Development and Optimization of Nutritionally Enhanced Wheat Breads Supplemented with tef (Eragrostistef (Zucc.) Trotter) Grain Flour

The aim of this study was to develop nutritionally enhanced and healthy bread through incorporation of tef flour to wheat flour. Accordingly, the influence of tef (brown and white) flour incorporation (0 -40% tef flour) on wheat-tef blend flour physicochemical  properties, baking characteristics and bread nutritional and sensorial qualities was evaluated. Incorporation of tef flour significantly increased water absorption capacity (57.67 to 97.00%), reduced both wet gluten (29.33 to 13.89 %) and dry gluten (12.03 to 6.37 %) content of composite flour. All the tef containing bread showed better mineral content (Ca, Fe, Mn, Zn, Mg, K and Na), good fiber and fat content as compared to wheat flour bread. Moreover, the bread specific volume decreased from 3.80 to 2.91 and the bake loss content increased from 17.52 to 30.94 with increase in tef flour from 0 to 40% in composite blends. The sensory attributes scores of the color, aroma, odor,  texture and overall acceptability decreased. However, it could be concluded that breads supplemented with 15% tef flour showed acceptable sensory quality and enhanced nutritionally properties. Keywords : tef; composite flour; gluten; nutritional composition; bread quality DOI: 10.7176/FSQM/102-03 Publication date: November 30 th 2020


Introduction
Bread is the most widely consumed foodstuff and a substantial part of many cultures and traditions and many people's diets throughout the globe. In Ethiopia, the consumption of wheat based product (bread) is expanding as a result of urbanization. The number of wheat milling and baking industries is increasing.
Bread wheat (Triticum aestivum Desf.) is the preferable raw material for manufacturing baked products with the most desirable end product characteristics. Because of its functional protein (gluten), large loaf volume and fine texture requires formation of well developed and elastic dough structure for making bread. This leads good dough machinability and suited to continuous commercial production which makes good consumer acceptance.
Current consumer preference for baked product has shifting to select healthy food. The need to avail nutritionally enhanced products to make the society nutritionally secure could be addressed through evaluating the feasibility of alternative food crops as a substitute for wheat flour from traditional crop production.
Tef is an indigenous cereal stable crop in Ethiopia which is widely grown in the country ranking the first in terms of area coverage (CSA, 2017). In addition, tef is considered as suitable ingredient in the bakery industry for its good starch gelling properties with small and uniform size of tef starch granules which provides larger surface area and the higher water absorption (Bultosa, 2007;Abebe and Ronda, 2014).
In this respect the enrichment of bread with tef flour show a favorable mineral composition (high in calcium, magnesium and iron), and high in fiber. Tef has high content of dietary fiber and mineral (Stojceska et al., 2010); good balance of all essential amino acid (Gebremariam et al.,2012); high amount of unsaturated fatty acids (mainly Linoleic acid (18:2,9,12) and α-Linolenic acid (18:3 9, 12, 15) (Hager et al., 2012a); gluten free nature and tef is also known for its low glycemic index. Therefore, tef is an ideal candidate to substitute bread wheat partially for the mentioned goals.
The incorporation of tef reached into 20% and 30% with wheat for making bread causing lowering the sensory acceptability scores and decreasing physical properties of resulted bread in (Mohammed et al., 2009) and30% (Ben-Fayed et al., 2008;Alaunyte et al., 2012) studies, respectively. The addition of tef flour increasing the proportion from 0 to 40% resulted in decreases in bread volume (Coleman et al., 2013). Hager et al., 2012 observed that 100% whole-grain white tef flour presents compromised bread sensory quality, and they recommended using it as part of a composite formulation. In addition to using tef for making traditional foods (injera), utilization of tef for other in bakery products (bread) could create alternative product for the consumers and new market for the farmers and processors. The study aims to develop an optimal formulation for producing nutritionally enhanced and healthy bread through incorporation of tef flour by using mixture design without significantly affecting the required bread quality parameters.

Gluten quality
The gluten quality of the composite flour sample was evaluated by AACC standard method (AACC, 2000 Method No 38-10). Ten gram of composite sample was weighed and transferred into the glutomatic washing chamber. 4.8 ml of the 2% sodium chloride solution was added and allowed for 10 min in the chamber. Then mixing and washing procedures were proceeded simultaneously. Wet gluten was removed from the washing placed in the centrifuge holder and centrifuged to stop automatically. The passed gluten through the sieve was weighed. The wet gluten content of the composite flour sample was expressed as a percentage of the mass of the original sample. The gluten residue retained inside the screen was weighed and then dried in a Glutrok 2020 heater to give dry gluten. The dry gluten was then weighed.
Water absorption capacity of the flour was determined following methods adopted by Oyeyinka et al., (2013). One gram of flour sample mixed with 10 mL distilled water for water absorption capacity. The mixture was allowed to stand at room temperature for 30 min and then centrifuged (Philips Drucker, Oregon, USA) at 3000 g for 30 min. Water absorption capacity was expressed as gram of water bound per gram flour.

Bread making
Breads were prepared by a straight-dough production process as indicated in the AACC Method 10-10B (AACC, 2010). The following formula was used for making bread: 300g composite flour, 3g yeast, 3g sugar, 3g baking powder and variable water on basis of composite flour water absorption. Bread dough was prepared by mixing 100% composite flour, 3% yeast, 2% salt, 3% sugarand 60-110% of water by weight. The dough was then immediately divided and put into baking pans and allowed to ferment or proof for 45 min in a fermentation cabinet (30 o C, 85% RH) followed by baking (220ºC, 15 min) in a preheated baking oven (XFT 115 ). Bread physical properties were measured after cooling the breads.

Loaf bread volume
The volume of the breads was determined by the rape seed displacement method as stated in AACC method 10-05.01 (AACC, 2010). The loaf was placed in a container of known volume into which rapeseed seeds were run until the container was full. The volume of seeds displaced by the loaf, expressed in cubic centimeters,was considered as the loaf volume.

Nutritional composition
Proximate composition (moisture, ash, fat and carbohydrate content) of the breads were determined using methods standard (AOAC, 2010). Determination of protein content was performed following the Kjeldahl method procedure. The total dietary fiber was determined based on AOAC Official methods (AOAC 985.29). Mineral composition of the breads were determined using the method described in AOAC (1990).The samples were analyzed from nitric acid-hydrogen peroxide digest using Inductively Coupled Plasma Atomic Emission Spectrometry (ICPAES).

Sensory evaluation
The sensory qualities of the breads prepared from the different composite flours were evaluated by using 20 untrained participants. A five point hedonic scale was employed to estimate the sensory acceptability and Food Science and Quality Management www.iiste.org ISSN 2224-6088 (Paper) ISSN 2225-0557 (Online) Vol.102, 2020 23 preferences of the participants. The attributes assessed included: visual color, taste, aroma, mouth feel, texture and over all acceptance.

Statistical analysis
Design Expert 7.1 was used to generate surface response plots that permitted to quantify the effects of different independent variables on the selected dependent variables (p < 0.05). All the measured parameters were replicated three times. Analysis of Variance (One-way ANOVA), followed by Duncan post hoc test, was performed to determine significant differences between bread samples made from composite flours by SPSS 20.

Results and Discussion Composite flour characteristics
The composite flour characteristics wheat flour and whole tef flour blends are shown in Table 1. In baking, optimum quantity of wet gluten is desirable for developing the required viscoelastic dough leading to a bread with the desired physical and sensorial quality. Wet and dry gluten content in the composite flours significantly decreased by 53% and 47% with increasing tef flour levels from 0-40%, respectively. Control (100% wheat flour) had the highest wet gluten (29.83%) and dry gluten (12.03%) content while maximum tef substitution (40% tef flour) had the lowest wet (14.00%) and dry (6.37%) gluten levels of composite flour. This could be due the fact that tef flour is gluten free which diluted of the gluten in the wheat (Hopman et al, 2007;7 Mohammed et al., 2009).
Water absorption capacity was lowest in control wheat flour (57.65%), while increased levels were detected in highest tef flour (35-40%) incorporation levels (97%). The tiny size of the tef starch granule which has larger bulk surface area could have contributed for the higher water absorption capacity of the tef incorporated composite flours (Bultossa et al., 2007;Abebe et al., 2015). White and brown tef in the composite flours were no significant differences in water absorption capacity, wet and dry gluten through 10% and 20% incorporation levels.

Bread physical characteristics
Incorporation of tef flours to the wheat flour significantly (p<0.05) affected the measured bread characteristics ( Table 2). The specific volume of tef breads containing different blending proportions are shown in Table 2. The composite bread differed statistically in weight, volume, specific volume and bake loss for both brown and white tef flour. Bake loss was lowest in bread wheat flour, while increased levels were detected with 20% tef flour incorporation levels.
The specific volume values of the 0-40% tef incorporated breads narrowly ranged from 2.91-3.80 mL/g. There were no statistical difference (p<0.05) between the white bread and 25% tef whole grain flour incorporate bread samples, which had an average specific volume of 3.10 mL/g, and 3.11 mL/g, respectively. Incorporated 10% and 20% of white and brown tef in bread making were significant difference in specific volume, in which brown tef higher than white one. The largest effect on specific volume reduction was observed in the use of highest tef incorporation levels, presenting 2.91 mL/g.  This effect reflects the greater impact on the gluten network due to the action of the fibers, which leads to a decrease in gas retention capacity, resulting in reduction in the specific volume of the breads (Gómez et al., 2003).The mean specific volume found in this study is consistent with those reported by Mohammed et al.,(2005), who found a mean value of 3.88 mL/g from 0-20% incorporated tef breads. The substituted tef flour contain high amount of fiber that may also affect the specific volume. The addition of whole grain generally weakens the structure of the bread by reducing the volume and elasticity of the crumb (Salmenkallio-Marttila et al., 2001).

Bread proximate composition
The proximate composition of breads made from refined wheat flour and whole tef flour blends are shown in Table  3.The moisture content increases from 5.15 to 6.31% with increase in percentage tef grain flour (0 to 40%) in composite bread. The increase in moisture content with increase in tef grain flour could be due to the extremely small particles size of the flour and due to high fiber content in tef flour which enhances water absorption in tef flour (Bultosa and Taylor, 2004;Abebe et al., 2015).
The formulated bread ash and fiber content doubled upon blending wheat with tef flours (0-40%) in Table 3. This could be due to the higher ash and fiber content of tef flour (1.85-3.06 g/100g and 2.6-3.8 g/100g, respectively) and this is because of the fact that tef has small grain size and it is always whole floured (Bultosa, 2007;Baye, 2014) The results are also in agreement with the earlier works reported by Mohammed et al (2009) andHager et al.(2012).  The fat content of the produced bread prepared from the composite flours significantly increased to 72.64% from control wheat flour to maximum tef substitution levels. This could be due to whole floured which contain the germ in the tef and significantly higher fat content in tef than wheat flours (Abebe et al., 2015).
The mean carbohydrate contents of the formulated breads significantly decreased by 4.8% when the proportion of tef in composite flour increased from 0-40%. This could be due to higher carbohydrate composition of refined wheat flour than tef flours. This lowering carbohydrate in the composite flours selected for blending of tef with wheat emphasize decreasing glycemic response of the bread to be obtained.

Bread mineral content
The mineral content of the blend bread made with the composite flour from wheat and tef was shown in Table 3. Blending of tef and wheat had a significant effect (P<0.05) on the mineral content of bread. Control bread samples (100% wheat flour) had the lowest in iron, calcium, manganese, zinc, magnesium, potassium and sodium; and also the highest in all mineral content was obtained for bread with maximum tef proportion (40% tef ). The Ca, Fe, Mn, Zn, Mg, K and Na result shown that the variation percent is at 5% and 40% tef flour addition were 69%, 75%, 56.69%, 18.65%, 59.22%, 30.96% and 51.43%, respectively. Higher mineral contentment in the tef incorporated breads could be due to the availability of high mineral content in tef flour than the wheat flour (Bultosa, 2007;Abebe et al., 2015). Bread Sensory Quality Blend ratio had a significant impact (p < 0.05) on the sensory acceptance of the composite bread among the 15 experimental formulations based on panelist preference. As shown in Table 5, as the proportion of tef flour increased, bread color, aroma, taste, mouth feel, texture and overall acceptability of the breads decreased significantly. With increasing tef incorporation levels (0-40%) bread color scores decreased from 5.00 to 3.16 with increase in added tef grain flour. Such detrimental effect could be related to the fact that the tef flour was a whole flour (Bultosa, 2007).
Similarly composite bread flavor mean score decreased from 4.63 to 2.95 (aroma) and 4.47 to 3.05 (taste). The different flavor of the composite breads could be because of the different intrinsic flavor difference that tef flour has (Mohammed et al;2009 andHager et al., 2012). The mean texture score of the composite breads decreased from 4.74 to 2.95 as percentage of tef grain flour (0-40%) increased. However, the tef incorporation level was significant (p<0.05) at above 10% incorporation level. The decrease could be due to of the decrease in bread volume caused by the dilution of the gluten level increased fiber content in the corresponding dough that led to poor rising of the dough and lower gas retention capacity prior and during baking.
The overall acceptability score of the composite bread ranged from 2.95 to 4.84. Most composite bread samples showed a good degree of overall acceptance, it was observed that control sample (100% refined wheat flour) was very well accepted because it exhibited the characteristics desired by the consumers. In contrast, maximum tef incorporated (40% tef and 60% refined wheat flour) was less accepted .

Conclusion
Loaf volume, color, texture and overall acceptability were deeming as common optimum parameter for bread formula. Tef grain flour can be used up to 15% in production of tef -wheat composite bread without significant effect on physical and sensory qualities acceptable by consumers. The formula containing 85.46% wheat farina flour and 14.56% tef flour was selected as the best formulation to produce a nutrient rich bread product with desirable texture and sensory quality.