Tropical Sources of Starches

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4. Modification of starches

It is obvious that different starches have different physicochemical and functional characteristics. Some of these may be desirable whereas others make them unsuitable for specific applications. So attempts have been made to modify the starches so that the undesirable properties are eliminated while the desirable ones are retained. The modifications can be achieved by different techniques which include physical and chemical treatments and biotechnological techniques. Among the physical treatments thermal treatments are most common, but radiation has also been tried. In view of the availability of a large number of hydroxyl groups in the starch molecules, it is possible to prepare a large number of derivatives. The most common chemical derivatisation include esterification, etherification and crosslinking. These alter the starch properties significantly depending on the level of modification and the nature of the derivative. Use of microorganisms to alter the properties is a simple method, but with advent of gene technology it has been possible to completely alter the starch structure

Cassava starch. The physical methods tried to modify the starch include heat-moisture treatment, steam pressure treatment and gamma radiation. Heat-moisture treatment under low levels of moisture improved the product quality as observed by Raja [97]. When the flour was sieved and heat moisture treated, the stickiness associated with untreated flour during production of various food items could be drastically brought down. Treatment with dilute phosphoric acid improved the textural quality of cassava flour and suitable for food products. [98] Steam pressure treatment of starch was attempted at various pressures and period of treatment. It was found that the viscosity came down considerably depending on the severity of treatment [99]. Along with reduction of viscosity, the breakdown was lowered. Reduction in breakdown is very desirable since it will be useful in reducing the cohesiveness of the starch paste. Heat-moisture and steam pressure treatments bring about reduction on viscosity and breakdown by strengthening the associate bonds among the starch molecules in the granules. This leads to lower swelling of the granules and less breakdown under heat and shear. The food products prepared from heat-moisture or steam pressure treated starches were actually found to be superior in texture and acceptability. Another important effect of heat-moisture treatment has been to impart fat-like characteristic to the starch and hence can find use as fat-substitute in dietary foods. Radiation modifies viscosity properties by bringing about partial breakdown of starch and leading to easy handling of starch-based food product.

Chemical treatments can be either simple complexation with a chemical or chemical reaction. It is well documented that lipids and surfactants can form strong complexes with starch. Originally it was thought that only amylose complexexs with the reagents, but the role of amylopectin has also been established. These chemicals can modify the starch properties considerably. The interaction of different starches with lipids and surfactants has been widely studied. Hoover and Hadziev [100] used Glyceryl Mono Stearate to reduce the stickiness of mashed potato and attributed the effect to the complexation of soluble amylose portion of the starch with GMS and preventing its leaching out. DSC studies using native lipids on tuber starches have shown that though the tuber starches do not have inherent lipids associated them, there is no hindrance for the starches to complex with lipids. So lipids at very low levels can be used to modify starch properties for possible food applications. The effect of various type of surfactants on the rheological properties of cassava starch has been examined and it was found that different types of surfactants affected the viscosity properties differently. Sodium stearate and sodium laurate stabilised the viscosity of the starch even at 0.3M concentrations. So these surfactants can be useful in improving the texture of the cassava paste in food applications. It was also observed that when cetyl trimethyl ammonium bromide as used , the surfactant complexed with the soluble amylose portion of cassava starch and thus this surfactant can be used to suppress the cohesive nature of the starch paste [101].

Chemical Derivatisation. Various ester and ether derivatives have been prepared from cassava starch. Among the esters, the most common one is starch acteate prepared by reaction of starch with acetic anhydride in dilute alkali or pyridine. Whereas use of pyridine results in high DS, alkali gives only low levels of DS. However, use of pyridine is not desirable in production of derivatives for food application. Starch acetate has a lower gelatinisation compared to native starch and this has advantage in food products in which heat-labile components are to be incorporated. Another advantage of the starch esters is the slowing down of retrogradation. Esterification tends to weaken the associative forces by reducing the available hydroxyl groups [102]. The bulky ester groups prevent parallel association of the starch molecules thus reducing the tendency of the starch to settle. This phenomenon helps in improving the freeze thaw stability of the starch paste, useful in canned and stored food products. When derivatives of cassava starch were compared, acetylated and propylated derivatives had better clarity, though only to a small extent. The clarity was best when pyridine-acetic anhydride was used for acetylation, probably due to the high D.S. achieved. Similarly starch propionate and starch succinate derivatives have been prepared which also have applications in food products. Cassava starch phosphate prepared by dry heating of starch-phosphate mixture has very high viscosity and finds use in instant puddings and similar products.

Crosslinking is a very useful modification technique to improve starch properties. Cassava starch paste tends to undergo breakdown when subjected to heat and shear. By use of crosslinking the starch molecules are strengthened preventing easy disintegration of starch. Some of the common crosslinking reagents used include phosphorus oxychloride, sodium tri polyphosphate and epichlorohydrin. Knight[103] has found that crosslinking improved the viscosity stability of cassava starch and can be incorporated into bread products, The freeze thaw stability could be improved by esterification of the crosslinked starch. Starches with multiple modifications are available for specific applications. These products have improved viscosity stability and better freeze thaw stability. Heat moisture treatment of starch derivatives can also yield products useful as fat substitutes.

Use of microorganisms to modify cassava starch has been practised since long . Sour starch obtained by natural fermentation has been used for preparation of bread in Brazil and Colombia and the fermented starch has better raising properties compared top native starch. Attempts have been made to reduce the fermentation time by using better and more efficient cultures [104]. Use of inoculum provided fermentation also improved the baking potential of cassava starch [33]

Modification of other starches

Unlike cassava starch , derivatives of other starches have not been commonly reported. Acetylated, succoynilated and hydroxypropylated amaranthus starch have improved freeze thaw stability [105,106] . Steam pressure treatment of the D. alata and D. rotundata starch reduced the viscosity depending on the pressure and time of treatment The peak viscosity came down to nil value at 15 psi for 60 minutes for both the starches [107] and hence steam pressure treatment can be attempted for the modification of yam starch. Cold water solubility has been incorporated into banana starch by alcohol-alkali treatment [109]. There is scope for modification of the lesser known starches which possess some inherent properties like high gel strength, viscosity and digestibility so that they can replace the chemically modified starches for specific applications.

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