Functional properties of taro flour are variety dependent and affect the rheologi-cal and sensory properties of reconstituted achu, particularly the hardness and overall acceptability. A significant effect of variety on the texture of achu and force of adhesion (FOA) is examined, with the yellow variety exhibiting a higher value compared to the red variety. Achu, dried electrically, exhibits a lower force of relaxation (FOR) and rate of relaxation (ROR) and a higher viscoelasticity index (VEI) as compared to solar drying. An increase in particle size induces significant decrease in FOA, hardness and FOR, while no consistent variations are reported on VEI and ROR (Njintang et al., 2007). Achu exhibits viscoelastic behaviour, varying from more elastic to more viscous (Njintang et al., 2008). The hardness, fracturability, force of adhesion and force of relaxation of achu increased significantly with increase in moisture content, whereas reduction of unrelaxed stress was noticed (Njintang et al., 2007).
In all cases, the instantaneous compliance, both for creep and recovery, was small compared to the retarded elastic compliance and the Newtonian compliance. Significant differences were found in the zero shear viscosity, Newtonian compliance, mean elastic compliance and the viscoelastic index (Njintang et al., 2006). Precooking time induces significant reduction in penetrometric index, foam capacity and increase in least gelation concentration (LGC), emulsion stability and water absorption capacity (WAC) of achu (Njintang and Mbofung, 2006). The drying temperature reduces emulsion capacity and stability and affects the other properties in a similar way to pre-cooking time. In-vitro carbohydrate digestibility of taro achu is reduced significantly when exposed to long pre-cooking times (>45 min) and drying temperatures (>60 °C) (Njintang and Mbofung, 2006). The flour which absorbs more water produces achu with Newtonian compliance and retard elastic compliance. The creep steady state shear compliance and the viscoelasticity index of achu is found to correlate to the sensory hardness (Njintang et al., 2007). The overall acceptability, hardness and the rheological properties of achu are significantly influenced by the variety (Njintang et al., 2007).
Irrespective of variety, reconstituted achu is less acceptable in terms of browning compared to traditional achu. But the flours obtained from the taro, Ibo Ekona and Ibo Ngdere had lower susceptibility to browning during reconstitution (Njintang et al., 2007). Table 9.1.8 shows the physicochemical properties of TFA, TCA and TTA. In either case, the water content of achu was not very different. On the other hand, the BVI (Blue value index) of reconstituted achu was very high and may be responsible for affecting the overall acceptability of achu. TFA represents a higher value for water absorption capacity indicating substantial differences in the texture of achu prepared from the flour as compared to TTA.
Table 9.1.8 Some physicochemical and textural properties of traditional and reconstituted achu
Characteristics | Raw taro flour | TFA | TTA | TCA
BVI (%) | 58.44 ± 9.10c | 427.20 ± 14.50a | 259.15 ± 6.43b | 298.20 ± 5.11b
Moisture content (%) | 9.02 ± 1.6b | 74.71 ± 0.90a | 74.46 ± 0.60a | 75.61 ± 055a
Flour Bulk density (g/mL) | 0.68 ± 0.01c | 0.85 ± 0.01a | 0.67 ± 0.01c | 0.71 ± 0.02b
Paste Bulk density (g/mL) | 1.11 ± 0.03a | 1.07 ± 0.01a | 1.09 ± 0.05a
Temperature of gelatinization | 70–72 | ― | ― | -
WAC (%) | 290.05 ± 7.64d | 739.68 ± 9.80a | 535.44 ± 4.00c | 612.08 ± 9.09b
WSI (%) | 16.69 ± 2.18a | 10.78 ± 1.61b | 12.25 ± 0.42b | 11.05 ± 0.60b
TTA = Traditional taro achu; TFA = Taro flour achu; TCA = taro chips achu, BVI = Blue value index; WAC = Water absorption capacity;, WSI = Water solubility index
Mean ±SD;n = 4; Figures in row followed by different superscripts indicate significantly (p < 0.05) different values determined by Fischer multiple comparison
Source: Njintang (2015b)
Gelatinization curves for TTA and TFA are shown in Figure 9.1.5. The curves show faster gelatinization of TTA as compared to TCA. The faster gelatinisation temperature of TTA may be due to the variations in structural and molecular configuration of starch.
Figure 9.1.5 Gelatinization curves of fresh taro macerate and processed taro flour (Njintang, 2015a).
Generally, TFA and TCA represent poor sensory score for colour (whiteness). Figure 9.1.6 clearly shows the difference in colour of achu, which is usually creamy white (Njintang et al., 2007). The processing of achu results in change of colour from white to brown. The responsible factors like phenolic compounds, free sugars and amino acids may individually or collectively be involved. The extent to which each substrate will contribute to the browning reaction is cultivar dependent (Njintang et al., 2001b). Browning is the major challenge in achu preparation, especially from the flour and chips.
Figure 9.1.6 Traditional and reconstituted achu (Njintang, 2015).
9.1.6.4 Storage
Achu is easily degraded and readily deteriorated by microorganisms if not properly stored. Staling is the major problem during the storage. The retrogradation of the starch (predominantly the amylopectin fraction) may be the primary reason for staling of taro paste during long-term storage (Krog et al., 1989). The hardness of achu from the flours decreases with the storage period. The particle size also affects the texture of achu. The highest value of gumminess is observed in the paste made from flours with particle size above 250 m (Aboubakar et al., 2010).
9.1.7 Taro Starch
Taro has a higher amount of starch than potatoes or sweet potatoes (Masalkar and Keskar, 1998). The starch in taro itself is about four-fifths amylopectin and one-fifth amylose. The amylopectin has 22 glucose units per molecule, while the amylose has 490 glucose units per molecule (Raksaphol, 2009). Taro contains 13–29 % starch, 63–85 % moisture and other compounds in minor amounts such as riboflavin, vitamin C, ash, etc. (Karmakar et al., 2014). Starch accounts for approximately 78 % of the total carbohydrate (Table 9.1.9). Taro is rich in gums (mucilage) and up to 9.1 % crude taro mucilage has been extracted from taro corms (Hong and Nip, 1990). Phosphorus, which is covalently linked to the starch and affects its properties (Takeda et al., 1986), ranges from 0.76 mg/100 g (variety CE) to 1.36 mg/100 g (variety KW2). These values are higher compared to the phosphorous content for Colocasia esculenta (0.01 mg/100 g), Xanthosoma sagittifolium (0.07 mg/100 g) and Manihot esculenta (0.05 mg/100 g) (Perez et al., 2005). For starch, a relative negative linear correlation is established between the hydrolysable level and the phosphorus content. This indicates that native starch with high levels of phosphorus exhibits low digestibility. The lower phosphorus content in starch granules may result in higher resistant starch (Liu et al., 2007). The levels of phosphorous in tuber starches are typically less than 500 mg/100 g (Thomas et al., 1997).