The Hydration Kinetics of Selected Ghanaian Maize ( Zea mays L.) Hybrids

Water absorption characteristics of maize (Abotem, Abeleehi, and Dorke SR) hybrids grown in Ghana were investigated using models obtained from Fick’s law of diffusion. The maize kernels were soaked in water at four different temperatures of 30˚C, 40˚C, 50˚C and 60˚C to evaluate their water absorption behaviours. The results indicated that temperature and variety were the major factors controlling water absorption patterns. The diffusion coefficients of the hybrids were found to differ in the order of Dorke SR > Abeleehi > Abotem and increased as the soaking temperature increased. The calculated water diffusivities varied from (2.54 - 3.49) × 10 −10 m 2 /s for Abotem, (2.64 - 3.59) × 10 −10 m 2 /s for Abeleehi, (3.21 - 4.20) × 10 −10 /s for Dorke SR. The Arrhenius-type equation was able to describe the strong effect of temperature on the diffusion coefficient of the hybrids. The activation energy values obtained were7.50 kJ/mol for Abeleehi, 6.27 kJ/mol for Dorke SR and 9.09 kJ/mol for Abotem. Results indicated that temperature and variety were the major factors controlling water absorption patterns.


Introduction
Maize is an essential member of the cereal family and important food crop produced worldwide including savannas of Sub-Saharan Africa (SSA). Maize is a major source of protein and energy and can be used as a substitute for major food staples. Many people believe that maize could become the future food for Africans. In SSA, maize contributes 40% to cereal production, where more than 80% is used as food [1]. At least 30% of the calorie intake in SSA is supplied by maize [2]. In Ghana, more than 1 million (20%) of estimated 5 million small scale farming households gain main income from the production of maize.
Generally, it is often necessary to soak maize kernel in water for a period of time before further processing takes place [3] to enhance processing operations such as milling, fermentation and to reduce cooking time as well as achieving desired palatability. Thus, absorption of water by agricultural materials is of practical importance to maize-based processing industries [4].
Demirhan et al. [5] [6] and Hsu et al. [7] gave different factors influencing amount of water absorbed by seed during soaking as the kernel size and variety, soaking time, temperature, initial moisture content and acidity level of the water.
Shafaei and Masoumi [8] supported the assertion by indicating that temperature and soaking time largely influence the rate at which seeds absorb water.
From engineering perspective, maize processors are not only interested in determining water absorption capacity and rates, but also the effects of processing parameters such as temperature [4] [9] and also how soaking time can be pre-

Preparation of Samples
Three samples of the maize hybrids (Abotem, Abeleehi and Dorke SR) were obtained from the Crop Research Institute's (CRI) experimental farm situated at Fumesua, Kumasi. The samples collected were graded and only good kernels were used for the experiment. Each sample was packed separately in an air-tight polythene bag, sealed and stored at 4˚C to maintain moisture and prevent recontamination.

Determination of Moisture Content
The initial moisture content of the sampled kernels were estimated using the Standard Oven drying method proposed by [10]. Three samples of 5 g each were measured, placed in dishes and numbered. The dishes containing the samples were heated for 4 h at a temperature of 103˚C in the oven. For each sample, the method was repeated three times and the mean value determined.

Determination of Length, Width and Thickness
The principal dimensions of each maize kernels were determined following [11] approach with slight changes. Hundred (100) kernels were randomly selected for each sample and their dimensions taken using a digital micrometer screw gauge. Thus, the major diameter was taken as length, the intermediate diameter as width, and the minor diameter as thickness of the kernel. Length was taken on 100 kernels and width and thickness on 50 kernels as specified by [12].

Determination of Equivalent Radius (r)
The equivalent radius for each variety was measured based on the assumption that the volume of the maize kernel could be approximated to the volume of a sphere with radius being half of the diameter of the kernel. Fifty kernels of each variety were immersed in the water and volume of water displaced was recorded.
The average volume obtained was equated to the volume of a spherical object [13] and r in mm calculated from Thus,

Soaking Experiment
Water absorption behaviour of the hybrids was evaluated essentially by immersing the kernels into water at four different temperatures of 30˚C, 40˚C, 50˚C, and 60˚C in a thermostatic water bath. Three replicates of sample size 5 ± 0.02 g each was separately placed in nylon mosquito net, tied and labelled. The mosquito net containing the samples were separately placed into the bath and removed at predetermined time interval of 1 h. The moisture content (Mc) of the kernels at each interval, was determined from the difference between the initial weight and the final weight. The moisture content at saturation point (SMC) was calculated when there was no significant change between the initial weight and the final weight.

Hydration Kinetics
At constant temperature, the diffusion process was assumed to follow the Fick's second law of diffusion, where Fick's three-dimensional equation given by: where, M represent instantaneous Mc at time t and D f the diffusion coefficient. A solution proposed for a spherical shaped object with radius r presented by [13] and [14], as where, MR represent moisture ratio, M i and M e being the initial and the equilibrium moisture content respectively, i the number of terms in the summation, Di, the effective water diffusion coefficient, r is the radius of the kernels and t the soaking time.
The problem was then formulated as a root finding equation: Activation energies (Ea) of the hybrids were evaluated using an Arrheniustype equation [15]: where:

Physical Properties
The physical properties, initial moisture contents of the maize hybrids investigated are presented in Table 1.
Generally, the values presented in Table 1 agree with those reported by [16].
From Table 1, the surface (SA) varied from 521.34 mm 2 (Dorke SR) to 537.62 mm 2 (Abeleehi). In general, for different cultivars of a cereal, one will expect an inverse relationship between kernel size and rate of water absorption, since a large kernel produce smaller surface area per unit mass [7].

Saturation Moisture Content (SMC) and Diffusion Coefficient (D f )
Water absorption curves of dry kernels at four soaking temperatures are presented in Figures 1(a)-(c).  Characteristically, the samples showed a normal moisture absorption behaviour. The initial water absorption was rapid, and was followed by a slower rate before it asymptotically reached the SMC at later stage. The SMC was reached at 12 h for water temperature of 30˚C but the soaking duration was reduced to 8 h when the temperature was raised to 60˚C as shown in Table 2.
The water absorption rate of the kernels increased as soaking temperature increased. A. Addo et al. [4] generated similar curves for two varieties of maize-Obatanpa and Mamaba. Studies by [13] [17] and [18] documented similar curves when they investigated the soaking behaviour of red kidney beans, wheat, selected Turkish legumes, and barley respectively.
As the soaking temperature increased, water absorption capacity also increased for all the hybrids although the extent and rates of increment were not the same. This could be attributed to differences in chemical composition of the hybrids used. This assertion is in line with reports by other researchers such as [19] and [20] after they studies on water absorption behaviours of rice. Dorke SR recorded a relatively higher moisture gain at most of the temperatures. This may be due to higher carbohydrate as reported by [4]. Abotem recorded the lowest moisture gain at 30˚C and 40˚C whiles Abeleehi recorded the lowest moisture gain at temperature 60˚C. The rate of water uptake was influenced by temperature. The greater the soaking temperature, the higher the absorption rate due to made similar observation when they studied water absorption behaviours of chickpea and soybean.
J. Tang et al. [24] indicated that small kernels have larger water absorption capacity because of an increased specific surface area available for absorption.
This phenomenon was exhibited in the present study.
Dorke SR, with a smaller kernel size compared to Abeleehi, and Abotem had a higher absorption capacity of water as expected. On the contrary, Abeleehi kernel is relatively larger and should have had lower water absorption capacity, but had the highest water absorption capacity at 40˚C. The current observation may be attributed to nutrient composition differences and the severity of dis-orderness resulting from differences in levels of temperature treatment. The present observation suggest that Abeleehi may possibly have higher starch content than the other two maize hybrids. A. Addo et al. [4] documented similar pattern when they reported that higher starch content in Obatanpa was responsible for its higher water absorption capacity although Obatanpa kernel is bigger than Mamaba. This observation confirms earlier report by [25] who indicated that temperature and grain characteristics are the major factors influencing SMC values.

Water Absorption Rates of Selected Maise Hybrids
The generated data on water soaking of the maize hybrids were plotted against the square root of soaking time. Based on Equation (4), the effective water diffusivities of the hybrids during soaking were evaluated.
The SMCs used in Equation (4) and the evaluated values of D f at various temperatures are presented in Table 2. Generally, the experimental data for the grain component was able to fit the diffusion equation with high accuracy. The analysis of the linear regression is also presented in Table 3 [4] for Mamaba and Obatanpa varieties are lower than values obtained in this study. Kernel's physical characteristics, internal structure, physiochemical and nutrient composition may have accounted for the differences. This observation confirms earlier findings of other researchers such as [4] and [26]. In addition, earlier report by [27] indicated that physiochemical and nutritional components as well as amylose content play key role in predicting rate of moisture gain in rice.
The rates of water absorption in Abotem, Abeleehi, and, Dorke SR are illustrated in Figures 2(a)-(c).
The figures indicate that increasing soaking temperature produces a higher water absorption rate. This phenomenon may probably be due to the increased temperature resulting in more open structures which favoures rapid hydration. Also, the figures show the existence of linear relationship between moisture increase and the square root of time (Time 1/2 ) during water absorption. This observation confirms earlier report by [28] that the rate of water absorption is a function of time and temperature.

Activation Energies (Ea) of Maize Hybrids
The D f of Abotem, Abeleehi and Dorke SR hybrids were fitted to an Arrhenius-type equation (Equation (5)). The relationship between D f and reciprocal of the temperatures are presented in Figure 3. Figure 3 illustrated that Arrhenius-type equation sufficiently described the temperature effect on water absorption during soaking of Abotem, Abeleehi, and Dorke SR. From Table 3, the Ea values as estimated from the curves, for Abotem, Abeleehi, and Dorke SR were 9.09; 8.27 and 7.50 kJ/mol respectively. The results show that the Ea of Dorke SR was relatively smaller. This may probably be due to small surface area of kernel. The higher Ea value recorded by Abotem gave an indication that temperature influence on Abotem was greater as higher Ea correspond to greater influence of temperature.
The Ea values reported in current study were similar to the one published by [4] for Obatanpa (6.54 kJ/mol) and Mamaba (6.82 kJ/mol). The value obtained for Dorke SR (7.60 kJ/mol) is also similar to the results documented by [29] for Asontem (7.27 kJ/mol); Hewale (7.26 kJ/mol); and Asomdwee (6.26 kJ/mol) of cowpea varieties. In contrast, Ea values observed in this study were significantly lower than other reported values. Verma and Prasad [9] and O. Bolaji et al. [30] reported Ea range of 33.5 -41.56 kJ/mol and 31.49 -96.06 kJ/mol for certain varieties of maize respectively.

Conclusion
The water absorption characteristics of maize hybrids, Abotem, Abeleehi and Dorke SR were investigated using models derived from Fick's second law of diffusion. The time of reaching SMC during soaking was reduced from 12 h to 8 h as the soaking temperature increased from 30˚C to 60˚C. Experimental data fitted into Fick's law of diffusion satisfactorily and was able to simulate and predict water absorption during soaking of Abotem, Abeleehi, and Dorke SR hybrids. Open Journal of Applied Sciences The values of D f for Abotem, Abeleehi, and Dorke SR ranged from (2.54 -3.49) × 10 −10 m 2 /s; (2.64 -3.59) × 10 −10 m 2 /s and (3.21 -4.2) × 10 −10 m 2 /s respectively.
The effect of temperature on D f was sufficiently described by Arrhenius-type equation with Ea values of 9.09 kJ/mol; 8.27 kJ/mol and 7.50 kJ/mol for Abotem, and Dorke SR respectively. Temperature had greater influence on Abotem variety as compared to Dorke SR and Abeleehi. Surface area and Temperature played a key role in water absorption rate of the cereals. Data presented in the current study on effective water diffusivity for the selected hybrids can help in better understanding and designing of equipment and sorption process