Isolation using Sodium Chloride According to the method of Riley et al. (2006), the edible portion of sweet potato was cut into small pieces and homogenized with 1 M NaCl solution using a blender. The mixture was filtered through a triple-layered cheese cloth and the starch was further washed with distilled water. The granules were allowed to settle and water was decanted. The sediment was centrifuged at 3,000 × g for 10 min. Starch was removed, allowed to dry overnight at room temperature and the dried starch was ground with a mortar and pestle into a fine powder.
Isolation using Distilled Water As described by Wickramasinghe et al. (2009), with slight modification, the edible portion of sweet potato was cut into small pieces and homogenized with distilled water for 1–2 min. The slurry was then passed through a double-layered cheesecloth and the filtrate was allowed to settle for a minimum of 3 h at room temperature. The precipitated starch was washed 3 times with distilled water, dried at room temperature for 2 days and then kept in an oven at 50 °C for 3 h and ground with a mortar and pestle into a fine powder.
11.1.8 Recent Developments for Extraction of Sweet Potato Starch
The chronological progression of the recent developments for the extraction of sweet potato starch is given in Table 11.1.5.
Table 11.1.5 Chronological progression of the recent developments for the extraction of sweet potato starch
S. No. | Method | References
1 | Selection of sweet potato tubers, peeling, cutting (4–6 cm), Mais (2008) soaking (0.2 % sodium metabisulfite solution for 5 min), extraction of juice (5 min), starch slurry, filtration of starch slurry (firstly through 200 micron and then through 100 micron), collection of filtrate, resting of filtrate (1 h), collection of white starch fraction, re-suspension in water, settling, oven drying of collected starch (40 °C/24 h), grinding, sieving (120 micron), packaging (polypropylene bags), storage (room temperature) |
2 | Selection of sweet potato tubers, washing with water, peeling, slicing, rewashing and grating to obtain the pulp, sieving, filtrate (starch milk) was allowed to stand for sometime for the starch to settle, decanting the supernatant to obtain wet starch cake, sun drying of wet starch cake, grinding, packaging (transparent polyethylene bags) | Oladebeye et al. (2009)
3 | Selection of sweet potato tuber, washing, peeling, cutting, homogenization (1–2 min), passing of slurry through sieve, settling of filtrate (4 °C/3 h), precipitated starch washed 3 times with water, drying (room temperature for 3 days and then in oven at 50 °C/3 h) | Fetoh and Salwa (2010)
4 | Selection of sweet potato roots, washing, peeling, cutting (4 × 4 × 4 mm), soaking (0.2 % sodium metabisulfite solution with ratio of 1: 2 w/w for 15 min), grinding (5 min), filtration of slurry (100 mesh sieve), resting (3 h), collection of starch, re-suspension of starch with water, settling of starch, repeated 3 times, drying of starch (hot air oven at 50 °C for 12 h until ~10 % MC wet basis), grinding, packaging (polypropylene bags) and storage (cold room, 4 °C) | Thao and Noomhorm (2011)
5 | Selection of sweet potato tubers, washing, peeling, cutting, weighing, grinding, dilution of grinding material with water, sieving, 0.1 N NaOH was added to the slurry filtrate and allowed to stand for ~3 h. Supernatant water was decanted off carefully, and fresh water was added again to wash the starch, the supernatant water was then decanted off after 3 h. The starch sediment was then air-dried | Jubril and Mohammed (2012)
11.1.9 Physicochemical Properties of Sweet Potato Starch
The physicochemical properties of starches dictate their functionality in various applications. For instance, starches with low amylose content gelatinize easily and produce clear pastes, suggesting its usefulness in paper-manufacturing industries. The amylose content of the starch granules varies with the botanical source of the starch and is affected by the climatic conditions and soil type during growth (Morrison et al., 1984). The activity of the enzymes involved in starch biosynthesis may be responsible for the variation in amylose content among the various starches (Krossmann and Lloyd, 2000). Amylose content, WAC and OAC of sweet potato starch was observed to be 15.8, 92 and 130 %, respectively (Table 11.1.6):
Table 11.1.6 Amylose content, water absorption capacity and oil absorption capacity of sweet potato starch
Component | Value
Amylose content (%) | 15.8
WAC (g/g) | 92
OAC (g/g) | 130
Tsakama et al. (2011) reported amylose content in the range 7-11.5 % for different sweet potato cultivars. The formation of hydrogen bonds between the hydroxyl groups of different starch chains lowers the water binding capacity (Hoover and Sosulski, 1986). High amylose starches are characterized by their high gelling strength, which suggests their usefulness in the production of pasta, sweets, bread and in the coatings of fried products (Hung et al., 2005; Vignaux et al., 2005). Differences in amylose content of sweet potato starches have been reported and ascribed to genotypic differences, environmental factors and starch-processing methods (Garcia and Walter, 1998; Oduro et al., 2000).
Amylose complexes with fats and emulsifiers in foods such as mono- and diglycerides can alter the temperature of gelatinization and the texture and viscosity profile of the paste, and can limit retrogradation (Thomas and Atwell, 1999). Das et al.
(2010) reported the water and oil binding capacity of sweet potato starch as 77.38 and 66.77 %, respectively. The water absorption capacity is observed to be higher in the starch where the amylose and amylopectin are loosely associated. The differences in availability of water binding sites among the starches may have also contributed to difference in water absorption capacity (Wotton and Bamunuarachchi, 1978).
Chibuzo (2012) reported that the water absorption capacity (WAC) and oil absorption capacity (OAC) of the starches ranged from 0.80-4.80ml/g and 0.35-4.14 ml/g, respectively. The oil-binding capacity of native sweet potato starch was 66.77 %, whereas it varies from 77.6-82.6 % and 74.3-78.9 % for acetylated and dual-modified starches, respectively (Das et al, 2010). It was observed that the oil-binding capacity of modified starches was higher as compared to its native starch sample, but decreases with the increase in substitution level. In acetylated starch samples, the oil-binding capacities gradually decrease because of reduction of the amorphous region in the starch granules. This reduces the number of available binding sites for oil in the starch granule.
Swelling power and solubility pattern of the starches have been studied to understand the interactions between the water molecules and the starch chains in the crystalline and amorphous regions during heating. When starch molecules are heated in excess water, the crystalline structure is disrupted and water molecules become linked by hydrogen bonding to the exposed hydroxyl groups of amylose and amylopectin, which causes an increase in granule swelling and solubility (Singh et al., 2003). The swelling power of sweet potato starch (SPS) at different temperatures was observed in the range between 1.1 and 24.4 g/g (Table 11.1.7). Abegunde et al. (2013) reported swelling power of the starches from the various sweet potato cultivars in the range from 13.46–26.13 g/g. The swelling power of starch depends on the water-holding capacity of starch molecules by hydrogen bonding (Lee and Osman, 1991).