The interaction plot showing the change in firmness with the levels of taro and gum in the composite is presented in Figure 9.2.1. This figure revealed that the effect of gum on bread firmness varied with the level of taro. In fact, at a taro level of less than 10 %, increase in gum induced an increase in firmness, while at taro level of more than 10 %, inconsistent change was observed. This controversial role of gum on the firmness probably reveals the interaction of gum with water on the one hand, but also the interaction of gum with other components such as starch and proteins on the other hand. In fact, we observed that Grewia gum, like other hydrocolloids, has a high ability to absorb water. The water absorption capacity of Grewia gum powder was 1220 g water/100 g gum, about 11 times that of wheat flour and 3 times that of taro flour. The decrease in firmness is probably a result of competition of gum and gluten/carbohydrates with water in the favor of gum, which probably reduced the gas retention during proofing. As reported by Shittu et al. (2009), the softening effect of gum could be attributed to the increasing hindrance of gluten-starch interaction.
Figure 9.2.1 Effect of incorporation of Grewiamollis gum and taro flour on firmness of composite bread.
Chewiness has been reported as the most indicative characteristic of bread (Abdelghafor et al., 2011). In concordance with other reported works on wheat/cereal composite bread (Abdelghafor et al., 2011), our results revealed that chewiness values are highly dependent on hardness (r = -0.75; p = 0.01). In this respect, denser bread are generally harder to break and more difficult to chew. The chewiness of composite bread varied from 0.67 (100 % wheat flour) to 6.25 (10 % taro and 0 % gum).
Sensory Characteristics of Bread The different bread samples were rated as acceptable by the panel of Africans (N = 30; 18–25 years old), except for the sample with 5 % Grewia gum. We observed a negative significant (r = -0.67; 0.05) linear correlation coefficient between the overall acceptability and the level of gum in the composite. Though not significant, we equally observed a negative correlation (r = -0.21) between the overall acceptability of composite bread and the taro level. Generally, the literature revealed a decline in bread preference with the increasing level of taro flour in the composite.
The overall preference of bread with variation in taro and gum levels is presented in Figure 9.2.2. The graph revealed that the acceptability varied mostly with the change in gum levels in the composite. An increase in gum level up to 3 % had no significant effect on the acceptability of composite bread irrespective of the taro level in the composite; beyond this value a significant decline in overall acceptability was observed. At the gum level 3 %, the maximum acceptability was observed for taro levels of 17–20 %.
Figure 9.2.2 Effect of incorporation of Grewiamollis gum and taro flour on overall acceptability of composite bread.
Colour and Structure of Bread The appearance of the crust and crumb of bread are the first parameters that are responsible for the acceptance or rejection of bread by consumers. The color of the crust and crumb are partly determined by the extent of the Maillard reaction during baking (Shittu et al., 2009). The variation in the Whiteness index (WI) of the crumb and the crust of the bread are presented in Figure 9.2.3. Generally incorporation of taro flour and gum induced no significant change on the Whiteness index of the crumb and crust of the bread. In fact, the coefficients of variation of the Whiteness index were 6.0 and 1.8 % for the crust and crumb, respectively.
Figure 9.2.3 Whiteness of the crust (A) and crumb (B) of composite bread as affected by the levels of Grewiamollis gum and taro flour.
In addition, the structure of the crumb changed with the incorporation of taro and gum (Figure 9.2.4). The bread at 100 % wheat and 90 % wheat/10 taro showed very small and homogenous crumb cell largely dispersed, while in 30 % taro/70 % wheat, very large crumbs cells appeared. The size of the cell crumb diminished in 65 % wheat/30 % taro and 5 % gum. The increase in crumb size upon increase in taro flour is probably a consequence of the weakness of the dough, which does not resist to the increase in volume during fermentation.
Figure 9.2.4 Some pictures of bread as affected by level of taro and gums in the composite: (A) 100 % wheat; (B) 70 % wheat, 30 % taro and 0 % gum; (C) 65 % wheat, 30 % taro and 5 % gum.
The optimal levels of taro and gum flour in the bread were then determined to 17 % taro flour and 2.5 % Grewia gum. The optimal substitution level of precooked taro flour was not far from 15 % substitution with taro used by Payne et al. (1941) for bread production on an industrial scale at Hawaii. According to these authors, the use of taro flour in bread preparation presents no difficulty under conditions where some precautions are respected. The first condition is general for all wheat composite flour, that hard wheat flour is preferable and the taro flour should be carefully blended with wheat in order to avoid dark and sticky lumps in the dough. Second, in order to avoid the dough to be too slack, for 1 % of taro flour used, the water added to the blend should be increased for 1 % over normal. Third, the fermentation should take less time as compared to wheat flour doughs. Lastly, the temperature of the baking oven should be lower as wheat taro blends take color much more quickly.
Payne et al. (1941) equally observed that wheat taro composite bread has superior keeping quality based on the high water absorption capacity of taro flour. Five days after baking, it was still possible to butter slices of the taro bread without crumbling, as compared to wheat bread which crumbled badly and could hardly be considered edible. As a recipe for bread formulation, they proposed mixing 794 g taro flour, 3,969 g white wheat flour, 57 g of shortening, 57 g of sugar, 114 g of salt, 85 g of yeast and 4.5 L water. The dough was produced in 2 steps of 90 and 30 min. To complete the fermentation the dough is left at room temperature for 10 min.
Payne et al. (1941) also reported the successful bread-baking test in a large scale done by the US Army. Taro flour was used in the proportion of 15 % and more precisely, 6,800 g taro flour were mixed with 38 556 g of wheat flour, 907 g of salt, 1,134 g shortening, 907 g of yeast, 1,814 g powdered milk, 1,361 g of sugars, 113 g of arkady and 34 020 g of water. They reported that the taro wheat composite bread obtained had high keeping quality as a result of its initial high moisture content, and not from any greater capacity for retaining its moisture. In using taro flour for bread preparation, they observed that taro flour was found harder to handle because it was softer. In addition, more time (10 min for each 454 g loaf and 15 min for 908 g loaves) was needed for baking.