Starch or amylum is a carbohydrate consisting of a large number of glucose units joined

by glycosidic bonds. This polysaccharide is produced by most green plants as an energy

store. It is the most common carbohydrate in human diets and is contained in large amounts

in such staple foods as potatoes, wheat, maize, rice, and cassava.

Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or

alcohol. It consists of two types of molecules: the linear and helical amylose and the branched

amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75

to 80% amylopectin by weight. Glycogen, the glucose store of animals, is a more branched

version of amylopectin. Starch is processed to produce many of the

sugars in processed foods. Dissolving starch in warm water gives wheatpaste, which can

be used as a thickening, stiffening or gluing agent. The biggest industrial non-food use

of starch is as adhesive in the papermaking process. Starch can be applied to parts of

some garments before ironing, to stiffen them.

Etymology The word "starch" is from sterchen, meaning

to stiffen. "amylum" is for starch, from the Greek αμυλον, "amylon" which means "not

ground at a mill". The root amyl is used in biochemistry for several compounds related

to starch. History

Starch grains from the rhizomes of Typha as flour have been identified from grinding stones

in Europe dating back to 30,000 years ago. Starch grains from sorghum were found on grind

stones in caves in Ngalue, Mozambique dating up to 100,000 years ago.

Pure extracted wheat starch paste was used in Ancient Egypt possibly to glue papyrus.

The extraction of starch is first described in the Natural History of Pliny the Elder

around AD 77-79. Romans used it also in cosmetic creams, to powder the hair and to thicken

sauces. Persians and Indians used it to make dishes similar to gothumai wheat halva. Rice

starch as surface treatment of paper has been used in paper production in China, from 700

AD onwards. In addition to starchy plants consumed directly,

66 million tonnes of starch were being produced per year world-wide by 2008. In the EU this

was around 8.5 million tonnes, with around 40% being used for industrial applications

and 60% for food uses, most of the latter as glucose syrups.

Energy store of plants Most green plants use starch as their energy

store. An exception is the family Asteraceae, where starch is replaced by the fructan inulin.

In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The

glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts

and especially amyloplasts. Toward the end of the growing season, starch accumulates

in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare

for the next growing season. Glucose is soluble in water, hydrophilic,

binds with water and then takes up much space and is osmotically active; glucose in the

form of starch, on the other hand, is not soluble, therefore osmotically inactive and

can be stored much more compactly. Glucose molecules are bound in starch by the

easily hydrolyzed alpha bonds. The same type of bond is found in the animal reserve polysaccharide

glycogen. This is in contrast to many structural polysaccharides such as chitin, cellulose

and peptidoglycan, which are bound by beta bonds and are much more resistant to hydrolysis.

Biosynthesis Plants produce starch by first converting

glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase.

This step requires energy in the form of ATP. The enzyme starch synthase then adds the ADP-glucose

via a 1,4-alpha glycosidic bond to a growing chain of glucose residues, liberating ADP

and creating amylose. Starch branching enzyme introduces 1,6-alpha glycosidic bonds between

these chains, creating the branched amylopectin. The starch debranching enzyme isoamylase removes

some of these branches. Several isoforms of these enzymes exist, leading to a highly complex

synthesis process. Glycogen and amylopectin have the same structure,

but the former has about one branch point per ten 1,4-alpha bonds, compared to about

one branch point per thirty 1,4-alpha bonds in amylopectin. Amylopectin is synthesized

from ADP-glucose while mammals and fungi synthesize glycogen from UDP-glucose; for most cases,

bacteria synthesize glycogen from ADP-glucose. In addition to starch synthesis in plants,

starch can be synthesized from non-food starch mediated by an enzyme cocktail. In this cell-free

biosystem, beta-1,4-glycosidic bond-linked cellulose is partially hydrolyzed to cellobioase.

Cellobiose phosphorylase cleaves to glucose 1-phosphate and glucose; the other enzyme—potato

alpha-glucan phosphorylase can add glucose unit from glucose 1-phosphorylase to the non-ruducing

ends of starch. In it, phosphate is internally recycled. The other product—glucose—can

be assimilated by a yeast. This cell-free bioprocessing does not need any costly chemical

and energy input, can be conducted in aqueous solution, and does not have sugar losses.

As a result, cellulosic starch could be used to feed the world because cellulose resource

is about 50 times of starch resource. Degradation

Starch is synthesized in plant leaves during the day, in order to serve as an energy source

at night. Starch is stored as granulates. These insoluble highly branched chains have

to be phosphorylated in order to be accessible for degrading enzymes. The enzyme glucan,

water dikinase phosphorylates at the C-6 position of a glucose molecule, close to the chains

1,6-alpha branching bonds. A second enzyme, phosphoglucan, water dikinase phosphorylates

the glucose molecule at the C-3 position. A loss of these enzymes, for example a loss

of the GWD, leads to a starch excess phenotype. Because starch cannot be phosphorylated, it

accumulates in the plastid. After the phosphorylation, the first degrading

enzyme, beta-amylase is able to attack the glucose chain at its non-reducing end. Maltose

is released as the main product of starch degradation. If the glucose chain consists

of three or less molecules, BAM cannot release maltose. A second enzyme, disproportionating

enzyme-1, combines two maltotriose molecules. From this chain, a glucose molecule is released.

Now, BAM can release another maltose molecule from the remaining chain. This cycle repeats

until starch is degraded completely. If BAM comes close to the phosphorylated branching

point of the glucose chain, it can no longer release maltose. In order for the phosphorylated

chain to be degraded, the enzyme isoamylase is required.

The products of starch degradation are to the major part maltose and to a less extensive

part glucose. These molecules are now exported from the plastid to the cytosol. Maltose is

exported via the maltose transporter. If this transporter is mutated, maltose accumulates

in the plastid. Glucose is exported via the plastidic glucose translocator. Now, these

two sugars act as a precursor for sucrose synthesis. Sucrose can the be used in the

oxidative pentose phosphate pathway in the mitochondria, in order to generate ATP at

night. Properties

Structure

While amylose was traditionally thought to be completely unbranched, it is now known

that some of its molecules contain a few branch points. Although in absolute mass only about

one quarter of the starch granules in plants consist of amylose, there are about 150 times

more amylose molecules than amylopectin molecules. Amylose is a much smaller molecule than amylopectin.

Starch molecules arrange themselves in the plant in semi-crystalline granules. Each plant

species has a unique starch granular size: rice starch is relatively small while potato

starches have larger granules. Starch becomes soluble in water when heated.

The granules swell and burst, the semi-crystalline structure is lost and the smaller amylose

molecules start leaching out of the granule, forming a network that holds water and increasing

the mixture's viscosity. This process is called starch gelatinization. During cooking, the

starch becomes a paste and increases further in viscosity. During cooling or prolonged

storage of the paste, the semi-crystalline structure partially recovers and the starch

paste thickens, expelling water. This is mainly caused by retrogradation of the amylose. This

process is responsible for the hardening of bread or staling, and for the water layer

on top of a starch gel. Some cultivated plant varieties have pure

amylopectin starch without amylose, known as waxy starches. The most used is waxy maize,

others are glutinous rice and waxy potato starch. Waxy starches have less retrogradation,

resulting in a more stable paste. High amylose starch, amylomaize, is cultivated for the

use of its gel strength and for use as a resistant starch in food products.

Synthetic amylose made from cellulose has a well-controlled degree of polymerization.

Therefore, it can be used as a potential drug deliver carrier.

Hydrolysis The enzymes that break down or hydrolyze starch

into the constituent sugars are known as amylases. 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; chimpanzees have very few amylase genes. It is possible that

turning to a high-starch diet was a significant event in human evolution.

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. Dextrinization

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.dextrins are mainly

yellow to brown in color and dextrinization is partially responsible for the browning

of toasted bread. Chemical tests

Iodine solution is used to test for starch; a dark blue color indicates the presence of

starch. The details of this reaction are not yet fully known, but it is thought that the

iodine fit inside the coils of amylose, the charge transfers between the iodine and the

starch, and the energy level spacings in the resulting complex correspond to the absorption

spectrum in the visible light region. The strength of the resulting blue color depends

on the amount of amylose present. Waxy starches with little or no amylose present will color

red. Starch indicator solution consisting of water,

starch and iodine is often used in redox titrations: in the presence of an oxidizing agent the

solution turns blue, in the presence of reducing agent the blue color disappears because triiodide

ions break up into three iodide ions, disassembling the starch-iodine complex. A 0.3% w/w solution

is the standard concentration for a starch indicator. It is made by adding 3 grams of

soluble starch to 1 liter of heated water; the solution is cooled before use.

Each species of plant has a unique type of starch granules in granular size, shape and

crystallization pattern. Under the microscope, starch grains stained with iodine illuminated

from behind with polarized light show a distinctive Maltese cross effect.

Food Starch is the most common carbohydrate in

the human diet and is contained in many staple foods. The major sources of starch intake

worldwide are the cereals and the root vegetables. Many other starchy foods are grown, some only

in specific climates, including acorns, arrowroot, arracacha, bananas, barley, breadfruit, buckwheat,

canna, colacasia, katakuri, kudzu, malanga, millet, oats, oca, polynesian arrowroot, sago,

sorghum, sweet potatoes, rye, taro, chestnuts, water chestnuts and yams, and many kinds of

beans, such as favas, lentils, mung beans, peas, and chickpeas.

Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta,

porridge and tortilla. Digestive enzymes have problems digesting

crystalline structures. Raw starch will digest poorly in the duodenum and small intestine,

while bacterial degradation will take place mainly in the colon. When starch is cooked,

the digestibility is increased. Hence, before humans started using fire, eating grains was

not a very useful way to get energy. Starch gelatinization during cake baking can

be impaired by sugar competing for water, preventing gelatinization and improving texture.

Starch 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. Historically,

Florida arrowroot was also commercialized. 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 pregelatinized starch.

Starch sugars Starch can be hydrolyzed into simpler carbohydrates

by acids, various enzymes, or a combination of the two. The resulting fragments are known