Effect of drying methods and blending ratios on dough rheological properties, physical and sensory properties of wheat–taro flour composite bread

Abstract The study was conducted to evaluate the effect of taro drying methods and blending ratios on the physical quality attributes and sensory quality of wheat–taro bread and rheological properties of the blend dough. Farinographic properties like water absorption capacity, dough development time, dough stability time, time to break down, mixing tolerance index, and farinographic quality number were significantly (p < .05) affected by drying methods and blending ratio and their interaction. Increased taro flour (10–20 g) per 100 g of wheat flour resulted in an increased water absorption capacity (57.38%–58.23%) and mixing tolerance index (67.33–70.21 FU). The sensory analysis had revealed that as taro flour blending ratio increased the acceptability of blended breads were reduced. With respect to physical and sensory properties, the control bread had better acceptability than that of 10, 15, and 20 g taro flour‐mixed bread. The study revealed that there is possibility of incorporating taro flour up to 15 g per 100 g of wheat flour with acceptable sensory attributes of the composite bread.

and potassium bind with oxalic acid) cause skin irritation and a pungent odor in unwashed taro corms (Kaushal et al., 2015;Lee, 2002).
Methods of drying affects the properties of the agricultural products such as color, texture, density, porosity, and sorption characteristics of materials (Krokida, Tsami, & Maroulis, 1998). Several drying methods reported in literature such as tray drying, drum drying, and spray drying used in taro flour production are not only unavailable in most developing countries but they are also expensive and require special equipment. In the face of these drawbacks, the use of other available drying methods such as oven, sun, and solar dryer have been considered as better alternatives (Whitfield, 2000).
Different drying methods were reported to produce taro flour (Agoreyo et al., 2011). The methods of drying have been reported to influence chemical composition, for example, reduction in moisture content, calcium oxalate, protein, and lipid, but ash and fiber contents were increased. Taro flour increase the moistness and keeping quality of taro blended bread and high viscosity, high thickening power, and small particle size starch is useful for noodle and bread making (Kaushal et al., 2015;Njintang, Mbofung, & Kesteloot, 2007). In the bread making, low retrogradation tendency of taro flour could reduce the bread stalling, which in turn could increase the shelf-storage of bread (Taggart, 2004). Despite its nutritional, industrial, and health importance, taro has not gained sufficient research attention to enhance its potential (Aboubakar, Scher, & Mbofung, 2007).
Substitution of taro flour to wheat flour in bread making is an important avenue toward utilization of this crop. This, however, calls for the use of proper flour production methods and suitable taro flour blending ratios through research. The objective of this study was to evaluate the effect of taro drying methods and blending ratios on the physical quality attributes and sensory quality of wheat-taro bread.

| Experimental design
A 3 2 factorial with three replications was used (Table 1). The two factors were wheat-taro flour blending ratio and drying methods; each factor was used at three levels. The upper and lower levels of variables were selected based on different composite to wheat flours studied in the past for bread making (Ikpeme-Emmanuel, Osuchukwu, & Oshiele, 2010;Njintang, Mbofung, Balaam, Kitissou, & Scher, 2008).

| Experimental materials
Wheat and taro, both grown in 2010 cropping season, were obtained from Debre Zeit and Areka Agricultural Research Centers, respectively, Ethiopia. The selection criteria of wheat (Kubsa) and taro varieties (Boloso I) were based on bread-making potential (Habtu, 2010) and bulk production (Adane, 2009), respectively.

| Sample preparation
Wheat was milled to particle size of less than 750 μm using the procedure described in the cereal grain processing manual, using the local miller (Bizzarri and Morelli, 1988). Taro roots were, weighed, washed, peeled, sliced (0.6-1.0 cm thick), and soaked in 120 ml lemon juice solution (1/2 cup lemon juice) and 2 L (2.2 quarts) cold water for 45 min to suppress oxidation while they dry (Nelson & Elevitch, 2011). The treated slices were removed, well drained, dehydrated using oven dryer (60°C for 12 hr), solar dryer, and sun drying until moisture reached 14% (Asha and Nair, 2002). The dried taro was milled into flour using a commercial miller. The flour was sieved by 0.75-mm mesh size sievers and finally packed in air-tight plastic.

| Rheological properties of wheat and taro blended flours
Dough strength was measured by Farinograph (Brabander Farinograph ® E OHG, 2002, Germany) according to AACC (2000) method No.54-21 of constant dough weight method at 30 ± 0.2°C using a 300 g mixing bowl, operating at 63 rpm. Each flour sample in the range of 284.5-300 g on a 14% moisture basis was weighed and placed into the corresponding Farinograph mixing bowl. Water from a burette was added to the flour and mixed to form dough.

| Bread making
Bread was baked using straight-dough methods as described in the AACC (2000).

| Loaf weight, loaf volume, and specific volume
The weight of bread samples were determined after sufficient cooling using a digital balance (0.01 g accuracy) and the loaf volume was determined using rapeseed displacement method (Chopin, 2000) and referred to 100 g of flour on 14% moisture base. The calculation of bread volume was adopted from Sangnark and Noomhorm (2003) as follows: Where, V 100 = Volume calculated for 100 g of the bakery product V sr = Reading of volume in cm 3 . G = Weight of one piece of bakery product.
The specific volume of each loaf was calculated as follows:

| Crumb water holding capacity
The bread sample was cut into slices of 1.5 cm thick using a sharp knife.
The outer crust of samples was carefully scrapped with kitchen-type bread knife. The 1 g cuts from each point were combined to make a final weight of about 5 g. The moisture content was determined using connective oven set at 130°C for 1 hr (Shittu, Raji, & Sanni, 2007).

| Sensory evaluation
Fifty member judges were selected from staff and graduate students of Haramaya University Department of Food Science and Postharvest Technology. The sensory attributes: visual color, taste, flavors, appearance, and over all acceptability were evaluated using a 5-point hedonic scale rated from 1 (extremely dislike) to 5 (extremely like). Bread was served at room temperature using the more widely used practice of three digit code during sensory analysis (Resurrection, 1998). Just before each test session, orientation was given to the judges on the procedures of sensory evaluation.

| Statistical analysis
The data collected on chemical composition, physical characteristics, and sensory properties were subjected to analysis of variance (ANOVA) with three replications using Statistical Analysis System (SAS, 1990) software version 9.0. Means were compared using Duncan's multiple range test (DMRT) at p < .05.

| Loaf weight
The results of the effect of blending ratio and drying methods on loaf volume are presented in Table 2. The loaf weight was significantly (p < .05) affected by the drying methods, blending ratio and their interaction. The highest was observed in the 20 g and 15 g taro per 100 g wheat flour and the lowest was between the solar-dried taro flour of 10 g per 100 g wheat flour. In general, with increase in the taro flour, an increase in the loaf weight was observed. Blending ratio appears to be dominant factor compared to the drying methods. Loaf *, **, and ns represent significant at 5%, significant at 1%, and nonsignificant at 5% probability level. Mean values followed by the same letter in the column are not significantly different at 5% probability level. DMRT (p < .05), Duncan's multiple range taste at α equal to 0.05; D 1 B 1 , D 2 B 2 , and D 3 B 3 , Oven-dried taro flour blended bread at 10, 15, and 20 g taro flour, respectively; D 3 B 1 , D 3 B 2 , and D 3 B 3 , Sun-dried taro flour blended bread at 10, 15, and 20 g taro flour, respectively and D 2 B 1 , D 2 B 2 , and D 2 B 3 , Solar-dried taro flour blended bread at 10, 15, and 20 g, respectively, Mn, grand mean and CV, coefficient of variance.
weight is basically determined by the quantity of dough baked gluten functionality and the amount of moisture and carbon dioxide diffused out of the loaf during baking (Shittu et al., 2007). Loaf weight reduction during baking is an undesirable economic quality to the bakers, as consumers often get attracted to bread loaf with higher weight believing that it has more substance for the same price (Shittu et al., 2007).

| Loaf volume
The loaf volume was significantly (p < .05) affected by blending ratio and the interaction of drying methods and blending ratio. Drying method had no significant (p > .05) influence. The highest was observed for control bread (240.98 cm 3 ) and loaf volume decreased with increase in the substitution of taro flour. This is may be due to the gluten protein contents of wheat flour. Lack of the gluten protein contents of taro flour is responsible for the reduction in the loaf volume of leavened taro-wheat flour bread (Belderok, Mesdag, & Donner, 2000;Sidhu, Al-Hooti, & Al-Sagar, 1999). Gluten protein contributes the vital role for the increment of loaf volume and elasticity of dough.
Loaf volume is affected by the quantity and quality of protein in the flour (Ragaee & Abdel-Aal, 2006).

| Specific loaf volume
The results of the effects of drying methods and blending ratio on the specific volume are presented in Table 2. Drying methods, blending ratio and their interaction significantly (p < .05) affected the specific volume. As taro flour content increased, the specific loaf volume decreased. This is may be due to the high fiber contents of taro flour that affects the loaf volume of blended bread by diluting the gluten network, which in turn impairs gas retention rather than gas production (Dewettinck et al., 2008;Eiman, Amir, & Mustafa, 2008;Elleuch et al., 2011). The specific loaf volume of bread is the determinate factor for the consumer acceptance. If they are lower than the usual one, consumers are not attracted by it (Shittu et al., 2007).

| Crumb moisture
Crumb moisture is the moisture of bread which is found in interior parts of bread, contributes significant effect to estimate the shelf life of bread. The crumb moisture was significantly (p < .05) affected by the drying methods, blending ratio and their interaction ( Table 2). As the taro flour increased in the blend, the crumb moisture contents also increased. This is probably due to high water binding by starch. As taro flour increased, there is a tendency of moisture to increase, this is probably at large attributed to the high moisture binding nature of small starch granules of taro.
Crumb moisture is important to determine the firmness of fresh bread; if the moisture contents of bread crumb are very high, the firmness of fresh bread is very low (He & Hoseney, 1990;Piazza & Masi, 1995). This result was similar to 40.5%-44.20% and 32%-39% reported by Ognean et al. (2007) and Shittu et al. (2007), respectively.    control flour (7.20 min.). This may be due to the low gluten protein contents of taro flours which take long mixing times to make consistent and uniform blended dough. The difference in the time break down of blended flours due to blending ratio (

| Mixing tolerance index
Mixing tolerance index is used by bakers to determine the extent that dough will soften over a period of mixing. The mixing

| Farinograph quality number
The FQN indicates the quality of flour for bread making. If the flour has poor quality, it gets weakened early and quickly. The drying method, blending ratio and their interaction significantly (p < .01) influenced the FQN. An increase in taro flour generally showed an improvement in the FQN. Even though there is such improvement, the bread quality may be not high because this may not necessarily indicate the leavened products produced. The highest the farinographic quality numbers the better dough handling features. Such positive contribution to the blend may be contributed by the high small starch granules in the taro flour.

| Color
The sensory scores for color are presented in Table 4. The drying method significantly affected the color of the composite bread. The color difference of taro-wheat bread due to drying methods (Table 4) were significant (p < .05). There was no significant (>0.05) difference in color of bread due to blending ratio and the interaction between drying method and blending ratio. However, there was a general de-

| Taste
The results of the sensory taste scores are presented in Table 4.
There was a significant difference (p < .05) in the taste of bread due to blending ratio and the interaction between blending ratio and the drying methods. Except taro sun dried, there was a general decrease in the taste score with increase in taro flour proportion. The highest score was 4.6 (close to extremely like) for taro flour dried in oven with taro proportion of 10 g/100 g flour. The least scores were for samples dried under solar and sun with taro proportion of 20 g/100 g.
Similar studies reported a decrease in the taste scores of wheat-taro flour composite bread with increased proportion of taro flour (Ammar et al., 2009). This might be due to poor taro flour odor, after taste flavor, and also the high calcium oxalate contents of taro flour which contributes to the salty taste to the blended breads (Kaushal et al., 2015).

| Flavor
The flavor of taro-wheat composite bread was significantly (p < .05) affected by the drying method, blending ratio and their interaction.

| Appearance
Appearance is the surface characteristics of food materials which attracts the consumer perception. The appearance of taro-wheat bread was significantly (p < .05) affected by drying method, blending ratio and their interaction. The appearance score for most of the treatment groups was around moderately like. However, composite bread from solar dried taro had higher appearance score whereas the control exhibited the highest appearance score. This might be due to the low gluten protein contents of taro flour which contributes to make less leavened characteristics of blended breads.

| Overall acceptability
The overall acceptability scores of wheat-taro composite bread are presented in Table 4. Drying methods and the interaction between drying method and blending ratio significantly (p < .05) influenced the overall acceptability. The score ranged from 3.95 to 4.55 which could be associated with like moderately and like very much. However, there was a general decreasing trend in the acceptability score with an increase in the proportion of taro flour.
T A B L E 4 Effect of drying methods and blending ratio on the sensory characteristics of taro-wheat bread *, **, and ns represent significant at 5%, significant at 1%, and nonsignificant at 5% probability level, respectively. Mean values followed by the same letter in the column are not significantly different at 5% probability level. DMRT (p < .05), Duncan's multiple range taste at α equal to 0.05; D 1 B 1 , D 1 B 2 , and D 1 B 3 are oven-dried taro flour blended bread at 10, 15, and 20 g taro flour, respectively, D 2 B 1 , D 2 B 2 , and D 2 B 3 are solar-dried taro flour blended bread at 10, 15, and 20 g, respectively, D 3 B 1 , D 3 B 2 , and D 3 B 3 are sun-dried taro flour blended bread at 10, 15, and 20 g taro flour, respectively, Mn, ground mean and CV, coefficient of variation.