PRODUCTION OF STARCH FROM CASSAVA (Maihot esculanta) AND ITS CROSS LINKED DERIVATIVES
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Starch can be obtained from cassava, sorghum, maize, sago and potatoes. But this project focused on the production of starch from cassava. Starch can be cross-linked a product that will be suitable for noodle, salad cream custard making. Normally it is easier to make this product from corn and potatoe starch, but cassava which is readily available and cheap can be employed to meet the demand of the people.
Other synthetic starch produced from cassava includes; carboxymethyl starch (which is produced when one of the hydrogen atom of the starch is replaced by carboxymethyl groups, starch acetate, starch xanthate and hydroxyl alkyl starch. These are used as thickening agents, sterbilizer and emulsifier in products. Cassava starch when treated with phosphate are used in frozen products when they are defrosted to prevent them from dripping. This study investigated the production of starch from cassava and preparation of cross-link derivatives.
1.1 MEANING AND COMPOSITION OF STARCH
Starch is one of the most abundant substances in nature, a renewable and almost unlimited resource with a chemical formula (C6H10O5)n. It is a polysaccharide, a chain of many glucose molecules. It is the most carbohydrate stored in roots and seeds of plants. There are two types of glucose chain in starch which are the amylose and amyloeptin.
1.2.1 COMPLEX BRANCH CHAIN (AMILOPECTIN)
Amylopectins are made up of several million glucose units. It forms branched structures with about 30 glucose units in a chain between branches. This makes the molecule somewhat stripped in appearance with the knotted branch point in all rows and smooth chain separating them. These molecules are so large that this stripped appearance show up under a light microscope forming what appears to be ‘growth rings’ in the starch grain.
1.1.2 AMYLOSE CHAIN
Amylose chain tend to curl up into tielice (spirals) with the hydrophobic part inside. This allows them to trap oil and fat inside the helix as well as aroma molecules.
1.1.3 STARCH GELATINIZATION
WHAT IS GELATINIZATION?
This is a colloidal structure that is, it has interparticle bonds (usually hydrogen bonds) or lower potential energy than starch in true solution .
Starch gelatinization is a process that breaks down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding and oxygen sites (the hydroxyl) to engage more water. This irreversibly dissolves in starch granules. Penetration of water increases randomness in the general granule structure and decreases the number and size of crystalline regions  . Hence crystalline region do not allow water entry. Heat causes such region to be diffused so that the chain begin to separate into an amorphous form. This process is used in cooking to make roux sauce, pastery custard or popcorn.
1.1.4 GELATINIZATION PROCESS
Gelatinization is also known as the thickening of a liquid, the starch or flour granules absorb the liquid. When heated, the grains/granules swell and burst releasing the starch into liquid
1.1.5 STARCH RETRO GRADATION
This is a reaction that takes place in gelatinized starch when the amylose and amylopectin chain realign themselves causing the liquid to gelatinize.
When native starch is heated and dissolves in water, the crystalline structure of amylose and amylopectic molecules are lost and they hydrate to form a viscous solution. If the viscous solution is cooled at lower temperature for long enough period, the linear molecule amylose and the lower part of amylopectin molecule retrograde and rearrange themselves again to a more crystalline structure. Hence, retrogradation can expel water from the polymer network. This is a process known as SYNERESIS. A small amount of water can be seen on top of the gel. Retrogradation is directly related to stalling or aging of bread 
1.2.0 SOURCES OF STARCH
1.2.1 CASSAVA STARCH (Manihot esculanta)
Cassava starch has many remarkable characteristics including high paste viscosity, high paste clarity and high freeze-thaw stability which are advantageous to many industries.
Several workers have reported the production of starch and its cross linked derivation  . This report described the principle of using cassava starch to produce some vital products because cassava are readily available and cheap. Hence, they are available at low cost  . Over the years, the ability to produce some synthetic starch from cassava has been reported in literature and this includes crosslinked starch, carboxyl methyl starch, starch acetate, starch xanthate etc.
1.2.2 ORIGIN, TYPES AND COMPOSITION OF CASSAVA STARCH
Cassava is a staple crop that is particularly important in South America, America and African countries . It is a perennial shrub that grows to approximately 2 meters tall and has the ability to grow on a marginal land in low-nutrient soil where other crops do not grow well. It is also fairly drought tolerant. It is grown for its enlarged starch rich tuberous roots.
Although cassava is a staple crop, it is poisonous in its raw state as the plant contains cyanogenic glucoside. These glucosides are converted to HCN by enzymes called Linamarase which is present in cassava and acts on the glucosides when the plant cell are ruptured during the consumption stage.
The amount of cyanide contained in cassava depends on the variety and stage of the cassava. There are two types of cassava  .
Bitter cassava (Manihot utilissiana)
Sweet Cassava (Manihot patinate)
The bitter cassava has a higher level of cyanide than the sweet cassava. The poison tends to be more concentrated in the skin of the root and can be readily removed during processing resulting in a safe and versatile product.
1.2.3 ADVANTAGES OF CASSAVA STARCH
High level of purity
Excellent thickening characteristics
A neutral taste
Desirble textural characteristics
A relatively cheap source of raw material containing a high concentration of starch that can equal or surpass the properties offered by other starches.
Appropriate composition of the cassava tube 
TABLE 1.0 APPROXIMATE COMPOSITION OF THE CASSAVA TUBER
20 – 30%
2 – 3%
Water 95 – 80%
Ash 1 – 1.5%
Nigeria is the world’s largest producer of cassava (F.A.O of the United Nations). However based on the statistics from F.A.O of the United Nation, Thiland is the largest exporting country of dried cassava with total of 77% of world export in 2005.
Fig 2.0 TABLE
COUNTRIES TONNES OF CASSAVA GROWN
Latin America 32.2million
1.3.0 TYPES OF STARCH
1.3.1 DEFINITION AND REASONS FOR STARCH MODIFICATION
This native starch that has been changed in its physical and chemical properties. This can be used for other industrial application through series of techniques, chemical, physical and enzymatic modification .
Modification may involve altering the form of granules or changing the shape and amylopectin molecules. It is carried out on the native starch to confer it with properties needed for specific uses. Which are;
To modify looking characteristics (gelatinization).
To reduce reterogradation.
To reduce paste’s tendency to gelatinize.
To improve adhesiveness between different surfaces, such as in paper application Some of these modified starches include;
Cross – link starch
Hydroxyl alkyl starch.
1.3.2 STARCH MODIFICATION
Starches can be modified in several ways to change their function as additives in products. They can be cross-linked, where the chain get stuck together into a mesh. They can be heated to break the long chains down into simpler molecules like dextrin , polydextrine , and malto-dextrin. These are simply short starches  .
Among carbohydrate polymers, starch is currently enjoying increased attention owning to its usefulness in different food products. Starches from various plant sources such as cassava, wheat, maize, rice and potatoes have received extensive attention in relation to structural and physical and chemical properties  .
Native starches is a good texture stabilizer and regulator in food system  , but limitations such as low shear resistance, thermal resistance, thermal decomposition and high tendency towards retrogradation limit to its use in some industrial food application. Starch modification which involves the alteration of the physical and chemical characteristics of the native starch to improve its functional characteristic, can be used to tailor starch to specific food application  . Starch modification is generally achieved through DERIVATION such as etherification, esterification, cross-linking and grafting of starch; decomposition (acid or enzymatic hydrolysis and oxidation of starch) or physical treatment of starch)or physical treatment of starch using heat or moisture etc.
Chemical modification involves the introduction of functional group into the starch molecules, resulting in markedly altered physic-chemical properties. Such modification of native granular starch profoundly alters their gelatnization, pasting and retrogradation behavior  .
1.4.0 CROSS – LINK STARCH
Cross – linking treatment is intended to add intra and inter molecular bonds at random locations in the starch granule  Starch pastes from cross – linked starches are less likely to break down with extended cooking times, increased acidity or severe shear  . Nutritional benefits of cross-linked starch as a new source dietary fiber have also been reported  .
Cross-linking is generally performed by treatment of granular starch with multifunctional reagent (phosphate of glycerol) capable of forming either ether or ester intermolecular linkages between hydroxyl groups on starch, molecules  . Dual modification (a combination of substitution and cross-linking) has been demonstrated to provide stability against acid, thermal and mechanical degradation of starch and to delay retrogradation during storage. The most common method of modification is cross-linking. When your cross-link starch, a chemical bond takes place between the starch chains.
The multifunctional reagents used in the treatment of starch introduces intermolecular bridges or cross – linking between molecules; thereby markedly increasing the average molecular weight. Because starch contains many hydroxyl groups, some intra molecular reaction which does not increase the molecular weight also take place. Intra molecular reaction is not significant on the usual granular reaction because the close packing of starch molecules favours intermolecular cross-linking  .