Abstract: The steeping process of yellow dent corn kernels was investigated at water temperatures ranging from 30 to 90°C for a period of 24 h. Two mathematical models were fitted to the moisture ratio values of steeping processes. Both models fitted to the experimental data with high degrees of correlations (R2>0.96) between the experimental and predicted moisture ratio values. The water absorption constants calculated for the models decreased with increase in temperature of water. An abrupt drop in the value of the absorption constant occurred when the temperature increased from 60 to 70°C, indicating the significant impact of gelatinization process on moisture absorption rate. Two general models were also developed for the water absorption process before and during gelatinization of the starch. Analysis of the two models indicted that the moisture ratios, at any steeping time, are almost the same for all water absorption processes before or during gelatinization process.
INTRODUCTION
Corn kernels are soaked in water for dry and wet milling process as well as to precook or cook the kernels for subsequent processes. During steeping, the kernel components are softened and the starch granules swell. Swelling is reversible, when steeping is done at low temperatures (below 60°C). When water temperature exceeds about 63°C, the starch granules are distorted causing an irreversible swelling which is known as gelatinization of starch (Sakanidou et al., 2003; Hoseney, 1994).
Water absorption during steeping is generally accelerated by an increase in the temperature of the water (Fan et al., 1963). Some chemical agents, such as sulfur dioxide or lactic acid may be dissolved, to a certain level, in soaking water to enhance moisture diffusion as well as to inhibit bacterial growth (Fan et al., 1965; Haros and Suarez, 1999).
The process of water absorption by corn kernels have been investigated and simulated by many researchers (Karapantsios et al., 2002; Laria et al., 2005; Syarief et al., 1978). In general, the modeling process of water uptake by agricultural products is approached using three general methods: a) theoretical methods which are generally based on principle of moisture diffusion (Muthukumarappan and Gunasekaran, 1994 a-c; Calzetta Resio et al., 2005; Sayar et al., 2001); b) semi-theoretical methods which are a modified form of moisture diffusion (Misra and Brooker, 1980) and c) empirical methods in which a suggested model is fitted to experimental data, i.e., Peleg model (Peleg, 1988; Badau and Jideani, 2005). The theoretical approaches give more information about the nature of moisture migration within the material, but these models usually do not fit well to the experimental data. On the other hand, the semi-theoretical and empirical models yield a better curve fit, but they generally lack a theory for explaining moisture absorption.
With a boost in production level of corn in Iran, the overall uptake of water by the kernel as a function of water temperature and time is of interest to local processors. Since the processes of water absorption below and during gelatinization process are quite different, general models describing the process of water absorption for these processes are useful for monitoring the process. In the literature, precise relations expressing the water uptake by corn kernel at different water temperatures, before and during gelatinization process have not been reported. This study was conducted to: a) investigate the water absorption characteristics of corn kernel at water temperatures ranging from 30 to 90°C; b) develop an appropriate model to predict the water absorption behavior of corn kernel as a function of temperature and c) develop two general moisture absorption models for corn kernel when steeped before or during gelatinization of starch within the kernels.
MATERIALS AND METHODS
Yellow dent corns dedicated for the experiments were harvested in the summer of 2004 in a farm in Kerman, Iran. The kernels were manually removed from the cobs and were spread on a tray at room condition (24°C and 45% RH) for a period of one week to reach a uniform moisture content. Then, the moisture content of the kernels was determined using the ASAE standard S352.2 (ASAE, 1997).
The experimental kernels were visually inspected for relatively uniform weight and sound appearance. Then, samples of corn kernels, about 20 g each, were weighed using an electronic balance with a resolution of 0.001 g. Each sample was placed in an Erlenmeyer flask containing 250 mL of distilled water. The flasks were placed in a stirred water bath controlled within 1°C of the testing temperatures. Individual flasks were removed from the bath at 1, 3, 5, 10, 20, 35, 60, 120, 300, 700, 1150 and 1440 min. The kernels were superficially dried using a paper towel to eliminate surface water. Then, they were weighed and the percentage of moisture content of the sample, M, at each time interval was determined by the following:
| (1) |
where m is the mass of the sample at a specified time interval, mi is the mass of the sample prior to soaking and IM is the initial moisture content of the corn kernels (wb, decimal). Experiments were performed at steeping temperatures of 30, 40, 50, 60, 70, 80 and 90°C and each was repeated three times.
Modeling procedure: Steeping corn can be considered as simultaneous water transfer into the kernels and reaction of starch granules with water. As water reaches the outer layer of starch granules, it simultaneously hydrolyzes the granules and fills the intergranular spaces. The reacted water is retained by starch molecules while moisture diffused proceeds toward the center of the kernel. A widely accepted mechanism for moisture diffusion within agricultural products is Fick’s second law of liquid/vapor diffusion (Crank, 1975; Bakshi and Singh, 1980; Muthukumarappan and Gunasekaran, 1994 a-c) described by:
| (2) |
where M (g) is the moisture, at location of x, y, z within the object at time t (s) and D is the diffusion coefficient (m2 sec-1). When the diffusion coefficient is known, the solution of Eq. 1 can be used to describe the change in moisture content of a point within an object as a function of time. The coefficient of diffusion of a material is not constant throughout a drying process and it depends on the temperature, moisture content and the structural changes of the material. For moisture absorption process, with a suitable set of assumptions for initial and boundary conditions (Muthukumarappan and Gunasekaran, 1994a), a solution for Eq. 2 is presented by:
| (3) |
where k is coefficient of moisture absorption and MR is the instantaneous moisture ratio defined by:
| (4) |
where M is the instantaneous (Eq. 1), Ms is the saturation and M0 is the initial moisture content of the object. Page (1951) added an exponent to time for modeling drying process and which resulted in a superior curve fit. Similarly, including an exponent to the time variable in Eq. 3 results the following equation:
| (5) |
Equation 3 and 5 were used for modeling water absorption by corn kernels at different water temperatures. The parameter k in Eq. 2 and parameters k and N in Eq. 5 were estimated using the PROC NLIN in SAS (SAS Inc. Cary, NC). The saturation moisture contents of the kernels at different temperatures, required for calculating MR values, were determined by keeping the kernels in flasks at the specified temperatures for 36 h. The goodness of fit for the models was evaluated by calculating the coefficient of determination (R2) and the Root Mean Square Error (RMSE) between the experimental and the predicted values of MR, using the following equations:
| (6) |
| (7) |
where N is the number of time intervals in an experiment, p is the number of estimated parameters for a model; MRexp and MRpre are, respectively the experimental and the predicted moisture ratios at a given time interval.
After investigating the goodness of fit values for different temperatures, two general models, one for water absorption before and one for water absorption during gelatinization of the starch within the kernels, were developed. The before gelatinization model was developed using the MR values of the 30, 40 and 50°C treatments. Similarly, the during gelatinization model was developed using the MR values of the 70, 80 and 90°C treatments. The prediction powers of these models were evaluated using the R2 values (Eq. 6) computed for the two models.
RESULTS AND DISCUSSION
Water absorption: The amount of moisture gained by corn kernels at different time/temperature are presented in Table 1. The first row lists the average initial moisture content and the last row presents the average saturation moisture contents of the kernels. The saturation moisture contents of the kernels are largely dependent on water temperature. Before gelatinization occurrence (below 60°C), the average difference in saturation moisture content for each 10°C is about 4% while in transition range (60 to 70°C) and during gelatinization, i.e., 70 to 80°C, a large increase in saturation moisture content is observed. The saturation moisture content increases from 80 to the 90°C treatment, but the increase is not very high indicating that at this stage, both time and temperature govern the moisture diffusion and gelatinization processes.
The trends of moisture absorption by kernels versus time are presented in Fig. 1. The figure indicates that with increased steeping water temperature, the amount of water absorbed by the corn kernels increased. The increase in the rate of water absorption with the increase in water temperature has been noted by many other researchers (Sayar et al., 2001; Bakshi and Singh, 1980).
| Table 1: | The average moisture content (wb) of corn kernels at different steeping time intervals |
| *Saturation time is 1800 min | |
| Fig. 1: | Trends of water absorption by corn kernels at different steeping water temperatures |
| Table 2: | Estimated parameters of the models represented by Eq. 3 and 5 |
| NA: Not Applicable | |
This increase is generally associated with disintegration of chemical bounds, expansion of capillaries and crack development within the kernel component which are accelerated by penetration and rise in temperature of water.
The high rates of water absorption during the initial stages of steeping are generally attributed to the natural capillaries present in the surface of the kernels which can be explained by the diffusion phenomenon. Initially, the rate of water absorption depends on the difference between saturated water and water content of capillaries present on the outer layer of the kernel. When these capillaries are exposed to water, the driving force (moisture gradient and capillary suction) is high and causing rapid increase in moisture absorption by kernels.
Figure 1 shows a gradual decrease in the rate of water absorption from about 20 min up to 300 min. At this stage, as water absorption proceeds, the capillaries are filled, causing a gradual decrease in the rate of water absorption. After 300 min, the curves show a linear trend for water absorption. The uniform slope indicates that after about 5 h of soaking, the process of water absorption is more time dependent. However, for the treatments above 60°C, the slope is slightly steeper indicating that in addition to simple water diffusion, some water is also utilized for gelatinization of starch.
Modeling results: The graphs of moisture ratio versus absorption time and their fitted models are given in Fig. 2. In general, the first model (Eq. 3) showed a good estimation of the absorption process up to 120 min. After this point, the model overestimated the process for all experiments. The overestimation is more visible for the treatments performed at lower temperatures. The second model showed a high degree of fit for all treatments.
The estimated parameters of the models for various treatments and the numerical comparison of their fitting power are presented in Table 2. Both R2 and RMSE values indicate that the second model had a better fit for estimating the water absorption rates by the corn kernels. The R2 values of the first model range from 0.96 to 0.99 while for the second model they are generally above 0.99.
A plot of k values calculated for the two model versus temperature of steeping water is presented in Fig. 3. The k values generally decrease with increase in temperature of steeping water which indicates the dependency of water diffusion on temperature of water. There is a sharp drop in k value when water temperature is increased from 60 to 70°C. This drop is as a result of gelatinization process which occurs around 64°C (Hoseney, 1994) and accelerates the water absorption process. The same observation has been reported by Calzetto Resio et al. (2005) and Sayar et al. (2001).
Since the second model (Eq. 5) resulted a better curve fit for all treatments, this model was used to develop water absorption models for before and during gelatinization of starch within the kernels. The average values of MR for 30, 40 and 50°C treatments were used for the before and the average values of MR for 70, 80 and 90°C treatments were used for the during gelatinization models. The least square curve fitting procedure resulted in the following equations for before and during gelatinization process, respectively:
| (8) |
| (9) |
Both of the above equations fitted to the calculated MR values from experimental data with R2 values greater than 0.98. The high coefficient of determinations for both models indicate that only these two models are sufficient estimating water absorption process before and during gelatinization of starch within the corn kernels. Further more, the high power of the two above models for estimating MRs indicates that even though the amount of moisture gain by kernels steeped at different temperature before or during gelatinization is different at any given time (Fig. 1), but their corresponding MR values can be considered constant.
| Fig. 2: | The calculated Moisture Ratio (MR) values versus absorption time and their corresponding fitted model. a) 30°C, b) 40°C, c) 50°C, d) 60 °C, e) 70°C, f) 80°C and g) 90°C |
| Fig. 3: | The changes in k value of the two models as affected by temperature of steeping water |
Yellow dent corn kernels were steeped in water at different temperatures and the amount of water absorption was measured at different time intervals for 24 h. In general, the moisture content and moisture absorption rate increased with increase in temperature of water. After about 5 h of steeping time, the rate moisture absorption stayed almost constant with time, but the rate was higher for the processes above gelatinization temperature range. The water absorption process was modeled using two different diffusion based models (Eq. 3 and 5). Two general models were also presented, one for water absorption before and one for water absorption during gelatinization of starch. Both models described the water absorptions process with high degree of accuracy. The result of this modeling indicated that MR values at different time intervals are almost constant when steeping is performed at temperatures prior to gelatinization of starch (60°C and below). The same conclusion is applicable to the corn kernels when steeped at above gelatinization temperature.