Physical Properties of Gluten Free Sugar Cookies Containing Teff and Functional Oat Products

Teff-oat composites were developed using gluten free teff flour containing essential amino acids with oat products containing β-glucan known for lowering blood cholesterol and improving texture. The teff-oat composites were used in sugar cookies for improving nutritional and physical properties. Teff and its composites had higher water holding capacities compared to wheat flour. The pasting properties were not significantly influenced by 20% oat product replacements in teff-oat composites. The pasting viscosities of teff-OBC and teff-WOF 4:1 composites were similar to teff flour, but they were all higher than wheat flour. The elastic properties of teff-OBC (oat bran concentrate) and teff-WOF (whole oat flour) doughs were slightly higher than teff dough. Differences were also found in geometrical and textural properties of the doughs and cookies. Overall, the teff-oat cookies were acceptable in colour, flavour, and texture.


Introduction
The important ancient grain Teff (Eragrostis tef) is finding resurgence in the modern age.Teff has an excellent nutritional profile with significant levels of minerals including calcium, iron, magnesium, phosphorus, potassium, and zinc.Also, teff is rich in vitamins, such as thiamin (B1), riboflavin, B2), vitamin A and K (USDA Nutrient Database, 2014).Furthermore, teff is high in proteins with an excellent amino acid composition including all 8 essential amino acids for humans that is superior in lysine than wheat or barley along with its high carbohydrates and fiber contents (Taha et al., 2012).
The suitability of teff flour in bread, layer cakes, cookies, and biscuits has been studies (Coleman et al., 2013).However, none of the prior studies included both teff and oat products.Although there are many cookie recipes using teff flour, none were scientifically studied and reported.Thus, oat products containing β-glucan and teff with its distinctive protein profile and gluten free uniqueness were used to produce teff-oat composites, and used in cookies for this research.
In this exploratory study, the teff-oat composites (4:1) were used and evaluated in sugar cookies.The objectives of this study were to evaluate the physical properties of teff-oat composites along with their formulated dough and cookies; to determine if the textures of dough and cookie will be changed by replacing teff with 20 % of oat products; and to determine if the teff-oat cookies are acceptable in the texture, colour, and flavor compared to wheat flour.The physical properties of teff-oat composites and cookies could provide useful information for functional food applications.

Preparation of Teff-Oat Composites
Teff flour was mixed with Nutrim, OBC, or WOF (4:1, w/w) using a mixer (KitchenAid, St Joseph, MI, USA) for 2 min, respectively.The mixtures were passed through a 20 mesh sieve followed by additional mixing in a mixer for 1 min to obtain the desired consistency.

Water Holding Capacity of Composites
The water holding capacity (WHC) of the samples was determined by a previous procedure with minor modifications (Ade-Omowaye et al., 2003).Each sample (2 g, dry weight) was mixed with 25 g of distilled water and vigorously mixed for 1 min to a homogenous suspension using a Vortex stirrer, held for 2 h, and centrifuged at 1,590 g for 10 min.Each treatment was replicated twice.Water-holding capacity was calculated by the difference between the weight of water added and decanted on dry basis (g of water absorbed /100 g of dry sample).

Pasting Property Measurement of Composites
The pasting properties of teff, oat composites, and wheat flour were evaluated using a Rapid Visco Analyzer (RVA-4, Perten Scientific, Springfield, IL).Samples (2.24 g, dry basis) were made up to a total weight of 28 g with distilled water in a RVA canister (8% solids, w/w).The viscosity of the suspensions was monitored during the following heating and cooling stages.Suspensions were equilibrated at 50°C for 1 min, heated to 95°C at a rate of 6.0°C/min, maintained at 95°C for 5 min, and cooled to 50°C at rate of 6.0°C/min, and held at 50°C for 2 min.For all test measurements, a constant paddle rotating speed (160 rpm) was used throughout the entire analysis except for 920 rpm in the first 10s to disperse sample.Each sample was analyzed in duplicate.The results were expressed in Rapid Visco Analyser units (RVU, 1 RVU = 12 centipoises).

Cookie Formulation and Preparation
Cookie flour formulations are teff flour; teff -Nutrim composites 4:1; teff-OBC composites 4:1; teff-WOF composites 4:1; and wheat flour.Cookies were prepared following AACC method 10-52 for sugar cookie as described by an earlier study with modifications (AACC International, 2000;Lee & Inglett, 2006).Sugar (72 g), brown sugar (22.5 g), Nonfat dry milk (2.3 g), salt (2.8g), and sodium bicarbonate (2.3 g) were mixed using a whisk in a bowl.The mixture was placed on top of shortening (100 g), and blended with a paddle beater in a mixing bowl using a KitchenAid mixer (St Joseph, MI, USA) at speed 2 for 3 min, scraping down every minute.The mixture of water (49.5 g) containing ammonium bicarbonate (1.1 g) was added and mixed for 1 min at speed 1.After scraping down, they were mixed for another minute at speed 2. Teff, or teff-oat composites (4:1), or wheat flour (225 g ) were added while mixing at speed 1, and continued mixing for 2 min at speed 2 with scraping every 30 s. Dough was flattened by using a rolling pin on a board with 7 mm gauge strips, and cut by a cookie cutter of 6 cm diameter.Cookies were baked at 205 °C in a convention oven (XAF-113 LineChef Stefania, Cadco, Ltd.Winsted, CT, USA) for 10 min, and cooled.The cookies were stored in a sealed plastic bag before measurements were taken.

Rheological Properties of Dough
A sample was loaded on a 2 cm diameter X-hatch parallel stainless plate with a 2 mm gap using a rheometer (AR 2000, TA Instruments, New Castle, DE, USA).The outer edge of the plate was sealed with a thin layer of mineral oil (Sigma Chemical Co., St Louis, MO, USA) to prevent dehydration during the test.All rheological measurements were carried out at 25°C using a water circulation system within ± 0.1°C.A strain sweep experiment was conducted initially to determine the limits of linear viscoelasticity; then a frequency sweep test was carried out to obtain storage modulus (G') and loss modulus (G") at frequencies ranging from 0.1 to 100 rad s -1 .A strain of 0.5 within the linear viscoelastic range was used for the dynamic experiments.All rheological measurements for samples were performed in duplicate.

Water Loss, Moisture Content, and Water Activity
Water losses during baking were measured by the weight of the difference before and after baking.After grinding three cookies with a pestle in a mortar, moisture content and water activity were measured.Moisture contents of cookies were determined by drying 5 g of sample at 105 °C to a constant weight (about 3-4 h).The water activity was measured using an Aqua Lab water activity meter (Decagon Devices Inc., Pullman, WA, USA).

Geometrical Properties
Six cookies were placed next to each other and the total diameter was measured.They were rotated by 90° and measured three more times.The average of four measurements was divided by six to calculate the average diameter of a cookie.To measure the height, six cookies were stacked, measured, restacked in different order, and measured again.The average cookie height was the mean of three readings divided by six.The spread ratio was calculated by dividing the diameter by the height.Nine measurements for each replicate were taken.

Colour
The colour of the cookies was measured with a Hunter Lab spectrocolorimeter (Labscan XE, Hunter Associates Laboratory Inc., Reston, VA, USA).The colorimeter was calibrated using a standard white plate.The colour values L*, a*, b* were measured with a C illuminant and a 10° standard observer.The dimension L* means lightness with 100 for white and 0 for black.The value a* indicates redness when positive and greenness when negative.The value b* indicates yellowness when positive.

Texture Analysis
Dough hardness was measured using a TA-XT2 Texture Analyzer equipped with 5 kg load cell in compression mode by penetrating with a flat probe of 5 mm diameter.The cookie dough (110 g) was gradually and evenly placed in a dough cell while compressing and flatting the surface by a plunger to avoid randomly distributed air pockets as a potential cause of variability in consistency measurements.The test was conducted at a pre-test speed of 2.00 mm s −1 and test speed of 3 mm s −1 , post-test speed of 10.0 mm s -1 , and distance of 20 mm.
Cookie hardness was measured using a recommended method of TA-XT2 Texture Analyzer (Texture Technology Crop., Scarsdale, New York, USA) equipped with 30 kg loading cell.The cookie hardness measurement was conducted by a cutting force using a three-point bending method with sharp-blade probe (6 cm long and 1 mm thick).The hardness of the cookies was indicated by the maximum peak force required to break the cookies.The slotted inserts were adjusted and secured on the Heavy Duty Platform to fit sample size and position centrally under the knife edge.The cookie was rested on two supporting beams spaced at a distance of 3cm.The instrument was set to 'return to start' cycle, a pre-test speed of 1.5 mm s −1 , test speed of 2.0 mm s −1 , post-test speed of 10 mm s -1 , and a distance of 5.0 mm.

Sensory Evaluation
Cookies were evaluated by 26 untrained consumers.A scorecard was evaluated for attributes including surface colour, texture, flavour, and overall quality.The panelist assigned scores for each parameter using a 9-point hedonic scale.

Statistical Analysis
All data from replicated samples were analyzed with analysis of variance using Duncan's multiple comparison to determine significant differences (P< 0.05) between treatments (SAS Institute, 1999, Rayleigh, North Carolina, USA).

Nutritional Composition
Teff has endured the ages from early civilizations as an important food of a highly nutritious gluten-free grain.Teff contains the highest levels of sugar, calcium, iron, potassium, and zinc whereas oat contains the highest magnesium and phosphorus contents among products (Table 1).Also, teff contains protein (13.3%) that is similar to protein in oat (13.7%) but higher than protein in whole wheat flour (13.2%), rice (5.95%) and corn (9.42%).In addition, teff and oat are rich in vitamins.Teff contains the highest vitamin B2 while oat has the highest thiamin (B1) among all products in the Table 1.All B vitamins help the body to convert carbohydrates into energy.Vitamin B2 is important for normal vision along with other nutrients.Some early evidence shows that vitamin B2 (riboflavin) might help prevent cataracts which can lead to cloudy vision (University of Maryland Medical Center, 2013).Vitamin B1 helps the body metabolize fats and protein, and is required for healthy skin, hair, eyes, and liver.It also helps the nervous system and brain function properly (University of Maryland Medical Center, 2013).Remarkably, only teff and oat contain vitamin K among all products in Table 1.Vitamin K is a fat-soluble vitamin that the body stores it in fat tissue and liver.It is best known for its role in helping blood clot and bone health (University of Maryland Medical Center, 2013).Oat was selected in this study because of its β-glucan content that is helpful for lowering blood cholesterol and improving food texture.Moreover, oat has high quality lipids including monounsaturated and polyunsaturated fatty acid.Therefore, the nutritional value of gluten free baked products could be improved by adding the ancient grain teff and oat products to recipes compared to cookies with wheat flour.
In general, the WHC of 4:1 composites for teff-Nutrim (135.3g/100g),teff-OBC (139.3g/100g), and teff-WOF (137.7g/100g) were all higher than wheat flour (93.8 g /100 g).Also, the actual WHC values of teff-Nutrim (135.3 g/100g), teff-OBC (139.3 g/100g), and teff-WOF (137.7 g/100g) were all much lower than theoretical WHC values that were calculated by using the actual WHC values of the starting materials (teff-Nutrim, 228.5 g/100g; teff-OBC, 175.8g/100g; teff-WOF 158.2 g/100g), respectively.Teff flour contains 13.3% protein and is rich in calcium and magnesium (Table 1).It is possible that the calcium and magnesium in teff reacted with protein causing precipitation that resulted in a reduction in WHC.It could be a similar mechanism involved in making tofu by coagulating proteins in soymilk with calcium or magnesium sulfate.The proteins coagulate when bonding occurs between the positively charged calcium ions and negatively charged anionic groups of the protein molecules resulting in protein clumping and the removal of insoluble material from solution.Nutrim was produced by jet-cooking that may possibly produce anionic groups on the surface that could allow interactions with protein and calcium in teff.Those results suggested an interaction between Nutrim and OBC with teff.Teff-oat composites could be widely used in different applications in the food industry because of improved WHC compared to wheat flour alone.These composites are notable for thickening properties, syneresis control, and emulsion stabilization along with their nutrients.

Colour of Cookies
The cookies containing teff were dark in colour (L*: 35.2) compared with the other cookies since the dimension L* means lightness with 100 for white and 0 for black (Table 3).The value a* indicates redness when positive and greenness when negative.All the cookies showed the redness in different degrees.The a* value (14.09) of wheat flour cookies was the highest indicating more redness than the other cookies.All the cookies also showed the yellowness in different degree since b* indicates yellowness when positive.All the cookies showed the yellowness in different degrees.Wheat flour cookies (b*: 32.49) had the highest value of yellowness among the cookies.In general, ground cookie powders were lighter with less redness and more yellowness compared to the surface colour of cookies with some exceptions.

Water Loss, Moisture Content, and Water Activity
Cookies with teff flour had the lowest water loss (9.95%) that was similar to cookies with teff-Nutrim (10.05%) and teff-WOF (10.11%) composites but they were lower than cookies with teff-OBC (10.44%) and wheat flour (10.32%,Table 4).Water losses during cooking ranged from 9.95% to 10.47%.All the cookies had similar moisture contents from 3.55% to 3.84%.Moisture is an important aspect of food stability and shelf-life.The growth and metabolic activities of bacteria, molds, and yeasts are retarded and eventually inhibited as the water activities (aw) of foods are decreased.Water activity of the cookies ranged from 0.246 to 0.298 (Table 4).They were much lower than the limit of water activity for spoilage by bacteria, yeasts and molds, approximately 0.90, 0.85-0.88,and 0.80, respectively.The rate of chemical reactions in food decreases much more slowly with reduced moisture content.Also, the enzymatic activity in foods may be significant at water activities as low as 0.30 (Smith, 2007).Additionally, water activity level affects lipid oxidation rates.Lipid oxidation is typically lowest when almond water activity (a w ) was ~0.25 to 0.35 (~3-4% moisture content), and increased above or below that water activity range (Huang, 2014).Overall, the differences were not intensive in water losses, moistures, and water activities among all cookies.

Texture of Dough and Cookies
The texture evaluations of dough and cookies were presented in Table 5.The cookie dough containing teff-Nutrim had the highest texture value (6.28 N), followed by cookie dough containing teff-OBC (4.51 N) tested by a penetrating force using a 5 mm diameter probe.The cookie dough using teff flour had the lowest hardness value (2.55 N).The dough hardness from teff-WOF composites (3.73 N) was higher than both doughs with wheat (3.14 N) and with teff flour (2.55 N).The cookie dough hardness appeared to be related to WHC.The highest dough hardness value was from cookies containing teff-Nutrim composites with the highest WHC.In contrast, the cookies containing teff-Nutrim required the least cutting force (28.44 N) to break the cookies.The cookies containing wheat flour required the maximum cutting force (94.34 N) to break the cookies.It indicated that dough texture properties were greatly changed after heating.Hoseney and Rogers reported that hardness of the cookies was caused by the interaction of proteins and starch by hydrogen bonding (Hoseney & Rogers, 1994).

Sensory Evaluation
The score for colour, texture, flavour, and overall ranged from 6.2 to 7.4, 6.4 to 7.2, 6.0 to 6.8, and 6.2 to 7.2, respectively based on the panelist assigned scores for each parameter using a 9-point hedonic scale (Table 6).The statistical results indicated that no significant differences were found in texture, flavour, and overall quality among cookies (Table 6).These results may be due to the fact that only a small portion (20%) was replaced by oat products, and also both teff and oat apparently had no distinct difference in flavour compared to wheat flour.In addition, the same formulation was used for all cookies.However, the scores of colour preference for wheat flour cookies were higher than cookies with teff and teff-oat composites.It may be due to the difference in colour between teff and wheat flour.Teff flour has brown colour while wheat flour has the light tan colour.If cocoa powder is used in the cookie formula for chocolate flavored cookies, it will make the colour of teff-oat cookies more attractive.Overall, sensory evaluation suggested that the cookies using teff-oat composites were acceptable in all respects..83± 0.58 a Means ± standard deviation; n=26; means followed by the same letter within the same column are not significantly different.The number 1 referred to dislike extremely; 2 to dislike very much; 3 to dislike moderately; 4 to dislike slightly; 5 to neither like nor dislike; 6 to like slightly; 7 to like moderately; 8 to like very much; and 9 to like extremely.

Conclusion
These composites improved nutritional value, water holding capacity, and the pasting properties along with gluten free quality.In general, teff-oat composites are very suitable for preparing cookies that were acceptable in colour, flavour and texture qualities compared to the cookies with wheat flour.OBC and WOF probably are more suitable to use for cookies by cost.Information on the physical properties of these innovative cookies provides useful information for new functional foods. Figure

Table 1 .
Comparison of teff composition with other cereals

Table 3 .
The colour profile of cookies

Table 4 .
The water loss during baking, moisture, and water activity

Table 5 .
The texture of cookies and dough Means ± standard deviation; n=3; means followed by the same letter within the same column are not significantly different (P>0.05).