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Starch in chemicals

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Starch in chemicals

Post   on Wed Jun 24, 2009 6:43 am

Starch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. Starch is produced by all green plants as an energy store and is a major food source for humans.

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.[1] Glycogen, the glucose store of animals, is a more branched version of amylopectin.

Starch can be used as a thickening, stiffening or gluing agent when dissolved in warm water, giving wheat paste.

Structure of starch

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 (about 2μm), potato starch have larger granules (up to 100μm). 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.

A typical feature of starch is that it 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. This process is called starch gelatinization. During cooking the starch becomes a paste and gets its viscosity. During cooling or prolonged storage of the paste, the semi-crystalline structure partially recovers and the starch paste thickens. This is mainly caused by the retrogradation of the amylose. This process is also responsible for the staling, hardening of bread and the water layer on top of a starch gel (syneresis).

Some cultivated plant varieties have pure amylopectin starch without amylose, known as waxy starches. The most used is waxy maize. Waxy starches have less retrogradation, the viscosity of the paste will be more stable.

Last edited by sunitas on Wed Jun 24, 2009 6:45 am; edited 1 time in total (Reason for editing : spell mistakes)

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discussion of ketone aldehyde

Post  sonooo on Wed Jul 29, 2009 6:45 am

A [url=] ketone aldehyde [/url] resin having a water

content of less than 0.4% by weight, obtained by condensing at least one ketone with at least one

aldehyde in the presence of at least one alkali metal compound and at least one phase transfer

catalyst in the absence of solvent or in a water-miscible organic solvent, wherein the aldehyde

is added (1) in an initial charge at the beginning of the condensation; and (2) in at least one

further substep following initiation of condensation in (1), wherein the ketone is selected from

the group consisting of 4-tert-amylcyclohexanone, 2-sec-butylcyclohexanone,

2-tert-butylcyclohexanone, 4-tert-butylcyclohexanone, 2-methylcyclohexanone,

3,3,5-trimethylcyclohexanone, and mixtures thereof.


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