The viscosity changes during the liquefaction of sweet potato starch were monitored using the RVA using similar systems as in the case of cassava starch. Tremendous reduction in viscosity was observed when 30 -90 mg Termamyl 60L was added to the 1:10 starch slurry (Table 10). Sweet potato starch required a higher enzyme concentration of 30 mg to reach this stage in RVA. This indicates the possible differences in the initial susceptibility to α-amylase attack of the starches i.e., cassava and sweet potato starch.
Post-harvest treatments such as the method of starch isolation, as well as genetic factors, may have a profound effect on the properties of sweet potato starch. The environmental conditions during growth of a plant, especially the temperature, constitute one of the important factors affecting the physicochemical characteristics of starch granules, besides genetic and endogenous factors (Hizukuri, 1969).
Table 9. Percentage conversion of starch after saccharification by AMG on sweet potato starch slurry (liquefied using two concentrations of Termamyl*
Liquefaction Saccharification
Starch conc. (g)
- amylase (mg/100ml slurry)
Percent conversion (after 1 h)
AMG conc.
(ml/100ml slurry)
Percentage conversion After 24 h 48 h 72 h
20 30 11.79 0.05 85.08 82.73 86.83
20 240 26.43 0.05 80.40 84.69 82.33
*S.N. Moorthy, unpublished results.
Table 10. Viscosity reduction of sweet potato (variety-S10) starch by adding - amylase as measured using Rapid Visco Analyser*
Sl. No. -amylase
conc. (mg) Peak 1
(cP) Trough
1 (cP) Breakdown
(cP) Final
Viscosity (cP)
Set back (cP)
Peak time (Sec)
Pasting (C) 1. Control 4020.00 2254.00 1766.00 3056.00 802.00 4.07 77.40
2. 1.2 1081.00 16.00 1065.00 23.00 7.00 3.47 75.80
3. 2.4 757.00 10.00 747.00 14.00 4.00 3.47 75.80
4. 3.75 496.00 13.00 483.00 13.00 0.00 3.40 75.80
5. 7.5 122.00 2.00 120.00 5.00 3.00 3.40 76.75
6. 15 63.00 -1.00 64.00 9.00 10.00 3.33 76.00
7. 30 17.00 -1.00 18.00 -1.00 0.00 3.33 Error
8. 60 10.00 -3.00 13.00 0.00 3.00 3.73 Error
9. 90 5.00 -2.00 7.00 -2.00 0.00 3.80 Error
*S.N. Moorthy, unpublished results.
Starch content and dry matter content are the main properties of raw material for starch production. A new sweet potato breeding line, Kanto 116 was developed, featuring low gelatinisation temperature and an altered starch fine structure and having pasting temperature of 20 V (viscosity) lower than those of ordinary cultivars (Katayama et al., 2002). Starch granule from Kanto 116 showed an abnormal morphology characterized by cracking into granules. A number of studies on the distinctive properties of sweet potato starch have been undertaken in the last two decades (Tian et al., 1991; Moorthy, 2002). Kitahara et al. (1996, 1999) reported a new line with low amylose content and two lines having approximately 10°C lower pasting temperatures than ordinary cultivars. Some new sweet potato lines were developed from progenies of a new cultivar, Quick Sweet, having a low pasting temperature (Katayama et al., 2004). The results indicated that this ‗Quick Sweet‘ is a useful breeding material for improving pasting and retrogradation properties in sweet potato starch. The
potential chemopreventive properties of dietary fiber prepared from sweet potato roots were examined to promote the demand of this residue from the starch industry (Yoshimoto et al., 2005; Yoshimoto, chapter 3 in this Volume).
Sweet potato starch has some unique characteristics and is mostly used by the food industry as an ingredient in products such as cakes, breads, biscuits, cookies and noodles (Zang and Oates, 1999). Noodles, bread, boiled rice and pasta have played an important role in the human diet, especially in the Asian countries (Japan, China, Taiwan, Korea, Vietnam and Thailand). Based on the raw materials, various types of noodles are produced throughout the world. In terms of food products, four quality attributes are important being nutritional, phytosanitary, shelf-life and organoleptic. These qualities depend on flour and starch quality.
Thus starch properties largely influence noodle quality. Starch with high amylose content and with C-type pasting profile characterized by the absence of a peak viscosity and a constant or increased viscosity during continuous heating and shearing, i.e., good hot paste stability is reported to be suitable for noodle processing (Collado and Corke, 1999).They reported that the textural attributes of sweet potato noodles show high positive correlation with some starch paste properties. It is also reported that the smaller particle size of the granule improves the strength of uncooked noodles without affecting the firmness of cooked noodle (Oh et al., 1985).
C
ONCLUSIONSweet potato is therefore one of the worlds most important starch producing crops, with 95 % of all roots produced in Asia and Africa. Sweet potato is used as direct food, processed foods, industrial starch and animal feed. The utility of sweet potato is primarily determined by its physicochemical properties, being the amylose/amylopectin ratio, the molecular structure, granule size and inorganic constituents. Pasting properties influence the quality of food processing materials and industrial products. Being a nontraditional source of starch, the characterization of genetic variation and interrelationships of sweet potato starch physical properties that can guide utilisation is therefore essential. An awareness of their potential uses can help in large scale cultivation of these crops and extraction of starch from them. It is also possible to modify the starch properties by simple physical methods such as hydro- thermal or steam-pressure treatments. The latest developments in biotechnology are also being evaluated for their potential to modify the starches. These include fermentation of starch by the use of selective organism or enzymatic modification, which can bring about specific substitutions (Sair, 1967; Raja, 1990; Moorthy, 1999).
Current research is seeking to produce a new cultivar and breeding materials with distinctive amylose content and pasting properties. The role of dietary fibre in human nutrition has attracted growing interest in recent years. Most of the research programmes carried out on sweet potato are attempting to reduce the content of crude fibre for improved eating quality. Furthermore, the production of ethanol from biomasses is a growing industry in this continuously developing society. The sweet potato having high starch yield and low gelatinisation temperature may be effective in reducing these production costs. The various improvements in starch properties are useful for providing consumers with starch products and spreading the demand for sweet potato starch.
R
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Chapter 5