Physicochemical and Functional Properties of Tropical Tuber Starches: A Review

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  1. Introduction
  2. Extraction Techniques
  3. Other components in Starch
  4. Colour and Appearance
  5. Granule Shape and Size
  6. Spectral Features
  7. X-Ray Diffraction Pattern
  8. Molecular Weight
  9. Amylose Content
  10. Thermal Characteristics
  11. Gelatinisation and Pasting Temperatures
  12. Viscosity
  13. Swelling Power
  14. Solubility
  15. Clarity
  16. Sol stability
  17. Digestibility
  18. Conclusions

9. Amylose Content

The linear component of starch viz., amylose imparts definite characteristics to starch. Amylose content varies considerably among different starches and genetic modifications have been carried out to obtain starch of amylose contents varying from 0 to >75%. Pan of amylose can exist as soluble amylose in the amorphous regions of the starch granules.

  1. Total Amylose

    1. Cassava Starch

      The total amylose content in cassava starch has been reported to range from 13.6-23.8% (Tab 8) [33].The Blue Values corresponding to total amylose varied from 0.50 to 0.55 for seven cassava varieties, indicating only very minor variation among the varieties [81]. Only insignificant differences in the amylose content were noticed during the growth period of six cassava varieties [62]. The gel permeation chromatographic analysis of five varieties of cassava carried out after debranching with isoamylose also did not show any noticeable difference among the varieties supporting the earlier results (Fig. 6)[41] . in a comparative study of five varieties from Nigeria Olorunda et al [113]have reported that higher amylose could be correlated with tuber meanliness. Kawabata et al [95] did not observe any noticeable effect of variety on the amylose content. Defloor et al. [75] reported that the amylose content of five cassava varieties harvested at different seasons was not related to the genotype. Asaoka et al. [36] in their study observed noticeable effect of season on amylose content. Examination of the effect of surfactants on the amylose content in cassava starch revealed that though the surfactants reduced the Blue Values [55]. Maximum reduction was obtained with cetyltrimethylammonium bromide, having bulky hydrophilic groups, might be blocking the entry of iodide into the amylose helix. Sriroth et al [114]reported that SO2 treatment didi not affect the amylose content of cassava starch and the DP of the resultant amylose changed only slightly from 1020 to 1050 by the treatment.

      Table 8 : Phsicochemical and functional properties of Cassava and sweet potato starches

      Cassava Sweet potato
      Amylose content [%] 13.6-27[37]; 18.3-23.6[75]; 18[94]; 8-16[89]; 17.9[38] 8.5-38[21]; 20.5-25.5[28]; 17.5-38[23,46,116,117]; 13.4-19[79]; 20[94]; 18[89]
      Pasting temp. [0C]

      58.5-70[70.93]; 55-64[97]; 49-73[146]; 58.5-68.5[86]; 60-70[32]; 68-90; 73-90[132]; 58-70[70]; 57-65[74]; 55-64[97]; 58.5-70[91]; 49-64.5; 62.73[146]; 58.5-68[96] 65-80[76]; 65-90[27]; 63-70[94]; 67-75[79]; 58.5-73[28]
      Sw. vol. [mL/g]

      71[37]; 49[22]; 46-52[97]; 42[93] 46[37]; 27.5-33.3(950C)[21,100]; 24.5-27.4(850C)[77]; 60-79(950C)[77]; 32-46(800C)[77]
      Solubility [%]

      48[37]; 35[22]; 25-28[97]; 31[93] 18[37]; 13.2-14.4(950C)[21,100]; 11.4-12.9(850C)[77]; 60-79(950C)[77]; 30-50(800C)[77]

      68.50[133]; 64.0[134]; 65-69[35]; 50.7-57.7[129]; 64[120]; 64.10[80]; 65.6[131]; 53.9-62.1[75]; 68[127]; 62.4[132] 61.3[135]; 58-64[28]; 67-75[100]; 65.6-68.21[15]; 67.3[137]

      71.2[133]; 69.2-73[41]; 54.7-61.3[129]; 78.0[120]; 69.0[80]; 70.8[131]; 78.0[127]; 69.3[132] 65.68[133]; 70.2-77[135]; 63-74[26]; 73-79[100]; 72.8-74.3[115]; 72.7[137]
      Tend[0C] 74.72[133]; 76.9[134]; 73.7-77.8[35]; 60.2-67.2[129]; 100[120]; 76.4[80]; 75.2[131]; 76.3-83.2[75]; 92[127]; 84.1[132] 80.7-88.5[135]; 78-83[28]; 81.4-84.8[100]; 84.6-86.8[115]; 13.6[137]
      ΔH [J/g] 12.4[133]; 16.6[134]; 10.8-13.6[35]; 6.8-8.8[129]; 16.0[120];15.6[131]; 9-15[75]; 22[127]; 4.8[132] 14.8-18.6[28]; 10-12.3[100]; 15.1-16.3[115]; 13.6[137]
    2. Sweet Potato Starch

      For sweet potato starch also, considerable variation in amylose content has been reported (Tab. 8). Madamba et al [46] working on six varieties of sweet potato from Philippines found only very little variation in amylose content. Garcia and Walter [26] have obtained by potentiometric titration values ranging from 20-25% for some Peruvian cultivars and location did not have any effect. Our studies showed that sweet potato starch has an amylose content of 20-25% depending on the variety. Noda et al. [63] did not observe any effect of fertilization on amylose content. Similarly there was no change in the amylose content of sweet potato varieties during the growth period [84]. Ishiguro et al. [118] studied the retro gradation tendencies of starch isolated from ten sweet potato cultivars having different amylose contents and chain length distribution. Starches having fewer amylose molecules and amylopectin molecules with higher content of short chains (DP 10) retrograded slower compared to others. Curing was reported to have either no effect on amylose content [117] or resulted in a slight increase [119].

    3. Other Starches

      Colocasia esculenta starch showed a wide range in the amylose content and a noticeable relationship between the amylose content and granule size was observed. The varieties C-9 which had the largest granule size, also possessed the highest amylose content [64]. In a study on a number of cultivars of taro. Strauss and Griffin [87] found that the maximum value for amylose content was 43% and minimum 3% and a mean value of 24.04%. They could not observe any good correlation between amylose content and granule size. Amylose content in yam starches also varied considerably according to various reports. Farhat et al.[80] have obtained the following values for amylose content in starches of different Dioscorea species : D alata 25%, D. rotundata and D. cyanesis 23.8% and D. dumetorum 12.6%. A values of 29.7% was reported for the amylose content of D. abyssinica starch from Ethiopia [39], while Soni et al [35] obtained 24.1% for D. ballophylla starch. Only very little variation in the amylose content was observed with the age of the crop for D. esculenta, D. alata and D.rotundata starches. Xanthosoma starch also had a similar amylose content viz. 15-25% and very little varietal variation was observed. The amylose content of ten cultivars of Amorphophallus paenoiifolius was found to vary very little [91]. Starch from these three accessions of Canna edulis had an amylose content ranging from 24-30%, the highest being observed for purple accession [66]. However, Soni et al. [29] reported a still higher value of 38%. Coleus starch ahd an amylose content of 33% [42] while Abraham and Mathew [30] found the amylose content in Coleus starch to be 34% by colourimetry and 18 % by amperometry. Based on all reports, it can be inferred that among the various tuber crops. Canna edulis and Coleus starches have the highest amylose contents (Tabs 9,10 Tabs 9,10)

      Figure 6

      GPC Pattern of Isoamylase Debranched Starch from Five Cassava Varieties on a Fructogel Column

  2. Soluble Amylose

    Soluble amylose can play a significant role in determining the textural properties. This fraction is easily leached out and hence considered responsible for cohesiveness in cooked tubers [52]. The soluble amylose contents in the tuber crop starches determined using iodometry ranged from 10-40% of total amylose. Soluble amylose content in different varieties of cassava did not vary during the growth period. Leelavathi et al [34] found that the soluble amylose content in cassava starch to be 4.8% out of a total amylose content of 17.9%. A similar trend was observed for Colocasia, D. alata and Xanthosoma starches. For Amorphophallus starches from different accessions, the soluble amylose content ranged from 9-11% forming nearly 45% of the total amylose content [91]. The soluble amylose content in Coleus starch was 12.8% [42], while it ranged from 10-12% for Canna edulis starch [66].

  3. Effect of Surfactants on Soluble Amylose

    The studies on complexation of tuber starches with the surfactants indicate that the soluble amylose is suppressed to different levels with different starches and surfactants (Tab. 11). In case of cassava starch, the surfactants had variable effect on soluble amylose portion. The anionic surfactants, potassium palmitate and potassium stearate and the neutral surfactant glyceryl monosterate had little suppressive action, whereas the cationic surfactants cetyltrimethylammonium bromide and sodium lauryl sulphite had a significant suppressive effect. Increase in concentration of these surfactants led to reduced Blue Values for soluble amylose [55].
    Though the cationic surfactant cetyltrimethylammonium bromide reduced the Blue Value for soluble amylose of all the starches (Tab. 11) its effect on Colocasia iand D. esculanta starches was more prominent indicating that the soluble amylose of these two starches may be more anionic in nature or that the amylose helix of these two starches is able to sterically favour the complex formation. Though these two starches have the lowest granule size, no correlation between complexing properties and granular size could be observed for the other starches.

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