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Starch is a plant polysaccharide synthesized in chloroplasts during photosynthesis and performs an energetic function. The starch formula is similar to the cellulose formula - (C 6 H 10 O 5) n.
Structure
Starch has a complex chemical structure, being a mixture of two main polysaccharides:
- amylose - 10-20%;
- amylopectin - 90-80%.
Each polysaccharide consists of a monomer, α-glucose. The amylose and amylopectin units are linked into chains by α (1 → 4) -glycosidic bonds.
The amylose molecule has a linear structure consisting of 200-1000 structural units. The chain is twisted into a spiral. There are six glucose residues for each loop.
Rice. 1. Structural formula of amylose.
Amylopectin is a branched chain of six to 40 thousand links. The branching of the chain is due to α (1 → 6) -glycosidic bonds through 20-25 glucose residues.
Rice. 2. Structural formula of amylopectin.
In addition to polysaccharides, starch includes inorganic substances (phosphoric acid residues), lipids, fatty acids.
Being in nature and getting
Starch is formed during photosynthesis as a result of glucose polymerization:
- 6CO 2 + 6H 2 O (light, chlorophyll) → C 6 H 12 O 6 + 6O 2;
- nC 6 H 12 O 6 → (C 6 H 10 O 5) n + nH 2 O.
Starch is the main constituent of plant seeds. It is used as a reserve of energy. Most of the starch is contained in the endosperm of cereals (up to 85%) and in potato tubers (20%).
Starch is found in cells in the form of grains, the shape of which depends on the type of plant. Starch grains are layered grains. They grow by putting new layers of starch on top of old layers. Grains are deposited in special plant cells (varieties of leukoplasts) - amyloplasts.
Rice. 3. Examples of starch grains.
In food and industrial chemistry, starch is most often isolated from potatoes. For this, the tubers are crushed, washed and defended. The starch floating on the surface is collected, washed and dried until crystals are formed.
Starch is not synthesized in animals. An analogous energy substance of animal cells is glycogen.
Properties
Starch is a white, crystalline, tasteless powder. The powder is insoluble in cold water. When interacting with hot water, amylose dissolves, and amylopectin swells, forming a paste. If you rub the crystals between your fingers, you can hear a creak.
When heated, starch undergoes hydrolysis by catalysts. Hydrolysis proceeds in steps. Dextrin is formed from starch, which is hydrolyzed to maltose. As a result of hydrolysis of maltose, glucose is formed. General equation:
(C 6 H 10 O 5) n + nH 2 O (H 2 SO 4) → nC 6 H 12 O 6.
A qualitative reaction is blue staining under the influence of iodine.
The reactions of the silver mirror and the reduction of copper hydroxide do not go on.
Starch is eaten together with plant products - potatoes, flour, corn. Also used to make glue.
What have we learned?
Starch is a complex substance of vegetable origin. Consists of organic and inorganic substances, includes two polysaccharides - amylose and amylopectin. Each polysaccharide consists of the same glucose residues. Formed in plants as a result of photosynthesis and accumulates in the form of grains. When interacting with water, it swells, forming a paste. It hydrolyzes when heated in the presence of a catalyst to glucose.
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This means that it consists of monosaccharides linked in long chains. It is actually a mixture of two different polymeric substances: starch is composed of amylose and amylopectin. The monomer in both chains is a glucose molecule, but they differ significantly in structure and properties.
General composition
As mentioned, both amylose and amylopectin are alpha-glucose polymers. The difference lies in the fact that the amylose molecule has a linear structure, while the amylopectin molecule is branched. The first is a soluble fraction of starch, amylopectin is not, and in general starch in water is a colloidal solution (sol), in which the dissolved part of the substance is in equilibrium with the undissolved one.
Here, for comparison, the general structural formulas of amylose and amylopectin are given.
Amylose is soluble due to the formation of micelles - these are several molecules assembled together in such a way that their hydrophobic ends are hidden inward, and their hydrophilic ends are hidden outward for contact with water. They are in equilibrium with molecules not assembled into such aggregates.
Amylopectin is also capable of forming micellar solutions, but to a much lesser extent, and therefore is practically insoluble in cold water.
Amylose and amylopectin in starch are in a ratio of approximately 20% of the former to 80% of the latter. This indicator depends on how it was obtained (in different starch-containing plants, the percentages are also different).
As already mentioned, only amylose can dissolve in cold water, and even then only partially, but in hot starch a paste is formed - a more or less homogeneous sticky mass of swollen individual starch grains.
Amylose
Amylose consists of glucose molecules linked by 1,4-hydroxyl bonds. It is a long, unbranched polymer, with an average of 200 individual glucose molecules.
In starch, the amylose chain is coiled: the diameter of the "windows" in it is approximately 0.5 nanometers. Thanks to them, amylose is able to form complexes, inclusion compounds of the "guest-host" type. These include the well-known reaction of starch with iodine: the amylose molecule is the "host", the iodine molecule is the "guest", which is placed inside the spiral. The complex has an intense blue color and is used to detect both iodine and starch.
V different plants the percentage of amylose in starch can vary. In wheat and corn, it is typically 19-24% by weight. it contains 17%, and apple contains only amylose - 100% mass fraction.
In the paste, amylose forms a soluble part, and this is used in analytical chemistry to separate starch into fractions. Another method, fractionation of starch, is the precipitation of amylose in the form of complexes with butanol or thymol in boiling solutions with water or dimethyl sulfoxide. In chromatography, the property of amylose to adsorption on cellulose (in the presence of urea and ethanol) can be used.
Amylopectin
Starch has a branched structure. This is achieved due to the fact that, in addition to 1 and 4-hydroxyl bonds, glucose molecules in it also form bonds at the 6th alcohol group. Each such "third" bond in the molecule is a new branch in the chain. The general structure of amylopectin resembles a bunch in appearance, the macromolecule as a whole exists in the form of a spherical structure. The number of monomers in it is about 6000, and the molecular weight of one molecule of amylopectin is much higher than that of amylose.
Amylopectin also forms an inclusion compound (clathrate) with iodine. Only in this case the complex is colored red-violet (closer to red).
Chemical properties
Chemical properties amylose and amylopectin, excluding the already discussed interactions with iodine, are exactly the same. They can be conditionally divided into two parts: reactions characteristic of glucose, that is, occurring with each monomer separately, and reactions involving bonds between monomers, for example, hydrolysis. Therefore, further we will talk about the chemical properties of starch as a mixture of amylose and amylopectin.
Starch belongs to non-reducing sugars: all glycosidic hydroxyls (hydroxyl group at the 1st carbon atom) are involved in intermolecular bonds and therefore cannot be present in oxidation reactions (for example, Tollens test - a qualitative reaction to an aldehyde group, or interaction with Felling's reagent - freshly precipitated copper hydroxide). The preserved glycosidic hydroxyls, of course, are available (at one of the ends of the polymer chain), but in small quantities they do not affect the properties of the substance.
However, just like individual glucose molecules, starch is capable of forming esters with the help of hydroxyl groups not participating in the bonds between monomers: a methyl group, an acetic acid residue, and so on can be "hung" on them.
Also, starch can be oxidized with iodic (HIO 4) acid to dialdehyde.
Starch hydrolysis is of two types: enzymatic and acidic. Enzyme hydrolysis belongs to the field of biochemistry. The enzyme amylase breaks down starch into shorter polymer chains from glucose - dextrins. Acidic hydrolysis of starch is complete in the presence of, for example, sulfuric acid: starch is split immediately to a monomer - glucose.
In wildlife
In biology, starch is primarily a complex carbohydrate, and therefore is used by plants as a way of storing nutrients. It is formed during photosynthesis (first in the form of individual glucose molecules) and is deposited in plant cells in the form of grains - in seeds, tubers, rhizomes, etc. (to be later used as a "food warehouse" by new embryos). Sometimes starch is found in the stems (for example, y - mealy starch core) or leaves.
In the human body
The starch in the food first enters the oral cavity. There, an enzyme contained in saliva (amylase) breaks down the polymer chains of amylose and amylopectin, converting molecules into shorter ones - oligosaccharides, then breaks them down, and in the end maltose remains - a disaccharide consisting of two glucose molecules.
Maltose breaks down maltase to glucose - a monosaccharide. And already glucose is used by the body as an energy source.
A polysaccharide containing monosaccharide residues of the same type is called homopolysaccharide.
According to their functional purpose, homopolysaccharides can be divided into two groups: structural and reserve polysaccharides. Cellulose is an important structural homopolysaccharide, and the main reserve ones are glycogen and starch.
Starch is a mixture of 2 homopolysaccharides: linear - amylose and branched - amylopectin, the general formula of which is (C6H10O5) n. As a rule, the content of amylose in starch is 10-30%, amylopectin - 70-90%.
Amylose- starch polysaccharide, consisting mainly of linear or weakly branched chains formed by α-glucose residues, connected by glycosidic bonds between the first and fourth carbon atoms. The amylose chain contains 200 to 1000 monosaccharide units. Due to the axial position of the glycosidic bond, the amylose macromolecule is coiled. Its colloidal particles (micelles) give a characteristic blue coloration with iodine.
Amylopectin- a branched polysaccharide built from α-glucose residues, which are linked in the main chain by α-1,4-glycosidic bonds, and at the branch points by α-1,6-glycosidic bonds:
Amylose and amylopectin are formed in plants as starch grains.
Starch is used as a filler, and in surgery - for the preparation of fixed dressings. It is widely used in powders, ointments, pastes, together with zinc oxide, talc. Inside, starch is used as a coating agent for gastrointestinal diseases.
Glycogen- a branched glucose homopolymer (animal starch), in which glucose residues are linked in linear sections by an α-1,4-glycosidic bond. At the branch points, the monomers are linked by α-1,6-glycosidic bonds. In structure, it is similar to amylopectin, but it has even more branching of the chains, which contributes to the fulfillment of the energy function. It is deposited as an energy reserve in the cells of mainly animal organisms, it is also found in small quantities in the tissues of plants and fungi; Glycogen is found in almost all organs and tissues of animals and humans, but most of all in the liver and muscles. It is a reserve carbohydrate.
Cellulose Is the most common plant polysaccharide. Serves as a support material for plants. It is a linear polysaccharide built from β-glucose residues linked by β-1,4-glycosidic bonds. Structural element cellulose is cellobiose.
Cellulose is one of the structural components
Glycogen- the main energy reserve of humans and animals. It is especially abundant in the liver (up to 10%) and muscles (up to 4% of the dry mass). It also consists of amylopectin, only the molecule is more compact, because has a more branched structure. n - the formula is the same as for starch. Mr 10 5 - 10 8 Yes
During acid hydrolysis, starch and glycogen break down first into dextrins, then into disaccharides - maltose and isomaltose, then into two GLA.
Cellulose (fiber ) Is a structural polysaccharide of plant origin, consisting of ß - D - glucopyranose residues connected by 1 "4 glycolysidic bonds. Mr = 1-2 mln Da. It is not digested in humans and animals; because there is no enzyme ß-glucosidase... In the presence of an optimal amount of cellulose in food, feces are formed.
Carbohydrate metabolism
Consists of
1) the splitting of polysaccharides in the gastrointestinal tract to monosaccharides, which are absorbed from the intestine into the blood;
2) synthesis and breakdown of glycogen in tissues;
3) anaerobic and aerobic breakdown of glc;
4) interconversion of hexoses;
5) aerobic metabolism of PVC;
6) gluconeogenesis - the synthesis of glc from non-carbohydrate components - PVC, glycerol lactate, AA and other sources.
The main metabolite in carbohydrate metabolism is glucose.
Its sources: 1) food carbohydrates
2) glycogen
3) PVC, AK, glts, etc.
Digestion of carbohydrates (starch).
1... Oral cavity. Saliva contains f-t amylase α, ß, γ (differ in the end products of their enzymatic action).
α-amylase is an endoamylase that acts on 1 "4 internal bonds in polys."
ß- and γ-amylases are exoamylases - they cleave the terminal 1 "4 bonds
ß – amylase - disaccharide maltose;
γ amylase - one by one terminal residues of glc.
Amylase of saliva is represented only by α-amylase, therefore, the result of its action is large fragments of glycogen and starch - dextrins and a small amount of maltose.
2. Stomach... Further, food, more or less moistened with saliva, enters the stomach. As a result of the acidic environment of the stomach (pH 1.5 - 2.5), salivary α-amylase is inactivated. In the deep layers of the food bolus, the action of amylase continues and the polysaccharides are cleaved to form dextrins and maltose. There is no decay in the stomach itself, because there are no specific enzymes.
3... The main stage of Y cleavage occurs in the duodenum 12.
Into the lumen of the intestine is released pancriatic α-amylase(pH - 7). Pancreatic amylase cleaves only 1 "4 glycosidic bonds. But, as you know, the glycogen molecule is branched. At branch points 1" 6 glycosidic bonds, it is affected by specific f-you: (glucose) oligo-1,6-glucosidase and (starch) a cute-1,6-glucosidase... In the intestine, under the action of these 3 f-tov, Y are broken down to disaccharides (maltose, etc.). These enzymes do not affect the bonds in disaccharides. For these purposes, the intestines have their own enzymes: their name is the root of the disaccharide + aza: maltase, sucrase, etc. As a result of the total effect of these E, a mixture of monosaccharides is formed - glc, galactose, fructose. The bulk is glucose.
4.Glc absorption occurs due to active transport with Na +. Glc + Na + forms a complex that enters the cell, here the complex disintegrates, Na + is excreted. Other monosugars are diffusely absorbed (i.e. by concentration gradient). Glc coming from the intestinal lumen for the most part (> 50%) with the blood of the portal vein enters the liver, the rest of the glc is transported through the general bloodstream to other tissues. The concentration of glucose in the blood is normally maintained at a constant level and is 3.33 - 5.55 μmol / l, which corresponds to 80-100 mg per 100 ml. blood. GLA transport into cells is facilitated diffusion but in many cells regulated pancreatic hormone insulin m (the exception is the brain and liver - here the content of glucose is directly proportional to the concentration of glucose in the blood). The action of insulin leads to the movement of carrier proteins from the cytosol to the plasma membrane. Then, with the help of these proteins, glc is transported into the cell by hail. concentration. Insulin thus increases the permeability of the cell membrane for GLA.
Starch(C 6 H 10 O 5) n - amorphous white powder, tasteless and odorless, poorly soluble in water, in hot water forms a colloidal solution (paste). Starch macromolecules are built from a large number of α-glucose residues. Starch consists of two fractions: amylose and amylopectin. Amylose has linear molecules, while amylopectin is branched.
Biological role.
Starch is one of the products of photosynthesis, the main nutrient reserve of plants.
Starch is the main carbohydrate in human food.
Receiving.
Starch is most often obtained from potatoes.
To do this, the potatoes are crushed, washed with water and pumped into large vessels, where settling takes place. The resulting starch is washed again with water, settled and dried in a stream of warm air.
Chemical properties.
1. With iodine, starch gives a violet color.
Starch is a polyhydric alcohol.
3. Starch is relatively easily hydrolyzed in an acidic medium and under the action of enzymes:
(C6H10O5) n + nH2O → nC6H12O6
starch glucose
Depending on the conditions, starch hydrolysis can proceed stepwise, with the formation of various intermediate products:
(C6H10O5) n → (C6H1005) x → (C6H1005) y → C12H22O11 → nC6H12O6
starch soluble dextrins maltose glucose starch
There is a gradual degradation of macromolecules.
The use of starch.
Starch is used in confectionery production (obtaining glucose and molasses), is a raw material for the production of ethyl, n-butyl alcohols, acetone, citric acid, glycerin and so on.
It is used in medicine as fillers (in ointments and powders), as an adhesive.
Starch is a valuable nutritious product. To facilitate its absorption, foods containing starch are exposed to high temperatures, that is, potatoes are boiled, bread is baked.
Starch. Chemical properties, application
Under these conditions, partial hydrolysis of starch occurs and dextrins, soluble in water. Dextrins in the digestive tract undergo further hydrolysis to glucose, which is absorbed by the body. Excess glucose is converted to glycogen(animal starch). The composition of glycogen is the same as that of starch - (C6H10O5) n, but its molecules are more branched.
Starch as a nutrient.
Starch is the main carbohydrate in our food, but it cannot be absorbed by the body on its own.
2. Like fats, starch is first hydrolyzed.
3. This process begins already when the food is chewed in the mouth under the action of an enzyme contained in saliva.
5. The resulting glucose is absorbed through the intestinal wall into the blood and enters the liver, and from there - into all tissues of the body.
Excess glucose is deposited in the liver in the form of a high molecular weight carbohydrate - glycogen.
Features of glycogen: a) in structure, glycogen differs from starch by the greater branching of its molecules; b) this storage glycogen between meals is again converted into glucose as it is consumed in the cells of the body.
The intermediate products of starch hydrolysis (dextrins) are more easily absorbed by the body than starch itself, since they consist of smaller molecules and dissolve better in water.
8. Cooking is often associated with the conversion of starch to dextrins.
The use of starch and its production from starch-containing products.
Starch is used not only as a food product.
2.In Food Industry glucose and molasses are prepared from it.
3. To obtain glucose, starch is heated with dilute sulfuric acid for several hours.
4. When the hydrolysis process is over, the acid is neutralized with chalk, the formed calcium sulfate precipitate is filtered off and the solution is evaporated.
If the hydrolysis process is not completed, then the result is a thick sweet mass - a mixture of dextrins and glucose - molasses.
Features of molasses: a) it is used in confectionery for the preparation of certain varieties of sweets, marmalade, gingerbread, etc.
NS.; b) with molasses, confectionery does not seem sugary-sweet, like cooked with pure sugar, and remains soft for a long time.
6. Dextrins obtained from starch are used as glue. Starch is used for starching linen: under the action of heating with a hot iron, it turns into dextrins, which stick together the fibers of the fabric and form a dense film that protects the fabric from rapid contamination.
Starch is most often obtained from potatoes. The potatoes are washed, then crushed on mechanical graters, the crushed mass is washed on sieves with water.
8. Small grains of starch freed from the tuber cells pass through the sieve with water and settle at the bottom of the vat. The starch is thoroughly washed, separated from water and dried.
Homopolysaccharides: starch (amylose and amylopectin), glycogen, cellulose - structure, properties, hydrolysis, biol.
Starch. This polysaccharide consists of two types of polymers built from D-glucopyranose: amylose (10-20%) and amylopectin (80-90%). Starch is formed in plants during photosynthesis and is “stored” in tubers, roots, and seeds.
Starch is a white amorphous substance.
It is insoluble in cold water, swells in hot water, and some of it gradually dissolves. With the rapid heating of starch due to the moisture it contains (10-20%), the macromolecular chain is hydrolytically split into smaller fragments and a mixture of polysaccharides called dextrins is formed. Dextrins are more soluble in water than starch.
This process of breaking down starch, or dextrinization, is carried out in baking.
Flour starch converted to dextrins is easier to digest due to its greater solubility.
Amylose- a polysaccharide in which the D-glucopyranose residues are linked by α (1,4) -glycosidic bonds, i.e.
Starch: formula, chemical properties, application
the disaccharide moiety of amylose is maltose.
The amylose chain is unbranched, includes up to a thousand glucose residues, molecular weight up to 160 thousand.
According to X-ray structural analysis, the amylose macromolecule is coiled. There are six monosaccharide units for each turn of the helix. The inner channel of the spiral can accommodate molecules of the same size, for example, iodine molecules, forming complexes called inclusion compounds.
The complex of amylose with iodine is blue. This is used for analytical purposes to discover both starch and iodine (starch iodine test).
Rice. 1. Helical structure of amylose (view along the axis of the helix)
Amylopectin unlike amylose, it has a branched structure (Fig.
2). Its molecular weight reaches 1-6 million.
Rice. 2. Branched amylopectin macromolecule (colored circles - branching sites of side chains)
Amylopectin is a branched polysaccharide, in the chains of which the D-glucopyranose residues are linked by α (1,4) -glycosidic bonds, and at the branch points - by α (1,6) -links.
There are 20-25 glucose residues between the branching points.
Hydrolysis of starch in the gastrointestinal tract occurs under the action of enzymes that break down α (1,4) - and α (1,6) -glycosidic bonds. The end products of hydrolysis are glucose and maltose.
Glycogen. In animal organisms, this polysaccharide is a structural and functional analogue of plant starch.
In structure, it is similar to amylopectin, but has even greater chain branching. Usually, there are 10-12, sometimes even 6 glucose units between the branching points. Conditionally, we can say that the branching of the glycogen macromolecule is twice that of amylopectin.
Strong branching contributes to the fulfillment of the energetic function of glycogen, since only with a multitude of terminal residues can the rapid cleavage of the required number of glucose molecules be ensured.
The molecular weight of glycogen is unusually high and reaches 100 million. This size of macromolecules contributes to the performance of the function of a reserve carbohydrate.
So, the glycogen macromolecule, due to its large size, does not pass through the membrane and remains inside the cell until the need for energy arises.
Hydrolysis of glycogen in an acidic medium proceeds very easily with a quantitative yield of glucose.
This is used in the analysis of tissues for glycogen content by the amount of glucose formed.
Similar to glycogen in animal organisms, the same role of the reserve polysaccharide in plants is played by amylopectin, which has a less branched structure. This is due to the fact that metabolic processes are much slower in plants and a rapid flow of energy is not required, as is sometimes necessary for an animal organism (stressful situations, physical or mental stress).
Cellulose. This polysaccharide, also called fiber, is the most abundant plant polysaccharide.
Cellulose has great mechanical strength and serves as a support material for plants. Wood contains 50-70% cellulose; cotton is almost pure cellulose. Cellulose is an important raw material for a number of industries (pulp and paper, textile, etc.).
Cellulose is a linear polysaccharide in which D-glucopyranose residues are linked by β (1,4) -glycosidic bonds.
The disaccharide moiety of cellulose is cellobiose.
The macromolecular chain has no branches, it contains 2.5-12 thousand glucose residues, which corresponds to a molecular weight of 400 thousand to 1-2 million.
The β-configuration of the anomeric carbon atom leads to the fact that the cellulose macromolecule has a strictly linear structure.
This is facilitated by the formation of hydrogen bonds within the chain, as well as between adjacent chains.
Such packing of chains provides high mechanical strength, fibrousness, insolubility in water and chemical inertness, which makes cellulose an excellent material for building cell walls of plants.
Cellulose is not broken down by the usual enzymes of the gastrointestinal tract, but is necessary for normal nutrition as a ballast.
Essential cellulose derivatives are of great practical importance: acetates (artificial silk), nitrates (explosives, colloxylin) and others (viscose fiber, cellophane).
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B). Starch function
Starch is a plant polysaccharide with a complex structure. It consists of amylose and amylopectin; their ratio is different in different starches (amylose 13-30%; amylopectin 70-85%).
Amylose and amylopectin in plants are formed in the form of starch grains.
Starch properties, starch molecule
Thickener. 2. Binding agent in products. Is present in raw materials or added.
Gelatinization and other properties... Intact starch grains are insoluble in cold water, but can absorb moisture reversibly and swell easily. The increase in the diameter of the grains upon swelling depends on the type of starch. For example, for ordinary corn starch - 9.1%, for waxy - 22.7%.
As the temperature rises, the vibration of starch molecules increases, intermolecular bonds are destroyed, which leads to the release of binding sites for interaction with water molecules through hydrogen bonds.
This penetration of water and the increasing separation of large and long segments of starch chains increases the disorder in the overall structure and reduces the number and size of crystalline regions. With further heating in the presence of a large amount of water, a complete loss of crystallinity occurs, accompanied by a loss of the outline of starch grains. The temperature corresponding to the destruction of the internal structure of starch grains is called the gelatinization temperature. It depends on the source of the starch.
During gelatinization, the starch grains swell very strongly, at first an increase in temperature leads to a steep rise in viscosity, which is associated with the swelling of starch grains.
The swollen starch grains then rupture and disintegrate, causing a drop in viscosity.
Factors affecting the starch gelatinization process:
1. Temperature.
2. Water activity ( the higher, the faster the process, the water activity is influenced by the water-binding components).
3. High sugar content reduce the rate of starch gelatinization, reduce the peak viscosity.
Disaccharides are more effective in slowing down gelatinization and lowering peak viscosity than monosaccharides. In addition, sugars reduce the strength of starch gels by acting as a plasticizer and interfering with the formation of binding zones.
For starch gelatinization during production food products influenced by lipids - triglycerides (fats, oils), mono- and diacylglycerides. Fats, which can form complexes with amylose, inhibit the swelling of starch grains. As a consequence, in white bread, which is low in fat, 96% of the starch is usually completely gelatinized.
In the production of baked goods, these two factors (high fat concentration and low aw) contribute greatly to the non-gelatinization of starch.
Monoacylglycerides of fatty acids (C | 6-C18) lead to an increase in the gelatinization temperature, an increase in temperature corresponding to the peak of viscosity, and a decrease in the strength of the gel.
This is due to the fact that the components of fatty acids in monoacylglycerides can form inclusion compounds with amylose, and, possibly, with long outer chains of amylopectin. Lipid-amylose complexes also interfere with the formation of binding zones.
5. Low concentration salts generally have no effect on gelatinization or gel formation.
The exception is potato amylopectin, which contains phosphate groups. In this case, salts can, depending on from conditions, either increase or decrease the swelling.
6. Acids are found in many foods where is used starch as a thickener. However, most foods has a pH in the region 4-7, and these concentrations of H + ions do not have a large effect on the swelling of starch or its gelatinization.
At low pH (salad dressings, fruit flavors), there is a noticeable decrease in the peak viscosity of starch pastes and a rapid decrease in viscosity on heating. At low pH, intense hydrolysis takes place with the formation of non-thickening dextrins, it is necessary to use modified cross-linked starches as a thickener in acidic products to avoid acid dilution.
The presence of proteins. This is primarily important from the point of view of the formation of the structure of bread, which is associated with the formation of gluten (with stirring during the dough preparation process), gelatinization of starch and denaturation of protein, due to heating in the presence of water. However, the exact nature of the interaction between starch and protein in food systems remains unclear.
8. In the production of frozen foods in which starch acts as a thickener, it is necessary to reckon with the possibility of retrogradation of amylose during thawing. If in this case ordinary starch is used, then during thawing the products acquire a fibrous or grain-like structure.
It is preferable for such products to use waxy corn starch, which is practically free of amylose, or phosphate cross-linked starches.
9. In many starch-containing food products (first of all, bakery products), during storage, staling is observed, associated with the association of amylose molecules. To prevent staling in such products, it is advisable to use fats as additives that form complexes with amylose, warming up, wetting with water.
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Polysaccharides.
Polysaccharides - these are natural high-molecular carbohydrates, the macromolecules of which consist of monosaccharide residues.
Main representatives - starch and cellulose - built from the remains of one monosaccharide - glucose.
Starch and cellulose have the same molecular formula: (C6H10O5) n but completely different properties. This is due to the peculiarities of their spatial structure.
Starch consists of α-glucose residues, and cellulose consists of β-glucose, which are spatial isomers and differ only in the position of one hydroxyl group (highlighted in color):
Starch called a mixture of two polysaccharides built from residues cyclic α-glucose.
It includes:
- amylose (inner part of starch grain) - 10-20%
- amylopectin (shell of starch grain) - 80-90%
The amylose chain includes 200 - 1000 α-glucose residues (average molecular weight 160,000) and has an unbranched structure.
The amylose macromolecule is a helix, each turn of which consists of 6 α-glucose units.
Starch properties:
Starch hydrolysis: when boiled in an acidic medium, starch is successively hydrolyzed.
2. Starch does not give a “silver mirror” reaction and does not reduce copper (II) hydroxide.
Qualitative reaction to starch: blue staining with iodine solution.
