During storage, starch may become cloudy and an insoluble white precipitate may be formed. This may be due to the re-crystallization of starch molecules. Initially the amylose forms double helical chain segments followed by helix-helix aggregation (Whistler et al., 1984). This phenomenon is termed retrogradation. The retrogradation of the taro starch, as measured by its enthalpy changes, appears to be more severe than that of corn starch (Jane et al., 1992).
Pasting properties are used to represent the behaviour of starches during heating and cooling cycles in excess of water. The properties can mainly be affected by amylose, branch chain length distribution of amylopectin and lipid content (Jane et al., 1999). The pasting characteristics play an important role in the selection of a starch for its use in the industry as a thickener, binder or for any other purposes. The viscosity of the gel formed during and after heating is an important factor in the selection of starch (Aryee et al., 2006). Pasting temperature (PT) is the minimum temperature required to cook the starch. Pasting temperatures of the taro flour pastes, 76–80 °C, are significantly higher than those of the taro starch pastes, 70–75 °C (Jane et al., 1992). The higher pasting temperature may be due to mucilage. A rapid increase in viscosity, due to swollen granules, is referred as peak viscosity (PV). The increase in viscosity with increase in temperature may be attributed to the removal of water from the exuded amylose by the granules as they swell (Ghiasi et al., 1982). PV can indicate the viscous load, likely to be generated in a mixing cooker and often correlates with the quality of the end product (Ragaee and Abdel-Aal, 2006).
Trough viscosity, measures the ability of paste to withstand breakdown upon cooling which is influenced by the rate of amylose exudation, granule swelling, amylose-lipid complex formation and competition between exuded amylose and remaining granules for free water (Sodhi et al., 2009). Breakdown viscosity measures the ease by which the swollen granules can be disintegrated. Final viscosity indicates the ability of the material to form a viscous paste. Final viscosity is largely determined by the retrogradation of soluble amylose upon cooling (Olkku and Rha, 1978). Setback viscosity measures the syneresis of starch upon cooling of cooked starch pastes. Setback value is the recovery of the viscosity during cooling of the heated starch suspension. Taro starch shows lower breakdown viscosity (5.53 cP), lower final viscosity (13.92 cP) and lower setback viscosity (4.50 cP) as compared to starches such as like yam, sweet potato, canna, arrowroot, konjac and cassava (Aprianita, 2010). The lowest breakdown of taro starch indicates paste stability. The lowest setback value of taro starch possibly indicates its lower tendency to retrograde.
9.1.7.4 Morphological Characteristics and Crystallinity of Taro Starch
Morphological properties of starch granules using Light Microscopy (LM) and Scanning Electron Microscopy (SEM) have recently been taken to characterize starch granules. LM has provided vital information about the internal structure of starch granules, giving information about the various shapes and sizes of starch grains, their refractive indices, as well as their transparent and colourless nature. SEM can potentially study the morphological changes due to the various effects and additives.
The crystallinity of starch granules is disrupted during chemical modification, and this leads to a greater degree of separation between the outer branches of adjacent amylopectin chain clusters in modified starches compared to those in native starches (Karmakar, 2005). According to the X-ray diffraction pattern, native starch granules can be classified as A, B and C-types (Eliasson and Gudmundsson, 1996). Starch is generally classified into three types (A, B, and C) according to the wide-angle X-ray diffraction (WAXD) pattern given by their amylopectin crystalline structures. Most cereal starches (e.g. normal corn, rice, wheat and oats) display the A-type, while tuber starches (e.g. potato, lily, canna, tulip) exhibit the B-type; C-type is the mixture of A-and B-types. It is believed that amylopectin is constituted of crystalline domains with the double helices arranged in the A-, B- or C-pattern (Yu et al., 2013). Starches with amylopectin of short average branch chains display the A-pattern, whereas those with long branches give the B pattern. The average chain length in between forms the C-pattern (Hizukuri et al., 1983). Crystallinity in native starch can be correlated with amylose content.
Taro starch granules show a variety of shapes with sizes between 1 and 5 pm and presents an A-type XRD pattern with a crystallinity level of 38.26 % (Acevedo et al., 2011). The X-ray diffraction of the starches to investigate the changes in crystallinity due to maturity stage of taro tubers (Sosso variety) indicates similar diffraction curves, suggesting a unique type of taro starch during growing, irrespective of the maturity stage (Himeda et al., 2012).
9.1.7.5 Modified Starches
Starch is tailor-made to meet the requirements of the end-user, giving rise to a range of speciality products but starches in their native form have limited application in industry. Native starches have low shear stress resistance, thermal decomposition, high retrogradation and syneresis (Fleche, 1985). In order to promote utilization of starches and to widen their application, there is a need to improve their functional properties. Such improvement can be achieved through modification. Modified starches usually have functional properties that native starches do not provide. Another purpose of starch modification is to stabilize starch granules during processing and make the starch suitable for many food and industrial applications (Schmitz et al., 2006).
By applying different reaction conditions, such as temperature and pH, additives and strict process control speciality products with unique properties are made. These speciality products are named modified starches, because they still retain their original granular form and thereby resemble the native (unmodified) starch in appearance. But the modification has introduced improved qualities into the starch when cooked. Recently, the modification of taro starch has been carried out by heat-moisture treatment (Eerlingen et al., 1996) and the physical method (Karmakar et al., 2014).
In the heat-moisture treatment method, the moisture contents of the taro starch samples are brought to 15,20,25 and 30 % (on a dry weight basis) by spraying appropriate amounts of distilled water. The samples are then heated in an oven, kept in air-tight bottles for 15 h at 100 °C and subsequently air dried at room temperature (Alam and Hasnain, 2009). In another method, 1 % taro starch slurry in water is treated for 30 min at the desired temperatures (60,70, 80 or 95 °C), cooled and air dried (Alam and Hasnain, 2009).
In the physical modification method by Karmakar et al. (2014), starch, 100 g (db) in 200 ml saturated Na2SO4 solution, is heated up to 120 °C, which corresponds to the calculated osmotic pressure of 341 atm (345 bar) (assuming Na2SO4 dissociated completely) for 15, 30 and 60 min. After the heating process is over, the flasks are cooled down to room temperature and the starch is removed and washed with water (8 × 500 mL) to remove Na2SO4 by centrifugation at 4,552 g for 10 min, which is repeated twice. Then the starch is dried at 60 °C in a hot air oven overnight (Pukkahuta et al., 2007).