Quality and sensory properties of maize flour cookies enriched with soy protein isolate

This study was carried out to assess the quality and sensory properties of maize flour cookies enriched with soy protein isolate with the aim of encouraging the use of soy beans in producing value-added products. Soy protein isolate was produced from soy beans and then incorporated at 5, 10, 15, 20, 25 and 30% levels in maize flour. The results revealed that substitution of maize flour with soy protein isolate significantly (p ≤ 0.05) increased the protein from 8.69 ± 0.19 to 29.11 ± 0.34 while the moisture, crude fibre, fat and sugar contents of the cookies from the composite flour decreased as the percentage of soy protein isolate in the blends increased from 8.76 ± 0.22 to 6.96 ± 0.13, 3.81 ± 0.07 to 2.87 ± 0.02, 2.63 ± 0.06 to 1.69 ± 0.01, 9.41 ± 0.24 to 8.45 ± 0.13 and 6.21 ± 0.20 to 4.98 ± 0.11 respectively. The calorific value of the cookies decreased from 468 ± 1.59 to 383 ± 1.01 cal/100 g as the percentage of soy protein isolate increased in the cookies. Sensory evaluation results showed that the colour, texture, taste and overall acceptability changed significantly (p ≤ 0.05) with increase in soy protein isolate in the composite flour. The optimum substitution level with soy protein isolate was 20%. Above this, the product consumer acceptability decreases. Subjects: Environment & Agriculture; Bioscience; Food Science & Technology


Introduction
Malnutrition is estimated to contribute to more than one third of all child deaths, although it is rarely listed as the direct cause (World Food Programme [WFP], 2000). Malnutrition as a state in which the physical function of an individual is impaired to the point where he or she can no longer maintain adequate bodily performance process such as growth, pregnancy, lactation, physical work and resisting and recovering from disease (WFP, 2000). Contributing to more than half of deaths in children worldwide; child malnutrition was associated with 54% of deaths in children in developing countries in 2001 (Monika & Mercedes, 2005;World Health Organization, 2013). Protein-energy malnutrition (PEM), first described in the 1920s, is observed most frequently in developing countries but has been described with increasing frequency in hospitalized and chronically ill children in the United States (Hendricks, Duggan, & Gallagher, 1995).
The consumption of cereal snacks foods, such as biscuits, cookies, wafers and short bread, has become very popular in Nigeria, especially among children (Abayomi et al., 2013). They are ready-toeat, convenient and inexpensive food products, containing digestiveand dietary principle of vital importance. In Nigeria, the consumption of ready-to-eat baked products is continually growing, and there has been an increase in reliance on imported wheat (Abayomi et al., 2013;Adeyeye & Akingbala, 2015;Akpapunam & Darbe, 1999).
Moreover, Nigeria grows staple crops other than wheat such as sweet potato, cassava or yam and cereals that can be used for baked foods. It would therefore be of economic advantage if wheat flour can be replaced with flour from tubers, legumes and cereals, hence reducing the reliance on its importation and thus enhance the industrial utilization of local crops (Abayomi et al., 2013;Adeyeye & Akingbala, 2015;Onabanjo & Ighere, 2014).
Maize (Zea mays) has the highest world-wide production of all grain crops, yielding 875 million tonnes in 2012 (http://faostat.fao.org/). Maize is a major source of starch. Maize flour is a major ingredient in home cooking and in many industrialized food products. Maize is also a major source of cooking oil (corn oil) and of maize gluten (Food and Agriculture Organization of the United Nations, Statistics Division [FAO Statistics Division], 2009; Thompson, Manore, & Vaughan, 2010). Maize starch can be hydrolyzed and enzymatically treated to produce syrups, particularly high fructose corn syrup, and a sweetener; and also fermented and distilled to produce grain alcohol. Grain alcohol from maize is traditionally the source of Bourbon whiskey.
Maize meal is made into a thick porridge in many cultures: from the polenta of Italy, the angu of Brazil, the mămăligă of Romania, to cornmeal mush in the US (and hominy grits in the South) or the food called mealie pap in South Africa and sadza, nshima and ugali or ogi in other parts of Africa. Maize meal is also used as a replacement for wheat flour, to make cornbread and other baked products. Masa (cornmeal treated with limewater) is the main ingredient for tortillas, atole and many other dishes of Central American food (FAO Statistics Division, 2009;Vallabhaneni & Wurtzel, 2009;Winkel-Shirley, 2001).
Soybean (Glycine max) is a leguminous vegetable of the pea family that grows in tropical, subtropical, and temperate climates. Soybean was domesticated in the 11th century BC around northeast of China (Food and Agriculture Organization of the United Nations [FAO], 2005;Gibson & Benson, 2005). It is believed that it might have been introduced to Africa in the 19th century by Chinese traders along the east coast of Africa. It consists of more than 36% protein, 30% carbohydrates, and excellent amounts of dietary fiber, vitamins, and minerals. It also consists of 20% oil, which makes it the most important crop for producing edible oil (Edema, Sanni, & Sanni, 2005;FAO, 2005;Gibson & Benson, 2005).
Malnutrition, particularly protein deficiency, is prevalent in many parts of Africa as animal protein is too expensive for most people in Africa. Many leguminous crops provide some protein, but soybean is the only available crop that provides an inexpensive and high quality source of protein comparable to meat, poultry and eggs (Edema et al., 2005;FAO, 2005).
More than 216 million tons of soybeans were produced worldwide in 2007, of which 1.5 million were in Africa. Africa imports nearly as much soybean as it produces. Africa exports about 20,000 tons annually. Nigeria is the largest producer of soybean in sub-Saharan Africa (SSA), followed by South Africa.
The key benefits of soya are its high protein content, vitamins, minerals and insoluble fibre. The high fibre content makes soya beans and other soya containing foods valuable in cases of constipation, high cholesterol and type-2 diabetes (Edema et al., 2005;FAO, 2005;Gibson & Benson, 2005).
Protein energy malnutrition (PEM) is one of the major nutritional dilemmas in developing countries (Edema et al., 2005;FAO, Statistics Division, 2009;Kadam, Salve, Mehrajfatema, & More, 2012). As cereals are the staple diet of these nations and they mostly rely on plant-based food sources, so the utilization of pulses to supplement maize flour can be an effectual approach to combat PEM. Soy bean is rich in protein with well-balanced amino acid profile, so in search for nutritious food products, it provides an opportunity to be used in baked products such as cookies, bread, pasta, soups and snack foods (Edema et al., 2005;Kadam et al., 2012).
The aim of this study, therefore, was to assess quality and sensory properties of maize flour cookies enriched with soy protein isolate with the aim of encouraging the use of these under-utilized food crops in producing value-added products with nutraceutical potential.

Procurement of raw materials
Yellow maize and yellow soy beans varieties were procured from Bodija market in Ibadan, Oyo State, Nigeria. All chemicals and reagents used were of food and analytical grade.

Preparation of maize flour
The maize flour was prepared by the method reported by Houssou and Ayemor (2002). Maize grains were sorted, conditioned by spray 35 cl of water into the grains and allowed to stay for 15 min. This is done for easy de-hulling and de-germing. The de-germed and de-hulled maize was milled in a disc attrition mill and allowed to pass through 250 μm opening. The flour was packaged in cellophane bag until used.

Preparation of defatted soy flour
Defatted soy flour was produced as described by Meyer (1971) with modification. One kilogram of cleaned and sorted soybean was cracked in a rice sheller into 4 or 5 cracks. The cracked beans were then winnowed to remove the hulls. One part of the dehulled soybeans was then milled with a moulinex blender and the other part with a hammer mill (Christy and Norris Ltd., England) with speed of 8,000 rpm. The flour was then defatted in a soxhlet extractor for 6 h using n hexane with boiling point of 50-55°C. The defatted flour was desolventised by leaving to dry at room temperature for about 24 h. The flour from the moulinex blender was autoclaved at 6.9 kpa pressure at 121°C for 20 min.

Processing of soy protein isolate
Soy protein isolate was processed by using the method of How and Morr (1982) with modification. Twenty grams of the autoclaved, moulinex blended defatted soy flour was extracted in 200 ml water at pH 8.9. The pH of the distilled water was adjusted using 1 N NaOH. The slurry was stirred continuously for 30 min. The mixture was then centrifuged using Eppendorf 30,000 ×g system comprising of Safe-Lock Tubes and Centrifuge 5430 R with high-speed rotor for 30 min.
The extract was decanted and the residue re-extracted with 100 ml of distilled water for 30 min. The extract was mixed with the first extract while the residue was discarded. The pH of the combined extracts was then adjusted to pH 4.5, the isoelectric point of the major soybean globulins. The extract was centrifuged using Eppendorf 30,000 ×g system comprising of Safe-Lock Tubes and Centrifuge 5430 R with high-speed rotor for 30 min. The supernatant wasdiscarded while the residue, that is, the precipitate was given two 50 ml washes with distilled water and re-centrifuged. The precipitate (ASPI) was then neutralized to pH 7 using 1 N NaOH and then spread thinly in a crucible and dried in an air-draught oven at 40°C for 12 h.

Composite flour
Soy protein isolate was incorporated at different levels into maize flour for the development of composite flour as shown in Table 1.

Preparation of cookies
Cookies were prepared by the method reported by Mohamed (2000) and Abayomi et al. (2013) with modification. Two hundred grams of flour from each sample of different flour blends was used for the experiment. Eighty grams of sugar was creamed with one hundred grams of margarine until light and fluffy constituency was obtained using Kenwood chef with initial minimum speed, and the speed increased stepwise until the mark of 6 on the chef indicator was attained. Whole egg (60 g) was added, followed by flour (200 g), powdered milk (20 g), baking powder (0.1 g) and salt (1 g) were added and mixed until a stiff paste (batter) was obtained. The batter was rolled on a floured board using rolling pin to a thickness of 0.2-0.3 cm. The rolled batter was cut into circular shapes with a cutter and arranged on a greased tray and baked at 150°C for 20 min. The cookies were brought out, cooked and packaged in cellophane bag until used for laboratory analysis.

Moisture content of the flour and cookies samples
Moisture content of maize grains, maize flour, their blends and cookies samples were determined by oven method described in by Association of Official Analytical Chemists (AOAC, 1990) method.

Protein content of the flour and cookies samples
Crude protein was determined by the Kjeldahl nitrogen method (AOAC, 1990) where:

Fat content of the flour and cookies samples
Fat content was determined by the Soxhlet extraction method described by AOAC (1990) and Adeyeye and Akingbala (2015). About 2 g of each sample was extracted with 240 ml of petroleum ether in a Soxhlet extraction apparatus for 8 h. The ether was distilled off and the flask dried: where, W3 is the weight of the flash with the extracted oil; W2 is the weight of the empty flash; and W1 is the weight of the sample.

Ash content of the flour and cookies samples
Ash was determined by AOAC (1990) method. About 2 g of each sample was ignited at 600°C in a muffle furnace for 6 h. The residue was cooled in a desiccator and weighed. % Crude protein = % Total nitrogen × 6.25

Crude fibre content of the flour and cookies samples
Crude fibre was determined by AOAC (1990) method. About 1 g of each sample was weighed into 600 ml of Erlenmeyer flask; 100 ml of trichloroacetic acid digestion reagent was added. The solution was brought to boil and reflux for exactly 40 min at 50-60°C counting from the time boiling started. The flask was removed from the heater, cooled a little bit and was filtered through a 15.0 cm, Whatman filter paper of 1.0 g, the residue was washed six times with hot water and once with methylated spirit. Residue was removed by spatula from the opened-out filter paper, and the fibre was transferred into a porcelain dish and was dried overnight at 500°C. The sample was transferred to a desiccator and weighed when cool; it was ashed in a muffle furnace at 600°C for 6 h. This was cooled again and reweighed:

Bulk density of the flour samples
The bulk density of the flour samples was determined by method of Beuchat (1977) and Olapade and Ogunade (2014).

Water binding capacity of flour samples
Water binding capacity was determined by method described by Beuchat (1977) and Olapade and Ogunade (2014).

Dispersability of flour samples
The method described by Padmashree, Vijayalakshmi, and Puttaraj (1987) was used.

Swelling power for samples
The swelling power of starches of the maize flour and their blends was determined by the method described by Beuchat (1977).

Sieve analysis of the sample
Particle size distribution of the flour samples was determined by the method reported by Okaka and Isieh (1990).

Cookie flow and break strength
Cookie flow was determined by method described by Okaka and Isieh (1990).

Starch content determination
Starch was described by the method by Hassid and Newfield (1964). Each of the flour sample was refluxed with boiling ethanol at 80°C for 6 h in a Soxhlet extractor. The ethanol insoluble residue was refluxed with 10% HCl for 4 h in a Soxhlet extractor. The resulting hydrolysates were neutralized with NaOH and quantitatively estimated by the anthrone-sulphuric acid method (Carroll, Longley, & Roe, 1956). The value of glucose was multiplied by 0.9 to obtain the starch value (Hassid & Newfield, 1964).

Sugar content determination
This was determined by a refractometry method with the ABBE 60 Refractometer (Bellingham-Stanley Limited, Kent, England) (Van Hal, 2000).

Calorific value determination
Calorific values of the cookie samples were determined by using Gallenkamp Ballistic Bomb Calorimeter (Coded CBB 330). One gram of each sample of the cookies was ignited and burnt in excess oxygen in the bomb. The maximum temperature rise of the bomb was measured with the thermocouple and galvanometer system (Van Hal, 2000). By noting the change in the temperature of the water, one can calculate the energy value of the food by applying the definition for a calorie.
% Crude fibre = Weight after drying − Weight after Washing Sample weight × 100 The unit of energy in the International System of Units (SI) is the joule (J). The conversion factors for joules and calories are: 1 kJ = 0.239 kcal; and 1 kcal = 4.184 kJ.

Sensory evaluation
Cookies prepared from maize flour and maize flour with various level of soy protein isolate substitution were compared to cookies from 100% maize flour sample (R) as control. Two different groups of (10 members; 5 men and 5 women) consumer type untrained sensory panellists were selected from undergraduate students of University of Agriculture, Abeokuta, who eat cookies regularly. Each panellist was provided with separate sensory booth. Water was provided to rinse the palate between two tasting sessions. Also, quality attributes, colour, texture, taste, appearance, crispiness and overall acceptability were evaluated on a nine-point Hedonic scale where 9 = Like extremely and 1 = Dislike extremely.

Statistical analysis
The data were collected in triplicates and means were compared statistically with IBM SPSS (Version 21.0) using Duncan's Multiple Range Test. One-way ANOVA was done on colour, texture, taste, appearance, crispiness and overall acceptability using Duncan's Multiple Range Test (p ≤ 0.05) to study the difference between means.

The proximate composition of maize grain and soy beans isolate
The proximate composition of the maize grains and soy protein isolate used for the production of cookies agreed with the previous works of David and Dickerson (1991), Adedeji, Oyinloye, and Ocheme (2014) and Adeyeye and Akingbala (2015). The values of 5.68 ± 0.11, 2.92 ± 0.08, 0.46 ± 0.01, 1.63 ± 0.08 and 89.31 ± 0.46 respectively for moisture, crude fibre, ash, fat and crude protein contents of soy protein isolate were also consistent with the range of those components reported in the literature (Mizrani, Zimmermann, Beak, & Cogan, 1967;Urbanski, Wei, Nelson, & Steinberg, 1982). The carbohydrate contents of the maize grains and soy protein isolate were obtained by difference. The carbohydrate and soy protein isolate values of 72.31 ± 0.23 and 51.96 ± 0.12 for 10% maize flour and 70% maize flour respectively agreed with the values of 70.1-83.9% reported by Adedeji et al. (2014); Abayomi et al. (2013); and Onabanjo and Ighere (2014) for maize flour and soy protein isolate respectively.

The proximate chemical composition of maize flour, soy protein isolate and their blends
The proximate composition of maize flour, soy protein isolate and their blends are presented in Table 1. The moisture, crude fibre, ash, fat and crude protein contents of the maize flour were 10.09 ± 0.17, 1.36 ± 0.07, 1.33 ± 0.08, 1.58 ± 0.07 and 8.06 ± 0.10 respectively while that of the soy protein isolate were 5.68 ± 0.11, 2.92 ± 0.08, 0.46 ± 0.01, 1.63 ± 0.08 and 89.31 ± 0.46 respectively. These values agreed with 9.6-13.5, 9.6-10.7, 1.4-9.4 and 5.0% reported by Oboh, Ologhobo, and Tewe (1989), Adedeji et al. (2014), Abayomi et al. (2013) and Onabanjo and Ighere (2014) respectively. The moisture, crude fibre, ash, fat, starch and sugar contents of the mixture decreased as the percentage substitution of soy protein isolate increased. While the protein content increased as the soy protein isolate substitution increased. This might be due to high protein content of soy protein isolate.

Effect of soy protein isolate substitution on the proximate composition of maize flour cookies
The effects of soy protein isolate substitution on the proximate composition of maize flour cookies are presented in Table 2. The moisture, crude fibre, fat and sugar contents of the cookies from different flour blends decreased as the percentage of soy protein isolate in the blends increased from 8.76 ± 0.22 to 6.96 ± 0.13, 3.81 ± 0.07 to 2.87 ± 0.02, 2.63 ± 0.06 to 1.69 ± 0.01, 9.41 ± 0.24 to 8.45 ± 0.13 and 6.21 ± 0.20 to 4.98 ± 0.11 respectively. The protein content, however, increased from 8.69 ± 0.19 to 29.11 ± 0.34%, respectively, as the percentage of soy protein isolate in the blends increased. This might be due to dilution of soy protein isolate and the fact that soy protein isolate has high protein content.

Effect of soy protein isolate substitution on the functional properties of the maize flour
The effects of soy protein isolate substitution on the functional properties of maize flour are presented in Table 3. The bulk density, dispersability and swelling power capacities of maize flour decreases as the level of soy protein isolate in the composite flour increases. These ranged from 4.6 ± 1.59 to 3.1 ± 1.32, 43.3 ± 2.59 to 18.1 ± 1.91 and 10.2 ± 1.41 to 5.8 ± 1.14 respectively. While the water holding capacity increases from 1.1 ± 0.12 to 2.0 ± 0.19. The incorporation of soy protein isolate increased water holding capacity and decreased bulk density, dispersability and swelling power capacities of maize flour.

Cookie flow and break strength properties
Data on cookie flow after baking and the break strength of the cookie samples are presented in Table  4. The cookies from 100% maize flour had the highest increase in volume of 57.6 ± 1.58%. Substitution of maize flour with soy protein isolate reduced cookie flow. Addo, Akinola and Yusuff (1987) and Okaka and Isieh (1990) reported similarly findings.

Calorific value of soy protein isolate substituted maize flour cookies
The calorific values of samples from soy protein isolate substituted maize flour cookies are presented in Table 4. Cookies made from maize flour (control) had the highest energy (468 ± 1.59 cal/100 g). This value agreed with the value of 493 Cal/100 g reported by McCance and Widdowson (1991) for digestive biscuits. The calorific value of maize flour cookies decreased with increase in concentration of soy protein isolate substitution ( Table 2). The calorific values of the cookies from maize flour and soy protein isolate substituted maize flour showed that they are comparable with similar products from wheat, barley and sorghum (McCance & Widdowson, 1991). The lower calorific values and increase in protein content of the cookies may be due to the soy protein isolate.

Sensory evaluation of cookies (hedonic scale)
For the hedonic scale shown in Table 5, all the samples were not significantly different (p < 0.05). The 100% maize flour cookies (control) with the value of 7.2 ± 0.17 rated the highest in terms of overall acceptability. About 25% level of soy protein isolate substitution reduced overall acceptability to 5.6 ± 0.11. The results showed that increase in the level of soy protein isolate in the maize flour was detected at 25% and became less acceptable as the percentage of substitution increases above 25%. About 30% soy protein isolate cookies had least acceptability level of 5.3 ± 0.10. For the organoleptic Key: MF = maize flour; SPI = soy protein isolate.
Notes: All data are means of three replicates expressed on dry weight basis; means with the same superscripts are not significantly difference at p ≤ 0.05; 95:5 = maize flour: soy protein isolate ratio.