Starch derivatives have huge potential in various food and industrial applications because of the broad range of functional properties they exhibit. Maize and cassava are the main commercial sources of starch. Cassava starch shows unique structural and functional properties which are different from cereals such as rice, wheat and maize and also from tubers like potato. Cassava starch is known as the most bland flavour starch in the world. Tuber starches swell more rapidly in a narrower temperature range than the common cereal starches. In addition to bland flavour and taste, cassava starch possesses many desirable characteristics like easy extractability, high paste viscosity, and high paste clarity and less tendency to retrograde. In spite of these desirable properties, it has a major limitation that the hot gels of cassava starch shows high instability and exhibits drastic fall in viscosity at high temperature conditions. It is possible to modify the properties of starch without altering the desirable characteristics by different physical and chemical treatments. Native and modified starches find a large number of applications in textile, paper, adhesives, pharmaceutical and food industries. Modified starches are currently the most functional, useful and abundant of food additives available. These are also used in the development of starch-based detergents and biodegradable plastics. In addition to the traditional applications, starches are being used in mining and oil industries as flocculating agents. Encapsulation of pesticides, fertilizers and herbicides for slow release effect is another recent area of starch application. Now most of the Indian industries depend on modified maize starches. However, cassava starch and its derivatives have very good potential as a substitute for more costly maize starch. Chemical modifications of starch are usually done in aqueous medium and in most cases the reaction takes 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. The use of pyridine is not desirable in the production of derivatives for food application. The toxic nature and high cost of these solvents are problems in the commercialization of these processes. Recently, there are some reports on alternative techniques such as microwave heating, extrusion etc for starch modification. Microwave technology is emerging as a potential technique in Green Chemistry for organic synthesis in view of solvent free synthesis, very fast reactions and cleaner products. Very little work has been carried out on the modification of starch by microwave technique. An attempt has been made to explore the feasibility of microwave technique and phase transfer catalysis in cassava starch modification reactions and the products were characterized structurally and functionally. Statistical models were also developed for the substitution level and reaction efficiency in different modification techniques which
help to predict the optimal reaction conditions for the synthesis of modified starches with specific properties. Cassava starch has been subjected to chemical modifications, which included esterification with succinic anhydride, octenyl and dodecenyl succinic anhydrides, citric acid and sodium orthophosphates; cross-linking with epichlorohydrin; hydroxypropylation and pyrodextrinisation. Heat-moisture treatment of cassava starch was carried out by microwave irradiation of the starch with different levels of moisture. Microwave-assisted succinylation could produce derivatives in very short reaction time with higher DS than those produced by conventional method and these derivatives exhibited modified swelling and pasting properties. Microwave irradiation of starch with succinic anhydride in presence of N,N-dimethyl formamide (DMF) resulted in the formation of high DS succinate derivatives which exhibited very high viscosity, high paste stability, low gelatinization temperature, favourable textural properties and low enzyme digestibility. Esterification with citric acid and sodium orthophosphates were also successfully carried out under microwave conditions and the duration of reaction could be reduced to a few minutes. Starch phosphates synthesised by this technique were found to be cold swelling. Derivatives with hydrophobic side chains were synthesised by the esterification of cassava starch with octenyl and dodecenyl succinic anhydrides by microwave technique and the products were amphiphilic in nature exhibiting hydrophilic as well as hydrophobic characters. Pyrodextrinization of cassava starch was also highly effective by microwave heating and the dextrinization took place in a very short reaction time. The use of a phase transfer catalyst (PTC), tetrabutylammonium bromide (TBAB) was found to be effective in the reaction of cassava starch with epichlorohydrin and propylene oxide in aqueous medium. The hydroxypropyl derivatives synthesized by reaction with propylene oxide exhibited very good freeze-thaw stability on low temperature storage. There was alteration in granule morphology also. The affected granules appeared with their outer sides drawn inwards and showed a depression in the central region and some of the highly affected granules appeared as a gelatinised mass with their boundaries fused together.Some promising derivatives with very desirable properties such as cold swelling, stable paste viscosity, good freeze-thaw stability, water binding capacity and amphiphilic properties were developed, which can be further exploited for commercial applications. Many of these chemically modified starches were found to exhibit lower in vitro enzyme digestibility than native starch which increases their importance as a source of resistant starch. The feasibility of scaling up of microwave heating as a means for starch modification offers very good scope since it can tremendously reduce the duration of starch modification reaction from hours in conventional methods to a few minutes.