STARCH AND STARCH TECHNOLOGY.
STARCH is a polysaccharide material found in cereal grains of wheat, corn and rice, and in root and stem tubers of cassava, yam and potato. As a polysaccharide polymer starch is composed of a large number of glucose monosaccharide units.
Starch is the storage form of carbohydrates in plant seed endosperm and tubers where it exists as granules. Each granule usually contains millions of amylose molecules and a lower number of millions of amylopectin molecules. Amylose molecules are always smaller than amylopectin molecules.
Starch is also metabolized for energy in plants and animals, and is used to produce a large number of industrial products.
AMYLOSE AND AMYLOPECTIN OF STARCH.
Starch is composed of two fractions – amylose and amylopectin. Natural starches (i.e unmodified starches) are mixtures of amylose (10-20%) and amylopectin (80-90%).AMYLOSE forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble in it. A colloid is a type of chemical mixture in which one substance (amylose) is dispersed evenly throughout another (hot water).
A colloidal system (i.e, a colloidal solution or colloidal suspension) consists of two separate phases: a dispersed phase (or internal phase) and a continuous phase (or dispersion medium).
Colloids may be translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid, or they may be opaque.
Amylose consists of long chains of glucose units connected by alpha (α) acetal linkage. All of the monomer units are α -D-glucose, and all the α acetal links connect carbon no. 1 (C1) of one glucose to carbon no. 4 (C4) of the next glucose.
C1 is called an anomeric carbon because it can either have the α configuration of attached groups or the beta (β) configuration. C1 is an acetal carbon because it has two ether oxygens attached to it.
The α configuration is when the ether oxygen on the C1 is on the opposite side of the ring as the C6. In the chair structure this results in a downward projection. This is the same as the -OH in a hemiacetal. Thus a glycosidic bond, which joins the hemiacetal group of a carbohydrate molecule to the hydroxyl of another group, and which may or may not be another carbohydrate, is an acetal bond.
Amylose characteristically forms a helix (a spiral that looks like a coiled spring). Amylose is specifically responsible for the formation of a deep blue colour of starch with iodine. This is because iodine molecules can penetrate into the amylose coil.
Amylose molecule (beween 300 – 3000 molecules of glucose)
AMYLOPECTIN is formed by α (1 – 4) acetal bonds between α –D-glucose units, as in amylose, but additionally there are branches formed by α (1 – 6) acetal linkages between glucose molecules. Amylopectin has 12-20 glucose units between the branches.
Amylopectin molecule (several thousands)
CLASSIFICATION OF STARCHES
There are basically two types of starches: native starches and modified starches.
NATIVE STARCHES
Native or natural starches are produced by simply separating naturally occurring starch from grain or root crop such as rice, maize, wheat, potato or cassava. Native starch products retain the original structure and characteristics as in the crop. They are used for the purpose of food texturizing and thickening, in pharmaceuticals and as an industrial raw material.
Native starches are insoluble in cold water, and they can swell to different degrees, depending on the temperature applied. They have good thickening, gelling, adhesion and moisture retention properties.
MODIFIED STARCHES
Any process that brings about change in any of the properties of native starch is known as modification of starch, and the product is called modified starch. This is often done in order to obtain some industrial benefits that are impossible with native starches. Physical, chemical and enzymatic methods may be used to modify starch. These procedures bring about changes in the physical and chemical properties of starch. Modifications may involve changing the form or structure of the granule, or changing the shape and composition of the amylose and amylopectin molecules of starch. Modified starches are used in food, pharmaceutical, paper pulp and textile industries.
When starch is modified, any of the following properties can be altered, depending on the specific modification procedure:
1. thickening.
2. gelatinization.
3. water retention.
4. palatability.
5. adhesion.
6. opacity.
PRODUCTION OF STARCH FROM CASSAVA.
Starch is the main component of cassava, with only a small amount of secondary substances such as protein and lipid.
Starch is used to produce diverse products such as food, textiles, adhesives, beverages, paper, building materials, and pharmaceuticals. Cassava starch has high paste viscosity, and high freeze-thaw stability, which makes it very desirable in many industries.
The procedure for cassava starch production is summarized as follows and illustrated in the chart below:
1. Cassava tubers are peeled and washed.
2. They are milled in a machine into a paste.
3. The paste is put in a sieve-bag (shasha bag) and about 3 to 4 times volume of water is added.
4. It is vigorously stirred, and the bag pressed to squeeze out liquid from it into containers. Of course starch will also go out with this liquid into the containers.
5. The containers are allowed to stand for about 1 to 2 hours to allow starch to settle as a thick hard white deposit at the bottom.
6. The liquid on top is decanted, while the starch deposit is sun-dried, weighed and packaged.
About 25% starch yield may be obtained from cassava tubers.
STEPS IN THE PRODUCTION OF CASSAVA STARCH:
USES OF CASSAVA STARCH.
1. Production of medical and industrial alcohol.
2. It used as food especially in Nigeria
3. It is used in confectionery as jellys and gums.
4. It is used to make monosodium glutamate valued for enhancing food flavour in Asia . Example is Ajinomoto.
5. It is used as sweetener.
6. It is used to make glues or adhesives.
7. It is used for production of tablets in pharmaceutical industries as binders or fillers.
8. It is used to manufacture plywood.
9. It is used in the paper making process.
10. It is used in the textile industry.
PRODUCTION OF GLUCOSE SYRUP FROM STARCH.
Glucose syrup is a concentrated aqueous solution of glucose (dextrose sugar), maltose and other nutritive saccharides. Although it is found in nature in grapes, it is produced and used in industry to make bakery products, pharmaceuticals and liquors.
The process for making glucose syrup from cassava consists of gelatinization, liquefaction, saccharification, and purification, which are described and illustrated below:Gelatinization
At room temperature, the microscopic granules of natural starch are insoluble in water. But if slurry of starch is heated above 60oC, the granules swell and rupture. This is called gelatinization of starch, which increases the viscosity of starch. Gelatinized starch is next reacted with the enzyme amylase to hydrolyze starch. Actually gelatinization of starch and its partial hydrolysis with heat-stable amylase can be carried out in practise as a one-step process called liquefaction. The partially degraded starch chains are called dextrins.
Liquefaction
Starch slurry is made with 30-35% dry starch solids and its pH is adjusted to 6.0-6.4. Calcium, which helps to stabilize the enzyme, is added as calcium hydroxide or calcium chloride. A heat-stable α-amylase is mixed into the slurry, which is then quickly heated to 100 oC and held at this temperature for 10 min before cooling it to 90 oC. This is maintained for 1-3 h to further hydrolyze the starch.
Dextrins produced at the end of liquefaction have a dextrose equivalent (DE) between 8 and 15, and DE, which is the total reducing sugar in the syrup expressed as dextrose on a dry weight basis determines the physical properties of the syrup.
Saccharification
After liquefaction, pH is reduced to 4.2 – 4.5 and the solution is cooled to 60 oC. An α glucomylase is added immediately. Reaction time for saccharification is usually between a day or two depending on the amount of enzyme added. Glucoamylase releases single glucose monomers from the ends of dextrin molecules. Syrups of 95% glucose are often manufactured in this way.
PRODUCTION OF GLUCOSE SYRUP.
REFERENCES
INTERNET: www.elmhurst.edu/~chm/.../547starch.html
INTERNET: http://www1.lsbu.ac.uk/water/hysta.htmlINTERNET: en.wikipedia.org/wiki/Colloid
INTERNET: cheng.cam.ac.uk/research/groups/.../Starchstructure.html
INTERNET: en.wikipedia.org/wiki/Glycosidic bond
INTERNET: http://www.cassavabiz.org/postharvest/gsyrup01.htm
ASSIGNMENTS
1. What is starch? Discuss.
2. Compare and contrast amylose and amylopectin components of starch.
3. Differentiate between an hemiacetal bond and an acetal bond in a starch molecule.
4. What are native starches?
5. What are modified starches?
6. Outline the uses of cassava starch in various industries.
7. Describe the processes involved in starch production from cassava.
8. Describe the processes involved in glucose syrup production from starch.
9. Outline the changes that can be introduced when starch is modified.
10. Compare cassava starch with starches from other natural sources.
11. Compare and contrast amylose and amylopectin components of starch.
12. What are modified starches?
13. Describe the processes involved in glucose syrup production from starch.
