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Sakinah Ghazali

on 18 November 2014

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Transcript of STARCH

Hydrolysis of starch
The amylases break down and hydrolyze starch into the constituent sugars.
Alpha-amylases are found in plants and in animals.
Human saliva is rich in amylase, and the pancreas also secretes the enzyme.
Individuals from populations with a high-starch diet tend to have more amylase genes than those with low-starch diets.
Beta-amylase cuts starch into maltose units.
This process is important in the digestion of starch and is also used in brewing, where amylase from the skin of seed grains is responsible for converting starch to maltose.

Starch Retrogradation
Phenomenon following in the dissolution of starch.
To describe changes that occur in starch granule when heating in continued after gelatinization.
Involve granular swelling, exudation of molecular component from the granule, and disruption of granule.

What is starch?
Starch Gelatinization
The disruption of molecular order within starch granule manifested in irreversible changes in properties such as granular swelling, native crystalline melting, loss of birefringence, uncoiling, and dissociation of the double helices and starch solubilisation.
Associated with a sudden increase in solution viscosity because the hydrodynamic radius of the swollen granule are many times larger than the native granule. When it mixed or allowed to heat for a prolonged period, and under acid condition, viscosity will decrease as the swollen granule are ruptured.
It is transformation that occur when starch is heated in water, thus break the hydrophobic and hydrogen bond. Starch granule start to swell and loses order.
Gelatinization initially occur in the amorphous region because of weak hydrogen bonding.

If starch is subjected to dry heat, it breaks down to form dextrins, also called "pyrodextrins" in this context.
This break down process is known as dextrinization. (Pyro)dextrins are mainly yellow to brown in color and dextrinization is partially responsible for the browning of toasted bread.
Carbohydrate intermediates between starch and the sugars produced from starch by hydrolysis by dilute acids, amylase or dry heat are usually referred to as dextrins .

30 ºC - Heating begin, water absorbed on granule surface, granule still clumped together.
40 ºC - More water absorbed, granule start separating.
50 ºC - Granule start swelling, hydrogen bond within granule broken, amylose leak out.
65 ºC - Granule start rupturing.
70 ºC - More rupturing and leakage of starch granule.
90 ºC - Optimum gelatinization, granule may disintegrate or implode.

Starch become less soluble.
Occurs faster in amylose which is smaller compared to amylopectin. It is due to restriction imposed by the branched structure of the molecules.
Can be reversed by gently heating.
Examples : refrigerated pudding, stale bread
A polysaccharide derived from plant sources (corn, potato, wheat)
A starch molecule is a polymer of 200 or more glucose unit, having both amylopectin and amylose.
Provide 70-80% calories consumed by human
The amount of amylose and amylopectin in starch is important because the behaviour of heated starch in water, a common food processing step, depends on whether the source is relatively high in amylose or in amylopectin.
Because starches often lose functionality when subjected to food processing conditions.

In most instances, modifications involve the alcohol groups on the starch polymer, or glycosidic bond cleavage.
In starch granules, amylose is present in the amorphous structure.
Molecular weight ranging from 10^5-10^6
Very highly branched molecules.
Branches very close giving amylopectin a cluster structure.
Dextrin is an oligomer of the glucose monomers, of which starch is a polymer.
It is an amorphous, yellow or white powder, which is (partly) soluble in water.
Dextrins are used for numerous industrial applications. Some examples of relevant areas are the pharmaceutical industry, the mining industry, the food industry, and the textile industry.

Chemical test
(Iodine test)
Iodine is used to test the presence of starch while iodine solution occur when the iodine dissolved in the aqueous solution of potassium iodide and react with starch producing purple black colour.
 The intensity of the colour decreases with increasing temperature and with the presence of water-miscible, organic solvents such as ethanol.
Besides, waxy starches with little or no amylose present will form red colour.
Meanwhile, starch indicator solution which consisting water, starch and iodine is used in redox titration.
When the presence of oxidizing agent, the solution will turn to blue colour.
Then, when the presence of reducing agent, blue colour will disappear because triiodide ions are break up into three iodide ions.

Iodine Test
Starch Pasting
Factor affecting gelatinization
- Increasing in temperature, more easy to gelatinize.
Type of starch
- Potato is the most effective as thickening agent.
Shear or Agitation
- Facilitate the swelling and rupturing of starch granules.
Effect of other ingredient
- Presence of sugar, acid, fat and others could reduce the gelatinization temperature.

The regular structure of starch molecule that lost during gelatinization, is restored during storage.
When a hot starch is cooled, it generally forms a viscoelastic, firm, and rigid gel or precipitate.
Realigning of starch molecule ( amylose ) and forming double helixes that then will aggregate.
Hydrogen bonds break and re- reform into more orderly crystals.

Factor affecting retrogradation
Size of linear starch molecule – small amylose has more tendency.
Amylose to amylopectin ratio – more amylose, faster retrogradation.
Temperature – cooling accelerate, freezing stop retrogradation.
Infering molecule
fatty acid ( form insoluble complex with starch )
emulsifier/ surfactants ( inhibit crystallization, commonly used in dough for bread.

Application of starch in food industry
Food Industry
The starch industry extracts and refines starches from seeds, roots and tubers, by wet grinding, washing, sieving and drying.
Today, the main commercial refined starches are cornstarch, tapioca, wheat, rice and potato starch.
To a lesser extent, sources include rice, sweet potato, sago and mung bean.
To this day, starch is extracted from more than 50 types of plants.
Untreated starch requires heat to thicken or gelatinize.
When a starch is pre-cooked, it can then be used to thicken instantly in cold water. This is referred to as a pregelatinize starch.

Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two and resulting fragment known as dextrin.
The extent of conversion is typically quantified by dextrose equivelence (DE), which is roughly the fraction of the glycosidic bond in starch that have been broken.

They include:
Maltodextrins a lightly hydrolyzed (DE 10–20) starch product used as a bland-tasting filler and thickener.
Various glucose syrups (DE 30–70), also called corn syrups in the US, viscous solutions used as sweeteners and thickeners in many kinds of processed foods.
Dextrose (DE 100), commercial glucose, prepared by the complete hydrolysis of starch.
High fructose syrup, made by treating dextrose solutions with the enzyme glucose isomerase until a substantial fraction of the glucose has been converted to fructose.
Fructose also has better microbiological stability.
One kind of high fructose corn syrup, HFCS-55, is sweeter than sucrose because it is made with more fructose, while the sweetness of HFCS-42 is on par with sucrose.
Sugar alcohol, such as maltitol, erythritol, sorbitol, mannitol and hydrogenated starch hydrolysate, are sweeteners made by reducing sugars.

Modified starch, also called starch derivatives, are prepared by physically, enzymatically, or chemically treating native starch to change its properties.
It used in practically all starch applications, such as in food products as a thickening agent, stabilizer or emulsifier in pharmaceuticals as a disintegrant as binder in coated paper.
Starches are modified to enhance their performance in different applications.
Starches may be modified to increase their stability against excessive heat, acid, shear, time, cooling, or freezing to change their texture to decrease or increase their viscosity to lengthen or shorten gelatinization time or to increase their visco-stability.

Food starches are typically used as thickeners and stabilizers in foods such as puddings, custards, soups, sauces, gravies, pie fillings, and salad dressings, and to make noodles and pastas.
Resistant starch is starch that escapes digestion in the small intestine of healthy individuals.
High amylose starch from corn has a higher gelatinization temperature than other types of starch and retains its resistant starch content through baking, mild extrusion and other food processing techniques.

It is used as an insoluble dietary fiber in processed foods such as bread, pasta, cookies, crackers, pretzels and other low moisture foods. It is also utilized as a dietary supplement for its health benefits.
In the pharmaceutical industry, starch is also used as an excipient, as tablet disintegrant or as binder.

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