Injera Making Quality Evaluation of Tef and Cassava Composite Flour

: Tef is indigenous Ethiopian cereal crop while cassava is a high carbohydrate-containing crop recently introduced to Ethiopia. Injera is the staple food to Ethiopians mostly prepared from tef as the main ingredient. This was a research done by partially substituting tef, relatively expensive flour, by cassava flour and evaluated its injera making quality. Tef and cassava flours were mixed in all possible ratios at 10% intervals. Functional properties, sensory evaluations and proximate compositions were estimated using standard methods. Except water absorption capacity, all other functional properties were significantly varied with changing proportion of tef and cassava flours. The sensory acceptability of tef-cassava injera was significantly decreased in all parameters with increasing proportion of cassava flour. Tef-cassava injera contained 7.59 to 9.41% moisture, 0.65 to 1.87% ash, 0.40 to 1.02% crude fat, 3.79 to 11.89% protein, 1.10 to 3.05% crude fiber, 75.73 to 83.54% carbohydrate and 349.45 to 364.45 energy/100g. As cassava flour substitution levels increase, most flour functional properties increased, whereas sensory characteristics and proximate composition parameters (protein, fat, ash and energy) decreased. It was concluded that up to 40% cassava could be incorporated with tef to make injera with slightly acceptable sensory quality and fair nutritional value. Further research is required with regard to quality of injera as affected by other factors like maturity stages of cassava and the like.


Introduction
Injera, a fermented flatbread common in Ethiopian culture is mainly made from flour of grain tef (Eragrostis tef (Zucc.) Trotter). This food product is popular in almost whole regions of Ethiopia though the level of consumption may differ to some extent from place to place. It is said to be a staple food in the central, western and northern parts of Ethiopia as well as among the urban community [1]. Though tef is the mainly preferred cereals for injera making, there are other possible sources like wheat, barley, sorghum, millet, maize, and rice to be used just based on the availability. These types of flour sources for injera making can either be tef alone or the combination of the above-listed cereals in addition. From the recent research trends roots and tuber flour can also possibly to be used in the development of this product by partially substituting the above cereals flour.
Cassava among these roots and tuber crops is a flour source with good potential to be evaluated for this partial substitution.
Tef considered superior due to its nutritional merits and the crop is rich in carbohydrate, fiber [2] and contains more iron, calcium and zinc than other cereal grains, including wheat, barley and sorghum [3]. Tef has physico-chemical properties with great potential to be used in a broad range of food applications. Tef's flour has a high water absorption capacity, which relates to the higher degree of swelling of its starches, which have a small and uniform granule size, hence, providing larger surface area [4,5], Tef starch has a slow retro gradation tendency and hence, it could have a potentially positive impact on the shelf life of baked products. In addition, tef is preferred for making injera in terms of flavor quality, texture and softness [4,6].
On the other hand, Cassava (Manihotesculenta) also known as yucca or manioc is originated in South America. It is now widely grown in many parts of Asia and Africa, mainly for its starchy tuberous roots, which may grow to an enormous size. Readily established from cuttings, it is grown in poor soil, requires relatively little attention, and withstands adverse weather conditions. Cassava is a less expensive source of carbohydrate and the energy yield from cassava is often very high, even higher than energy from cereals. It should be encouraged in non-arid areas where nutrition and the main food problems arise from a shortage of total food and deficient energy intake, because of its high yields and other agricultural advantages.
Cassava is recently introduced to Ethiopia, its utilization as a portion of food is not well known and limited to local tradition, cooking the root similar to other roots and tuber crops. Despite this, cassava is among food security crops due to its high yield per unit land which could be as high as 36.5t/hec as reported in literatures [7]. Thus, cassava is a crop with high potential to improve food security issues in developing countries including ours. On the contrary to that, tef is of high demand both in domestic and international market due to its high quality including nutritional and health benefit. Therefore, its price is escalating from time to time. Traditional products like injera are almost solely prepared from the crop [8]. In order to minimize the price of products like injera, at least partially substituting tef, highly expensive cereal with relatively less expensive flour sources like cassava is very important. This study was therefore initiated and conducted to evaluate injera making quality of cassava through partially substituting tef flour.

Cassava Flour Preparation
Fresh cassava roots (Kello variety) were collected from the horticulture research program of Jimma Agricultural Research center (JARC). The root was washed and the outer layers of the root were peeled by using a sharp stainless steel knife and chopped and soaked in pure water for 48 hours. After the water was drained off, the tuber was sun-dried on mesh for five days till brittle enough for grinding. The dried cassava was then milled into flour and sieved in order to obtain flour with uniform particle size. Finally, the flour was packaged in polypropylene bags until processing and analysis [9].

Tef Flour Preparation
Tef (Kuncho variety) was collected from the cereal crop research program of JARC. Tef grain sample was cleaned for any foreign materials (dust, stone, earth tags, chuff, etc) and milled by disk attrition mill to whole flour to the fineness level used traditionally for injera making at the cottage tef grain-milling house (Jimma, Ethiopia). The milled powder was sieved, collected, and packed in dry polyethylene bags until blending.

Formulation of Blends
Tef and cassava flours were mixed in all possible ratios at 10% intervals as shown in table 1.

Water Absorption Capacity (WAC)
Water absorption capacity was computed as described by [10]. A gram of the blended flour was added into 10ml distilled water in a weighed centrifuge tube and stir six times by 10-minute intervals. The sample was centrifuged at 3000rpm for 25 minutes and the clear supernatant was decanted and re-weight. Change in weight was expressed as a percent of water absorption.
Where W1=weight of the sample, W2=weight of the empty centrifuge tube, W3=Weight of residue after decanting supernatant in centrifuge tub

Swelling Power
The swelling power of flour was determined based on the modified method of [11]. A gram of flour sample was transferred to a weighed centrifuge tube and 10ml of distilled water was added. The sample in the centrifuge tube was stirred gently for 10minutes by a stirrer and then heated at 90 °C for 30 minutes. After cooling to room temperature, the samples were centrifuged at 5000rpm for 10minutes and weighed after decanting all supernatant from the tube.

Oil Absorption Capacity (OAC)
To determine the OAC, 1g of samples was added to 10ml of soybean oil in 25ml of the centrifuge tube. The sample in the tube was stirred for two minutes and then centrifuged at 4000rpm for 20 minutes. The oil formed as the supernatant was separated in a 10ml cylinder and recorded [11].

Dispersibility
In order to determine the dispersibility, 10g of flour sample was added into a 100ml measuring cylinder and filled by distilled water up to 100ml following the method used by [12]. The sample in the measuring cylinder was stirred vigorously and allowed to settle for 3hrs. The Dispersibility was calculated by subtracting the volume of the settled particle from 100ml.

@ A
A B CD = 100ml − Volume of settled particle

Injera Making Procedure
The injera was made as described by the modified method of [8]. To make injera, 200g of composite flour from each treatment was mixed with 180 ml of water and kneaded for about 5 min. A starter culture (Irsho, 5% dough saved from previous works) was added and fermented for 48 hrs. at room temperature. Part of the dough named Absit (80 g or 10%) was mixed with 30 ml of water and cooked in 200 ml boiled water for 1 min, cooled to 45°C and added back to the fermenting dough and mixed well. A 100 ml of water was added and the batter fermented for 3-4 hours at room temperature until foam and bubbles formed (second fermentation). About 500 ml of the batter was poured in a circular manner; on a 50cm diameter hot clay griddle (Metal) and baked covered for about 2-3 min. the hot fresh injera was drawn from metad. The fresh baked injera was cooled and presented for sensory evaluation. Injera samples were dried for 105°C in an oven for proximate analysis.

Sensory Evaluation
A semi-trained panel (a panel briefed about the scoring of degree of liking for sensory attribute) of 50 peoples from JARC (Jimma Agricultural Research center), who have experienced consuming injera has participated. Injera prepared from each treatment were evaluated using a 9 points hedonic scale (1-extremely like, 9 -extremely dislike) [10] in duplication. A rolled piece of injera was presented in two digit-coded plates for panelists on a tray at room temperature within 2 hours after baking. The attributes evaluated were top and bottom surface color, description of eyes, texture, taste and overall acceptability.

Proximate Analysis
International method, Association of Official Analytical Chemists [13] were used to determine crude protein, crude fat, crude fiber, moisture and ash contents of injera samples. Protein content was determined by Kjeldahl (Kjeltec 8400, Auto Sample Systems, Foss, Sweden) using the N conversion factor of 6.25 following the same official method of AOAC. Soxhlet method (Soxtec 8000, Foss, Sweden) was used to extract crude fat. The moisture content was determined by the air convection oven drying method as described by the same method. Ash was determined using the combustion method by a muffle furnace at 550°C. Total carbohydrate was calculated by difference: 100 -(% Moisture +% Crude protein +% Crude fat +% Crude fiber + % Ash) (Onwuka, 2005). Energy value per 100 g was calculated using the Atwater conversion factors, where E (kcal per 100 g) = [9 X crude fat (%) + 4 x crude proteins (%) + 4 x total carbohydrates (%)] (Maclean et al., 2003).

Statistical Analysis
The data were analyzed by using analysis of variance (ANOVA), at a 95% level of confidence by the SAS 9.3 version. Separation of the mean values was carried out using Fischer's test, and a significant difference (LSD) between any two means was accepted at p < 0.05.

Functional Properties of Tef-cassava Composite Flour
Functional properties of tef-cassava composite flour are presented in Table 2. Except water absorption capacity (WAC), all other functional properties were significantly varied with changing proportion of tef and cassava flours. The water absorption capacity of the flour was ranged from 105.35 -156.90% and the finding was in agreement with [8]. WAC of the tef-cassava composite flour generally was high and flours with high water absorption capacity have more hydrophilic constituents such as polysaccharides. The high WAC of the composite flour helps the processors to add more water during processing, and might be important to maintain freshness of the product [14].
Swelling power ranging from 3.60% -6.10% were reported as indicated in table 2 below. According to Eriksson (2014), swelling power expressed as the weight of sediment per gram of starch; the maximum increase in volume and weight the starch undergoes when heating in excess water. This property of flour is reported to be related with the eating quality of food and hence the higher the swelling power the better will be eating quality of food [11].
Oil absorption capacity is the indication of the emulsifying capacity and amount of oil uptake by a sample during food processing like frying [11]. It is the property mostly related with sensory property of a product like flavor, taste and aroma. The oil absorption capacity reported in this study varied from 111.0% to 160.0%. The oil absorption capacity of tef flour alone was score lowest value (115.0) and the highest oil absorption capacity was recorded for 70% cassava flour in the blended ratio. Authors in [15] reported that high oil absorption capacities of flour could improve the property of other flour during formulation development.
Dispersibility is a measure of the reconstitution of flour or starch in water. The higher the dispersibility, the better the sample reconstitutes in water [16]. The lower the dispersibility of flour samples is probably the indication of lump formation tendency during preparation [12]. The dispersibility of blend flour in this study ranged between 68.5 and 76.5. Tef flour without cassava had the highest dispersibility (76.50) and blending ratio of 30% tef and 70% cassava flour had the lowest dispersibility (68.50). Tef flour had the best reconstitution ability than all-composite flour proportions with cassava flour.

Sensory Properties of Tef-cassava Injera
The sensory acceptability of tef-cassava injera was significantly decreased in all parameters with increasing proportion of cassava flour. A similar report by [8] also indicated that the sensory quality of injera declined with an increasing proportion of cassava flour. Even though the difference in sensory acceptance is significantly declining with addition of cassava, incorporation of cassava flour as high as 30% was moderately acceptable. Even the 40% inclusion level is slightly liked in color and eye description, but neither liked nor disliked in taste, texture and overall acceptability. These indicate that it is possible to prepare injera by incorporation of up to 40% cassava flour with tef flour with acceptable sensory quality.

Proximate Composition of Tef-Cassava Injera
The proximate composition of tef-cassava injera is presented in table 4.
Moisture content of tef-cassava injera was in the range of 7.59-9.41% and had shown a slight increment as the proportion of cassava increased. However, the increment was not significant statistically. This might be due to the high water absorption capacity of cassava flour (Table 2).
Ash content of injera prepared from 100% tef flour was 1.87% and the same parameter reduced to 1.04% at 10% cassava substitution. This result was in line with [8] where the author reported tef flour had higher ash content than cassava flour. The fat content estimated in this study is generally low (0.23-1.54%), and the value had shown a decreasing trend with increasing level of cassava flour which might be due to higher fat content of tef than cassava.
The blending of cassava flour had a significant (p<0.05) effect on the protein content of blended injera. The addition of more cassava flour had reduced the protein content of injera. The protein content of injera had decreased from 11.89 for 100% tef to 3.79% for 90% cassava containing injera that might be due to the higher protein content of tef than cassava. This may be attributed to the general low protein content of roots and tuber crops including cassava.
The fiber content of blend injera was in the range between 1.10 -3.05%. There was also a decreasing trend in fiber content as level of cassava increase in the proportions. The carbohydrate content of injera had an increasing trend for every 10% addition of cassava flour. However, the increment was not linear, and the lowest carbohydrate (75.26%) was recorded for control injera (100% tef) while the highest (83.54%) was recorded at 80% cassava. This increase in carbohydrate content may be attributed to high carbohydrate composition of cassava as is in other roots and tuber crops.
Similarly, there was a decreasing trend in energy value of injera as the proportion of cassava in the blend is increased. This is attributed to the higher fat and protein content of tef as compared with that of cassava. Nevertheless, the energy contents of the blended injeras were high in all proportions because of the high carbohydrate of cassava as it is true in other roots and tuber crops. In general, the effect of cassava flour addition is not as such a concern on fat, fiber, carbohydrate and energy contents of tef injera. rather, sensory acceptability is more important factor to determine the substitution level of cassava in injera making.

Conclusion and Recommendation
This study showed that cassava flour can be a good ingredient to prepare injera together with tef and diversify Ethiopian diet. As cassava flour substitution levels increase, most flour functional properties including water absorption capacity, swelling power and oil absorption capacity increased, whereas sensory characteristics and some proximate composition parameters (protein, fat, ash and energy) were slightly decreased. From this study, it may be concluded that up to 40% cassava could be incorporated with tef to make injera with slightly acceptable sensory quality and fair nutritional quality.
In the current study standard tef injera preparation procedure was used to process the tef-cassava flour blend. However, some process modification might be required to adjust cassava incorporation. Further investigation is also required to assess the influence of various maturity stages during cassava harvesting and its post-harvest storage on injera quality.