As a consequence of the absence of gluten, it has been demonstrated that above 10 % taro flour substitution, the elasticity index of the composite dough falls within the interval of 35–60 % required for the use of wheat flour in French bread manufacture (Njintang et al., 2008). The fact that some bread was accepted with up to 50 % substitution of wheat suggested that the maximum level of taro incorporation into the composite depends mostly on the acceptability of the end product. In this respect, the type of bread and the habit of population are determinant parameters in this acceptability. One positive characteristic of taro-wheat bread quality is its low retrogradation index, and the capacity to keep its softness over a long period of time as compared to 100 % wheat bread.
Background The acceptability of the above-mentioned studies on bread taro blends did not reveal the irritating limit of the uncooked taro flour. Our recent findings on biscuit production from raw taro flour revealed that irritation induced by raw taro when it comes into contact with the body remained unchanged in the biscuits when tested organoleptically, suggesting that baking had no significant effect on taro acridity (Himeda et al., 2014). These findings were reinforced by an earlier study on Japanese taro, which reported that baking significantly increased the concentration of total oxalate (Catherwood et al. 2007). On the contrary, boiling has been shown to significantly reduce the oxalate content and rapidly annihilate irritation (Aboubakar et al., 2009 Catherwood et al., 2007; Nip 1997). It is in this respect that we recently tested the use of pre-cooked taro flour and also the incorporation of Grewia mollis gum in bread-making.
Method The pre-cooked taro flour was obtained by 25 min boiling taro corms of size of more than 284 g followed by peeling and slicing 0.5 cm thick, drying at 45 °C to constant water content before milling into flour to pass through a mesh size of 500 pm using a hammer miller. The rheological and acceptability of the bread were evaluated. Taro flour, wheat flour and Grewia gum were mixed following a mixture design plan, as shown in Table 9.2.1.
Table 9.2.1 Mixture design plan for the preparation and some characteristics of taro-wheat-gum blend flour
Flours percentage in the mixture | Bread parameters
Wheat flour | Taro flour | Grewia gum | Volume(mL) | Firmness (kg Force) | Chewiness | Overall acceptability (/5) | WI of the crust | WI of the crumb
65 | 30 | 5 | 315 | 1.93 | 3.46 | 2.0 | 61.8 | 80.1
81.25 | 15 | 3.75 | 345 | 0.76 | 1.36 | 3.6 | 59.4 | 80.2
90 | 10 | 0 | 275 | 1.71 | 6.25 | 3.4 | 70.8 | 85.9
70 | 30 | 0 | 235 | 1.31 | 1.74 | 3.1 | 65.5 | 82.5
83.75 | 15 | 1.25 | 265 | 1.65 | 2.33 | 3.1 | 57.3 | 83.7
73.75 | 25 | 1.25 | 235 | 1.11 | 2.44 | 3.4 | 65.3 | 82.3
71.25 | 25 | 3.75 | 285 | 0.88 | 1.26 | 3.0 | 63.1 | 80.4
77.5 | 20 | 2.5 | 285 | 1.59 | 1.61 | 3.2 | 66.3 | 81.2
85 | 10 | 5 | 245 | 4.20 | 5.63 | 2.0 | 62.7 | 81.3
100 | 0 | 0 | 385 | 0.28 | 0.69 | 3.3 | 65.0 | 81.2
Mean value | ― | ― | 287 | 1.54 | 2.68 | 3.01 | 63.7 | 81.9
Standard deviation | ― | ― | 49 | 1.06 | 1.88 | 0.56 | 3.8 | 1.8
WI = whiteness index
In the procedure of bread preparation, 100 g of each blend was thoroughly mixed with 1.7 g salt, 5 g of leavening agent yeast and 150 g of water in a Kenwood food Mixer Moulinex, France. The mixing was done to ensure homogeneity of the dough. After fermenting for 40 min at 37 °C and 75 % humidity, the dough was knocked and moulded into a loaf, proofed for 20 min at 37 °C before being baked at 210 °C for 20 min in a baker model Arthur martin pyrolyst under an automatic vapour injection system.
The texture profile characteristics of bread were evaluated by using a double cycle compression test protocol on a Lloyd type testing machine (Lloyd instrument LRX, Germany) provided with the software Nexigen, equipped with a 500 N load cell and a cylindrical probe with a diameter of 10 mm. The probe was allowed to compress bread (2.50 ± 0.02 cm thickness) to 80 % depth, at 10 mm/min speed test, with no delay between first and second compression. All trials were done in triplicate. Firmness, chewiness and other parameters were generated automatically by the software.
The colour whiteness WI = 100 ― √((100 ― L)†2 + a†2 + M†2) index of the bread crush and crumb was calculated from L, a and b colour attributes measured with a Chromameter CR210 (Minolta France S.A.S., Carrieres-sur-Seine) (Nguimbou et al., 2013).
Physical Properties of Bread The effect of taro and gum flours on volume of wheat bread is presented in Table 9.2.1. Bread specific volume varied from 235–385 mL. In agreement with many other authors (Ade-Omowaye et al., 2008), all the volumes of bread made from composite flours were lower than those made from pure wheat flour.
In addition, flours containing higher levels of taro flour (30 or 25 %) associated with lower gum levels (0 and 1.25 %) resulted in lower bread volume. The reduction in bread volume with increasing levels of non-wheat flour has been attributed to lower levels of gluten network in the dough (Abdelghafor et al., 2011). The positive action of gum in bread-making has also been long demonstrated in many studies (Shittu et al., 2009).
Generally, hydrocolloids such as sodium alginate, k-carrageenan, xanthan gum, hydroxypropyl methyl cellulose and carboxy methyl cellulose have been found to affect the stability of dough during the proofing stage. But this varied with the type of gum. No significant linear relationship was observed between the bread volume and the gum or taro levels. However, the bread volume was positively linear correlated (r = 0.45; p = 0.19) with wheat flour levels in the mixture, while a negative correlation was observed with taro flour (r = -0.49; p = 0.15). We equally observed that the volume of bread was linearly correlated, though not significantly, with the texture parameters firmness (r = -0.51; p = 0.13) and chewiness (r = -0.41; p = 0.24). This observation show that as the volume of bread increased, the hardness decreased as well as the ability to chew. A similar relationship has been observed in previous studies where wheat was substituted with cereal (Abdelghafor et al., 2011).
Firmness is the first texture parameter perceived by the consumer when he comes into contact with bread. In addition, the chewiness is also an important parameter along with the color perception of bread. In the texture profile, the firmness or hardness is defined as the peak force value corresponding to the first two successive deformations. Firmness varied from 0.22 kg force to 4.2 kg force. The bread sample having 100 % wheat flour shows a lower value of firmness, while bread made from 10 % taro and 5 % gum had higher firmness (Table 9.2.1). Firmness was the only texture parameter significantly correlated with the overall acceptability. Generally breads with higher firmness had lower acceptability scores (r = -0.75; p = 0.01). Though not significant, linear relations were observed between firmness and gum level (r = 0.49; p = 0.15). This general view suggested that as the level of gum increases in the mixture, the firmness increases, but this hid the interaction between the taro and gum levels on the firmness.