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Water absorption characteristics of paddy, brown riceand husk during soaking

Abhay Kr. Thakur, A.K. Gupta *

Department of Processing and Food Engineering, Punjab Agricultural University, Ludhiana, Punjab State 141 004, India

Abstract

Water absorption characteristics of paddy, brown rice and husk were measured at three temperatures ranging from 30 to 60 C.From the water absorption characteristics curve, it was observed that the husk was a significant barrier in the water absorption pro-cess by brown rice. Using the measured moisture data, a non-linear regression method was applied to an approximate solution of thediffusion equation MR = A1exp(kt) for an infinite cylinder shape. The geometrical shape factor was estimated using the value ofconstant A1 and the characteristics length. The predicted value of moisture content at any time was in good agreement with theobserved data. The mean values determined for the diffusion coefficients were 4.91 1011 m2/s for paddy, 9.56 1011 m2/s forbrown rice and 1.16 1008 m2/s for husk. Analysis of variance showed that soaking temperatures did not have significant effecton diffusion coefficients. An Arrhenius-type equation was used to relate the diffusion coefficient of paddy, brown rice and huskto absolute temperature (K) and the energy of activation was estimated. The values determined were 31.50 kJ/mole for paddy,37.32 kJ/mole for brown rice and 19.25 kJ/mole for husk.

Keywords: Paddy; Brown rice; Water absorption; Non-linear regression; Diffusion coefficient; Activation energy

1. Introduction

Rice, the most widely grown food grain crop, servesas the staple food for about half of the population inworld. The rice crop forms the basic economic activitydirectly or indirectly for about 150 million rural house-holds in India (Krishnaiah & Janaiah, 2000). After har-vesting, paddy (rough rice) normally goes through two

moisture treatments; one is drying that may be requiredfor safe storage, and the other is water absorption inpreparation for further processing. Soaking of paddyis one of the important activities in the processing linefor parboiled, puffed and flaked rice. Soaking causesextensive quantitative (leachate loss, kernel bursting)

and qualitative (colour, smell) changes (Pillaiyar,1988). Each variety of paddy has its own optimal soak-ing time. Freshly harvested paddy absorbs water at alower rate than stored grain (Bhattacharya & SubbaRao, 1966). Though differences in moisture uptake areseen initially in paddy having different initial moisturecontent, it levels off as the soaking progresses to practi-cally the same level, irrespective of initial moisture con-

tent (Ali & Ojha, 1976).Extensive research work has been done on modeling

diffusion of moisture during drying of different grains.Research work on water-vapour transport mechanismhas also been conducted for various grains. Water-vapour adsorption by dry corn and rice in jute andwoven polypropylene bags was studied by Gurinto,Haque, and Chung (1991). Diffusion of moisture is gen-erally enhanced by the temperature of the fluid mediumand it has an exponential relationship with the inverse

mailto:akgupta_pau@rediffmail.com

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of the fluid temperature (Muthukumarappan &Ganesekaran, 1990; Steffe & Singh, 1980; Walton, White,& Ross, 1988). Thin layer moisture adsorption data were

measured for barley at different temperatures and rela-tive humidities and fitted to Pages model (Basunia &Abe, 2003). The model gave a very good fit for the mois-ture content with an average standard error of 0.176%dry-basis. Studies conducted on soaking behaviour ofthree varieties (IR-8, Patni-23 and Sitsal) of paddyand reported that Beckers model fits best (Ali, 1974).Lu, Siebernmorgen, and Archer (1994) conducted soak-ing tests on Newbonnet long-grain rough rice and foundthat Beckers model fits best to the experimental data.Water absorption characteristics of wheat and barleyduring soaking were measured at five temperatures rang-

ing from 10 to 50 C and it was reported on the basis ofthe water absorption curve that water absorption was insecond falling rate period (Tagawa et al., 2003). Moisturecontent distribution within a wheat kernel was predictedfrom a finite element diffusion model, and moisture diffu-sion coefficients for wheat during isothermal moisturesoaking were determined (Kang & Delwiche, 1999). Pad-dy grain is composed of two major physical compo-nentsbrown rice and husk with a bran layer inbetween the two. This gives paddy grain a more complexstructure than wheat. The objective of the experimentwas to study the liquid water absorption characteristicsof paddy, brown rice and husk and determine the diffu-sivity of liquid water transport at different soaking tem-peratures as well as their required energy of activation.

2. Materials and methods

The paddy (cultivar PR116) used in this study washarvested in November, 2003 at experimental farm ofthe College of Agricultural Engineering, Punjab Agri-cultural University, Ludhiana. It was packed in poly-propylene bags and stored in an air tight metallicstore-room for about eight months. Before experiments,

the paddy samples were taken out, cleaned thoroughlyand defective grains removed. Only sound grains wereused for the experiment. The initial moisture content

was determined by the static air-oven drying methodat 120 2 C for 24 h. A part of paddy samples were de-husked in a rubber roll laboratory sheller (Satake Eng.Co., Japan) to obtain brown rice and husk. The brokengrains and dust were separated from the rice and husksamples using a standard mesh. The initial moisture ofthe samples was 13.614% (w.b.).

Physical characteristics like 1000 grain weight, graindimensions and bulk density of paddy and brown ricewere determined using normal suggested methods(AACC, 1976). Linear dimensions and weight of grainswere measured with digital vernier calipers having a res-

olution of 0.01 mm (Mitutoyo Corporation, Japan) andan Afcoset electronic balance (FX-400) having a resolu-tion of 0.001 g, respectively. Geometric mean diameter,sphericity and surface area were measured as suggestedin McCabe and Smith (1984) for non-spherical particles.Density was determined by the toluene displacementmethod. Porosity percentages were calculated from thedifference between density and bulk density and dividedby density. The soaking experiments for paddy, rice andhusk were conducted at 30 1 (normal temperature ofwater); 45 1 and 60 1 C water temperatures. Sam-ples of 20 g paddy and rice and 5 g husk were placedseparately in fiber glass nets (screen size 16 18) andplaced into separate glass beakers containing potablewater which were already preset to the desired tempera-ture in a water bath. During soaking experiments, thesamples were removed at predetermined time intervals(initially 30 min and then 1 h) and the soaked samples(without net) were quickly blotted with paper towels45 times to remove residual surface moisture (Luet al., 1994) and then reweighed. The increase in samplemass during soaking in water was considered to be anincrease in sample moisture content. Another set of sam-ples were kept in water for 50 h in order to find the sat-uration moisture content (Ms).

Nomenclature

A1 constant of an approximate solution of diffu-sion equation (decimal)

D diffusion coefficient (m2/s)

D0 diffusion constant (m

2

/s)Dp geometric mean diameter (mm)Ea activation energy (kJ/mole)k coefficient of water absorption rate (h1)l characteristic length (radius of infinite cylin-

der, m)M moisture content at any time during soaking

(% d.b.)

M0 initial moisture content (% d.b.)Ms moisture content at saturation (% d.b.)MR moisture ratio (decimal)

R gas constant (kJ/mole/K)s surface area (m2)T soak-water temperature (K)t soaking time (h)v volume (m3)k characteristics value/ sphericity

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2.1. Application of diffusion equation

The infinite series diffusion equation (Crank, 1975)was used to model the water absorption process by pad-dy components

MR Ms M=Ms M0 X

1

i1Ai expDk2i t 1

where MR = moisture ratio, dimensionless, M0 = initialmoisture content, % d.b., M= moisture content at anytime of soaking, Ms = saturation moisture content, %d.b., Ai = constant for a given solid shape.

For an infinite cylinder,

Ai 4=k2il

2

where l= radius of infinite cylinder, and ki = geometri-cal shape factor.

The infinite series of the right hand side of Eq. (1)converges rapidly to the first term when the Fouriernumber (F0 = Dt/l

2) becomes large (Tagawa et al.,2003) and it will give

M Ms Ms M0A1 expDk21t 2

where Dk21 k is defined as the coefficient of waterabsorption rate. Thus the appropriate model could beestimated from the experimental values of A1 that wereevaluated by applying a non-linear regression procedure(SPSS 8.0) to Eq. (2). The diffusion coefficients for graincomponents (paddy, brown rice and husk) were calcu-

lated after fitting the absorption data to Eq. (2) and esti-mation of A1 and k (water absorption rate constant,h1).

3. Results and discussion

3.1. Physical characteristics

The variety PR116 (semi dwarf) has long slender,clear translucent grains with good cooking quality.Observations on length, width, thickness, geometricmean diameter, sphericity and surface area (average of50 sound kernels), 1000 kernel weight, bulk density, den-sity and porosity of paddy and brown rice kernel aregiven in Table 1. The ratio of brown rice and paddywas found to be 0.78. The bulk density and density ofpaddy were 29.87% and 13.20% less, respectively, thanbrown rice kernel. The low density of paddy is due tothe presence of husk. This was also resulted in bulk pad-dy being more porous than rice. Bhattacharya, Sow-bhagya, and Indudhara Swamy (1972) reported highervalues of porosity for paddy than for brown rice andmilled rice. The average thickness of husk was observedto be 0.12 mm.

3.2. Soaking characteristics

The relationship between moisture gain by the paddy,brown rice and husk samples and duration of soaking atdifferent temperatures is shown in Figs. 13. The rate of

moisture migration was dependent on the temperatureof soaking. The higher the soaking temperature,the higher was the rate of moisture absorption. Atthe beginning of the soaking period, a higher rate of

Table 1Physical characteristics of paddy and brown rice of cultivar PR 116

Parameter Value (mean SD)

Paddy Brown rice

1. Length (L), mm 9.127 0.464 7.011 0.2732. Width (W), mm 2.364 0.093 2.173 0.072

3. Thickness (T), mm 1.884 0.065 1.732 0.0704. Sphericity, / = [(L W T)1/3]/L 0.377 0.011 0.425 0.0095. Surface area, s pD2p /, mm

2 13.998 0.741 11.834 0.7686. 1000 grain weight, g 25.424 0.244 19.875 0.0827. Bulk density, g/ml 0.601 0.025 0.857 0.0158. Density, g/ml 1.157 0.005 1.333 0.0099. Porosity, % 48.055 1.219 35.731 0.005

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12

Time, h

Moisturecontent,%d.b

30C45C

60C

Predicted

Fig. 1. Moisture gain by paddy at different soaking temperatures:observed and predicted values (Eq. (2)).

0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7

Time, h

Moisturecontent,%d

.b

30C

45C

60C

Predicted

Fig. 2. Moisture gain by brown rice at different soaking temperatures:observed and predicted values (Eq. (2)).

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absorption was observed in the case of brown rice andhusk compared to paddy over the entire temperaturerange. The results show that the husk layer provides

an initial barrier to rapid absorption of water.

3.3. Fitting of model equation

In order to fit experimental data of paddy and brownrice to Eq. (2), the value ofl was set by considering halfof the perimeter divided by p as the effective radius ofthe nominal cylinder. The estimated values of parame-ters of Eq. (2) for paddy and brown rice obtained bynon-linear regression analysis are given in Table 2.The value ofk (coefficient of water absorption, h1) be-comes larger as the soaking water temperature rises

from ambient temperature to 60 C.The average value for A1 for paddy and brown rice

found after regression analysis of the observed soakingdata were 0.917 and 0.942 with standard deviations of0.0495 and 0.0335, respectively, which on further calcu-lation gives values ofk1l as 2.09 and 2.06 which are lessthan the standard table value of 2.4048 (Tagawa et al.,2003). The increasing value of A1 is explained by thechange in the dimensional shape factor (k1) with increasein soaking water temperatures (Table 2). It was reported

in earlier studies that the starch molecules swell duringthe process of liquid water soaking.

Using the value of characteristics length (l) =

1.352 mm for paddy and 1.243 mm for brown rice(Table 1), the shape characteristics (k1) value were eval-uated and the diffusion coefficients (D, m2/s) at differentsoaking temperatures were calculated (Table 3). A simi-lar procedure was applied for water absorption processby the husk. The shape of the husk was regarded as asheet which a thickness of 0.12 mm. The parameters ofEq. (2) for rice husk are also given in Tables 2 and 3.The comparison of observed and calculated moistureratio for paddy, brown rice and husk was in good agree-ment (Figs. 13).

The values of diffusion coefficients for paddy, brownrice and husk (Table 3) as obtained lead to an Arrhenius

relationship of the type

D D0 expEa=RT 3

where D = diffusion coefficient (m2/s), D0 = constant(m2/s), Ea = activation energy (kJ/mole), R = gas con-stant (8.314 J/mole/K) and T= absolute temperature.The parameters (D0) and activation energy (Ea) forpaddy, brown rice and husk are presented in Table 3.The average value of diffusion coefficient (D) for paddy,brown rice and husk were obtained as 4.91 1011,

Table 2Estimated values of parameters of Eq. (2) for paddy, brown rice andhusk

Component Ms Temperature(C)

A1 k (h1) r2 MSE

Paddy 44.2 30 0.860 0.210 0.951 0.003545 0.949 0.352 0.984 0.0015

60 0.942 0.662 0.982 0.0017

Brown rice 45.2 30 0.908 0.399 0.947 0.003745 0.943 1.005 0.972 0.002460 0.975 1.399 0.986 0.0014

Husk 107.0 30 0.990 2.042 0.990 0.001745 0.992 2.259 0.991 0.001960 0.999 4.049 0.999 0.0001

Table 3Diffusion coefficient and energy of activation for paddy, brown rice and husk

Components Temperature (C) Diffusion coefficientD (m2/s)

Activation energyEa (kJ/mole)

Relationship r2

Paddy 30 2.56E11 31.50 D = 6.72E06exp(31,500/RT) 0.9945 4.24E1160 7.92E11

Brown rice 30 3.89E11 37.32 D = 1.15E04exp(37,320/RT) 0.9545 1.02E1060 1.46E10

Husk 30 8.42E09 19.25 D = 1.62E05exp(19,250/RT) 0.8445 9.34E0960 1.69E08

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3

Time, h

Moisturecontent,%d.b

30C

45C60C

Predicted

Fig. 3. Moisture gain by husk at different soaking temperatures:observed and predicted values (Eq. (2)).

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9.57 1011 and 1.16 1008 m2/s, respectively. Theeffective diffusion coefficient of brown rice is approxi-mately double the value of diffusion coefficient of paddyand 8 103 times the diffusion coefficient of husk. Thisshowed that the husk layer was a significant barrier inthe process of water absorption by paddy grain. Theaverage moisture diffusivity of paddy (cultivar Parijata)at an initial moisture content 20.2% d.b. and in the soak-ing temperature range of 4060 C was reported as3.48 1012 m2/s (Mohanty, Bal, Das, & Panda,

2002). It was also reported that the average mass diffu-sivity of milled rice decreased from 1.78 1010 to8.33 1011 m2/s as the moisture content increased from13% to 50% (Zhang, Bakshi, Gustafson, & Lund, 1984)during finite element analysis of the milled rice soakingprocess. On the basis of a three dimensional transmis-sion line matrix, the average moisture diffusivities forendosperm and bran were found to be 1.09 1010

and 0.22 1010 m2/s, respectively, during soaking oflong-grain brown rice at 30, 40 and 50 C (Hendrickx,Lauwerens, & Tobback, 1988).

The values obtained for effective diffusion coefficient

for paddy, brown rice and husk at different soak watertemperatures were statistically analyzed to see the signif-icance of the rice components and different soaking tem-peratures. The results of the analysis of variance(ANOVA) at a 1.0% confidence level are given in Table4. Results indicated that the values of diffusion coeffi-cient obtained for components i.e., paddy, brown riceand husk at different soaking temperature were non-sig-nificant (p < 0.01). However, the mean value of the dif-fusion coefficient for paddy, brown rice and husk differsignificantly at the 1% level of confidence.

The value of activation energy was maximum(37.32 kJ/mole) in brown rice and minimum (19.25kJ/mole) in husk. The low value of activation energy(31.50 kJ/mole) in the case of paddy as compared tobrown rice (37.32 kJ/mole) could explain the effect ofhusk on the water absorption process. Soaking of paddyand brown rice showed that it is the husk layer that pro-vided the primary barrier to rapid absorption of mois-ture. The characteristics of husk like thickness, porespace etc., may be important factors in water diffusivityof paddy in addition to other varietals parameterskernel size, shape etc. It was also suggested in earlierstudies that if the hulls of the paddy in bulk grain couldbe slightly opened by some kind of mechanical manipu-

lation, the soaking steps required in various processingoperations could be very much simplified.

4. Conclusions

The water absorption characteristics of paddy, brownrice and husk (cultivar PR116) were studied duringsoaking at three temperature levels (30, 45 and 60 C)and modeled with a solution of the diffusion equationto develop guidelines for various pre-processing opera-tions. The rate of moisture gain increased with increasein soaking temperatures and it was in the second fallingrate period. For modeling the water absorption process,an infinite cylindrical grain shape was considered with acharacteristic length of half of the perimeter divided byp. The shape characteristic value (k) was evaluated usingthe constant (A1) of the diffusion equation as obtainedthrough non-linear regression. The comparison of mea-sured data with the calculated value from the diffusionmodel was in good agreement. The soaking temperaturedid not show a significant effect (p < 0.01) on diffusioncoefficients of paddy, brown rice and husk. However,the difference in effective diffusivity of paddy, brown riceand husk were statistically found significant. The diffu-sion coefficient of brown rice was approximately twotimes greater than the paddy and 8000 times less thanthat of husk. The diffusion coefficients for paddy, brownrice and husk at different temperatures followed anArrhenius-type relation and the energy of activationwas estimated using parameter the Ea/R of the

Arrhenius equation. The activation energy of brown ricewas approximately 1.2 times higher than the activationenergy of paddy and 1.9 times that of the husk. Huskimparted effective resistance to water absorption processby paddy grain.

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Table 4ANOVA for diffusion coefficient of paddy, rice and husk at differenttemperatures

Source of variance SS df MS Fcal. Fcrit.

Temperatures 1.49E17 2 7.45E18 1.048 17.999Components 2.64E16 2 1.32E16 18.554 17.999Error 2.84E17 4 7.1E18

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