Starch extraction and starch preparation
In ancient times, starch was extracted from wheat and used as an additive to medicines and as an adhesive.
Thousands of years ago, ripened grain was soaked in water, pounded with stones to form a grain porridge, mixed with clear water, and after a resting period, the floating components were poured off. The light sediment on the bottom of the vessel was mainly starch.
This sediment was then dried in the sun. In a similar way, our ancestors later obtained potato starch from crushed potatoes. In Europe, the starch industry developed as an agricultural sideline. People worked with the simplest equipment, which was only further developed into special process engineering machines in the course of industrialization. This improved purity, yield and cost structure.
Crops with their approximate starch and amylose contents
In Europe, the raw materials for starch production are potatoes, wheat and corn; outside Europe, starch is also obtained from the crops tapioca and rice. Today, the commercial product starch (C6H10O5)n may contain a maximum of 3% foreign substances. Internationally, the following rule has become established: The maximum permissible protein content in the dry matter of starch is limited. It is 0.58% for cereal starch and 0.13% for potato starch.
Low-protein wheat starch is obtained for dietary nutrients (e.g., celiac disease). The Codex alimentarius (international food regulations) defines starch as gluten-free if the protein content (gluten) is less than 20 mg/kg. Current analytical methods can detect residual protein levels below 5 mg/kg.
The chemical composition of cellulose and starch is very similar.
Starch, an energy carrier, is produced by photosynthesis and is stored in the tubers and seeds of all plants. Thousands of glucose molecules cross-link helically to form a starch molecule, which is incorporated into the starch granule.
Enzymes determine the composition and structure of starch granules. They can link glucose molecules into a long glycosidic strand. This structure is called amylose. If side chains are attached to the strand, it is called amylopectin.
The molecular similarity of starch and cellulose molecules is interesting. Starch is the energy store of plants, while cellulose forms the cellular skeleton of plants - with impressive strength and elasticity, just think of wood or 4 m high hemp stalks.
Depending on the type of starch, the starch grains are of different sizes. The diameter of starch particles can be over 100 µm in potatoes, 2 to 35 µm in wheat, 5 to 25 µm in corn, and only 0.5 to 3 µm in amaranth. Wheat starch has a bimodal distribution of starch grains. This is utilized to produce a high-purity A wheat starch (20 - 35 µm) on the one hand and a small-grain B wheat starch (2 - 10 µm) with higher impurities on the other.
Depending on the origin, starch usually has an amylose content of 14% to 27% and an amylopectin content of 73% to 86%. However, special plant varieties also provide starch with an amylopectin content of up to 99% or an amylose content of up to 85%.
Starch is insoluble in cold water, but starch granules can swell easily and reversibly, increasing their volume by up to 28%. If water is removed from the starch, swelling decreases.
Native starch can bind water or moist goods well, but usually not permanently and especially not at changing temperatures. Starch is therefore modified to accelerate, control or stabilize gel formation. Modified starch can solidify liquids. It can stabilize the consistency of liquids. The consistency is then independent of heat-cold influences and independent of shear stress.
Modified starches can have both hydrophilic and hydrophobic properties. This adapts the starch to the needs of the producers. In the food sector, these are in particular convenience products and the baking agent industry with the process steps cooking, baking, roasting, shock freezing, thawing, influencing Maillard reactions.
In the modification of starch, a distinction is made between 3 main types
by heat treatment, grinding, pregelatinization, roller drying, extrusion or agglomeration. This treatment is not subject to food declaration requirements. Depending on the desired degree of cold solubility, either the cost-effective roller drying or the more expensive spray drying process is used. The latter is usually combined with fluidised bed agglomeration if the starch is supposed to have particularly good instant properties.
The starch is suspended in water in a stirring kettle and, after adding small amounts of acid or lye, is carefully heated without reaching gelatinisation temperature. Once a certain pH value has been achieved, a modifying reagent is added. Once the starch has been neutralised, washed, filtered and dried, it has completely different properties. If starch is chemically transformed, degraded, dextrinized, esterified, etherified or oxidized, it must be declared in foods as an additive with an E number or as modified starch.
If the crystalline structure of the starch granules should remain largely intact despite effective modification, the method of chemical cross-linking of the starch molecule groups with suitable hydroxyl groups such as ethylene or propylene oxide or dicarboxylic acids can be used. This reduces the solubility of the starch, increases the gelatinization temperature and, depending on the degree of crosslinking, suppresses retrogradation.
The enzymatic hydrolysis of starch is a highly efficient process for saccharifying starch into sweeteners. Just as enzymes organise and link molecular structures, form molecular chains and side chains, enzymes can also split such starch molecules. In contrast to chemical cleavage, the enzyme-catalytic hydrolysis of starch proceeds more slowly, but also with less warming. Suitable enzymes can be obtained from moulds, bacteria or the pancreas of cattle. The enzymes are completely washed out of the starch derivative or inactivated after the reaction. The degree of starch degradation can be precisely adjusted so that a variety of products (starch syrup) are available. This form of modification is not subject to declaration. Alternatively, the starch can also be broken down with an acid (acid hydrolysis).
Maltodextrin, e.g. for instant beverages, spice preparations, fruit preparations and ice cream, can be efficiently obtained in this way. The starch is suspended in water with alpha-amylase under slow stirring and is heated slowly. Defined stirring and residence times at different temperatures are observed to achieve the most complete enzymatic degradation possible. The suspension is then washed several times, centrifuged and thermally dried.
The integrity of native starch is easily demonstrated when the starch granules are viewed microscopically under polarized light. Due to the double refraction, the native starch grains appear iridescent with a dark cross, while the treated starch grains appear monochrome without a cross - obviously because their crystal structure is destroyed.
If starch is heated as an aqueous suspension, the starch grain is destroyed above a certain temperature, swelling increases further and amylose emerges from the grain. This process is called gelatinization. The viscosity increases, as does the clarity of the starch-water mixture and its electrical conductivity. It is a structurally viscous solution whose viscosity decreases the more the solution is stirred or sheared. As the solution cools, it clears, the glucose chains align in parallel and form new hydrogen bonds. Depending on the type of starch, a more or less stable gel is formed.
This structurally viscous behavior is exactly opposite to the initial state. Moist starch (water in starch suspension) is dilatant. The higher the shear stress, the more the viscosity increases.
amixon® synthesis reactor / vacuum mixer dryer
In the amixon® process reactor, starch can be modified, mixed and thermally treated in various ways. Under pressure, inert gas atmosphere or vacuum, even highly complex process sequences are realized with excellent mass transfer.
In many cases, the final drying step takes place in a vacuum mixer dryer. In the amixon® vacuum contact dryer, drying of suspensions is particularly effective - even at low temperature levels. In individual cases, the dilatancy of starch suspensions can hinder the flow process. amixon® has extensive experience in handling highly viscous products (dilatant, structural viscous or tixotropic). amixon® equipment stands for safe reaction control. Even if processing (mixing, reacting, suspending, deagglomerating, coating, conditioning, vacuum contact drying) is done in large batches.
Starch has a wide range of applications.
Starch influences texture and opacity, film formation, gel formation and retrogradation. Wherever starch is used in the food industry, it should be neutral in taste and improve the end-use experience for the consumer. In addition, it is said to positively support the mouthfeel, depending on the food.
- A cake spread should be cold swelling, quick to prepare, support creamy flavor, but have lasting elasticity and dimensional stability when cut into cake pieces
- A spray-dried baby food should have good instant properties and a suitable liquid consistency
- A milk-based fruit dessert or yogurt should feel refreshing and cooling in the mouth, but not at all sticky or furry, and on the other hand it should be easy to dose and drip-free in the high-performance filling machine.
- A barbecue sauce should be easy to dispense from the bottle and wet the barbecue food in a thick and highly viscous manner despite the effect of heat, but in the mouth it should naturally develop the spice aromas when chewed.
- A breading or batter powdered with starch should evenly wet the food and adhere firmly. Regardless of whether the dish is consumed immediately or first frozen, packaged and stored.
- An instant beverage powder should disperse quickly and without lumps in the liquid phase even after prolonged storage.
- In a multi-stage fluidized bed process, even highly volatile or oxidation-sensitive liquids can be microencapsulated using starch.
Retrogradation is usually undesirable. Previously bound water is released again with a delay. Gels can liquefy. Such processes occur especially during temperature changes, such as cooling after cooking or thawing of frozen food. Undesirable retrogradation can be reduced by using modified starches or by adding suitable emulsifiers.
Significance and outlook
In Germany, most starch types are interchangeable, especially when used in modified form. Potato starch is generally more expensive than cereal starch, as potato tubers are only available seasonally and there are few by-products in the production process. The supply of wheat starch is currently increasing, as wheat gluten in particular is gaining in importance as a co-product. Wheat gluten / wheat protein has always been a valued co-product, available in dried form for the bakery industry or for animal feed. Today, wheat gluten also serves as the basis for meat substitute products and has become comparatively expensive. The extraction of starch from wheat is therefore becoming increasingly interesting.
Total starch consumption in Europe is about 12 million tons with an annual increase of about 2%. In the USA, the increase is around 4%, in South America around 4.5% and in Asia as much as 7% per year. About 10% of the starch produced worldwide is used in the chemical industry, about 30% in the paper and corrugated board industry, 30% in the food industry and about the same amount is modified or saccharified for the beverage and confectionery industry. Demand for starch and starch derivatives is expected to continue to grow in all industries.
Solid liquid separation, thermal drying is a costly process step.
If the starch or starch derivative is to be marketed as a powder, the major process engineering challenge lies in solid/liquid separation. In the first step, this is done mechanically in horizontally rotating peeler centrifuges or vertically rotating separators. Separation occurs due to density differences between water and solids. This is followed by thermal drying. Continuously operating roller contact dryers or convection dryers such as flash dryers, ring dryers or even grinding dryers are usually used here. The water is evaporated by hot air, while the moist starch is pneumatically swirled and discharged as dry dust.
The food industry is an important driving force for the use and further development of starch derivatives.
Food production worldwide benefits from technological developments in the starch industry.
Starch and its derivatives
- as an additive for instant foods
- as a filler for dietary supplement tablets
- as viscosity regulator and opacifier for instant beverages
- to increase the creaminess in the preparation of desserts
- for conditioning sauces for frozen ready meals
- as a basis for oleoresins in aroma and spice refining
- to increase the water binding capacity in sausage and meat processing
- as a binder in large kitchens and canteens
- as a filler for flavor enhancers
- as an additive to sugar to coat the particles with fat
- as an additive for baking agents and ready-to-bake flours
- as a conditioning agent for breading
As diverse as starch and its derivatives are used in the food industry, so is their use in the pharmaceutical industry:
- as lubricant for medical gloves
- as a filler to bring tablets to a manageable size
- as coating and disintegrating agent for tablets
- as a base material for medicinal powders and deodorants
- as a binder for active medical ingredients
- for stretching cosmetic blush
- as a release agent and lubricant for the trouble-free operation of tablet presses
- as a powdering agent to permanently separate sticky particles from each other
- as viscosity regulator for creams, emulsions, ointments and even aerosols
Starch derivatives are also used in heavy industry:
- in the production of flocculants and defoamers for water treatment
- for the production of cooling lubricants for tunneling and earth drilling
- for adjusting the flowability of concrete for concrete pumps
- for the conditioning of molding sands in the foundry industry
- for smoothing cotton threads in order to be able to weave them without wear, in textile production
- as an adhesive for stamps and corrugated board
- for the production of wood glues
- as a smoothing and conditioning agent in paper production
Starch as a carrier substance for aromas and flavor formers
When starch and starch derivatives are used as carriers for liquid flavors, oleoresins, food colorants, baking extracts, oils and fats, great experience is required to achieve fast and uniform wetting. After all, the primary goal must be to keep energy input as low as possible. The cooler the overall mixture leaves the mixing plant, the better filling, storage, quality preservation and freshness consistency will function later. The production of flavors and aromas, convenience foods, soups, dips and sauces is often a multi-step process.
Specific coating effects are designed to enclose and protect the liquid active ingredients. There is a major conflict of objectives here between the highest possible loading of liquid active ingredients on the one hand and the best possible flowability of the finished mixtures on the other.
Trials in the amixon® test center
Powdered starches and starch derivatives are in most cases irreplaceable additives for the production of powder mixtures for the above-mentioned products. In this context, the mixer is of crucial importance. It must produce an ideal mixing quality in a short time - without heating the mixing goods - which can no further be improved in practice. This process becomes more difficult when liquid additives are incorporated into the formulations, which is often the case. Starches and their derivatives are usually finely dispersed, adherent and not very free-flowing. As products of organic origin, they are also moderately to heavily dusting. They often tend to stick to the mixing tools and to the walls of the mixing chamber.
amixon® has more than 30 precision mixers and synthesis reactors available. We invite you to conduct trials at our facility and offer you proven solutions based on our 40 years of experience.
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