Proceedmqs of the 7th Iniernoiumal Worktng Conference on Stored-proiuct Protection - Volume 2

Study on rice fissuring during intermittent drying

Y. B. LI1, C. W. Cao2 and J. LI1

Abstract

The fissuring of nee dunng intermittent drying was studiedthrough experimental and numencal methods The moisturedistnbution inside the nee kernel dunng drying andtempering was obtamed by diffusion model. The moisturegradients were used to analyzethe hydro stresses m the neekernel during intermittent dryingDiscontmumg the drying process With tempenng can

decrease the hydro stresses in the nee kernel. Decreasedunit drying time or increased intermittent ratio causeddecrease of the stresses m the rice kernel durmgmtermittent drying Less fissured nee was also observed bydiscontmumg the drying process in the expenments. HIghermterrruttent ratio or lower umt drying time caused lowerpercentage of fissured nee m the expenments

Introduction

Artificial drying of nee has become an accepted practice mmost areas of Chma. But more and more people haverealized that the artificial dried nee has worse taste andhigher breakage dunng mIllmg than tradi banal solar drIedrice. The fissurmg of nce will cause nce gram breakagedurmg mIllmg, more susceptible to insect and mIcrobIalattacks and decreased ratio of seed germmatIon As thecooking quality of broken nce is very poor, the market prIcewIth broken grams IS much lower than that for whole grams(Sarker, 1994).RapId drymg With hIgher air temperature IS preferable m

terms of reducing drymg cost, but it WIll mcrease thepossibIlIty of grain fissunng. The temperature and mOIsturegradient WIthm the gram kernel dunng drymg Will result involumetrIC changes. These non-umform expanSIOn andcontractIOn result m faIlure when the induced stressesexceed the failure strength of the matenal (IrudayaraJ andHaghighl, 1993) Kunze (1972) proposed that the mOIsturegradIents created during the drying perIod proVIde the

IFeed Research InstItute, Chmese Academy of Agncultural ScIences,No 30 Bmshlqlao Road, BellIng, 100081, Chma

2 Chma AgnculturaI Umverslty, Qmghuadonglu, BeIJIng, 100083 ,Chma

potential for later fissunng. When the moisture gradientreclines after drying, moisture from the central portion ofthe gram diffuses to the surface, causing it to expand whilethe mternal portion contracts due to moisture loss As aresult, tensile stresses were created in the mner portion andcompressive stresses in the outer portion of the gram. Whenthe maximum tensile stresses m the grain center exceeds itsfailure strength, the kernel will be fissured. Yamaguchi(1984) proposed that the thermal stresses In the nee kernelare negligibly small compared with the hydro stressesbecause the coefficient of thermal expansion IS remarkablysmaller than that of hygroscopic expansion for neeendosperm.The fissuring of nee IS influenced by several factors.

Besides rice vanety, drying and tempering conditions alsoaffect the extent of kernel damage. Tempering of nceduring drying operation has become a common practice toreduce the fissuring of nee. During the tempering penod ,the moisture m the kernel IS equalized through moisturediffusion, ThIS moisture equalization increases the dryingrate of grain in the next cycle of drying, and decreases thestresses caused by moisture gradient WIthin the kernel, thusthe drying caused rice-fissunng to decrease. The tempenngtime and the time when the drying should be intermitted bytempermg depend on the drying condItions. The mfluence oftempermg time and coohng on nce mIllmg yIeld andmOIsture removal was studIed by Steffe (1979), atheoretical model for predlctmg the reqUIred tempenng tImebased on drymg vanables was proposed.The ObjectIve of thIS work was to study the fIssuring

mechamsm of rough nce dunng mtermittent drying, analyzethe effects of tempenng tIme, mtermIttent ratIO and umtdrying time on nce flssunng through expenmental methodand develop SImulation program to analyze the effects ofmtermlttent parameters on the stresses m the nce kernelduring mtermlttent drymg.

Materials and Methods

The common nce vanety m the suburb of BeIJmg was usedm thIS study The fresh nce WIth a moisture content around18.5% (w. b ) was sealed wIth plastic bags and stored m arefrigerator at 2°C. The samples were removed from therefrigerator 2 hours before the drymg experiments to warmup m room temperatureThe expenmental drymg apparatus was equipped WIth a

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Proceedings of the 7 th Intemaiumal Workmg Conference on Storei-proiuc: Protectwn - Volume 2

fan, an electric heater, a temperature controller, and 2 Wiremesh trays with an area of 18 x 18cm. The air temperaturewas automatically controlled and measured by thethermocouples before the air reached the samples. Four airtemperatures (35, 45, 50 and 60°C) and two air velocities(2.6 and 3.4 m/s) were used in the expenments.Unit drying time (drying time of one cycle) and

intermittent ratio (tempering time over drying time of onecycle) were defined to study the intermittent dryingprocess. Three unit drying time (5, 10 and 15 min) and 3intermittent ratio (0, 2 and 8) were used m theexpenments.RIce fissurmg percentage was determined by observing

the samples on a lighted glass board. The fissunngpercentage of nee was checked at different time after dryinguntil there was no more change of fissurmg. DIfferent typesof cracks (transverse cracks, surface cracks, and smallcracks) were also recorded. The fissuring expenments wererepeated at least three times. The average value wasreported as the result. The relative error for the fissuringexperiments was about 1% .

Mathematical Models

Rough nee IS a composite matenal that is made up of starchyendosperm, bran and husk. As these components havedifferent properties such as moisture diffusivity andequilibrium moisture content, the composite modeldeveloped by Steffe (1980) was used to SImulate themoisture diffusion process Within a rough nee kernel, whererough rice was considered as a sphere

aM = D m = 1 2 3 {a2M + 2... aM Iat m' "ar2 r orDurmg drying, the moisture content at the gram surface

was assumed reachmg the equihbnum moisture contentinstantaneously.

M(t,r = R) = Me (2)During tempenng, the average moisture content of the

grain kernel is not changed, and only the internal moisturecontent is redistributed (Sabbah, 1972 ), 1. e the surface ofgram kernel during the tempenng penod can be consideredas water impermeable. The boundary condition fortempering penod can be descnbed as:

aMI -0ar r=R

Solution of Equations (1) to (3) by fmIte dIfferencemethod can establish the mOIsture distribution within ncekernel durmg drymg and tempenng cycles.It is assumed that the uneven expanSIOn and contraction

induced by mOIsture gradient are the mam reason for ricefissuring. The VIscoelasticIty for sphere is adequatelydescribed by several authors such as Rao (1975) andYamaguchi (1984). The following equations assume thesphere to be Isotropic, continuous and homogenous. As a

(1)

decrease in moisture corresponds to an mcrease in the timescale, the time- moisture shift factor aM is used to definethe reduced time:

~(t)=Jtdr (4)o aM

The mtegral form of the constitutive equations can be

written as:

Suet) = 2 J: G(e- na~Al'J (t' )dt' (5)

a( t ) = 3 J K(E- E') a~~ [e( r) - 3aMI( t~) ]dr (6)

The deviatoric strains are:

e = -2em= -2emm=2r ~(Ur) (7)rr TT 3 ar rThe deviatoric components of the stresses are:Srr= -2Sm= -2Sepep=2/3(arr - am) (8)

S1tI= Seep= Sepr=O (9)Ignoring the forces of inertia and gravity, the equilibrium

equation IS given by:

~a + 2... (a - a ) (10)ar rr r rr m

After the moisture distnbutions withm the kernel wereobtained from Equations (1) to (3) under grven conditions,the moisture distributions withm the brown rice was thenused to calculate the deviatonc stresses Srr incorporatingwith Equations (5), (6) and (7) The radial stress arr andthe tangential stresses am were calculated from Equations(8) and (10). The finite difference method was used tosolve Equations (4) to (10).After the stresses during drying and tempering period are

calculated, the internuttent drying process can be simulatedaccording to different mtermrttent schedule. TheVIscoelastic properties given by Yamaguchi et aI. (1984)were used in the models to calculate the stresses.

Results and Discussions

(3)

Experimental results

The changes of nee fissuring after drying were shown inFig. 1. The different curves in the figures designate thefissunng percentage of nee that dned for 30, 45 and 60mm, respectively. It can be seen that the mcrease of dryingtime and air temperature caused mcrease of the percentageof fissured nee. Most of the rice fIssuring occurred shortlyafter drying. Its percentage increased rapidly m the fIrst 4 hof holding. Mter 48 h there was no more fissurmgobserved. The changes of the percentage of fissured riceafter drymg were analyzed by curve fIttmg the expenmentaldata using the least square method.F= alnth + b th~48h (11)

The values of a and b were shown m Table 1.The effects of tempering time on the fIssuring of nce

were shown m Fig 2. FIrstly the gram was drIed for 15 mm

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Proceedings of the 7th International Worktng Conference on Stored-product Protection - Volume 2

30 1- 60 min 2- 45 mm 3- 30 min100 1- 60 mm 2-45 mm 3-30 nun

25 80----~ 'Z.

"$. ~Q)

'-' 20 ~Q)

if - 60- 55 ee 15 Q)

(J) 0..0.. ~.~ ·C 40

::s10 rn::s rn

en ~rn~ 20

5

0 040 50 60 70 80

th (h) th (h)

A T;= 45°C B T,= 60"CFig. 1. Changes of nee fissunng after drying.

using heated air at 45°C then held in a sealed vessel fortempenng. After different tempering time, the sampleswere dried for another 15 min. It can be seen that theincrease of tempering time results m a sharp dechne of thepercentage of rice fissured until the tempering time reaches2 h. Further increase of tempenng time shows insignificanteffect Under the drying condition of this test, thetempenng time can be selected between 1 hand 2 h.

The effects of intermittent ratio and unit drying time onthe percentage of fissured rice were shown in Fig. 3. It canbe seen that increased intermittent ratio caused a decrease offissuring, when the intermittent ratio increased from 0 to 8,the percentage of fissured rice was only about one fifth ofthat without tempering. It can be seen that decreased unitdrying time also caused a decrease of fissuring, but its effectwas not sigrufrcant.

Numerical analysis

The effects of tempering time, intermittent ratio and unitdrying time on the radial stresses at the center of nee werestudied usmg the program for stress calculation. The dryingconditions were as following- initial grain moisture content,24%; air temperature, 45'C , relative humidity, 20% Thepositive and negative values of the stresses designate the

Table 1. The values for a and b m equation (11).DrYillg temperature ('C) Dryillg time (mill. ) a b

60 30 6.92 7.1660 45 13.02 29.3360 60 13.30 35.4445 30 1.65 3.3945 45 2.61 3.9345 60 4.40 7.57

tensile and compressive stresses respectively.It can be seen from Figure 4 that the central portion was

contracted and greater tensile stresses were created duringthe holding period as the moisture was transported from thecentral portion to the surface. Discontmumg the dryingprocess with tempering period caused a decrease of themaximum tensile stresses at the grain center. Curve 1represents the radial stresses at the center of the rice kernelthat was dried for 1.0 h continuously. Curves 2, 3, 4 and 5are the radial stresses at the center of the rice when the 1.0h drying was discontinued with 0.5, 1. 0, 2.0 and 3.0 htempering after 0.5 h drying. It can be concluded thatincrease of tempering time would decrease the maximumtensile stresses at the center of the rice kernel initially.When the tempering time exceeds 1.0 h, the effect oftempenng time is small.

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Proceedings of the 7th International W01'king Conference on Stored-prcduct Protection - Volume 2

12 14

~ 10 12:of ~0

'n' <f- lOco 8

I.s" 8~ 6" OIlCo " 6co .~" 4.<:

'" 4" ~'"£ 22

0 00 0.5 2 4 0 0 2

Tempering time(h)

Fig. 2. Effect of temperature time on the percentage offissured rice.

Figure 5 shows the effects of intermittent ratio and unitdrying time on the radial stresses at the center of the ricekernel. Increased intermittent ratio or decreased unit dryingtime causes a decrease of the maximum tensile stresses atthe center of the rice kernel with the effect of temperingtime being more obvious. It reveals that increasedintermittent ratio or decreased unit drying time caused lessrice fissuring. This result agrees with the experimentalresults of rice fissuring shown in Fig. 3.

Conclusions

Most of the rice fissuring occurred shortly after dryingfinished. Its percentage increased rapidly in the first 4 h ofdrying and there was no further increase when the holdingtime exceeded 48 hours.Discontinuing the drying process with tempering can

decrease the stresses in the rice kernel. As a result, thefissuring percentage decreased.For the intermittent drying of rice, an increase of

intermittent ratio or a decrease of unit drying time candecrease the stress at the center of the kernel according tonumerical prediction. As a result, the rice fissuring can bedecreased.

Notation

aM time moisture shift factorD diffusion coefficient, D[, D2 and D, are the diffusioncoefficient of starchy endospenn, bran and husk of roughrice, m2/se deviatoric strainF percentage of fissuringG shear relaxation modulus. N/m2

K bulk modulus, N/m2

M moisture content, kg water/kg dry solids

ElDrying time-S min• Drying time- 10mino Drying rime- 15min

g

Intermittent ratio

Fig. 3. Effect of intermitted ratio and unit drying time on thepercentage of fissured rice,

M average moisture content, kg water I kg dry solidsR equivalent radius of rough rice, m

:~I II

o ~

'l I::E -2;t:

-4 1 - t=O2 - t=0.5 h3 - t=1.0 h4 - t=2.0 h5 - t=3.0 h

1 2 3 4 5"",./ ' ;' 0I .( -.j? r«: "<,, "<.r· -{ -,'i-.., ____. J......... -Jj"-'-\ I \ I\ I i Ii / I ii I :1 /

. / .

I . I III ::J \1-6

-8 +I--~--------,--o 234

Time(h)

5 6

Fig. 4. Effect of tempering time on radial stresses atthe center of rice kernel,

Me equilibrium moisture content, kg waterl kg dry solidsS deviatoric stress,N/m2

T, air tempera ture , "C1', r radial coordinate, mt,t time, st h holding timeGreek letters:" coefficient of linear hygroscopic expansionE strain'P , e sphere coordinates, m(J stress. N/m2

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Proceedings of the 7th Intemaiumal Workmg Conference on Stored-product Protection. - Volume 2

4..,-------------------.

2

3 2

o

1 - Umt drying tlme = 60 min2 - Unit drying time = 30 mm3 - UnIt drying time = 15min

-4

-6

-8 -r----.-------,------r-------jo 2

Tlme(h)3

Unit drying time

4 I

2 ~

2 3/I

r I

oJ/ <,

<,\ ---

-2 j I f,-..

I«1

Il:l...::.E'-G I Ivl'"

I I I-4 iI JI~I 1 - lnterrruttent ratio = 1

2 - Intermittent ratio = 23 - Intermittent ratio = 4

-8 I0 2 3 4 5

4Tlme(h)

Intermittent ratio

Fig. 5. Effect of unit drying time and mterrruttent ratio on radial stresses at the center of nee kernel.

e, e' reduced time.:1 different increment

Acknowledgement

The authors acknowledge the fmancial support from theNatural National Science Foundation of China.

References

Henderson, S.M. , 1954. The Causes and Characteristics ofRice Checking, Rice Journal, 57, 5, 16 and 18Irudayaraj , J. and Haghighi , K., 1993. Stress Analysis ofViscoelastic Materials during Drying: I. Theory and FiniteElement Formulation, Drying Technology, 11, 5, 901-927.Kunze, O. R. and Choudhury M. S. U , 1972. MoistureAdsorption Related to the Tensile Strength of RIce, CerealChemistry, 49(6) 684-698Rao, V. N. M., Hamann, D D. and Hammerle, J. R.,1975. Stress Analysis of a VIscoelastic Sphere Subjected to

Temperature and Moisture Gradients, Journal AgnculturalEngineering Research, 20, 283 - 293.Sabbah, M. A. , Foster, G. H., Redias, C. G and Peart,R M., 1972. Effects of Tempering after Drying onCoolmg Shelled Corn, Transactions of the ASAE 15, 4,763 -765.Sarker, N. S., Kunze, O. R. and Strouboulis, T , 1994.Finite Element SImulation of Rough Rice Drying, DryingTechnology, 12,4, 761-775.Steffe, J. F , Singh, R. P. and Bakslu, A. S., 1979.Influence of Tempering Time and Cooling on Rice MilhngYields and Moisture Removal, Transactions of the ASAE,1214 -1218, 1224.Steffe, J. F. and Singh, R. P. ,1980 Liquid Diffusrvity ofRough Rice Components, Transactions of the ASAE, 23,767-774.Yamaguchi, S., Wakabarashi, K. and Yamazowa, S.,1984. Change of cracked rice percentage and internalstress of brown rice kernels during drying operation,Proceedmgs of Fourth International Drying Symposium,IDS' 84, Kyoto, Japan. 640 - 647.

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