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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