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In this study, water absorption characteristics of some rice varieties (Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44) from Nigeria were studied at 30°C, 45°C, 60°C and 75°C by determining the increase in grain weight as a function of time during soaking. Differences in moisture content among the selected varieties of paddy during soaking were significant (P < 0.05) at all temperatures considered. Using the experimental moisture data, a non-linear regression procedure was applied to an analytical solution of the Fick’s second law of the diffusion for an infinite cylinder. The predicted values of instantaneous moisture contents were in good agreement with the experiential data with R^{2} of (0.834 - 0.997). Water absorption rate was found to increase with soaking temperature, while water saturation time decreased with temperature. Average values of diffusion coefficients of moisture during soaking of paddy rice at different temperatures (30°C, 45°C, 60°C and 75°C) were estimated as 6.25 × 10^{ -11}, 6.28 × 10^{ -11}, 7.02 × 10^{ -11}, and 5.51 × 10^{ -11} and 5.52 × 10^{ -11} m^{2}/s for Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44, respectively. The activation energies of the diffusivity through different varieties of rice grains werecalculated using Arrhenius-type equation for diffusion dependence on temperature and were determined as 41.96, 38.69, 40.16, 34.05 and 42.12 kJ/mole for Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44 for the respectively rice variety above.

Parboiling is a pre-milling hydrothermal (heating and hydration) handling of paddy (rough rice) which brings about substantial physical and chemical alterations in rice. The main purpose of this process is to gelatinize the starch granules, transforming the ordered structure of the starch into a disordered one. Starch gelatinization imparts additional hardness to the rice grains and makes them to withstand harsher milling [

Paddy parboiling process is made up of three stages: soaking of paddy to saturation moisture content (SMC), steam heat treatment of the soaked paddy to gelatinize the rice starch and drying the steamed product to moisture content adequate for milling. Soaking is an essential operation done to achieve fast and uniform distribution of moisture in grains required for efficient gelatinization of starch granules during steaming operation, but it is time-consuming. Complete soaking is attained when there is an optimal absorption of water in the center of the soaked grains and when no white belly is observed in kernels of parboiled paddy. The moisture content corresponding to this condition is the SMC and has been found to be approximately 30% (w.b.) [

Traditionally, soaking of paddy is done by steeping the product in water at ambient temperature, and allowing it to stay for 2 - 3 days before draining the water. Increasing the temperature of soaking water, generally to that of starch gelatinization, reduces the time required for paddy to reach saturation [

Since soaking is an essential operation, understanding the characteristics of water movement during the process is of practical importance. More so it affects quality of the final product. However, each variety paddy has a unique optimal soaking time at different temperatures, that is, the time required to reach saturation. Hydration characteristics of different varieties of paddy have been studied. Bhattacharya and Rao [

In a study by [

There are published data on hydration characteristics of South American and Asian rice grains in the literature [

1) Study hydration characteristics of different varieties of rice paddy (namely Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44) from Nigeria at 30˚C, 45˚C, 60˚C and 75˚C soaking temperatures.

2) Determine moisture diffusivity and activation energy during hydration of the rice paddies.

Samples of, four improved rice varieties (FARO 44, FARO 52, FARO 60, FARO 61) used in this study were harvested in December, 2012 and collected from the Breeding Unit of Rice Research Program, National Cereal Research Institute (NCRI) Badeggi Niger State, Nigeria. One local variety (Bisalayi) was also harvested in the same period and obtained from crop improvement unit of Kano State Agriculture and Rural Development Agency (KNARDA). They were packed in nylon jute bags and transported to the Bioresource Engineering Department, McGill University, Montreal, Canada and stored at room temperature. Prior to experiments, the unparboiled paddy samples were taken out, thoroughly cleaned and extraneous materials and flawed grains were removed. The moisture contents of the samples were determined by the fixed air-oven method drying at 120˚C for 24 h in duplicates and found to be 0.0713 ± 0.0023 g/g d.b.

Physical characteristics of the paddy varieties were determined according to Varnamkhasti et al. [_{G}) measured using a digital vernier caliper to the accuracy of 0.01 mm. Using the different values, the equivalent diameter (D_{p}) in mm considering a prolate spheroid shape for a paddy grain was calculated according to [

D p = ( L ( B + T G ) 2 4 ) 1 3 (1)

where D_{p} is the equivalent diameter, L, the grain length, B, the breath of the grain and T_{G}, the grain thickness

Samples of Paddy (≈10 g) in duplicates were placed in wire mesh and soaked in a 500 ml beaker containing 400 ml of distilled water. Before the run, the beaker was placed in a stirred water bath (T 1404 B Lauda, Lauda Dr. R. Wobser GMBH & Co., Germany) which was set to the desired temperature for the experiment. The temperature of the waterbath was controlled at (±0.2˚C) and the soaking temperatures were 30˚C, 45˚C, 60˚C, and 75˚C. Samples were drawn at different intervals (1, 2, 3, 4, 5, 6, 7, 8 and 9 h) and the soaked samples (without net) was surface-dried with paper towel to remove adhering surface water. The blotting procedure was repeated twice after which samples were reweighed in a balance with 0.0001 g accuracy to determine the increase in sample mass. Moisture content was rechecked by drying soaked sample a fixed air-oven at 105˚C for 24 h.

The hydration kinetics of paddy rices was analyzed according to Fick’s second law of diffusion. To simulate this, the following assumptions were made:

1) The effective diffusivity is independent of moisture content;

2) The volume of grain remains constant during water absorption;

3) The surface of the grain reaches saturation instantaneously upon immersion in water.

The analytical solution to the diffusion equation is an infinite series [

M R = M t − M e M o − M e = ∑ n = 1 ∞ B i exp ( − D e f f λ i 2 t ) (2)

where MR is the moisture ratio, M_{o} is the initial moisture content, M(t) is the instantaneous moisture content M_{e} is the equilibrium moisture content, B_{i} is constant depending on the shape of the product, D_{eff} is the diffusivity (m^{2}/s) and λ is the geometrical shape factor. As time gets larger, the infinite series on the right hand side of Equation (2) converges such that the momentary moisture content can be expressed as:

M ( t ) = M e + ( M o − M e ) B i exp ( − k t ) (3)

where; k = D e f f λ i 2 (4)

To determine the equilibrium moisture content another set of samples were soaked for 30 h at 30˚C, 24 h at 45˚C, 15 h at 60˚C and 10 h at 75˚C. These conditions were selected due to the fact that prolonged soaking in water temperature at 30˚C may result in germination of rice kernels [

Geometrically, rice grain could be considered as a cylinder, the infinite series solution infinite cylinder was evaluated to determine the model that best describes water absorption characteristics of rice.

The correct model could be estimated from the empirical values of B_{1} and this was evaluated by applying a non-linear regression procedure (PROC MODEL) using SAS 9.2 to Equation (3) (known as the Henderson and Pabis diffusion equation). The optimum soaking time was taken as the minimum time required reaching the saturation moisture content.

Temperature dependence of diffusion coefficient is generally described using Arrhenius-type relationship to obtain an agreement of the predicted value with experimental data:

D e f f = D 0 exp ( − E a R T ) (5)

where D_{0} is the pre-exponential factor (m^{2}/s), E_{a} is the activation energy (kJ/mole), R is the gas constant (8.314 J/mole/K) and T is the absolute temperature.

The relationship between moisture content of paddy rice and soaking period at different temperatures is shown in

During soaking up at 60˚C (30˚C, 45˚C, and 60˚C), paddies absorbed water at a slow rate and reached equilibrium. This observation was consistent with finding by other authors during soaking of other rice [

starches of FARO 44, 52 and 60 under the influence of high temperature. The increase in water uptake of FARO 44, 52 and 60 continued without flattening out in the time range of the experiment. A further increase in water uptake was observed accompanied by husk splitting. This observation could be due to the fact that the starch gelatinization temperatures of FARO 44, 52 and 60 fall within the temperature range of 60˚C to 75˚C. Igathinathane et al. [

Furthermore, FARO 61 and Bisalayi were found to have similar water absorption characteristic curves. At 75˚C soaking water temperature, husk splitting was observed after 5 hours of soaking for FARO 61 and Bisalayi. Bhattacharya et al. [

In order to estimate moisture diffusivity during soaking of different varieties of paddy, a non-linear regression procedure was applied to Equation (3) as stated in the material and methods. The estimated values of parameters for different varieties of paddy are given in

The characteristic lengths (radii) of the infinite cylinder model were calculated assuming that the model cylinder has the same volume and length as the rice kernel. The average values of radii for the infinite cylinder were 0.86, 0.87, 0.87, 0.83 and 0.84 mm for Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44, respectively. The average value of B_{1} for all varieties was observed to be 0.944 ± 0.0064 which is close to the theoretical value for diffusion model in infinite cylinder (0.69). Similar observation was made for other paddy rice and was reported to be 0.917 ± 0.0495 [

Varieties | Temperature | B | k | R^{2} | MSE |
---|---|---|---|---|---|

Bisalayi | 30 | 0.881 ± 0.001 | 0.159 ± 0.002 | 0.950 | 0.0005 |

45 | 0.915 ± 0.003 | 0.373 ± 0.011 | 0.944 | 0.0006 | |

60 | 0.969 ±0.013 | 0.629 ± 0.048 | 0.976 | 0.0004 | |

75 | 0.998± 0.002 | 1.464 ± 0.065 | 0.994 | 0.0002 | |

FARO 61 | 30 | 0.898 ± 0.003 | 0.149 ± 0.001 | 0.963 | 0.0004 |

45 | 0.920 ±0.009 | 0.406 ± 0.008 | 0.938 | 0.0007 | |

60 | 0.966 ± 0.010 | 0.583 ± 0.022 | 0.978 | 0.0003 | |

75 | 0.995 ± 0.002 | 1.187 ± 0.040 | 0.992 | 0.0001 | |

FARO 60 | 30 | 0.907 ± 0.0003 | 0.165 ± 0.005 | 0.834 | 0.0003 |

45 | 0.936 ± 0.011 | 0.400 ± 0.009 | 0.931 | 0.0008 | |

60 | 0.977 ± 0.002 | 0.655 ± 0.033 | 0.985 | 0.0003 | |

75 | 0.999 ± 0.0002 | 1.379 ± 0.114 | 0.999 | 0.0000 | |

FARO 52 | 30 | 0.887 ± 0.011 | 0.183 ± 0.006 | 0.951 | 0.0005 |

45 | 0.921 ± 0.002 | 0.418 ± 0.001 | 0.948 | 0.0006 | |

60 | 0.965 ± 0.009 | 0.569 ± 0.044 | 0.978 | 0.0004 | |

75 | 0.999 ± 0.001 | 1.145 ± 0.076 | 0.999 | 0.00002 | |

FARO 44 | 30 | 0.890 ± 0.004 | 0.133± 0.003 | 0.952 | 0.0005 |

45 | 0.904 ± 0.008 | 0.316 ± 0.002 | 0.943 | 0.0007 | |

60 | 0.962 ± 0.002 | 0.587 ± 0.014 | 0.972 | 0.0005 | |

75 | 0.999 ± 0.005 | 1.186 ± 0.089 | 0.997 | 0.0002 |

k = coefficient of water absorption (Equation (4)); B = shape dependent constant (Equation (3)); MSE = mean square error.

The variation of effective diffusivity of paddy rice with temperature and the values of activation energies for different varieties of paddy rice are shown in

The result of analysis of variance (ANOVA) at a 0.5% confidence level during soaking at water temperature of 30˚C and 45˚C shows no statistical difference in Bisalayi, FARO 61, FARO 60, and FARO 52 except for FARO 44. This observation suggests that the husk of FARO 44 provided more barriers to water penetration during soaking at 30˚C and 45˚C water temperatures than the husk of other varieties of paddy used in this study. During higher soak water temperatures (60˚C and 75˚C), Bisalayi and FARO 60 were found to have the highest diffusivity

Variety | D_{0} (m^{2}/s) | Temperature (˚C) | D_{eff} (m^{2}/s) | E_{a} (kJ/mol) | R^{2} |
---|---|---|---|---|---|

Bisalayi | 3.34E−4 | 30 | 7.23E−12 | 44.47 | 0.99 |

45 | 1.76E−11 | ||||

60 | 3.14E−11 | ||||

75 | 7.54E−11 | ||||

FARO 61 | 8.10E−5 | 30 | 1.15E−12 | 40.78 | 0.98 |

45 | 8.64E−12 | ||||

60 | 1.78E−11 | ||||

75 | 7.37E−11 | ||||

FARO 60 | 1.5E−4 | 30 | 7.83E−12 | 42.13 | 0.99 |

45 | 1.96E−11 | ||||

60 | 3.35E−11 | ||||

75 | 7.22E−11 | ||||

FARO 52 | 1.55E−5 | 30 | 7.83E−12 | 36.42 | 0.98 |

45 | 1.86E−11 | ||||

60 | 2.66E−11 | ||||

75 | 5.53E−11 | ||||

FARO 44 | 2.74E−4 | 30 | 5.8E−12 | 44.50 | 0.99 |

45 | 1.4E−11 | ||||

60 | 2.78E−11 | ||||

75 | 5.83E−11 |

D_{o} = pre-exponential factor; D_{eff} = effective moisture diffusivity coefficient; E_{a} = activation energy.

(P < 0.05), while FARO 44 and FARO 52 has the lowest diffusivity values. This could be that in addition to husk, the bran of FARO 44 and FARO 52 further retards water absorption during soaking. Kaptso et al. [_{eff}) against the reciprocal of absolute temperature show a linear relation and are presented in

Hydration characteristics of different varieties of paddy from West Africa (Nigeria) (FARO 61, FARO 60, FARO 52, FARO 44 and Bisalayi) were studied during soaking at low temperature (30˚C and 45˚C) and high temperature (60˚C and 75˚C) to suggest optimum soaking time for different varieties of paddy. Increase in temperature of soaking water increased the rate of water absorption and reduced the time for reaching saturation moisture content for paddy. However, during steeping at water temperature of 75˚C, FARO 44, 52 and 60 absorbed more water than Bisalayi and FARO 61, leading to husk splitting. Soaking in water temperature at 60˚C did not result in excess water absorption nor husk splitting in paddy rice. Optimum soaking time (time to reach saturation moisture content) was observed to be 7 h for Bisalayi, FARO 44 and FARO 52, while for FARO 60 and FARO 61 the values were 6 and 8 h, respectively. The observed excess water absorbed during soaking of FARO 44, FARO 52 and FARO 60 could be attributed to the swelling and gelatinization of their starch.

A suitable prediction of water absorption during soaking of FARO 61, FARO 60, FARO 52, FARO 44 and Bisalayi was possible by fitting experimental data to Fick’s diffusion equation with a high R^{2} value of 0.834 - 0.997. The diffusivity of Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44 was 7.23 × 10^{−12} - 7.54 × 10^{−11} m^{2}/s; 1.15 × 10^{−12} - 7.37 × 10^{−11} m^{2}/s; 7.83 × 10^{−12} - 7.22 × 10^{−11} m^{2}/s; 7.83 × 10^{−12} - 5.53 × 10^{−11} m^{2}/s and 5.80 × 10^{−12} - 5.83 × 10^{−11} m^{2}/s, respectively with temperature range 30˚C - 75˚C.

An Arrhenius type equation described the strong dependence of diffusivity on temperature with activation values of 44.47, 40.78, 42.13, 36.42 and 44.50 kJ/mol for Bisalayi, FARO 61, FARO 60, FARO 52 and FARO 44, respectively.

The authors declare no conflicts of interest regarding the publication of this paper.

Ejebe, C., Kwofie, E.M. and Ngadi, M. (2019) Hydration Characteristics of Selected Varieties of Paddy Rice from Nigeria. Advances in Chemical Engineering and Science, 9, 65-75. https://doi.org/10.4236/aces.2019.91005