Bread is the most widely consumed food product 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^[1]. 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^[2]. 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^[3,4] .

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^[5] ; good balance of all essential amino acid^[6] ; high amount of unsaturated fatty acids (mainly Linoleic acid (18:2, 9, 12) and -Linolenic acid (18: 3 9, 12, 15)^[7] ; 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^[8] and 30%^[9,10] studies, respectively. The addition of tef flour increasing the proportion from 0 to 40% resulted in decreases in bread volume^[11]. Hager et al., 2012^[7] 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.

Materials and Methods

Grain and Flour preparation

Tef (variety: DZ-01-96) used in this study was obtained from the 2018/19 main crop production season at DebreZeit Agricultural Research Center (DZARC). Similarly, bread wheat grain (variety: Ogolcha) was taken from the lot produced in the 2018/19 main crop production season at Kulumesa Agricultural Research Center (KARC). These DZ-01-96 and Ogolocha varieties were selected for the formulation because of their popularity and preference of farmer and consumers, and it’s very white color and high gluten content, respectively.

Both the tef and bread wheat grains were manually cleaned very carefully by winnowing, sifting and sorting to remove all chaffs, dust and other impurities. The tef grain was ground into whole flour with a laboratory mill (Perten mill 120, Finland) fitted with a 0.5 mm screen size , while the bread wheat was tempered to 17% and then milled using a Chopin laboratory mill (Moulin CD1 MILL, Chopin technology, France).

Experimental formulation and blending: Flour blend formulation of the wheat (60–100)% and tef (0–40)% flours was conducted by using mixture response surface methodology through D-optimal design (obtained 13 run) for making tef incorporated breads (Table 1). The proportion limit of wheat and tef flours were selected based on earlier reports and preliminary trial. The wheat and tef flours were mixed by rotating drum mixer (Chopin MR 10L, France) to ensure uniform blending. The composite flours were stored in refrigerator at 5°C after packing air tight polyethylene bags until needed.

Gluten quality: The gluten quality of the composite flour sample was evaluated by AACC standard method^[12]. 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 preceded 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)^[13]. 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^[12]. 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% sugar and 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 (30ºC, 85% RH) followed by baking (220ºC, 15 min) in a preheated baking oven (XFT 115, Korea ). 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^[12]. 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^[13,14]. 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 was determined using the method described in AOAC method. 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 preferences of the participants. The attributes assessed included: visual color, taste, aroma, mouth feel, and 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 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^[15,8].

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^[3,4]. 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.

Values followed by different letters with in a column indicate significant difference (p<0.05). All values are expressed as mean ± SE in triplicate; WAC: Water absorption capacity; WT- White Tef; BT- Brown Tef; BW- Bread Wheat

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.

Table 2: Measure bread physical parameters

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.

Values followed by different letters with in a column indicate significant difference (p<0.05). All values are expressed as mean ± SE in triplicate; WAC: Water absorption capacity; Spec. volume: Specific volume, WT: Wheat Tef; RT- Red Tef; BW- Bread Wheat

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^[16]. The mean specific volume found in this study is consistent with those reported by Mohammed et al.,(2009)^[8], 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^[17].

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^[18,4].

Table 3: Proximate composition of bread from composite tef and wheat flour

Notes: Values are mean and standard error of each run. Values followed by different letters with in a column indicate significant difference (p < 0.05).

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^[19,3]. The results are also in agreement with the earlier works reported by^[7,8] .

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^[4].

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 4. 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^[3,4].

Table 4: Mineral content of bread from composite tef and wheat flour

Notes: Values are mean and standard error of each run. Values followed by different letters with in a column indicate significant difference (p < 0.05).

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^[3].

Table 5: Sensory acceptability of formulated bread from composite tef and wheat flour

Notes: Values are mean and standard error of each run. Values followed by different letters with in a column indicate significant difference (p < 0.05).

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^[6,7] .

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^[20].

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.

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