The solubility of the starches from the various sweet potato cultivars has been reported in the range of 8.56–19.97 %, whereas the solubility of SPS at different temperatures was observed between 2 and 12 % (Table 11.1.7). The differences between swelling powers and solubilities of starches from different sources may also be due to differences in morphological structure of starch granules. It has been suggested that amylose plays a role in restricting initial swelling, because this form of swelling proceeds more rapidly after amylose has first been exuded. The increase in starch solubility, with the concomitant increase in suspension clarity is seen mainly as the result of granule swelling, permitting the exudation of the amylose.
Table 11.1.7 Swelling power and solubility of sweet potato starch
Temperature | 60 °C | 70 °C | 80 °C | 90 °C
Swelling power (g/g) | 1.1 | 12.5 | 21.6 | 24.4
Solubility | 2 | 6 | 10 | 12
The extent of leaching of soluble substances mainly depends on the lipid content of the starch and the ability of the starch to form amylose-lipid complexes. The amylose-lipid complexes are insoluble in water and require higher temperatures to dissociate (Morrison, 1988; Raphaelides and Karkalas, 1988). The differences of the swelling and solubility behavior of the starches between botanical sources and among the cultivars of any one botanical source are caused by the differences in the amylose and the lipid contents, as well as the granule organization (Singh et al., 2003).
X-ray diffraction diffractometry has been used to reveal the presence and characteristics of the crystalline structure of the starch granules (Hoover, 2001). In normal and waxy starches, the branched molecule, amylopectin constitutes the crystallites. The branches of the amylopectin molecule form double helices that are arranged in crystalline domains (Sarko and Wu, 1978). The “A”, “B” and “C” pattern are thus the different polymeric forms of the starch that differ in the packing of the amy-lopectin double helices. The amylopectin component inside starch granules crystallizes into either A- or B-type structures (Katz and Van Itallie, 1930). The A-type structure is characteristic of cereal starches and the B-type of many tuber and high amylose starches. The C-type structure, which is a mixture of A- and B-type X-ray patterns, is characteristic of legumes and some tuber starches. The A-type starch has a mono-clinic unit cell, which is closely packed. In contrast, the B-type polymorphic starch has a hexagonal unit cell, which is relatively loosely packed with an open channel of water in the unit cell (Imberty et al., 1991). Most of the tuber and root starches exhibit the typical B-type X-ray pattern (Zobel, 1998b) with the peaks that are both broad and weak and with two main reflections centered at 5.5 and 17o 20 angles.
11.1.10 Pasting Properties of Sweet Potato Starch
The pasting behaviour is helpful in understanding the textural change or retrogradation potency of the applied products (Chen et al., 2003). The viscosity parameters during pasting are co-operatively controlled by the properties of the swollen granules and the soluble materials leached out from the granules (Doublier et al., 1987; Eliasson, 1986). The pasting properties of SPS are shown in Table 11.1.8.
Table 11.1.8 Pasting properties of sweet potato starch
Pasting properties | Value
PV (cP) | 2 603
TV (cP) | 1888
BV (cP) | 185
FV (cP) | 2 660
SV (cP) | 772
PT (°C) | 74.8
In the rapid visco-analyzer, swelling of starch is a function of temperature and is continuously recorded as a change in viscosity. When the temperature rises above the gelatinization temperature, the starch granules begin to swell, and viscosity of the solution increases on shearing when these swollen granules squeeze past each other. The temperature at the onset of this rise in viscosity is known as the pasting temperature. Granule swelling is accompanied by leaching of amylose into the external matrix, resulting in dispersion of swollen granules in a continuous matrix (Bili-aderis, 1992; Noel et al., 1993).
When a sufficient number of granules become swollen, a rapid increase in viscosity occurs, known as peak viscosity. The increase in viscosity with temperature may be attributed to the removal of water from the exuded amylose by the granules as they swell (Ghiasi et al., 1982). Peak viscosity occurs at the equilibrium point between swelling and polymer leaching. As the mixture is subsequently cooled, viscosity will increase to a final viscosity. Miles et al. (1985) reported that increase in final viscosity might be due to the aggregation of the amylose molecules. The setback is the measure of retrogradation tendency or syneresis upon cooling of cooked starch pastes. The values of PV, TV, BV, FV and SV for sweet potato starch was 2,603,1,888,185, 2,660 and 772 cP, respectively. The pasting properties of starch are listed in Table 11.1.9.
Peak viscosity of SPS ranged from 403–473 RVU (Thao and Noomhorn, 2011). whereas Collado and Corke (1997) reported the peak viscosity of sweet potato starches in the range 377–428 RVU. They also reported that peak viscosity has a negative correlation with the amylose content, because amylose restricts the starch granules swelling, which may result in low peak viscosity. The tendency of starch retrogradation can be predicted by using the setback ratio obtained from the RVA curve. A higher setback value indicates higher rate of retrogradation.
Table 11.1.9 Pasting properties of sweet potato starch
No. | Method | Starch content (%) | Peakviscosity | Breakdown | Setback | Pasting temp. (°C) | References
21 | RVA | 10 | 143–288 | 29-163 | 15–79 | 73.5-87.7 | Aina et al., 2012
3 | RVA | 6 | 133–152 | 18–37 | 42–59 | 77.6-80.8 | Wickramasinghe et al., 2009
1 | RVA | 10 | 465 | ― | 78 | Tetchi et al., 2007
1 | RVA | 9 | 281 | ― | 73 | 72.4 | Peroni et al., 2006
20 | RVA | 7 | ― | 85-206 | 106–176 | 53.8-66.6 | Kartayama et al., 2004
2 | RVA | 10 | 381.9-433.4 | 197.5-237.3 | 125.4-176.4 | 74.8-80.5 | Jangchud et al., 2003
2 | BA | 10 | 240–275 | ― | 380–405 | 75–79.5 | Osundahunsi et al., 2003
7 | RVA | 8 | 275–330 | 124–154 | 32–68 | 73.5-77.6 | Noda et al., 2002
25 | RVA | 10 | 145-1260 | ― | ― | 73.5-81.1 | Waramboi et al., 2011
11 | RVA | 8.9 | 268–469 | 149–247 | 51–76 | ― | Zhu et al., 2011
5 | BV | 6 | 666–887 | 37-417 | 275–396 | 54.9-73.6 | Kitahara et al., 2005
8 | RVA | 8 | 348–385 | 128–189 | 69–98 | 74.1-77.2 | Toyama et al., 2003
4 | RVA | 710 | ― | 52.6-73.651.4-72.6 | Katayama et al., 2002
9 | RVA | 7 | 126–190 | 33–84 | 119–199 | 60.4-76.0 | Takahata et al., 2010
No., number of genotypes tested in the specific study; RVA, Rapid visco-analyzer; BA, Brabender amylograph; BV, Brabender viscograph; Viscosity unit of peak viscosity, breakdown, and setback for RVA, BA and BV are RVU, AU and BU, respectively.