Figures 9.2.5, 9.2.6 and 9.2.7 respectively show the variation in soluble proteins, soluble sugars and carbohydrate digestibility of the composite biscuits as affected by the level of taro. Generally soluble protein level increased with increase in taro level in the composite. The increase was higher for biscuits made from egg-like taro flour variety as compared to Red Ibo taro flour variety. This increase might possibly be due to soluble proteins present in the taro flour. According to Njintang et al. (2014). the soluble proteins in taro are mainly composed of mucilage, which the content varied from one variety to another.
Figure 9.2.5 Effect of variety and level of taro flour incorporation on soluble proteins of biscuits.
Figure 9.2.6 Effect of variety and level of taro flour incorporation on soluble sugars of biscuits.
Figure 9.2.7 Typical surface plot for the effect of digestion time and level of taro flour on the in vitro carbohydrate digestibility of biscuits (Egg-like variety).
In addition, Jiang and Ramsden (1999) closely associated the taro corms proteins with those of the mucilage. Based on this we can assume that the egg-like taro variety used in this work was higher in mucilage as compared to red Ibo coco variety. However this needs to be investigated. In addition to soluble proteins, soluble sugars of composite biscuit increased with level of taro flour incorporation, and particularly for Red Ibo coco variety (Figure 9.2.5). The soluble sugars of wheat-taro biscuits varied between 2.61-2.84 g for biscuits made from egg-like taro flour and 2.61-3.49 g for those from Red Ibo variety. The increase in sugars also reflected the high level of soluble sugars in taro flour. Figure 9.2.7 shows that dough formulated with taro flour substantially increased in vitro carbohydrate digestibility of biscuit compared to the 100 % wheat biscuit. Replacement of wheat with 30 % egg-like flour and 30 % Red Ibo increased the in vitro digestibility of carbohydrates by 50.8 and 59.0 % respectively. The improved digestibility suggests potentially improved sugar absorption and retention in humans. In this respect, increase in digestibility with taro flour substitution could be attributed to dilution of the less soluble and digestible wheat carbohydrate with the more soluble and digestible carbohydrate content of taro flour. The high digestibility of taro starch has been associated to the small size of its granules and encouraged in the preparation of infant food in Hawaii and other Pacific islands (Nip, 1997). Indeed our study and others on the microstructural characterization of taro starch revealed the size of taro starch granules between 1 and 5 pm, one of the smallest that exists in nature (Aboubakar et al., 2008).
Physical properties of wheat-taro composite biscuits The values of the physical properties of taro wheat composite biscuit are shown in Table 9.2.4.
Table 9.2.4 Some physical characteristics of taro-wheat composite biscuits
Level and type of flour in the composite | Weight (g) | Diameter (mm) | Thickness (mm) | Density (g/cm3) | Spread ratio | Volume (cm3)
100 % Wheat flour | 5.18 ± 0.41a | 4.96 ± 0.03a | 0.57 ± 0.03b | 0.47 ± 0.03b | 8.70 ± 0.56a | 10.99 ±1.03 a
Level of Egg-like variety
5 | 5.23 ± 0.36b | 5.06 ± 0.05d | 0.56 ± 0.03a | 0.47 ± 0.02ab | 9.03 ± 0.55b | 11.23 ± 0.87 a
10 | 5.38 ± 0.19c | 5.03 ± 0.05c | 0.56 ± 0.03a | 0.48 ± 0.04c | 8.98 ± 1.02c | 11.12 ± 0.78 a
15 | 5.34 ± 0.43a | 4.98 ± 0.06b | 0.56 ± 0.03b | 0.48 ± 0.05ab | 8.89 ± 1.09a | 11.07 ±0.79a
20 | 5.36 ± 0.37ab | 5.05 ± 0.08d | 0.56 ± 0.04a | 0.48 ± 0.02a | 9.02 ± 0.52ab | 11.22 ± 1.02a
25 | 5.39 ± 0.43ab | 5.03 ± 0.08c | 0.55 ± 0.04a | 0.48 ± 0.02ab | 9.15 ± 0.57b | 11.07 ±1.06a
30 | 5.40 ± 0.41c | 5.03 ± 0.08c | 0.55 ± 0.04a | 0.49 ± 0.05c | 9.15 ± 1.19c | 11.02 ± 0.99 a
Red Ibo variety
5 | 5.75 ± 0.52b | 5.10 ± 0.13de | 0.56 ± 0.04a | 0.51 ± 0.06d | 9.10 ± 0.90d | 11.24 ± 1.26b
10 | 5.92 ± 0.49cd | 5.08 ± 0.11cd | 0.56 ± 0.04d | 0.50 ± 0.04c | 9.07 ± 0.75c | 11.88 ±1.19d
15 | 5.87 ± 0.48c | 5.07 ± 0.11bc | 0.56 ± 0.03bc | 0.51 ± 0.03d | 9.05 ± 0.49d | 11.42 ±1.14bc
20 | 5.73 ± 0.36b | 5.06 ± 0.10b | 0.56 ±0.03d | 0.49 ± 0.03b | 9.03 ± 0.55b | 11.84 ±0.86d
25 | 6.01 ± 0.47d | 5.12 ± 0.11e | 0.55 ± 0.03b | 0.52 ± 0.03de | 9.31 ± 0.66e | 11.53 ±1.01c
30 | 6.27 ± 0.48e | 5.18 ± 0.10f | 0.55 ± 0.03c | 0.52 ± 0.05e | 9.42 ± 1.15f | 11.97 ±1.01d
Mean ± standard deviation; N = 25. Means in the same line within each variety followed by similar figure (a-e) in superscript are not significantly different at p < 0.05.
The mean weight of the control wheat biscuit was 5.18 g and the weight increased significantly (p < 0.01) with the increasing level of taro flour in the composite, irrespective of the taro variety. The weight of wheat-taro biscuits varied between 5.23 and 5.40 g for Egg-like taro flour and 5.73 and 6.27 g for Red Ibo taro flour. These results suggested that as the level of taro increased in the composite, the biscuit becomes denser as no significant variation was observed on the volume of the biscuit, irrespective of the taro variety.
It has been clearly established experimentally that biscuit density is strongly correlated to the water absorption capacity of flour (Doescher et al., 1987). We found in our earlier studies that increase in the level of taro in the composite led to significant (p < 0.05) increase, not only in water absorption capacity, but also in water solubility index and swelling index (Himeda et al., 2014; Njintang et al., 2008). The high water absorption capacity of taro flour could have contributed to the higher density of composite wheat-taro biscuit. According to Njintang et al. (2008) and in conformity with other studies on incorporation of navy beans, the observed increase in WAC could be ascribed to the high level of carbohydrate in taro flour, which was as high as 89 %. WAC also has a direct link with the dough consistency, and this was proved not only for whole wheat flour (Kitissou, 1995), but also for wheat-taro composite flour (Njintang et al., 2008). The densification of the dough with increase in level of taro in the composite was also shown recently (Himeda et al., 2014). In fact, as the level of taro in the composite increases from 0-30 %, we observed that the rupture pressure of the dough systematically increased from 99-166 mm H2O for Red Ibo, and from 99-146 mm H2O for Egg-like variety.