pre-gelatinized starch's good viscoelasticity and shape-retention properties can enhance the elasticity of glutinous rice balls (tangyuan) and help maintain their shape, preventing collapse.
5. Excellent Raw Material for Snack Foods like Rice Crackers and Crisp Biscuits
Pre-gelatinized starch is a superior raw material for snacks like rice crackers and crisp biscuits compared to ordinary starch. The reason is that when making the mixed dough/batter, part of the starch has already absorbed water. During baking, this water escapes from the starch granules, causing expansion. Sometimes, modified pre-gelatinized starch is used for even better results.
6. Thickening and Stabilizing Effect
Pre-gelatinized starch can swell and dissolve in cold water, forming a paste with a certain viscosity and good dispersibility. It has thickening and stabilizing effects and is convenient to use, hence its widespread application in various convenience foods. It eliminates the need for cooking/heating during consumption and also provides thickening and taste-improving functions. It is used in various nutritional pastes, instant soup mixes, jams, and apple pie fillings.
For example, pre-gelatinized starch is used to formulate instant oatmeal, almond paste, chicken essence, etc., which can be consumed by mixing with warm water. Its typical usage level in jams and sweet pie fillings is 5–10%. Pre-gelatinized starch gels have a certain strength and can replace pudding gels. In Europe and America, instant pudding powder, made from pre-gelatinized starch as a base with added amounts of starch sugar, nutritional fortifiers, and flavorings, is a very popular convenience food.
7. Application in Meat and Fish Products
Pre-gelatinized starch has strong water absorption, high gelatinization degree, and viscoelasticity. It can also be used in fish surimi products, ham, sausages, etc., to enhance elasticity, improve moldability, and prevent water loss. For instance, adding a certain proportion of pre-gelatinized starch when making fish balls acts as both a binder and prevents balls from sticking together. It also significantly improves the structure, color, flavor, and texture of the fish balls, making the product plump and tender. However, the addition level should be 10–40%; excess may lead to retrogradation.
Pre-gelatinized starch, with its characteristics of cold-water solubility, high viscosity, and high swelling power, has extremely wide applications. It is already extensively used abroad in food, pharmaceuticals, chemicals, feed, foundry, petroleum drilling, textiles, papermaking, and other industries. China's pre-gelatinized starch industry is developing rapidly, with current annual production capacity reaching about 200,000 tons. However, due to factors related to raw materials, equipment, and technology, the quality of pre-gelatinized starch produced in China has lagged behind international advanced levels. Both in terms of product quality and quantity, it cannot meet domestic market demand.
The choice of raw material for pre-gelatinized starch, the gelatinization method and conditions, and other components in the food all affect the gelatinization outcome. Different foods also have different requirements for the properties of pre-gelatinized starch, such as viscosity, translucency, and freeze-thaw stability. High-quality pre-gelatinized starch must be prepared in combination with specific requirements. Improving production technology, enhancing the quality of pre-gelatinized starch, and further promoting its application in the food industry are of great significance for the development of China's starch industry.
