Table 9.1.9 Fractional analysis of carbohydrate content of taro corm
Component | %
Starch | 78.0
Pentosans | 2.6
Crude fibre | 1.4
Dextrin | 0.5
Reducing sugars | 0.5
Sucrose | 0.1
Amylose | 17-28
Source: (Onwueme, 1999) in FAO (2003)
Taro starch forms a hard coating layer and its solution has clarity at even high solid concentrations. It has high swelling power, high gel strength and peak viscosity (Ade-bayo and Itiola, 1998). Taro starch, in view of its small granule size (0.5–5 pm), forms a smooth gel (Jirarart et al., 2006) and has been found to be easily digestible (Sug-imoto et al., 1986). Due to ease of assimilation, infants and persons with digestive problems can preferably use the food containing taro starch/flour, replacing other flours and starches (Moy and Nip, 1983). Taro starch has also been studied as a filling agent for biodegradable polyethylene film and as a fat substitute (Alam and Has-nain, 2009) and can be applied in puddings and white sauces (Fetoh and Salwa, 2010). The extracted starches show turbidity in the range 1.013-1.491 at zero time of storage. But the gelatinization temperature increased from 55–95 °C in the samples (Fetoh and Salwa, 2010).
9.1.7.1 Basic Steps of Production
The basic steps of taro starch extraction include:
1. Raw material selection: Loose dirt, sand and gravel are removed from the raw material. Fresh taro corms of a selected variety are chosen for the extraction of starch. The selection of the variety and harvesting methodology are important aspects in the selection, because these parameters will directly affect the starch yield.
2. Washing: The impurities adhering to taro on delivery depends largely on weather conditions and soil. The raw taro is washed with water to remove different impurities together with soil. The washed taro is then sliced into small pieces and suspended in 10-fold of 0.1 % NaOH aqueous solution for 48 hrs at 5 °C (Assefa and Admassu, 2013). Sometimes several washing steps are needed in order to remove slimy mucilage while extracting taro starch.
3. Extraction: Starch can be extracted by a combination of grinding, starch-rich material and wet separation techniques. The sedimentation of starch granules takes place in water due to higher density. Native starch is insoluble in cold water and is a white powder with a bland flavour. Powerful washing is needed to flush the taro starch granules out from the cells and to form a filtering mat that tries to retain the starch (www.starch.dk). The extraction of starch from taro can be carried out by:
3.1. Simple process: This method involves dispersion of taro flour in the required amount of water. The mixture is then homogenized for a specific period of time, followed by holding for the required period to separate out the solid and liquid phase. The liquid layer is decanted and sediment is adequately washed with water. Hereafter, the water is decanted and starch is obtained by filteration. The obtained starch is then dried.
3.2. Wet milling process: In this method, steeping of taro flour in a sodium metabisulphite solution (150 ml, 0.45 % W/V) is carried out overnight in a refrigerator. The slurry is milled and mixed with 450 ml NaCl solution (0.1 M) and 50 ml toulene. The mixture is stirred and allowed to stand until the starch granules settle to the bottom. The protein in toluene and NaCl solution layers are siphoned off. This procedure is repeated until all of the proteins are removed. The starch layer is washed repeatedly with water and then with ethanol. The wet milled starch is recovered by filtration followed by rinsing and air drying (Ahmed and Khan, 2013; Syahariza et al., 2010). A flow sheet showing the different unit operations to manufacture starch by wet milling process is shown in Figure 9.1.7.
3.3. Centrifugation process: Taro flour is steeped in sodium metabisulphite solution of desired concentration (150 ml, 0.45 % w/v) for a specific time and temperature to separate out the liquid phase (Ahmed and Khan, 2013; Ji et al., 2004). The separated slurry/paste is placed in a tube/container with distilled water and homogenized. The homogenized slurry is filtered followed by back-washing. Protein and fiber fractions are removed during filteration. The starch-protein mixture is further separated by centrifugation with repeated washing until free from protein. The cleaned starch is then air dried.
4. Drying: The drying of extracted starch is generally carried out in the range of 30–60 °C.
Figure 9.1.7 Wet milling process for taro starch extraction.
9.1.7.2 Recent Developments for Extraction of Taro Starch
The chronological progression of the recent developments for the extraction of taro starch is given in Table 9.1.10.
Table 9.1.10 Chronological progression of the recent developments for the extraction of taro starch
S. No. | Method | References
1 | Selection of taro corms, addition of 0.05 % (w/v) NaOH solution, centrifugation of suspension at 3,000 g and 4 °C for 10 min, and drying. | Tattiyakul et al. (2006)
2 | Selection of taro, washing, peeling, chopping (1 cm3), pulverizing (high-speed blender for 5 min), suspension of pulp (in ten times its volume of water), stirring (5 min), Alteration (using double fold cheesecloth), settling of filterate (2 h), decantation of top liquid, addition of water into the sediment, stirring (5 min) and drying (60 °C for 12 h). | Nand et al. (2008)
3 | Selection of taro corms, washing, peeling, cutting, homogenization (with water for 1–2 min), sieving, settling (3 h at 4 °C), washing of precipitated starch (3 times with distilled water) and drying (room temp for 2 days and then in oven at 50 °C for 3 h). | Fetoh and Salwa (2010)
4 | Selection of taro tuber, slicing, drying (convection at 40 ± 2 °C), milling (using hammer mill), screening (200 μm), steeping of taro flour (1 kg) (10 L of 2 % NaCl solution), mixing (40 °C/5 h), sieving (80 μm), holding (overnight), treating precipitate (with 10L NaOH 0.03 M), centrifugation (4,500 rpm/15 min), washing (water and ethanol), drying (30 °C), collection, grinding and storage (polyethylene bags)| Himeda et al. (2012)
5 | Selection of taro tuber, cleaning, peeling, slicing, washing, drying (room temperature) and crushing (grinder). Extraction from taro powder was carried out by simple method, wet milling and centrifugation. | Ahmed and Khan (2013)
6 | Selection of taro tuber, peeling, washing, dicing, placing in ice-cold water (100 ppm of NaHSO3), homogenization, filteration, washing the residue (with a small amount of water), centrifugation (3,000 g for 10 min), removal of sediment, further purification (by repeated suspension in water), centrifugation and drying (35 °C) | Karmakar et al. (2014)
7 | Selection of taro tuber, washing, peeling, cutting, weighing (100 g), grinding (90 s), mixing of slurry with 100 ml distilled water, enzymatic treatment of slurry (cellulase from Aspergillus Niger and xylanase from Thermomyces anuginosus and their combination), incubation (at different time and temp at 150 rpm), filteration, centrifugation (3,000 g/10 min), washing, drying (45 °C/24h, hot air drying oven), grinding and sieving (100 mesh). | Sit et al. (2014)