EFFECT OF SOAKING TEMPERATURE ON THE PHYSICAL PROPERTIES AND WATER DIFFUSION COEFFICIENTS OF SELECTED COWPEA VARIETIES

ABSTRACT
The purpose of this study was to investigate the effect of soaking temperature on the physical properties and water diffusion coefficients of three cowpea varieties (Asontem, Hewale and Asomdwee) grown in Ghana. 10 g of all three cowpea varieties were subjected to four soaking temperatures (30, 40, 50 and 60 ) for 10 h during which measurements were taken at 30 min interval. The initial values for moisture content, length, breadth, thickness, equivalent radius, and thousand seed weight of Asontem were 16.53% (d.b), 7.19 mm, 6.01 mm, 4.78 mm, 3.46 mm and 146.9 g, whereas the geometric mean diameter, surface area, seed volume and sphericity were 5.90 mm, 111.00 mm2, 341.06 mm3, and 0.83, respectively. The initial values for moisture content, length, breadth, thickness, equivalent radius, geometric mean diameter, surface area, seed volume, thousand seed weight and sphericity of Hewale were 12.40% (d.b), 6.88 mm, 5.54 mm, 4.45 mm, 3.26 mm, 5.53 mm, 96.44 mm2, 281.75 mm3, 122.27 g and 0.81, respectively. The initial average values for moisture content, length, breadth, thickness, equivalent radius, geometric mean diameter, surface area, seed volume, thousand seed weight and sphericity of Asomdwee were 13.63% (d.b), 7.20 mm, 5.44 mm, 4.63 mm, 3.08 mm, 5.66 mm, 100.81mm2, 299.99 mm3, 125.36 g and 0.79, respectively. As soaking temperature increased from 30 to 60 , the values of the physical properties of the three cowpea varieties decreased linearly. The length, breadth, thickness, geometric mean diameter, surface area, seed volume and sphericity of Asontem decreased linearly from 9.60 mm, 7.13 mm, 5.98 mm, 7.42 mm, 173.5 mm2, 678.2 mm3, 0.77 to 8.85 mm, 6.61 mm, 5.56 mm, 6.91 mm, 149.9 mm2, 542.7 mm3 and 0.77, respectively. The length, breadth, thickness, geometric mean diameter, surface area, seed volume and sphericity of Hewale also decreased linearly from 9.93 mm, 6.98 mm, 5.89 mm, 7.42 mm, 173.1 mm2, 673.6 mm3, 0.75 to 9.64 mm, 6.59 mm, 5.50 mm, 7.06 mm, 156.1 mm2, 576.6 mm3 and 0.73, respectively. The length, breadth, thickness, geometric mean diameter, surface area, seed volume and sphericity of Asomdwee decreased linearly from 9.69 mm, 6.59 mm, 5.52 mm, 7.07 mm, 156.9 mm2, 581.4 mm3, 0.73 to 9.42 mm, 6.34 mm, 5.20 mm, 6.77 mm, 144.0 mm2, 511.3 mm3 and 0.72, respectively. The water absorption kinetics of Asontem, Hewale and Asomdwee cowpea varieties followed the Fick’s law of diffusion during the first hours of soaking. The values of water diffusion coefficients determined for Asontem, Hewale and Asomdwee cowpea varieties, within the temperature variation from 30 ºC to 60 ºC during soaking ranged from 5.12 x 10-10 m2/s to 6.64 x 10-10 m2/s, 3.96 x 10-10 m2/s to 5.12 x 10-10 m2/s and 4.93 x 10-10 m2/s to 6.08 x 10-10 m2/s, respectively. The influence of temperature on the water diffusion coefficient was adequately described by an Arrhenius-type equation giving activation energy values for Asontem, Hewale and Asomdwee cowpea varieties as 7.27 kJ/mol, 7.26 kJ/mol and 6.26 kJ/mol respectively.


CHAPTER ONE
INTRODUCTION
1.0 Background to the Study
Legumes have considerable protein and soluble fibre content making them an essential fraction of the diet in many countries (Kabagambe et al., 2005). Legumes serve as chief sources of protein in many dishes prepared all over the African continent (El-Maki et al., 2007). Legumes are inexpensive and excellent providers of essential nutrients needed by the human body. In contrast to animal products, legumes contain reasonably low but right amounts of the essential amino acid methionine needed to manufacture protein in humans (Shafaeia et al., 2014). An essential member of the legume family Fabaceae is cowpea (Vigna unguiculata), which is widely consumed in many parts of the world including Ghana.

Cowpea is considered as one of the crops which have been in existence for so many years. Cowpea is grown in certain areas in all the five continents and more predominantly in the African, Asian and South American continents (Firouzi and Alizadeh, 2012). It has been indicated that cowpea on average contains 23.4%, 11%, 3.6%, 1.3% and 56.8% of protein, water, ash, fat, carbohydrate, respectively (Davies and Zibokere, 2011). Cowpea is a chief source of plant protein in the West African region, because it provides food for people, livestock and even other plants (Henshaw, 2008; Olotu et al., 2013). The leaves and undeveloped pods of cowpea serve as vegetables for some consumers.

Estimation of worldwide area of production of cowpea stands at approximately 10.1 million hectares with about 4.99 million tonnes representing annual global grain production (Hamid et al., 2014). This agricultural raw material is a vital source of carbohydrate, protein, iron, Vitamin B among others (Demirhan and Özbek, 2015).

In monetary terms, cowpea is also much cheaper than some grains available (Ayenlere et al., 2012). Therefore, anybody at all can purchase this commodity. Again, cowpea has the ability to mix well with other food ingredients (Muoneke et al., 2012).

In Ghana, cowpea is used in preparing foods such as cowpea fritters popularly referred to as ‘koose’, which is served with millet porridge; cooked beans with ‘gari’(roasted grated fermented cassava) and ‘tugbani’ (steamed bean cake) as well as in making of stews and soup (Appiah et al., 2011). In addition, cowpea, by virtue of its nutritional and functional qualities, is gradually becoming an important raw material for the production of cowpea flour for use in food formulations into other food products on industrial scale (Olotu et al., 2013; Aremu et al., 2014).

Generally, processing of cowpea first requires soaking of cowpea seeds in water to allow some level of water absorption for a period of time before additional processing of the cowpea seeds takes place (El-Syiad et al., 2014). The rate of water absorption by the cowpea seeds largely depends on the temperature of water for soaking and soaking time (Shafaei and Masoumi, 2014). It also depends on the initial moisture content of the seeds, variety of the seeds, soaking duration, acidity level of the water and the seed physical characteristics (Demirhan and Özbek, 2015).

At the industrial level of cowpea processing, soaking of seeds is done taking into account, if not all, most of the factors that affect the rate of water absorption by the cowpea seeds. Thus, the temperature of soaking water for the cowpea seeds, for example, is usually preferred above ambient temperature because high soaking water temperature increases the rate of water absorption by the seeds (Turhan et al., 2001). Consequently, the soaking time is reduced making it possible to process large quantities of cowpea seeds within the shortest possible time (Shafaei et al., 2014).

Contrary to the industrial level of processing cowpea, the domestic level of cowpea processing does not necessarily consider the factors which affect the rate of water absorption. It involves soaking the cowpea seeds in water at ambient temperature overnight or for 24 h or more.

1.1 Statement of the Problem
Soaking is an integral part of processing leguminous seeds. It clearly establishes a relationship between the seed structure and water absorption by the seed (El-Syiad et al., 2014). Thus, many researchers have sought to know how water moves into seeds. Many studies have further demonstrated the role of temperature in water absorption process by different seeds (Seyhan-Gürtaş et al., 2001; Turhan et al., 2001; Shafaei and Masoumi, 2014).

Extensive studies have been carried out by many researchers to study the water absorption patterns in a number of seeds of legumes and cereals (Tagawa et al., 2003). The effects of temperature, pH, physical properties, chemical properties as well as the nutritional composition of some grains and legumes, on their water absorption capacities have also been investigated (Agarry et al., 2014). In addition, some investigations conducted by researchers have revealed how temperature affects the diffusion of moisture into some legumes including soybean seeds, Egusi melon (Cucumeropsis edulis) seeds as well as cereals such as amaranth grains and maize kernels (Hsu, 1983; Addo and Bart-Plange, 2009).

However, effects of varietal variations and processing variables on the rate of water uptake and moisture diffusivity in some new varieties of legumes grown in Ghana, such as Asontem, Hewale and Asomdwee, all cowpea varieties have not been established. Thus, this research was undertaken to study the influence of temperature and variety on the water absorption characteristics of the three newly-developed cowpea varieties grown in Ghana.

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Item Type: Ghanaian Postgraduate Material  |  Attribute: 79 pages  |  Chapters: 1-5
Format: MS Word  |  Price: GH50  |  Delivery: Within 30Mins.
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