Comparative Study Between Hot Air and Infrared Drying Of

COMPARATIVE STUDY BETWEEN HOT AIR ANDINFRARED DRYING OF PARBOILED RICE: KINETICSAND QUALITIES ASPECTSO. BUALUANG1, Y. TIRAWANICHAKUL2 and S. TIRAWANICHAKUL1,3,*

1Department of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hatyai 90112, Songkhla, Thailand2Plasma and Energy Technology Research Laboratory, Department of Physics, Faculty of Sciences, Prince of Songkla University, Hatyai 90112,Songkhla, Thailand

3Corresponding author.TEL: +6674287306;FAX: +6674558833;EMAIL: [email protected]

*Present address: Department of ChemicalEngineering, Faculty of Engineering, PO Box15, Hatyai 90112, Songkhla, Thailand.

Accepted for Publication June 18, 2012

doi:10.1111/j.1745-4549.2012.00813.x

ABSTRACT

The objectives of this study were to analyze the effects of drying employing threedifferent heat sources on drying kinetics and to evaluate qualities of parboiled riceafter drying. Drying temperature was varied between 60 and 100C. Power of infra-red (IR) heat source was fixed at 1,000 and 1,500 W and air flow rate was fixed at1.0 � 0.2 m/s. The three drying strategies composed of hot air (HA), IR and com-bined HA + IR drying. The experimental results were simulated using variousequilibrium moisture content models and the mathematical drying model for pre-diction of drying kinetics and evaluation of effective diffusion coefficient (Deff)followed by Fick’s law of diffusion. The results revealed that Deff values of HA andIR drying were in the range of 10-12–10-11 m2/s and relatively depended on tem-perature. For quality evaluation, conclusions reached that head rice yield usingHA + IR drying had the highest value, while yellowness and whiteness of the par-boiled rice are significantly affected by drying condition (P > 0.05).

PRACTICAL APPLICATIONS

These results suggested that combined hot air and infrared (HA + IR) dryingoffers a great potential for preserving Leb Nok Pattani parboiled rice. Dryingkinetics and quality aspects determined that IR drying is more efficient than theother drying method. However, the color degradation in the grain kernel is anissue for quality measurement.

INTRODUCTION

Rough rice or paddy that is subjected to hydrothermal treat-ment prior to milling is defined as parboiled rice. Tradi-tional parboiling involves soaking the paddy in cold water,followed by steaming and drying (Bhattacharya 2004). Par-boiling is practiced in many parts of the world such as Asia,Europe and America (Pillaiyar 1981; Heinemann et al.2005). Moreover, parboiled rice products in Thailand tendto increase, especially in health food and green organicproducts. Parboiled rice has beneficial grain kernel becauseduring the parboiling process, the rice grain kernel changesits physical and physicochemical properties due to rice gela-tinization and leads to a high milling rice yield (Fan et al.1999; Bhattacharya 2004; Tirawanichakul et al. 2004a; Sopo-nronnarit et al. 2005). This is because during the parboiling

process, starch gelatinization takes place – a thermo-physical reaction between the starch granules and heatenergy in the presence of water. The starch gelatinizationbrings about changes in the physicochemical properties ofthe rice (Fan et al. 1999; Islam et al. 2001, 2002). In addi-tion, parboiled rice has lower nutrient loss during millingand cooking corresponding to percentage of ash enrichmentand high content of mineral and nutrient matter comparedwith the milled rice (Rao and Juliano 1970; Wu et al. 2002;Heinemann et al. 2005). In the southern part of Thailandlocated among the Andaman and Pacific oceans, the rain-falls are frequent so losses during postharvesting period arealso high because of the high humidity in the surrounding(Soponronnarit et al. 1998; Sun et al. 2002; Tirawanichakulet al. 2004b; Schluterman and Siebenmorgen 2007). Tostrive for reduced degradation of rice grain and for

Journal of Food Processing and Preservation ISSN 1745-4549

1Journal of Food Processing and Preservation •• (2012) ••–•• © 2012 Wiley Periodicals, Inc.

additional rice grain value, rice parboiling process is thus ofinterest. Leb Nok Pattani, a local medium-grain rice variety,is widely produced in the southern part of Thailand. It iseasily broken when its moisture is reduced through high-temperature drying or sun drying on the floor. The parboil-ing process not only reduces the serious effects of post-harvesting but can also enhance the quality of rice kernels.However, few researches have been made in Southeast Asiancountries for the optimum rice parboiling process. The par-boiling process consists of three steps, namely: soaking,steaming and drying. Some work studied the temperingeffect, which could maintain and improve head rice yield(Poomsa-ad et al. 2002; Tirawanichakul et al. 2004a).Drying is most essential because high moisture content ofsoaked rice (>35% dry-basis) leads to easy degradationbecause of many effects such as infection by microorgan-isms, yellowing by nonenzymatic reaction (Soponronnaritet al. 1998), etc. Drying process is an important processaffecting product quality, and there are many methods toreduce moisture content such as hot air (HA) drying (Ray-aguru et al. 2009), infrared (IR) radiation drying (Delwicheet al. 1996; Das et al. 2004, 2009; Maftoonazad et al. 2009)and microwave (MW) drying (Therdthai and Zhou 2009).The appropriate moisture content of grain kernel for a longshelf life is about 16 � 1.0% dry basis (Quitco 1982; Sopon-ronnarit 1997; Tirawanichakul et al. 2004b; Soponronnaritet al. 2005). Because there is little published literature on IRdrying of parboiled rice (Schroeder and Roseberg 1961; Abeand Afzal 1997; Das et al. 2004), the IR drying for parboiledrice is, thus, an interesting process. This is because the IRheating provides a rapid means of heating and drying.However, the IR drying is attractive only for surface heatingapplications of biomaterial. Application of combined elec-tromagnetic radiation and HA heating is considered to bemore efficient over radiation or HA convective heatingalone as it gives the synergistic effect (Umesh Hebbar et al.2004).

So, the objectives of this research were to investigate theeffects of drying methods: HA convection, IR and combinedHA + IR of Leb Nok Pattani parboiled rice on productkinetics in terms of equilibrium moisture content (EMC)and effective coefficient of diffusion; and qualities in termsof whiteness, yellowness, physicochemical, chemical prop-erty, enthalpy property, morphological structure and textureanalyses.

MATERIALS AND METHODS

Material

Local medium-grain Leb Nok Pattani paddy harvested inApril 2010 was provided from the Rice Research Institute inPhatthalung Province, Thailand. The fresh paddy was

cleaned and soaked in warm water at 70 � 1C for 3 h inorder to obtain fully water-saturated paddy kernels. Then,these soaked paddy kernels were steamed at 100 � 1C for30 min to provide parboiled rice without white belly andgetting high head rice yield (Cnossen et al. 2000, 2003; Sunet al. 2002; Tirawanichakul et al. 2004a). Finally, the par-boiled rough rice was dried with three drying strategies (HAconvection, IR and combined HA + IR heating).

Experimental Procedure

Drying of soaked paddy was carried out in tray dryer with0.55 ¥ 0.55 ¥ 0.50 m3 dimensions. The drying chamber wasmade of stainless steel and the drying chamber walls wereinsulated by microfiber flat sheet of 5 cm thickness. Thesoaked rice sample was placed in a perforated tray, whichwas made of stainless steel. This tray drying system con-sisted of 5 main parts as follows: electric heating unit, fourelectric infrared heating units (500 W ¥ 4), centrifugalblower driven by a 1.5-HP motor, air-recycled duct, dryingchamber and a proportional plus integral plus derivativeaction temperature-controlling unit (TF3-10, Keyence Co.Ltd., Osaka, Japan). The rice samples with a weight of 600 gwere uniformly provided on the perforated tray for eachexperiment; thus, the sample bed depth was about of0.50 cm. The drying temperature, grain temperature, dry-and wet-bulb temperatures, were continuously measured bythe K-type thermocouple wires connected to the data loggerwith an accuracy of �0.5C (Model FX100, Yogokawa,Tokyo, Japan). Sample weights were taken at regular intervalthroughout the drying time using an electronic balance(Model GF 3000, A&D, Tokyo, Japan) with an accuracy of0.001 g. The schematic diagram of tray drying system isshown in Fig. 1. The experiments were carried out underthe conditions of drying temperature ranging from 60 to100C, inlet air flow rate of 1.0 � 0.2 m/s and infrared powerof 1,000 and 1,500 W. The moisture contents of the ricesamples were evaluated by following the standard Associa-tion of Official Agricultural Chemists (AOAC) method(AOAC 2007). The average initial moisture content of thesamples was in the range of 54 � 1% dry-basis and thepaddy samples until it reached the final moisture content at22 � 1% dry-basis. The parboiled rice samples were takenout the dryer and were then ventilated by aeration until themoisture content of the paddy was about 16 � 1% dry-basis for prolonging shelf life (Soponronnarit 1997,Tirawanichakul et al. 2004a).

Determination of Equilibrium MoistureContent (EMC)

In this study, EMC values were evaluated at the controlledair temperature ranging from 40 to 65C correspondingto the relative humidity (RH) of 11–87% using static

HOT AIR AND INFRARED DRYING OF PARBOILED RICE O. BUALUANG, Y. TIRAWANICHAKUL and S. TIRAWANICHAKUL

2 Journal of Food Processing and Preservation •• (2012) ••–•• © 2012 Wiley Periodicals, Inc.

gravimetric method. The parboiled paddy was put in sealedglass bottles with five different saturated salt solutions com-prising of LiCl, KNO3, NaCl, Mg(NO3)2•6H2O, andMgCl2•6H2O. After a few days, the samples were weigheddaily until the sample and saturated salt solution was in theequilibrate state. This state was acknowledged when con-secutive weight measurement differences are lower than0.001 g. Then the moisture contents of the parboiled ricesamples were determined according to the AOAC standardmethod (AOAC 2007) The EMC values of the sampleswere calculated by means of duplication. The experimentalEMC values were simulated using the five selected EMCmodels. These five EMC models are Brunauer-Emmett-Teller (BET) (Brunauer et al. 1938), Guggenheim-Anderson-de Boer (GAB), Henderson (Henderson 1952;Chen 1990), Halsey and Oswin models as shown in Table 1.The EMC value was a function of temperature and RH. The

parameters of these models were determined using nonlin-ear regression analysis. Criteria for determination of thebest fitting EMC model was determined by the best value ofcoefficient of determination (R2) and the lowest root meansquare error (RMSE) value. Then the EMC model would beused for calculation of EMC value for moisture ratio in thenext section.

Mathematic Drying Modeling and EffectiveDiffusion Coefficient (Deff)

The evolution of moisture transfer can be explained by theFick’s law of diffusion. Description the transport of waterinside the sample surface in terms of diffusivity with theassumption that the water moves out in the directions ofradial and axial coordinates and parboiled paddy is an iso-tropic solid, which means that its structure is in a form offinite cylindrical shape. Additionally, the moisture is trans-ferred by liquid diffusion and the shrinkage is negligibleduring the drying. The partial differential equation of mois-ture diffusion for a single grain kernel, which is consideredgeometrically as a finite cylindrical shape, can be written asshown in Eq. (1).

∂∂

= ∂∂

+ ⎛⎝

⎞⎠

∂∂

+ ∂∂

⎡⎣⎢

⎤⎦⎥

M

tD

M

r r

M

r

M

zeff

2

2

2

2

1 (1)

where Deff is the effective moisture diffusion coefficientaccounting for the heterogeneous solid in the m2/s; M ismoisture content of material in % dry-basis; t is drying timein seconds; r is the radius of the cylinder material in meters;and z is height of the cylindrical material in meters.

When the soaked paddy with high moisture content isdried at the beginning, the concentration of water existingat the grain surface is suddenly evaporated and conse-quently closed to the moisture equilibrium with the dryingair. However, the moisture content inside the grain kernel isthe most difficult region to be reduced because the complexmorphology of kernel limits its mobility, so that the waterconcentration at the rice center has the highest valuethroughout drying period. The initial and boundary condi-tions for paddy drying are given by:

FIG. 1. TRAY DRYER FOR PARBOILED PADDYDRYING

TABLE 1. MATHEMATICAL MODEL FOR PREDICTING EQUILIBRIUMMOISTURE CONTENT OF PARBOILED RICE

Model Model equation References

BET M

MT

C RH

RH C RHeq

m

=

⎛⎝⎜

⎞⎠⎟ × ×

−( ) × + −( ) ×[ ]1 1 1Brunauer

et al.(1976)

GAB MM

CT

k RH

k RH k RHCT

k RHeq

m

=× ⎛

⎝⎜⎞⎠⎟ × ×

− ×( ) × − × + ⎛⎝⎜

⎞⎠⎟ × ×⎡

⎣⎢⎤⎦⎥

1 1Chen

(1990)

Henderson MRH

A B T

C

eq = −( )− × +( )

⎡⎣⎢

⎤⎦⎥

ln 11

Chen(1990)

Halsey MA TRH

B

eq = − ×( )

⎡⎣⎢

⎤⎦⎥ln

1

Halsey(1948)

Oswin M A B TRH

RH

C

eq = + ×( ) ×−

⎛⎝⎜

⎞⎠⎟1

1

Oswin(1967)

Note: Meq is equilibrium moisture content; A, B, C, k and Mm is modelparameter; T is temperature in K; RH is relative humidity in decimal.BET, Brunauer-Emmett-Teller; GAB, Guggenheim-Anderson-de Boer.

O. BUALUANG, Y. TIRAWANICHAKUL and S. TIRAWANICHAKUL HOT AIR AND INFRARED DRYING OF PARBOILED RICE


Initial condition:

t r r M M= ≤ ≤ =0 0, ,0 in

Boundary condition:

− ≤ ≤ =l z M M1, in

t r r M M> = =0 0, eq

z M M= ± =1 eq

t rM

r> = ∂

∂=0 0 0,

where l is half length of finite cylinder in meters; M is mois-ture content at any time t in decimal; Min is initial moisturecontent in decimal; Meq is equilibrium moisture content indecimal; r is co-ordinate along the radius of cylinder inmeters; r0 is the radius of the cylinder in meters; t is timein seconds; and z is the length of cylinder in meters. Theanalytical solution for the moisture inside a single kernel isexpressed by the following equation:

MR r l tM r l t M

M M

JJ

r

r

, ,, ,

( )e

( ) = ( ) −−

= ⎛⎝⎜

⎞⎠⎟

eq

in eq

m mm

4 22

10

0π λ λλ xxp

cos

exp

−⎛⎝⎜

⎞⎠⎟

× −( )+

+( )

−

=

∞

=

∞

∑

∑

λ

π

m eff2

02

1

0

1

2 1

2 1

2

D t

r

n

n z

l

m

n

n

ππ2 2

2

2 1

4

n D t

l

+( )⎛⎝⎜

⎞⎠⎟

eff

(2)

By the integration of Eq. (2) over the volume of cylinderand dividing by its total volume, the average moisturecontent can be expressed as:

MRM M

M M

D t

r

n

m

=−−

= ⎛⎝

⎞⎠

−⎛⎝⎜

⎞⎠⎟

+

=

∞

∑eq

in eq m

m eff8 4

1

2 1

2 2

2

02

1π λλ

exp

(( )− +( )⎛

⎝⎜⎞⎠⎟=

∞

∑ 2

2 2

20

2 1exp

π n D t

Ln

eff

(3)

Eq. (3) can be expressed by the following equation (Crank1975; Soponronnarit 1997; Tirawanichakul et al. 2004b),when Deff is the diffusion coefficient in m2s-1, l is grain halflength in meters; L is grain length in meters (0.00804 m); r0

is the radius of parboiled rice (0.00141 m for Leb Nok

Pattani); t is drying time in seconds; lm is the mth root ofBessel (l1 = 2.4048, l2 = 5.5201, l3 = 8.6537, l4 = 11.7915and l5 = 14.9309). The temperature dependence of theeffective diffusivity may be described by an Arrhenius-typerelationship as follows in Eq. (4)

D DE

RTeff = −⎛

⎝⎞⎠0 exp a

(4)

where D0 is the pre-exponential factor of the Arrheniusequation as following in unit of m2/s; Ea is the activationenergy in kJ/mol; R is the universal gas constant inkJ/mol K; and T is the absolute temperature in K.

The parameters of these models (Deff, D0 and Ea) wereevaluated using a nonlinear regression analysis. Statisticalprogram was developed by following the Quasi-Newtonestimation method and was carried out for best relationshipbetween the moisture ratios and drying time. Finally, suit-ability of equations was evaluated and compared using R2

and RMSE, which indicate the fitting ability of a model to adata set for selecting the best equation to describe theexperimental data. Following equations of these two param-eters were written in Eqs. (5) and (6), respectively:

RX X

X X

i

N

i

N2 1

1

1= −−( )−( )

=

=

∑∑

exp pre

expexp

(5)

RMSEN

X Xi

N

= −( )=∑1

1

exp pre (6)

where Xexp is the measured in the experiment; Xpre the calcu-lated using the models; and Xexp is defined as the mean ofmeasured experiment data and N the number of datapoints.

Quality Analysis

Head Rice Yield (HRY). Determination of the HRY valuewas performed according to the procedure set by the RiceResearch Institute, Phatthalung Province, Thailand. TheHRY value is usually expressed as a weight percentage ofwhole and broken white rice kernels that are longer thanthree-fourths of the rice kernel’s length. The HRY value wascalculated by dividing the head rice weight by the initialrough rice weight. This value was determined in duplicates.

Color Determination. The surface color of the driedparboiled rice was measured using a CIE Lab colorimeter(Chromameter model CR-300, Osaka, Japan). Measurementwas based on the CIE Lab system with color values of



brightness (L*), redness value (a*) and yellowness (b*)value. However, the yellowness value of parboiled ricemostly was recognized for evaluation its quality.

Whiteness of Parboiled Rice. The whiteness of milledrice samples was measured with a commercial whitenessmeter (Model C-300, Kett Electronic Co. Ltd., Tokyo,Japan). This meter measures the whiteness of rice kernels aswhiteness (W) in the linear range of 0–100, where 0 corre-sponds to perfect black surface and the 100 corresponds tothe whiteness of magnesium oxide fumes. The equipmentwas calibrated with the provided ceramic plate having awhiteness value of 85.8.

Physicochemical Property. The rice kernel sampleswere used for determination of physicochemical properties,namely, gel consistency (GC), alkali spreading (AS) valueand water absorption. The GC and AS values were first rec-ommended by Little et al. (1958) and Cagampang et al.(1973) also reported in their work as well as some relatedworks (Tirawanichakul 2004; Shubhneet et al. 2011). Waterabsorption was determined following Juliano (1985) andsome previous works (Tirawanichakul et al. 2004b; Tomo-chika et al. 2008). The AS value is an inverse indicator forevaluation the gelatinization temperature (GT) of milledrice flour and is used in some previous related works(Cnossen et al. 2000; Peisong et al. 2004). A 7-point scaleused in AS has been reported since 1985, and many previousresearches have followed on this scale (Juliano 1985; Khatunet al. 2003; Tirawanichakul et al. 2004b; Shubhneet et al.2011). AS value corresponded to GT as follows: 1–2 definedas high (74.5–80C); 3 defined as high-intermediate; 4–5defined as intermediate (70–74C); and 6–7 defined as low(<70C) GT. Additionally, GT is an important property ofrice because it is one of the features most closely related tothe overall rice cooking behaviors and to the texture ofcooked products. During rice cooking, starch granules takeup water and swell. Over a critical temperature range, theyundergo an irreversible process known as gelatinization,which is characterized by crystalline melting (loss of bire-fringence) and starch solubilization. Physicochemical prop-erties, in the terms of GC and water absorption, imply therice eating quality. The GC value measures the tendency ofcooked rice to harden when it cools down. Normally, thetotal acceptability of panelists prefer soft cooked rice, whichis correlated to higher GC value (> 60 mm of gel length),compared to hard cooked rice with low GC value (< 60 mmof gel length) (Quito 1982; Tirawanichakul et al. 2004a). Inaddition, the AS values were conventionally suggested byLittle et al. (1958), while this method was reported in manyprevious works (Juliano 1971; Cagampang et al. 1973;Tirawanichakul 2004; Tirawanichakul et al. 2004a).

Chemical Property. In this present study, chemical prop-erty in terms of amylose content (AMC), protein content(PC) and lipid content (LC) was determined. The evalua-tion of AMC was followed using AOAC standard method(AOAC 2007), which is so-called iodine colorimetricmethod. The PC was determined using Micro Kjeldahlapparatus and the LC was also determined according toAOAC (2007)).

Cooking Time. Cooking time is the time duration topartial starch gelatinization of 90% of the total starchkernels based on visual observation. By the cooking timetesting, 10 g of mature rice grain kernels were taken andwere boiled in 250 mL of distilled water. After a cookingperiod of 20 min, the rice sample was taken for visual obser-vation and followed up in every 2 min until the end of thecooked cycle. After that, the 10 grain kernels were removedfrom the water and placed over a Petri dish and compressedwith a spatula in order to visually observe and count thegrains that no longer had the opaque core (fully gelatinizedkernels). The same procedure was repeated every minuteuntil all the 10 kernels reached complete gelatinization fortwo successive cooking times.

Thermal Property. Thermal property of the rice flourwas determined using differential scanning calorimeter(DSC) (Model 821, Mettler-Toledo, Melbourne, Australia).The starch gelatinization brings about changes in the physi-cochemical properties of rice (Kimura et al. 1995; Islamet al. 2001, 2002; Miah et al. 2002). Rice flour weight of3.50 � 0.10 mg dry matter was put into a 15 mL aluminumpan and then distilled water was added by a micro-syringe.The aluminum pan was sealed hermetically. Each sealed panwas allowed to equilibrate the sample at room temperaturefor 1 h before scanning. Thermographs were obtained from30 to 120C at a scanning rate of 1C/min while an emptyaluminum pan of the same weight was used as a referencesample. The beginning and endpoints of the gelatinizationendotherm were obtained from the intersections of the twotangents around the inflection point (Sekine et al. 2000).The enthalpy (DH) was estimated by integrating the areabetween the thermogram and a baseline connecting thebeginning and endpoints of melting under the peak, andwas expressed in mJ per unit weight of the dry matter (mJ/mg). The degree of gelatinization of hydrothermally treatedrice was also calculated by the following equation (Marshallet al. 1993; Islam et al. 2002; Tirawanichakul 2004).

Degree of gelatinization parboiled rice

raw rice

%( ) = −⎡

⎣⎢

⎤

⎦⎥1

ΔΔH

H(7)

where DHparboiled rice is the enthalpy of parboiled rice sampleand DHraw rice is the enthalpy of untreated rice.



Morphological Study. Morphological study of the driedparboiled rice kernels were characterized with a scanningelectron microscope (Model JSM-5800 LV, JEOL, Bangkok,Thailand) at 10–20 kV. To prepare the sample for scanning,the parboiled rice kernels were broken out in half. Then, thehalf rice kernel samples were glued on metal cylinder stubsand the stubs with samples were coated with gold (~30 nmthick) by DC plasma sputtering to act as electrical conduc-tor for electron attachment during scanning.

Texture Analysis of Cooked Rice. Texture analysis interms of hardness, stickiness and adhesiveness of cookedparboiled rice were determined by a bench-top texture ana-lyzer model TA-XT2i (Stable Micro Systems Ltd., New York,NY). In an aluminum cylindrical cup, 30 g of each milledhead rice sample was placed. The sample was cooked withdistilled water at rice-to-water weight ratio of 1:2. The com-pression probe was set at the pretest speed, test speed andpost-test speed at 1.5, 0.5 and 10 mm/s, respectively. Themaximum force required for compressing cooked rice to90% of the initial height of 20 mm is an indication of thehardness of the cooked rice. The hardness, stickiness andadhesiveness value was presented by means of five replica-tions (in kg).

Specific Energy Consumption (SEC)

SEC was defined as the energy required for removing a unitmass of water in drying the parboiled rice from its initial

moisture content of 54 � 1% dry-basis to the final moisturecontent of 22 � 1% dry-basis. The SEC was calculated asfollow:

SECP

M M W=

−( )3 6.

in f d(8)

All experimental results were shown in average values byusing one-way analysis of variance (P < 0.05) while themodel suitability was determined by the highest value of theR2 and the lowest RMSE value.

RESULTS AND DISCUSSIONS

Determination of EMC

Comparisons of EMC models with the experimental dataare shown in Fig. 2. Results showed that for the temperaturerange of 40–65C and RH from 11 to 87%, the GAB’s modelyielded the highest correlation coefficient and the lowestvalue of least RMSE.

Hence, the EMC of the GAB’s model (see in Fig. 3) wasfound to be the best fitting model for predicting EMCexperimented values, as shown:

FIG. 2. EXPERIMENTAL AND PREDICTED DATA OF EQUILIBRIUMMOISTURE CONTENT FOR LEB NOK PATTANI PARBOILED PADDYWITH SURROUNDING RELATIVE HUMIDITY OF 11–87% ANDTEMPERATURE OF 40CBET, Brunauer-Emmett-Teller; GAB, Guggenheim-Anderson-de Boer.

FIG. 3. THE BEST FITTING MODEL OF EQUILIBRIUM MOISTURECONTENT FOR LEB NOK PATTANI PARBOILED PADDY WITHSURROUNDING RELATIVE HUMIDITY OF 11–87% AND TEMPERATUREOF 40–65CGAB, Guggenheim-Anderson-de Boer.



M

RH

TRH

RHRH

T

eq =

×⎛⎝

⎞⎠

− ×( )

× − × + ×⎛⎝

⎞⎠

11 804

1 00 828

1 0 828210 786

.

.

..⎡⎡

⎣⎢⎤⎦⎥

(9)

where the GAB’s constant value are Mm = 0.0557;C = 254.572; k = 0.828; and R2 = 0.928.

Deff and Mathematical Model

Deff curves evaluated according to Fick’s diffusion law(Crank 1975; Soponronnarit 1997; Tirawanichakul 2004)were illustrated in Fig. 4. The results showed that the Deff ofIR drying was slightly high compared with drying withcombined HA + IR and HA drying, respectively. This isbecause infrared waves can penetrate into the interior of thefood, where it is converted to thermal energy, providing arapid heating mechanism. The combined convective andinfrared drying gives shorter drying time because of thehigher heat and mass transfer coefficients compared withthe HA drying. Increase of IR heat flux results in shorterdrying time corresponding to the previous work (Mongpra-neet et al. 2002; Jaturonglumlert and Kiatsiriroat 2010). Deff

equations for individual drying strategies were evaluatedand presented in Table 2. The Deff of parboiled drying was inthe ranges of 7.89 ¥ 10-12 to 4.40 ¥ 10-11 m2/s. As expected,

the drying rate relatively depended on drying temperature.Moreover, the temperature gradient between grain kernelsand drying temperature affected to the drying rate of mois-ture transfer. The Deff has been found to increase when IRintensity increase corresponding to the previous works(Tirawanichakul et al. 2008a; Das et al. 2009).

Rice Qualities

Head Rice Yield (HRY). The HRY of control and par-boiled rice under different drying conditions is shown inTable 3. The HRY of reference sample was 50.0% while theHRY of control rice sample was 63.6%. As the results ofdried parboiled rice with five drying strategies, they showedthat the HRY value of the parboiled sample was maintainedand improved over the reference rice sample about 13.6%.This is because the high moisture content and high tem-perature from steam condensation on the grain surface inthe soaking and steaming processes yielded the occurrenceof partial gelatinization in rice starch. The result showedthat the degree of gelatinization of paddy was related to themoisture content and drying temperature corresponding tothe previous works (Rao and Juliano 1970; Poomsa-ad et al.2002; Taechapairoj et al. 2003; Tirawanichakul et al. 2004a;Soponronnarit et al. 2005; Swasdisevi et al. 2010). Addition-ally, because of gelatinization occurrence, the HRY was rela-tively high compared with the control sample (about of3.4–4.0%). explanted that in the gelatinization steps, whenhigh moist paddy is heated up and gain temperature reachesa GT between 65 and 70C, its starch cell swell with loss bire-fringence. This causes protein degradation, which will thenpenetrate into the void spaces among the starch granules,resulting in the reduction of fissures within the grainkernels (Atwell et al. 1988; Tirawanichakul et al. 2004a).

FIG. 4. EFFECTIVE DIFFUSION COEFFICIENT OF PARBOILED PADDYWITH VARIOUS DRYING METHODHA, hot air; IR, infrared.

TABLE 2. EFFECTIVE DIFFUSION COEFFICIENT EQUATIONS OFDIFFERENT DRYING STRATEGIES

Drying methodEffective diffusionequation R2 RMSE

HA Deff = 3.25 ¥ 10-8 exp(-23,032.10/RT)

0.9979 0.0296

IR 1,000 W Deff = 2.49 ¥ 10-7 exp(-26,816.54/RT)

0.8259 0.4417

IR 1,500 W Deff = 2.63 ¥ 10-7 exp(-26,947.60/RT)

0.9048 0.0187

HA + IR 1,000 W Deff = 6.69 ¥ 10-8 exp(-23,477.86/RT)

0.9995 0.0187

HA + IR 1,500 W Deff = 1.26 ¥ 10-7 exp(-24,923.39/RT)

0.9853 0.1209

Note: Deff is effective diffusion coefficient in m2/s; R is the universal gasconstant in kJ/mol K; T is drying temperature in K.HA, hot air; IR, infrared-radiation.



Comparative studying of the HRY value in drying strategies,It showed that HRY value of parboiled rice with HA andcombined HA + IR method was slightly higher than IRdrying. This may be because at the beginning of drying thefast drying rate of IR method causes high stress in grainkernel and takes more cracking in rice grain kernel.

Whiteness and Yellowness. The whiteness (W) andyellowness (CIE Lab b* value) of parboiled rice dried withtemperatures of 60–100C were evaluated and presented inTable 3. The change in the color of parboiled rice has amore serious effect as compared with the reference rice. Themain cause of the change in the color is during the soakingand steaming step, the bran covered around the kernel isdissolved and then absorbed by the endosperm. On theother hand, the drying temperature is also an importantfactor influencing the degree of whiteness and yellowness;the whiteness decreases while the yellowness increases withthe increasing drying temperature (Tirawanichakul et al.2004a,b; Swasdisevi et al. 2010). The whiteness of thesamples which was dried with IR is lower than it was driedwith the other sources. This is because IR can penetrate intothe rice kernel, which was heated up to a high temperature,thus more browning reaction occurs. This is in accordancewith previous works (Rao and Juliano 1970; Pillaiyar 1981;Quitco 1982; Soponronnarit et al. 2005). Yellowness valuesobtained from all drying strategies showed a pale yellow

appearance corresponding to b* value (CIE lab) in the rangebetween 18.5 and 19.6. For whiteness, it was in the rangebetween 17.7 and 23.3.

Physicochemical Property. Samples were determinedthree physicochemical properties, namely, GC, AS value andwater absorption. Physicochemical properties of the par-boiled rice are shown in Table 4. AS is an inverse indicator,which was used to evaluate the GT of milled rice starchgranules (Cnossen et al. 2000). Table 4 shows that the AS ofparboiled rice was lower than that of the reference rice, so,the parboiled rice had higher GT and required longercooking time. AS of parboiled rice was in the range of 3.0–4.0 and can be evaluated that GT would be over 74C andcooking time would be longer than 24 min. As for the refer-ence rice, AS was 4.8, implying a GT in the range of 70–74Cand cooking time in the range of 20–24 min. GC of par-boiled rice in the range of 98.5–84.3 mm decreased withincreasing drying temperature. This demonstrated that therice gel got harder, although all drying conditions producedsoft cooked rice. Water absorptions of grain kernels werenot significantly different. So, drying temperature anddrying strategy were not important factors influencing theAS, GC and water absorption of dried parboiled rice amongdrying temperature of 60–100C.

Chemical Property. Table 5 illustrates the amount ofchemical quality in terms of AMC, PC and LC of parboiled

TABLE 3. WHITENESS, YELLOWNESS AND HEAD RICE YIELD OF DRIED PARBOILED RICE

Drying methodDryingtemperature (C)

Dryingtime (min)

Dryingrate (kg/h)

Min (Mf) (%dry-basis) Whiteness

Yellowness

HRY (%)CIE Labb value

Reference rice – – 16.0 (16.0) 54.7a 12.3d 50.0e

Control rice – – 55.7 (16.1) 24.8b 18.7b 63.6d

HA 61.1 87 0.07 54.0 (22.0) 22.4c 18.2c 69.8b

78.6 54 0.11 54.0 (23.0) 19.6d 18.8b 69.6b

100 39 0.16 54.0 (22.1) 19.9d 19.8a 70.5a

IR 1,000 W 61.3 54 0.11 54.0 (23.7) 19.6c 19.2a 69.2b

80.9 30 0.21 54.0 (23.1) 18.6d 19.6a 63.7d

95.8 24 0.26 54.0 (22.5) 17.8e 19.2a 66.7c

IR 1,500 W 62.9 43 0.12 54.0 (23.1) 21.3c 18.9b 67.5c

78.4 27 0.24 54.0 (23.1) 19.0d 18.8b 67.3c

96.8 18 0.30 54.0 (23.1) 17.7e 19.5a 67.2c

HA+IR 1,000 W 61.3 54 0.11 54.0 (21.5) 23.2c 18.5b 68.6c

84.3 30 0.21 54.0 (22.3) 20.5d 19.1a 70.4a

99.7 24 0.26 54.0 (23.1) 19.4e 19.2a 70.6a

HA+IR 1,500 W 60.0 48 0.15 54.0 (22.0) 23.3c 18.7b 70.6a

78.1 27 0.24 54.0 (22.4) 21.2d 18.8a 67.0c

98.8 21 0.35 54.0 (22.4) 20.0e 19.0a 70.0a

Note: Min is initial moisture content (%dry-basis); Mf is final moisture content (%dry-basis); control rice is parboiled rice which was dried in ambientair; reference rice is raw rice which was not parboiled rice. In a column, means with common superscript letter(s) are not significantly different atP < 0.05 level based on analysis of variance test.HA, hot air; HRY, head rice yield; IR, infrared-radiation.



rice are shown in. It can be seen that chemical properties ofrice are not affected by drying temperature and dryingstrategy. LC, PC and AMC of parboiled rice are shown inTable 5. The hydrothermal process increased the LC of ref-erence rice because lipid from rice bran dissolved and thendiffused in to the rice kernel. PC did not alter between thereference rice and the parboiled rice at any drying condi-tion. It has been reported for rice that, parboiling decreasesthe extractability of the proteins (Devi et al. 1997). Thechanges in the composition of the constituent fractions maybe due to denaturing of protein bodies and possible forma-tion of protein-tarch and protein–polyphenols complexes.The hydrothermal treatment changed the amount ofamylose value of the parboiled rice. The AMC of the refer-ence rice decreased from 25.18 to 22.41–24.12 g/100 g com-pared with dried parboiled rice. These changes in thecomposition of amylose may be due to the retrogradation ofthe soluble component of the amylose (Dharmaraj andMalleshi 2011). These results correspond to the previouswork (Ali and Bhattacharya 1972). In addition, the increasein drying temperature yielded a slightly apparent AMCdecrease. Because of the part of amylose in starch exists in abound form with polar lipids, either as inclusion com-pounds or linkages through hydrogen bonding. Further-more, the dissociation and reaggregation of amylose chains

result in the alteration in the molecular conformation,leading to enhancement of the helical form and therebylowering the solubility of amylose (Raja and Sindhu 2000).

Thermal Property. Thermal properties of rice flour werestudied using a DSC to determine the GT of rice and thepercentage of gelatinization. Table 6 shows the degree ofgelatinization of rice obtained from DSC. It was found thatthe onset temperature (To), peak temperature (Tp) and con-clusion temperature (Tc) in gelatinization process of par-boiled rice were higher than the reference rice. The DHvalue of the reference rice was higher than that of the par-boiled rice. It was indicated that the partial gelatinizationoccurred in the parboiled rice. It was found that in theresults, the parboiled rice had a GT of 79C and cookingtime of 27 min while the reference rice was respectively 72Cand 23 min. This result is in line with the AS result. Thehigher GT is due to retrogradation of parboiled rice causingreassociation of starch molecules. Consequently, the reasso-ciated starch structure is resistant to water penetration andthus required more energy for water absorption during the

TABLE 4. ALKALI SPREADING, GEL CONSISTENCY, WATERABSORPTION AND COOKING TIME OF DRIED PARBOILED RICE

Drying source

Dryingtemperature(C) AS

GC(mm)

Waterabsorption(kg water/kg flour)

Cookingtime(min)

Reference rice 4.8a 100.0a 3.64a 23b

Control rice 3.3c 98.5b 2.80d 27a

HA 61.1 3.8b 92.4c 3.37b 29a

78.6 3.5c 91.3c 3.22b 27a

100 4.0b 94.2c 3.12c 28a

IR 1,000 W 61.3 3.5c 97.5c 3.30b 28a

80.9 4.0b 88.3d 3.34b 27a

95.8 3.0d 86.5d 3.24b 28a

IR 1,500 W 62.9 3.8b 97.3b 2.90c 27a

78.4 3.6b 98.0b 2.79d 29a

96.8 3.3c 84.3c 2.74d 28a

HA+IR 1,000 W 61.3 3.0c 98.5b 2.82d 28a

84.3 3.5b 94.6b 3.11c 27a

99.7 3.3b 86.1c 2.91d 28a

HA+IR 1,500 W 60.0 3.3c 95.0c 2.64d 27a

78.1 4.0b 92.1c 3.08c 27a

98.8 3.0c 87.3d 2.82d 28a

Note: AS is alkali spreading value; GC is gel consistency in mm;Control rice is parboiled rice which was dried in ambient air; Referencerice is raw rice which was not parboiled rice; in a column, means withcommon superscript letter(s) are not significantly different at P < 0.05level based on ANOVA test.AS, alkali spreading; HA, hot air; IR, infrared-radiation.

TABLE 5. LIPID CONTENT, PROTEIN CONTENT AND AMYLOSECONTET OF PARBOILED RICE DRIED WITH DIFFERENTDRYING STRATEGIES

Condition LC (%) PC (%) AMC (%)

Reference rice 1.05b 6.02b 25.18a

Control rice 1.40a 6.00b 23.35bc

HA61.1 1.10ab 6.12b 23.54b

78.6 1.40a 5.98c 22.87c

100.0 1.20c 6.20a 23.90b

IR 1,000 W61.3 1.10a 6.21a 24.01ab

80.9 1.30ab 5.98c 23.65ab

95.8 1.20ab 6.06b 22.94b

IR 1,500 W62.9 1.30b 5.98b 24.12b

78.4 1.20c 6.12b 23.21ab

96.8 1.40ab 6.44a 23.97ab

HA+IR 1,000 W61.3 1.36ab 6.10b 22.87b

84.3 1.50a 5.87c 23.53b

99.7 1.32b 6.17b 23.68b

HA+IR 1,500 W60.0 1.15b 6.12b 22.41c

78.1 1.35c 6.07b 23.73b

98.8 1.15a 6.33a 23.50b

Note: LC is lipid content in %; PC is protein content in %; AMC isamylose content in %; control rice is parboiled rice which was dried inambient air; reference rice is raw rice, which was not parboiled rice; ina column, means with common superscript letter(s) are not significantlydifferent at P < 0.05 level based on analysis of variance test.HA, hot air; IR, infrared-radiation.



heating period in the DSC test corresponding to the previ-ous work (Soponronnarit et al. 2005; Swasdisevi et al. 2010;Tananuwong and Malila 2011).

Morphological Study by SEM. The microstructure ofstarch granules is shown in Fig. 5. Microstructure of refer-ence rice is shown in Fig. 5a, where starch granules can beseen to spread throughout the cross sectional surface. Theparticle size of rice starch granules of reference rice wasapproximately 3–5 mm which is pentagonal shape asreported by the previous work (Singh et al. 2003). The pres-ence of air spaces and single granule, rather than compoundamyloplast, as well as disorganised cellular structure, offersthe opportunity for fast diffusion of water in the referencegrains during cooking and causes a decrease in cooking

time (Lisle et al. 2000).It can be seen that the starch struc-ture did not occur in control and parboiled samples. Incontrol, or parboiled rice dried in ambient air, the starchgranules absorbed surrounding moisture, swelled, and gela-tinized, leading to losses in their granular appearance. Therice kernels were thus observed to have smoother appear-ance as depicted in Fig. 5b–g. It was indicated that thedegree of gelatinization increased when the samples weresoaked at 70 � 1C for 3 h and steamed at 100 � 1Cfor 30 min. It was observed that the starch granules weredisappeared at this condition.

SEC. The SEC (energy required for drying 1 kg of par-boiled paddy) was found to decrease with drying tempera-ture increase at all drying method. Results are shown in

TABLE 6. GELATINIZATION TEMPERATUREAND ENTHALPY ENERGY OF REFERENCE ANDCONTROL RICECondition

Gelatinization temperature (C)Enthalpyenergy (J/g)

Degree ofgelatinization (%)To Tp Tc

Reference rice 71.61 76.33 80.97 9.03 –Parboiled rice 79.32 84.17 88.40 1.96 78.29

To, Tp, Tc is onset temperature, peak gelatinization temperature and conclusion temperature in C,respectively; Parboiled rice is the parboiled samples were dried in ambient air; reference rice isfresh rice.

cba

fe

g

d

FIG. 5. MORPHOLOGY OF REFERENCE ANDPARBOILED RICE UNDER DIFFERENT DRYINGCONDITION(a) reference rice; (b) control rice; (c) HA100.0C; (d) IR 1,000 W, 95.8C; (e) IR1,500 W, 96.8C; (f) HA + IR 1,000 W, 99.7C;and (g) HA + IR 1,500 W, 98.8C. HA, hot air;IR, infrared.



Fig. 6. Higher drying temperature is associated with shorterdrying time. Drying time depends slightly on drying airtemperature, but is highly dependent on energy source.Results from the three drying conditions indicated that,under drying temperatures ranging of 60–100C, IR dryingat power inputs of either 1,000 W or 1,500 W renderedlower SEC than HA+IR and HA dryings. This is becauseinfrared waves can penetrate into the interior of the par-boiled rice, where it is converted to thermal energy to assista more rapid heating mechanism. The higher IR powerinput gave a slightly higher SEC value than the lower. Infra-red intensity increasing yielded slightly decrease in SEC thisis because increased power of IR lamps increases emissionintensity and drying occurs in a shorter period. Thoughneeding least energy compared with other means, IR dryingproduced undesirable results regarding coloration of theproduct. Minimum value of SEC observed at 60C of HAdrying was 187.60 MJ/kg while the maximum value wasobserved at 95.8C of IR 1,500 W was 10.59 MJ/kg. The SECof IR 1,500 W drying decreased 0.99–1.31 times comparedwith IR 1,000 W and SEC of HA+IR 1,500 W dryingdecreased 1.03–1.06 times compared with HA + IR 1,500 Wdrying. While, the SEC value of HA + IR 1,500 W and HAdrying decreased 4.56–5.56 and 9.50–13.18 times comparedwith IR 1,500 W drying, respectively. This result was similarto previous works (Das et al. 2004; Sharma et al. 2005;Tirawanichakul et al. 2008b; Motevali et al. 2011).

Texture. Results of the hardness and stickiness of the par-boiled rice were compared with reference rice in Table 7. As

shown, the hardness of control rice and parboiled rice driedunder different drying strategies were significantly higherthan that of the reference rice. One reason is that partialgelatinization in the parboiled rice had led to an increase instrength of the rice. Hardness of the parboiled rice slightlyincreased with an increase in drying temperature wherehigher degree of gelatinization occurred. In all drying strat-egies, hardness of the parboiled rice did not differ much.

Stickiness and adhesiveness of the parboiled rice were sig-nificantly lower than that of the reference rice because ofthe occurred partial gelatinization. Nevertheless, dryingtemperature and drying strategy did not significantly affectthese two properties.

CONCLUSIONS

The parboiling process in this experiment consisted of aseries of activity. i.e., 70C water soaking for 3 h, ambient airtempering for 24 h and steaming for 30 min. The GAB’smodel was the best fitting to the experimental results. Theproducts were then dried under three drying strategies; HA,IR and HA + IR. Drying kinetics of the parboiled rice waswell explained by the diffusion model. Deffs for the parboiled

FIG. 6. SPECIFIC ENERGY CONSUMPTION OF PARBOILED RICEDRYINGHA, hot air; IR, infrared.

TABLE 7. HARDNESS, STICKINESS AND ADHESIVENESS OF PARBOILEDRICE DRIED WITH DIFFERENT DRYING STRATEGIES

ConditionHardness(kg)

Stickiness(kg)

Adhesiveness(kg.s)

Control rice 27.15b -0.193a -0.008a

Reference rice 25.65a -0.881b -0.082b

HA61.1 26.51ab -0.236b -0.012a

78.6 27.09b -0.165ab -0.008a

100.0 27.21b -0.138a -0.006a

IR 1,000 W61.3 28.21cd -0.215a -0.007a

80.9 27.47bc -0.203a -0.003a

95.8 28.72d -0.179a -0.007a

IR 1,500 W62.9 29.55b -0.245b -0.011a

78.4 29.25b -0.195ab -0.009a

96.8 29.29b -0.141a 0.005a

HA+IR 1,000 W61.3 27.72b -0.194a -0.012a

84.3 29.75c -0.166a -0.012a

99.7 29.58c -0.167a -0.015a

HA+IR 1,500 W60.0 26.76b -0.201a -0.008a

78.1 28.15c -0.177a -0.012a

98.8 28.56d -0.175a -0.007a

Note: Control rice is parboiled rice which was dried in ambient air; Ref-erence rice is raw rice which was not parboiled rice; in a column,means with common superscript letter(s) are not significantly differentat P < 0.05 level based on analysis of variance test.HA, hot air; IR, infrared-radiation.



rice were in the range of 7.89 ¥ 10-12 to 4.40 ¥ 10-11 m2/s. Inthe study of the effects of drying strategy and drying tem-perature of dried parboiled rice, it was found that HRYemploying combined HA + IR drying yielded a higher valuethan IR drying, while yellowness and whiteness of the riceare significantly affected by drying temperature and dryingstrategy. Chemical properties, and hence quality, however,were not affected by any means. The results suggested thatcombined HA+IR drying is an efficient method to improveHRY while maintaining acceptable whiteness of the rice.Nevertheless, the combined HA + IR drying process takeslonger operating time and needs a relatively high energyconsumption compared with IR drying. Although dryingunder IR requires the lowest energy consumption, it createsproblem of yellowness.

ACKNOWLEDGMENT

The authors would like to thank the Office of the HigherEducation Commission, Thailand, in providing financialgrant under the program Strategic Scholarships for FrontierResearch Network for the Thai Doctoral degree Ph.D.Program. We would like to thank also the Department ofChemical Engineering, Faculty of Engineering, the Depart-ment of Physics, Faculty of Science and the Graduate Schoolof the Prince of Songkla University for their supports. Inaddition, we appreciate the Phatthalung Rice ResearchCenter and Agricultural and Seafood Product in allowing usto access her laboratory facilities.

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Comparative Study Between Hot Air and Infrared Drying Of

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