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

3. Other components in Starch

The extracted starch is invariably accompanied by various other components viz., fibre, lipids, proteins and minerals, depending on a number of factors such as method of extraction, age of the crop, environmental conditions etc. Some of these impart desirable qualities to the starch, while others affect the quality.

1. Moisture Content

   The moisture content of dry starch varies from 6-16% depending on the process used for drying the starch. Higher levels of moisture can lead to microbial damage and subsequent deterioration in quality. The maximum moisture content prescribed for safe storage by most of the starch producing countries is 13% [18-20]. Considerable variation in moisture content among tuber starches has been reported by several workers as evident from the values in Tab. 2[21-30, 32]. Climatic factors also play a parting deciding the moisture content. For D.dumeforum starch, Nikala et al. [32] obtained 12% moisture for starch extracted during the wet season, but 13.5% for that extracted during the dry season. The result has been explained on the basis of higher granularity due to lower granule size during the dry season illustrating the effect of environment on the moisture content.
   Table 2 : Biochechemical contents of different tuber starches

   Starch	Moisture [%]	Fibre/ash [%]	Lipid [%]	Phosphorous [%]
   Cassava	14.8[21], 10-13[22]	0.02-0.49[33], 0.33[34], 0.22[35], 0.32[21], 0.1-0.8[22], 0.01-0.029[36]	0.1-1.54[41,33,34,43], 0.96[35], 0.1-0.4[22], 0.07-0.73[36]	0.007-0.012[33], 0.0075[35]
   Sweet potato	11-17[23,24-26] 9.8-15.3[26]	0.05-1.28[23-25], 0.7-1.3[26]	0.006-0.26[23,37,46]	0.009-0.022[23,24,37,46]
   C.esculenta	16.6-17.4[21]	0.81-0.92[21]	–	0.006-0.013[64]
   X.sagittifolium	15.5-16.5[21], 12.0[32]	0.19[38], 0.19-0.22[21]	0.39[38]	–
   P. erosus	10.9, [27]	0.06[27]	0.33[27]	–
   Arror root	10.10[22]	Tr[22]	–	–
   A.paeoniifolus	10.55[28]	0.58[28]	0.088[35], 0.1[28]	0.045[35]
   Canna edulis	11.0[29]	0.061[29]	0.30[29]	0.01[29], 0.05-0.08[66]
   D.alata	13.6, 18.2[21]	0.22[22], 0.26[21]	–	–
   D.esculenta	16.8[21]	0.46[21]	–	–
   D.rotundata	16.7-18.6[21]	0.19-0.46[21]	–	0.011-0.015[65]
   D.dumetorum	12-13.5[31], 16.5[21] 13,5[22]	0.16-0.3[31], 0.30[21]	37.3-39.6[31]	0.003[31]
   D.ballophylla	–	–	–	0.005[35]
   D.abyssinica	–	0.1[39]	1.0[39]	–
   Coleus	12.2[30], 15.1[21]	0.183[30], 0.4[42]	–	–
   Curcuma sp.	–	–	–	0.045[67]

2. Fibre Content

   The fibre content in starch varies to a great extent depending on the sieve used for removal of the fibrous material, varietal variation and age of the crop, especially for cassava and sweet potato, where the fibre content increases with the maturity. Wide variation in fibre and ash contents in different tuber crops is evident from various reports (Tab 2.). Cassava flour (containing 2-3% fibre) had different properties comparer to the isolated starch (having 0.1-0.15% fibre) and neither defeating nor ethanol extraction brought about any major change in the properties of the starch properties [40-41]. The total dietary fibre in cassava flour was reported to vary from 4.7 to 5.5%. In D.dumentorum starch, the ash content almost doubled during dry season. [31].

3. Lipid Content

   Lipids from another important component that has a strong effect on the starch properties [43-45, 47-51]. The formation of the starch-lipid or starch-surfactant complexes improves the textural properties of various foods [44, 52-54]. The starch-lipid interaction is particularly important in cereal starches, which harbour lipids to noticeable extent. The tuber starches contain much lower quantities of lipids so that the effect is not so pronounced. The lipid content in live cultivars of cassava varied from 0.11 to 0.22% in starch and 0.27-0.45% in flour [41]. Widely varying lipid contents have been reported for the different tuber starches (Tab 2). Lipid content in starch was also influenced by various pretreatments of the tubers of yams and aroids [8]. Viscosity stability of cassava starch could be enhanced by treatment with surfactants [55]. Since the root starches contain much smaller quantities of native lipids in them, the addition of lipids or surfactants would be desirable to obtain quality improvement and it was found that there is no hindrance for the tuber starches to complex with surfactants or lipids [56, 57]. The specific complexing ability of the amylose with surfactants has been utilized for the determination of amylose content in starches [58-60]. Recently modulated DSC has been used in determination of amylose content in eleven tuber starches [61].

4. Phosphorous Content

   Another important component invariably present in starch is phosphorous, which is associated in the synthesis of starch in the chloroplasts. Wide variation occurs in the phosphorous contents of different starches (Tab 2). No noticeable variation in P content of cassava starch was observed with age of the crop for six cultivars over a growth period of 2-18 months [62]. The phosphorous content in sweet potato starch is nearly similar to cassava starch [23, 24, 37, 46], but much less than that of potato starch. Takeda et al. [25] also found that sweet potato amylose contains less P (3-6 μg/g) than the amylopectin (117-144 µg/g). Noda et al. [63] did not observe any effect of fertilization on the P content in two sweet potato varieties. The P content in starch of different Colocasia cultivars varied from 0.006 to 0.013% [64]. Studies on the P content in six accessions of D. rotundata showed only very minor variability (0.011-0.015%) [65]. For D.dumetorum starch. Nkala et al. [31] observed that almost all of the phosphorous exists as bonded to starch and in the range 29-32 mg/100 g. Starch from three cultivars of Canna edulis from CTCRI contained P in the range of 0.05-0.08% which is even higher than that found in potato starch [66]. Curcuma starch also contained a high percentage of P (0.045 %) [67]. The high phosphorous content impart high viscosity to starch and also improves the gel strength. High P starches can find use in food applications requiring high gel strength, such as jellies etc. Cookies made using Canna starch are very popular in some Latin American countries.