There is increased advocacy on the consumption of functional foods by world human nutrition due to different health problems related with food consumption such as diabetes and coronary heart diseases (WHO/FAO, 2003). Food professional/industries might face challenges of producing food products containing functional ingredients in order to meet the nutritional requirements of individuals with health challenges. This is  because  of  the  effect  of added sugar and lipids in the industrial production of foods products. Alternative source of food production was advocated by Oke and Adeyemi (1991) in tackling food crises. The prospect of blending tubers, roots and plantain with cereals and legumes for the production of household food products is receiving considerable attention (Nnam, 2002; Onoja and Obizoba, 2009). This might make the products to be nutritious, relatively cheap and affordable to the rural poor to stem-off hunger and malnutrition.

Baked products provide an excellent opportunity to incorporate food-grade fractions from grains, legumes or other indigenous food sources. High cost of wheat flour in non-wheat producing countries such as Nigeria poses a problem to bakery industries and consumers of baked products (Chinma et al., 2012). Nigeria is currently one of the world’s largest importers of United State wheat flour (United States Department of Agriculture, 2014). The present high cost of baked products in Nigeria presents the need to further study on incorporation of indigenous food sources for baking, as this will help reduce total dependence on wheat flour.

Plantain is the common name for herbaceous plants of the genus Musa. Plantain (Musa paradisiaca) is an important staple food in Central and West Africa. It is a basic food crop and cheap source of energy in Nigeria (Faturoti et al., 2007; Adeniyi et al., 2006). Several food consumption surveys in Nigeria identified plantain among the major starchy staples (Odenigbo, 2012; Okeke et al., 2008; Ogechi et al., 2007). According to FAO (2005), over 2.11 million metric tons of plantains are produced in Nigeria annually. However, about 35-60% post-harvest losses had been reported and attributed to lack of storage facilities and inappropriate technologies for food processing (Olorunda and Adelusola, 1997).

An average plantain has about 220 calories and is a good source of potassium and dietary fiber (Randy et al., 2007). It is rich in carbohydrate, dietary fiber, irons, vitamins, and minerals. This nutritious food is ideal for diabetics, children, and pregnant women. It can also be a good supplement for marasmus patients. Plantain contains small amount of serotonin which has the ability to dilate the arteries and improve blood circulation. Its regular consumption helps to cure anemia (low blood level) and maintain a healthy heart (USDA Nutrient Database, 2010). A diet of unripe plantain is filling and can also be a good inclusion in a weight loss diet plan (Oke et al., 1998).

Plantain is widely grown in the Southern states of Nigeria and it is used both in Nigeria and many African countries as a cheap source of calories, excellent for weight control, slow in the release of energy after consumption with a low glycermic index (Mendosa, 2008), high in potassium and good for diabetic patients (Akubor, 2003?). Plantain is also a good source of Iron, and β – Carotene (Pro-Vitamin A) as reported by Ogazi (1988?). It contains 32% carbohydrate, 1%  protein,  0.02 fat, 60% water, some vitamins and mineral elements (Kure et al., 1998). With the progressive increase in the consumption of bread and related baked products in Nigeria, the composite flour program if adopted has the potential to add value to indigenous crops like plantain and at the same time conserve foreign exchange spent on wheat importation. The aim of the study therefore is to evaluate the effect of substitution on the functional properties of the wheat/ plantain composite flour and the proximate/sensory properties of wheat/plantain bread.

Plantain is rich in dietary fibre (8.82%), resistant starch (16.2%), and low in protein and fat (Ayodele and Erema, 2011). Dietary fibre in human diets lowers serum cholesterol, reduces the risk of heart attack, colon cancer, obesity, blood pressure, appendicitis and many other diseases (Rehinan et al., 2004). On the other hand, resistant starch assists in preventing and managing type 2-diabetes (Jideani and Jideani, 2011). Resistant starch has interesting functional properties for use in foods including: formation of products with high fibre content and low volume with improved sensory properties like texture and appearance (Nugent, 2005). Considering the health benefits of plantain, its incorporation as composite blend in the preparation of cake will help in enhancing the nutritional and health status of consumers, reduce total dependence on wheat flour and incidence of certain chronic non communicable disease.

The possibility of producing bakery products from wheat/plantain composite flour has been assessed (Bamidele et al., 1990; Mepba et al., 2007; Idoko and Nwajiaku, 2013). The water absorption capacity and dough development time of the composite flour decreased with increasing levels of supplementation with plantain (Bamidele et al., 1990). The percentage of wheat flour required to achieve a certain effect in composite flours depends heavily on the quality and quantity of wheat gluten and the nature of the product involved. Akubor (1998) has shown that plantain flour has a good potential for use as a functional agent in bakery products on account of its high water absorption capacity.

Aurore et al. (2009) used colour index according to the commercial peel colour to define 8 ripening stages of banana. At stages 1-3, banana is not usually eaten like fruit, because it is green, very hard, astringent, and rich in starch. At stage 8, it is overripe and muddy. Plantain can be utilized at all stages of ripening, and its nutritive value depends on their ripeness, variety, climatic conditions and soil of crop production (Baiyeri et al., 2011; Ogazi, 1986). Baiyeri et al. (2011) reported increased ash and carbohydrate contents with ripeness, whereas at unripe stages, fat, protein and dry matter were relatively higher. With changes observed in composition of plantain due to ripening it becomes imperative to assess the use of wheat and plantain flour at different stages of ripeness in cake production. This study was undertaken to evaluate the composition of cake produced from different ratios of plantain (at stages 2 and 5 of ripeness) and wheat flours.

Source and preparation of samples

The fresh plantains (Musa paradisiaca) used for this study were bought from Ogige market Nsukka in Enugu State, Nigeria. The plantain fingers were at stages 2 (unripe) and 5 (firm ripe) of ripeness using the colour index chart as described by Aurore et al. (2009).

Wheat flour and the cake ingredients (margarine, eggs, granulated sugar, vanilla and baking powder) were also bought from the local market.

The plantain fingers were washed, peeled, sliced, sun-dried for 96 h (during dry season) and milled into flour using Attrition Mill (Globe P 44, China). The flour samples were sieved through a 75µm sieve and stored in airtight plastic containers at room temperature (28±2°C).

Formulation of composite flour

The unripe plantain (stage "2" of ripeness) and firm ripe plantain (stage "5" of ripeness) flours were mixed with wheat flour separately at different proportions (25:75; 50:50 and 75:25) while 100% wheat flour was used as control. The flours were mixed using a B8 (Mega Best Industry Ltd, GuangDong, China) universal spiral mixer at 450 rpm for 20 min until uniform blends were obtained.

Cake making

The proportion of ingredients used consists of flour (100g), egg (100g), sugar (60g), vanilla (three drops), baking powder (1.7g), water (80 ml) and margarine (80g). The baking procedure described by Ceserani et al. (1995) was adopted.

Determination of functional properties of the flour samples

Determination of bulk density

The bulk density was determined using Onwuka (2005)’s method with slight modification. Fifty grams of each sample was measured into a clean 100 ml graduated measuring cylinder which was tapped gently several times until there was no further diminution. Its volume was recorded and the bulk density was calculated using the formula:

Determination of foaming capacity and stability

Foaming capacity and stability were studied as described by Narayana and Narasinga (1982). Two grams of each flour sample was blended with 50 ml distilled water at 30±2^oC. The whipped mixture was transferred into 100 ml graduated cylinder. The suspension was mixed and properly shaken to foam and the volume of the foam after 30 s was recorded. The foaming capacity was expressed as a percentage increase in volume. The foam volume was recorded 1 h after whipping to determine the foaming stability as a percentage of the initial foam volume.

Determination of water and oil absorption capacity

Water and oil absorption capacities were  determined  according  to the method described by Okezie and Bello (1988). Briefly, 1.0 gram of each sample was mixed with 20 ml distilled water (for water absorption capacity) and 20 ml of oil (for oil absorption capacity) in a flask shaker and centrifuged at 2,000 rpm for 1h. Water/oil absorbed by samples was calculated as the difference between the initial and final volumes of water/oil. Means of triplicates determination were reported.

Determination of least gelation concentration

The least gelation concentration was determined using the method of Coffmann and Garciaj (1977). Sample suspensions of 2-20% were prepared in distilled water. Ten milliliter of each of the prepared dispersions was transferred into a test tube. It was heated in a boiling water bath for 1 h, followed by rapid cooling in a bath of cold water. The test tubes were further cooled at 4^oC for 2 h. The least gelation concentration was determined as that concentration when the sample from the inverted test tube did not slip.

Determination of emulsion capacity

Emulsion capacity was determined using the procedure of Abbey and Ibeh (1988) with slight modification. One gram of each flour sample was dispersed in a beaker containing 5 ml distilled water and 5 ml of vegetable oil (corn oil) was added. The mixture was emulsified by centrifuging at 1,600 for 5 min. Emulsion capacity (%) was calculated as:

Determination of emulsion stability

Emulsion stability was studied by the method described by Sathe and Salunkhe (1981) with slight modification. 0.5 g of the sample was blended with 25 ml of distilled water, then 25ml of vegetable oil was added while blended for 30 s at high speed. The emulsion prepared was allowed to stand in a graduated cylinder and volume of water separated at time intervals of 0.5, 1, 2, 3 … 12h was noted in each case as the emulsion stability. Triplicate measurements were made and average results taken.

Emulsion stability (%) was calculated as:

Chemical analysis

Proximate composition

Proximate analysis of the samples was carried out using AOAC methods (AOAC, 1995). Moisture content was determined by air oven method at 105^oC. The protein content of the sample was determined using micro-Kjeldahl method. Fat was determined by Soxhlet extraction method using petroleum ether as extracting solvent. The ash content was determined by weighing 5 g of charred sample into a tarred porcelain crucible. It was incinerated at 600^oC for 6 hours in ash muffle furnace until ash was obtained. Crude fibre was determined by exhaustive extraction of soluble substances in a sample using H₂SO₄ and NaOH solution, after the residue was ashed and the loss in weight recorded as crude fibre. The carbohydrate content was determined by difference as follows:

% Carbohydrate = 100 – (% Moisture + % Ash + % Protein + % Fat + % Crude fibre).

Vitamins and mineral analyses

Provitamin A was determined using the method adopted from IVACG (1992) and vitamin B_1, B₂, vitamin C and folate were determined using the method of AOAC (1995).

Mineral compositions were determined using AOAC method (1995). The ash was digested with 3 cm³ of 3M HCl and made up to the mark in a 100cm³ standard flask with 0.36 M HCl before the mineral elements (calcium, zinc, magnesium, iron and potassium) were determined by atomic absorption spectrophotometer (PYE Unicam SP 2900, UK)

Determination of sensory properties

Thirty panelists consisting of staff and students of the Department of Home Science, Nutrition and Dietetics University of Nigeria Nsukka, Nigeria were selected for the sensory evaluation based on their familiarity with the quality of cake. Cakes prepared from the flour blends were coded and presented in white plastic plates. Water was provided to rinse mouth between evaluations. The samples were evaluated for texture, colour, taste, flavor and general acceptability. Panelists evaluated cake samples on a 9-point hedonic scale quality analysis (Ihekoronye and Ngoddy, 1985) with 9 = liked extremely, 8 = liked very much, 7 = liked, 6 = liked mildly, 5 = neither liked nor disliked, 4 = disliked mildly, 3 = disliked, 2 = disliked very much and 1 = disliked extremely .

Statistical Analysis

The data obtained were analyzed statistically by Statistical Package for Social Science (SPSS), version 18, using one way analysis of variance (ANOVA). Means were separated by calculating the least significant difference (LSD) at (P ≤ 0.05). Data reported on the tables are average values of triplicate determinations.

Table 1 shows the functional properties of ripe plantain, unripe plantain and wheat flours. The bulk density of the flour samples ranged from 0.64 to 0.82 gm³ for ripe plantain and wheat flour, respectively. However, there was no significant difference between the bulk density of wheat and unripe plantain flours. The foaming capacity, foaming stability and emulsion capacity of unripe plantain flour was found to be significantly higher than that of ripe flour (p<0.05). The ripe plantain and wheat flours had the highest least gelation concentration (35%) as compared to the unripe plantain flour (30.01%). The water absorption capacity of the ripe plantain (2.77g/g) and unripe plantain (2.71g/g) flours was significantly higher (p<0.05) than wheat (2.09g/g) flour.

Table 2 shows the proximate composition of the ripe, unripe plantain and wheat flours. The moisture content ranged from 18.48% for ripe to 20.43% for unripe plantain flours. Protein content ranged from 2.73 to 3.04% for unripe plantain and wheat flours, respectively. There was no significant difference in the protein content of wheat and ripe plantain flours. Wheat flour had the highest fibre (1.48%) and fat (1.28%) contents.  Carbohydrate content ranged from 72.87 to 74.56% for wheat and ripe plantain flours, respectively. Ripe (2.33%) and unripe plantain (2.11%) flours had significantly higher (p<0.05) ash content when compared to the wheat flour (1.11 %).

Table 3 shows the proximate composition of cakes prepared from the wheat-plantain composite flour and wheat flour (control). Table 3 shows that the carbohydrate content of the cakes prepared from plantain and wheat flour blends were significantly higher than  that prepared from 100% wheat flour. However, 100% wheat flour cake had the highest protein (26.41%) and ash (9.56%) content when compared to other cakes.

Table 4 shows the vitamin composition of the cake samples. The URP₅₀W₅₀, RP₅₀W₅₀, RP₇₅W₂₅ and URP₇₅W₂₅ cakes had significantly more pro-vitamin A, vitamin B₁ and vitamin C values. Cake prepared from 100% wheat flour was significantly low in pro-vitamin A (905 µg/100 g) and vitamin C (0.60 mg); high in vitamin B₁ (1.01 mg) and folate (235 µg/100 g).

The mineral composition of the cakes is presented in Table 5.  The URP₇₅W₂₅ cake had significantly more potassium content (2310 mg/100 g) but less calcium (449 mg/100 g), iron (9.84 mg/100 g), magnesium (262 mg/100 g) and zinc (6.02 mg/100 g).

The sensory attributes of cakes prepared from the flours are presented in Table 6. The URP₅₀W₅₀ was rated best plantain-base cake in terms of texture (7.80) and acceptability (7.82). The URP₇₅W₂₅ cake was rated lowest in texture (6.76), appearance (6.24), taste (6.44), flavor (6.67) and overall acceptability (6.42). However, the plantain-base cakes compared favourably with the 100% wheat cake in most the sensory attributes.

The foaming capacity of the flours ranged from 11.52 -28.81%. Wheat flour had the highest foaming capacity and stability. Foaming capacity is assumed to be dependent on the configuration and nature of protein molecules, as flexible proteins have good foaming capacity (Graham and Philips, 1976) (). Unripe plantain flour had higher foam stability (58.14%) and capacity (17.47 %) when compared to ripe plantain flour (54.78 and 11.52%, respectively). This may suggest the usefulness of the flour in improving textural and leavening characteristics. Akubor et al. (2000) reported that food ingredients with good foaming capacity and stability can be used in bakery products. The water absorption capacity of the flours ranged from 2.09 - 2.77 g/g with wheat flour having the lowest value (2.09 g/g). The major chemical composition that enhances the water absorption capacity of flours are carbohydrates and proteins, since they contain hydrophilic parts such as polar or charged chains (Lawal and Adebowale, 2004). The result of carbohydrate content of the plantain flours (Table 2) may have contributed to their water  absorption  capacity.  The emulsion capacity ranged from 2.30 to 4.00% with unripe plantain having the highest value. Emulsion capacity plays a significant role in many food systems where protein has the ability to bind to fat such as in batter and dough (Sathe, 2001). There was significant difference in the least gelation concentration of unripe plantain flour and wheat flour. According to Sathe et al. (1982) the variation in the gelling properties of flours is attributed to the relative ratio of protein, carbohydrates and lipids that made up the flours and interaction between such components.

The moisture content of the flours ranged from 18.48 to 20.43%. Unripe plantain flour had the highest moisture value (20.43%) which is still low when compared to other studies (Idoko and Nwajiaku, 2013; Ketiku, 1973; Asiedu, 1987) that reported a range of 49.40 to 62.0%. Low moisture content enhances keeping quality/shelf-life. The fat content of wheat flour (1.28%) was higher than the plantain flours (0.30% and 0.63%). The fat content of the plantain flour in this study is similar to those reported in other studies (Odenigbo et al. 2013; Egbebi and Bademosi, 2011). However, Idoko and Nwajiaku (2013) reported higher values  of  fat  for  firm  ripe  (2.10%)  and unripe plantain (2.30%) flour. This difference could be attributed to stage of ripeness, soil and climate condition under which the fruits were planted (Chandler, 1995; Baiyeri and Unadike, 2001).

This study showed that the protein contents of ripe and unripe plantain flours were 3.03 and 2.73%, respectively. This is in agreement with Egbebi and Bademosi (2011) who reported that protein content of ripe and unripe plantain flour varied from 2.18 - 3.15%. Ayodele and Erema (2011) also reported that plantain has low protein and fat contents. The increase in protein content of the plantain flour from unripe (2.73%) to ripe (3.03%) stage could be associated with amino acid uptake and incorporation into protein during fruit ripening (Brady et al., 1970). The crude fibre content of the plantain flours (1.31 and 0.49% for ripe and unripe plantain, respectively) is an indication that when incorporated in human diet would help in lowering serum cholesterol, reduction of risk of heart attack, colon cancer, obesity, blood pressure, appendicitis and many other diseases as reported by Rehinan et al. (2004). The ash contents of both ripe (2.33%) and unripe plantain (2.11%) flours reported in this study  are  comparable  with  the  work  of Odenigbo et al. (2013). Ash contents are indication of minerals that are contained in the flours. The carbohydrate content of the flours ranges from 72.87 - 74.56%, with ripe plantain having the highest value (74.56%). Carbohydrate is a source of energy for human daily activities.

The moisture content of the 100% wheat cakes was significantly lower than the composite cakes except URP₂₅W₇₅ cake. This could be due to the ability of the plantain flours to absorb more water than the wheat flour as shown in Table 1. Protein content of the cakes ranged from 18.91 - 26.41%. This shows that 100 g of the cake can provide more than one-third of recommended daily protein intake (IOM, 2005) of a healthy adult when consumed The protein content of the plantain base cakes increased with addition of wheat flour. This could be due to additive effect of wheat flour as it contains more protein than the plantain flour (Table 2). The fibre and fat content of the cakes varied between 0.36 – 9.44% and 4.23 – 12.32%, respectively with URP₂₅W₇₅ cake having the highest fibre and ash contents. The health benefits of fibre are enormous (Rehinan et al., 2004). The cakes had appreciable ash content which ranged from 2.81 – 9.56%. The plantain base cakes had more carbohydrate content than the 100% wheat cake. This is quite understandable as plantain flour had higher carbohydrate content than the wheat flour (Table 2).

The composite cakes contain significantly higher vitamins (pro-vitamin A, vitamin C, B₁, B₂ and folate) than 100% wheat cake. This demonstrates the beneficial effect of blending food in food product development. The result agrees with Akubor (2005) that nutritional enhancement is the advantage in the use of composite food products. The calcium and iron contents of cakes made from the composite flour varied from 449 – 454 mg and 9.84 – 12.39 mg, respectively. Calcium is essential for proper bone and teeth formation (Wardlaw and Kessel, 2002). The potassium, magnesium and zinc composition (mg/100g) of the composite cakes ranged from 1012 – 2310, 262 – 314 and 6.02 – 9.97, respectively. These minerals are quite beneficial to human health; potassium is crucial to heart function and plays a key role in skeletal and smooth muscle contraction, making it important for normal digestive and muscular function (Wardlaw and Kessel, 2002). Magnesium is an essential constituent of all cells and is necessary for the functioning of enzymes involved in energy utilization and it is present in the bone (ADA, 2002). Zinc is needed for the body's defensive (immune) system to properly work (Wardlaw and Kessel, 2002 ). It plays a role in cell division, cell growth, wound healing, and the breakdown of carbohydrates. The high mineral composition of the cakes shows that it might help to mitigate some mineral deficiencies.

The evaluated sensory attributes of 100% wheat cake are similar to the plantain base cakes up to 50:50 flour substitution levels. Cakes prepared from 25:75 and 50:50 plantain: wheat flour ratios were rated higher in appearance and overall acceptability than the 75:25 plantain: wheat flour cakes. This is because cakes made from 75% plantain and 25% wheat flour had dull colour, as a result of the activity of phenolase on the plantain during processing (Perez-Sira, 1997). However, colour and oxidative stability of plantain flour could be enhanced by blanching slices in 1.25% NaHSO₃ solution (Mepba et al., 2007).

Ripening affects the functional and nutritional properties of plantain flour as well as its products. Acceptable cakes can be produced using plantain: wheat composite flours up to 50:50 substitutions without adversely affecting the sensory quality. Cakes produced from these flour blends can serve as functional foods especially for hypertensive, diabetic and obese patients considering their high protein, magnesium, potassium, and relative high fibre content.

The authors have not declared any conflict of interests.