Tropical Tuber Crops

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4. Modification of starches.

   The studies on the basic properties of different tuber starches show a wide variability in their physicochemical and functional properties. However they have some undesirable properties. Therefore attempts have been made to modify the undesirable properties while maintaining the desirable ones. These include physical and chemical modifications. The physical methods tried is steam pressure treatment and blending with other starches. Chemical modifications include complexation with surfactants and dramatizations like esterifications, crosslinking and oxidation.

   1. Physical modifications.

      1. Steam pressure treatment

         Steam pressure has been used to modify the starch properties. The Blending with other starches. The methodology consists of taking the dry starch in a conical flask, plugging with nonabsorbent cotton and autoclaving at different steam pressures for different periods of time. After cooling the starch is removed and stored. The physicochemical and functional properties are modified by the treatments. The reducing values are hardly affected indicated that the starch structure is hardly affected. The viscosity was lowered depending on the pressure used and the treatment period. Swelling volumes are also lowered. There is some reduction in clarity and sol stability brought about by the compressive action of steam. In case of Dioscorea starch there was no change in the XRD pattern again showing that most of the changes take place in the amorphous regions of the starch molecules.

         The method thus offers a simple process of starch modification without use of chemicals. The level of viscosity reduction can be controlled by manipulating the conditions of treatment and workup is simple.

      2. Blending of starches

         The functional properties of starch vary considerably among the different starches. So blending of starches was examined. The blends prepared include binary blends cassva- mize, cassava-amorphophallus and cassava-D. rotundata and a tertiary blend of cassva-maize-potato. starches. The results showed that there is additive behavior in the properties and they are all mutually compatible. The blend can be useful in imparting specific properties to a starch which intrinsically does not possess it. For example, the clarity of amorphophallus starch paste can be improved by blending with cassava starch while viscosity may be stabilized by using maize starch. This is also a non-chemical method to modify starch properties especially for food applications.

   2. Chemical modifications.

      1. Incorporation of surfactants and lipids

         Surfactants and lipids can form complex with the amylose chains and free chains of amylopectin. The complexation brings about considerable changes in starch properties. The effect of different surfactants on cassava starch was examined. Cationic, anionic and neutral surfactants were examined. The amylose contents, viscosity, swelling properties were examined. The results revealed that different surfactants affected the properties differently. Whereas sodium lauryl sulphate increased the peak viscosity, especially at higher concentrations, effect of potassium palmitate and potassium stearate was not so pronounced. Glyeceryl monostearate reduced the peak viscosity at higher concentrations. Viscosity stability was imparted by potassium stearate and potassium palmitate even at 0.02% molar concentration.

         Thus incorporation of surfactants is another convenient process for starch modification. Here also the process is simple and workup is clean. The chemicals used are also cheap and harmless.

         The effect of starch complexation with polar and polar lipids has been detailed earlier. The results show that lipids can be used to modify the starch properties. The complexation will be helpful in production of Resistant Starches useful as dietary food for the diabetics and the obese in view of the lower digestibility imparted to the starch.

      2. Starch derivatives

         Undesirable properties of tuber starches like cohesive texture and poor viscosity stability of cassava starch, low clarity and sol stability of colocasia starch can be modified by chemical derivatisation. In addition, production of some products by degradation of starch was attempted. Starch is chemically a polymer of glucose units joined together by a(1,4) linkages and partly a(1,6) linkages. These linkages render the starch susceptible to breakdown by various chemicals and enzymes unlike cellulose which is much more stable to many chemicals and enzymes. Thus it is possible to derive various partially degraded products with special properties suited to various applications in food and industry. In addition the presence of a large number of hydroxyl groups makes it possible for reaction with various chemical reagents

         1. Starch esters

            Ester derivatives were prepared from starch by different procedures. Various acids and anhydrides were used for esterification. The reaction was attempted using the following systems.

            • Direct reaction with the acid
            • Reaction of acid/anhydride in presence of alkalies
            • Reaction of acid with starch in presence of catalysts
            • Reaction with acylating agents

            1. Direct reaction with acid

               The esters prepared by direct action of acid were (a) formic (b) isobutyric (c) glycolic (d) thioglycollic derivatives. The degree of substitution (D.S) obtained by using different acids are given in Table 21. Lactic acid did not give an ester on direct reaction with starch due to its weakly acidic nature.

            2. Reaction of acid/anhydride in presence of bases

               The acid or its anhydride was reacted with the starch in presence of different bases. The bases tried included dilute alkali, pyridine, triethanolamine and triethylamine. In case of dilute alkali, though the yield of the products was good, the degree of substitution was low. Further substitution could be achieved by treating the samples again with acid anhydride and dilute alkali. However such repeated treatments could not increase the degree of substitution above 0.3.

               It was observed that a high degree of substitution was obtained when pyridine was used and the maximum possible D.S= 3.0 was achieved when pyridine in combination with anhydride was used. By controlling the amount of acid/anhydride used, desired level of D.S could be obtained.

               The other bases tried for the reaction, viz. triethanolamine and triethylamine did not give good D.S. indicating that their basicity is not enough to form stable complexes with the acid.

               Table 21 : Yield , nature and properties of different ester derivatives of cassava starch

               	Yield g 25-1gStarch	D.S	Viscosity(Seconds)	Mode of preparation
               1. Formyl	20.7	0.23	50.0	Direct reaction with acid
               2. Glycollic	18.8	0.20	44.0	Do
               3. Thioglucollic	18.0	0.19	44.0	Do
               4. Isobutyric	19.3	0.22	51.0	Do
               5. Citric	24.2	0.05	58.5	Do
               6. Malic	22.5	0.06	56.5	Do
               7. Tartaric	22.9	0.02	60.0	Reaction with acid/anhydride in dilute alkali
               8. Succinic	23.5	0.10	57.0	Do
               9. Stearic	22.0	Very low	–	Do
               10. Phthallic	23.2	Very low	–	Do

            3. Catalytic esterification

               The esterification of cassava starch was tried with various catalysts. Metal halides were used in the reaction of the starch with acetic anhydride in acetic acid. The catalysts tried were ZnCl₂, SnCl₂, MnCl₂ and AlCl₃. The results showed that SnCl₂ gave highest D.S. followed by ZnCl₂.

               The catalytic effect of perchloric acid on acylation of cassava starch was tried The results showed that the optimum temperature for the reaction to give good yield and a reasonable level of substitution was 30°C. Propionic anhydride in presence of perchloric acid gave similar results. The D.S of the esters could be determined by finding out the intensity of the 1680 cm^-1 absorption in the IR spectrum.

               The gelatinization temperatures were slightly lowered by increasing the substitution. The associative forces are weakened by substitution of the hydroxyl groups and hence the earlier gelatinization. Viscosity at 75°C also showed a fall with increasing substitution, again due to the reduction of associative forces. Clarity was improved to a small extent by substitution by acetyl or propionyl groups. The paste stability is as high as 10 days when the D.S is around 0.10. This property is desirable for food purposes, where the tendency of starch to retrograde, especially on freezing and thawing , poses a problem.

               Ferrous sulphate and hydrogen peroxide catalysis did not give any notable level of substitution.

            4. Reaction with acylating agents

               Attempts to prepare esters by using sodium acetate-acetic anhydride gave a D.S of 0.05. The viscosity of the starch was 46.0 seconds (Redwood No.1) , gelatinization temperature was 48-65°C and it had acceptable clarity and good sol stability.

               The different ester derivatives prepared, their method and D.S are summarised in Table 22.. It was generally observed that higher levels of substitution were obtained when pyridine was used as the base and the anhydride of the acid was used for the reaction. However, as noted for acetyl derivatives, the sol stability was reduced when pyridine was used for the reaction.

               Among the various derivatives prepared, acetyl derivative was found to be the easiest to prepare and had desirable properties. It was found that phthallic and stearic esters could be obtained only in low levels of substitution, probably due to steric factors.

               Chloroacetic ester of starch was prepared by reaction of chloroacetic acid with starch suspended in KOH solution at low temperatures. However D.S was low and the product did not possess good thickening property.

         2. Cross linking

            Starch granule strength could be improved by cross linking with poly functional reagents. These reagents bridge the starch molecules and prevent the breakdown under heat and hence reduce the cohesive nature of starch and minimise viscosity fall during holding period.

            The crosslinking agents used were epichlorhydrin, phosphoric acid and phosphorus oxychloride. The crosslinked products were obtained by standard procedure and showed improved stability. A sample of phosphate crosslinked starch had higher viscosity and less cohesive texture. Though higher crosslinking levels are achieved by using epichlorihydrin or phosphorus oxychloride, these reagents are difficult to handle and the reaction conditions are very selective (pH and temperature should be kept within specific ranges). The phosphate ester on the other hand can be prepared easily using phosphates and has no toxic effects. Hence this reagent is preferred to bring about crosslinking.

         3. Oxidative reactions

            Oxidation of starch can lead to various products depending on the oxidising agent used. Oxidised starch gives a clear fluid and adhesive paste which does not form a hard gel on cooling but retains its free flowing, adhesive nature. Films formed from oxidised starch pastes are strong, tough and horny, in contrast to the weak and brittle films of acid modified starches or dextrins. Oxidised starch has maximum use in paper industry and also in drilling muds as dispersant.

            Dialdehyde starch which is obtained by cleavage of 2,3-diol bond is useful for we-end application in paper industry. The basic polymeric structure is maintained and hence dialdehyde starch can be useful for synthesis of polyols.

            Oxidation of starch was tried with bromine under steam pressure. The amount of bromine used, the pressure and time of treatment were varied. The final product obtained was analysed for reducing value and viscosity. It was found that starch is degraded to a large extent during the oxidation by bromine as indicated by increased reducing values and decreased viscosity. There was no effect of sodium peroxide or benzoyl peroxide on the products under different conditions showing that the reaction is not radical initiated.

            Dialdehyde starch was prepared by oxidation of cassava starch with potassium metaperiodate The dialdehyde starch did not give blue colour with iodine, indicating that it loses the helical structure required for iodide-starch complex. The loss in the coiled nature is probably due to absence of enough hydrogen bonds to stabilise the coiled structure and also due to formation of hemiacetal type of structure by internal bond formation. This is confirmed by the fact that the dialdehyde starch shows only a very weak peak at 1680-1700 cm^-1 in its IR spectrum. However, the dialdehyde starch gave condensation derivatives with hydrazine, urea and hydroxylamine, though to a small extent. These condensation products were formed in gelatinous form and were difficult to crystallise.

            Oxidised starch for possible use in drilling muds was made by permanganate and chromic acid oxidation. The starch was oxidized by using aqueous 0.05M permanganate and 0.5 M HNO₃ or 0.1 M sodium dichromate and concentrated HNO₃. The products stained blue with iodine and exhibited weak carbonyl absorption in the IR spectrum indicating that no 2-3 diol bond cleavage had taken place.

         4. Pregelatinised Starch

            Pre-gelatinized starch is used in various instant foods, since it is more miscible in water or milk compared to raw starch. Pre-gelatinized starch was prepared by heating, with continuous steady stirring the starch with minimum amount of water required to gelatinize the starch, until the starch was completely gelatinized (as evidenced by the translucency of the paste). After heating was stopped, the slurry was spread out into a thin film and dried in the sun or in an oven at 60-65°C. The dried sample was powdered while hot and stored by keeping out of contact with moisture. The finely ground material was used for various preparations. A formulation including the Pregelatinised starch, cocoa powder and sugar was found to be quite acceptable in taste and quality and was miscible in hot and cold milk and serve as an infant food.

         5. Modification by fermentation

            The properties of the starch extracted from the fermented tubers were studied for possible modification during fermentation. Apparent reduction in total and soluble amylose contents was observed. Differential Scanning Calorimetry of the samples indicated that the enthalpy of gelatinization was reduced, but the gelatinization temperature was enhanced. Marked reduction in Brabender viscosity values of starch from fermented tubers was observed, but he X-ray diffraction patterns remained unaffected. All these changes could be attributed to the presence of fibrous material and consequent reduction of starch content in unit volume rather than any major change in the granular structure of starch.

            The starchy flour has better functional properties and very useful in food applications. With the increasing application of Biotechnology, it should be possible to modify the starches to suit to different applications.\