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Starch is one among the three most abundant natural carbohydrate biopolymers; starch, cellulose and chitin. Starch is stored principally in the roots, piths and seeds of the plant. The starch granule is well suited to store energy in green plants, being insoluble in water and densely packed but still accessible to the metabolic enzymes of plants. Though starch occurs throughout the plant world, there are only a limited number of plants utilized extensively for the production of commercial starch. Maize and cassava form major sources of starch; cassava starch is produced mainly in the tropical countries. The manufacture of starch involves freeing of starch granules from other constituents such as fiber, protein and extraneous materials, purifying by screening, washing, dewatering and drying. Starch has been used for centuries as a thickener in food and is a versatile and widely used viscosifier. But more than that, it is a precursor for a very large number of ingredients used in food, textile, paper, pharmaceutical and adhesive industries.

Starch is made up of two types of polymers – amylose, the linear α-1,4 linked glucan and amylopectin, an α-1,4 linked glucan with α-1,6 branch points. The amylopectin molecule is the crystalline component in the granules, with the short branched chains forming local organisations. The diversity in starch arises from the levels of amylose and amylopectin, the granule size and characteristics, which vary depending on the starch source. The typical composition in most starches is 25% amylose and 75% amylopectin. In some starches, genotypes exist with either high amylopectin content (waxy starch varieties) or high amylose content (high amylose starches).

Cassava (Manihot esculenta Crantz) is a major staple in tropical countries. Among the non-cereal sources of starch, the most important ones are sago palm, potato, cassava and sweet potato. Cassava starch shows unique structural and functional properties which are different from the starches present in cereals such as rice, wheat and maize and also starches from the tubers such as potato. Compared to wheat, potato and maize starches, cassava starch contains less amylose and hence it displays characteristic differences in properties. Cassava starch is known as the most bland flavored starch. It possesses many desirable characteristics such as easy extractability, high paste viscosity, high paste clarity and less tendency to retrograde. In spite of these desirable properties, it has a major limitation that the hot gel of cassava starch shows high instability and exhibits drastic fall in viscosity at high temperature conditions. Hence the modifications of cassava starch have been mainly aimed at overcoming this drawback.

Among other tuber starches, sweet potato and arrowroot starches are other two commercially important starches. Sweet potato (Maranta arundiaceae) is primarily grown for its quality starch which is valued as a food stuff, particularly for infants and invalids. It is also used in making various bakery products, special glue and paste, base for face powder, ice cream stabilizer etc. Arrowroot starch is easily digestible and therefore finds wide use in convalescent foods, baby foods and bakery items.

Some properties of the native starches limit their usefulness in many commercial applications. The shortcomings, which limit the usefulness of starch in many commercial applications, include insolubility of the starch in cold water, lack of free-flowing properties or water repellency, excess viscosity after cooking, cohesive texture of the cooked starch particularly for waxy corn, potato and cassava starches, breakdown of the starch pastes under conditions of low pH, extended cooking and shear, lack of paste clarity and high retrogradation of the cooked starch on low temperature storage. Therefore, starch modifications have been used for customizing starch properties for a variety of applications. The presence of amorphous and crystalline regions in the starch granules offers more opportunities for starch modification. Physical, chemical or enzymatic modifications of starch results in the alteration of the physicochemical and functional characteristics of the native starch and enable the production of a variety of end-use specific products. Modified starches are currently the most functional, useful and abundant of food additives available. Starch can be used as a plating agent to aid dispersion or blending of oily or highly viscous ingredients into dry mixes. Modified starches play a significant role in fat-reduction products. Maltodextrins and other fat mimetics based on modified starches are used as fat-replacers in low-calorie and dietetic foods. Another new area of starch application is its use as dietary fiber in the form of resistant starch (RS), which is resistant to enzyme digestion and not digested in the small intestine, but can be fermented in the large intestine.

Physical Modification
The common physical modification methods of starch include heat-moisture treatment (HMT), annealing, heat treatment, extrusion and gamma irradiation. Physical modification includes simultaneous action of several conditions such as temperature, pressure, moisture and shear. Temperature and moisture conditions during processing of starch alter its functional properties. Hydrothermal treatment of starch includes the combined action of moisture and temperature which affects its properties. Incubation of starch at low water contents and temperature above the gelatinization temperature range is referred to as heat moisture treatment. At high water contents and at temperature below the gelatinization temperature range, the process is called annealing. In extrusion process, high temperature and low water content are combined with high shear forces due to the mechanical treatment.

Chemical Modification
The chemical treatments of starch for modification of its properties could either be a simple complexation with a chemical or chemical reaction, which involve the incorporation of additional functional groups in starch or the modification of those already present.  Chemical modification of starch involves the introduction of functional groups into starch resulting in altered physic-chemical and functional properties. Some of the different modes of chemical reactions are oxidation, crosslinking, esterification, hydroxypropylation, cationization and phosphorylation. Oxidation of starch with sodium hypochlorite causes depolymerization of starch molecules and produces a clear and thin paste. Oxidized starches are widely used in paper industry as a surface sizing agent.

Another important method of starch modification is esterification. The common chemicals used for this reaction are acetic anhydride, acetic acid, vinyl acetate, succinic anhydride, alkenyl succinic anhydrides, citric acid and formic acid. Depending on reagents and reaction conditions used it is possible to obtain wide variability in chemically modified starches.

Another important method of modification of starch is crosslinking. Crosslinking treatment is intended to add intra and inter molecular bonds at random locations in the starch granule that stabilize and strengthen the granule. Starch pastes from crosslinked starches are less likely to breakdown with extended cooking times, increased acidity or severe shear. Crosslinking is performed by treatment of granular starch with multifunctional reagents capable of forming either ether or ester inter-molecular linkages between hydroxyl groups and starch molecules.

Enzymatic Modification
Enzyme conversion is used to produce derivatives with varying viscosity, gel strength, thermoreversibility and sweetness. In enzymatic modification techniques, the gelatinized starch is subjected to degradation by enzymes resulting in various products.

Microwave technique in starch modification
Chemical modification of starch are usually done in aqueous medium and in most cases it takes several hours or even days to attain the required level of substitution. Higher levels of substitution can be attained by carrying out the reaction in pyridine or other solvents. But the use of pyridine is not desirable in the production of derivatives for food applications. Microwave technique for starch modification is especially useful in dry reactions, where the reactant and reagents are solids. Compared to conventional methods, microwave heating technology offers enhanced speed, reproducibility and scalability.
Central Tuber Crops Research Institute
Sreekariyam, Thiruvananthapuram, Kerala - 17