Journal of Food Engineering 88 (2008) 499–506

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Journal of Food Engineering

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Grinding characteristics and batter quality of rice in different wet grinding systems

Pankaj Sharma a,b,1, A. Chakkaravarthi a, Vasudeva Singh c, R. Subramanian a,*

a Department of Food Engineering, Central Food Technological Research Institute, Cheluvamba Mansion, Mysore, Karnataka 570 020, Indiab Centre for Food Technology, Jiwaji University, Gwalior, Madhya Pradesh 474 011, Indiac Department of Grain Science and Technology, Central Food Technological Research Institute, Cheluvamba Mansion, Mysore, Karnataka 570 020, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 November 2007Received in revised form 29 February 2008Accepted 10 March 2008Available online 16 March 2008

Keywords:Apparent viscosityAverage particle sizeBond’s work indexDamaged starchKick’s constantRice batterRittinger’s constantScanning electron microscopySpecific energy consumptionWet grinding

0260-8774/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jfoodeng.2008.03.009

* Corresponding author. Tel.: +91 821 251 3910; faE-mail address: subbu@cftri.res.in (R. Subramanian

1 Present address: M/s Hindustan Unilever ReseaWhitefield, Bangalore 560 066, India.

Grinding characteristics of raw and parboiled rice were evaluated in various wet grinding systems,namely, mixer grinder, stone grinder and colloid mill. The duration of grinding had inverse effect onthe particle size and direct impact on the starch damage as well as energy consumption in batch grinders.Stone grinder was the least energy efficient and specific energy consumption for grinding raw rice(160.6 kJ/kg) was nearly twice as that of mixer grinder (74.9 kJ/kg). Parboiled rice required longer dura-tion of grinding compared to raw rice, consequently specific energy consumption was higher (�220 kJ/kg). All the three classical laws of grinding (Kick’s, Rittinger’s and Bond’s) seemed to be applicable whileRittinger’s law showed better suitability than the other two followed by Bond’s law. Predominant com-pressive forces involved in stone grinder reflected in higher starch damage in batter which was also evi-dent in the micrographs. Parboiled rice slurry exhibited much greater viscosity than raw rice but bothdisplayed non-Newtonian pseudoplastic behaviour.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Wet grinding is a critical step in the preparation of batter basedtraditional food products. It involves both physical and chemicalchanges while dry grinding is a mere size reduction operation. Inwet grinding of cereals, the protein matrix holding the starch gran-ules is destroyed, releasing the starch granules from the proteinnetwork (Kent and Evers, 1994). Desikachar et al. (1960) reportedthat the flour presoaking method could be an alternative, however,wet grinding still remains as the practical method of batter prepa-ration for snack foods such as idli and dosa.

Solanki (2003) has reviewed the developments that had takenplace in the wet grinding implements in the country. There weresignificant attempts made at improving the existing types of wetgrinders and development of new types suitable for the domestickitchen. However, much effort is needed towards developing sys-tems for large scale operation owing to the steady increase in mar-ket demand for ready-to-use batter.

Grinding is an energy intensive process and therefore, there is aneed to look for avenues to save energy. Most of the published

ll rights reserved.

x: +91 821 251 7233.).

rch Centre, 64, Main Road,

information in this area is confined to 1940–1960 and a few re-search papers have been published in the last decade. However,excellent equipment for dry grinding are being designed andinstalled. It is difficult to determine the minimum energy requiredfor a given size reduction process but with the help of theories suchas Kick’s, Rittinger’s, and Bond’s laws, it has become possible tounderstand the grinding characteristics. But, unfortunately thereis no single law, which predicts the performance of various mate-rials during grinding (Chakkaravarthi et al., 1993). While someattempts have been made to improve the understanding on drygrinding, practically no such effort has been made towards wetgrinding of food materials.

Cereal batter industry could be developed as successfully asthat of wheat flour mills considering its vast potential. In a recentstudy from this laboratory, it was shown that the colloid mill has apotential for industrial adoption for batter production after com-parative evaluation of its performance with domestic wet grindingsystems (Solanki et al., 2005). Attempts were also made to under-stand the higher energy requirement in the case of wet grinding ofparboiled rice which revealed that slurry viscosity could be a majorfactor (Jagtap et al., 2008).

In the present study, three different wet grinding systems,namely, mixer grinder, stone grinder and colloid mill were evalu-ated in terms of particle size, starch damage, apparent viscosity,specific energy consumption and applicability of grinding laws

Nomenclature

CV coefficient of variationd particle size dimension (m)d1 initial particle dimension (m)d2 final particle dimension (m)E specific energy consumption (kJ/kg)

K constantKk Kick’s constant (kJ/kg)KR Rittinger’s constant (kJ/kg m)n constantWi Bond’s work index (kJ/kg)

500 P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506

while achieving a reference particle size. The study would be usefulin the design of wet grinding systems for cereal batter productionin terms of quality of batter and utilization of energy.

2. Materials and methods

2.1. Materials

Paddy (IR-64 belonging to indica variety) was procured fromthe Agriculture Produce Marketing Corporation, Mysore, Indiaand stored at room temperature. Paddy was parboiled by soakingin hot water (78–80 �C) overnight, decanting the excess waterand steaming the soaked grains at atmospheric pressure for20 min in an autoclave and drying at 25–30 �C for 48 h under shade(Bhattacharya and Swamy, 1967). Raw and parboiled rice were ob-tained by shelling and polishing of the raw and parboiled paddyusing a rice miller (M/s McGill mill, Houston, USA). Degree of pol-ishing was maintained between 6% and 8%.

Laboratory grade high-viscosity carboxy methyl cellulose (CMC)was purchased from M/s Loba Chemie Pvt Ltd., Mumbai, India.

2.2. Wet grinding systems

The following three wet grinding systems were used in thestudy: (1) a mixer grinder – slimline model, 0.5 L capacity,500 W, 18,000 RPM; M/s Chhaya Industries, India; (2) a stone grin-der – tabletop model fitted with three cylindrical roller stones, 2 Lcapacity, 500 W, 960 RPM; M/s Gandhimathi Appliances Ltd., Kan-cheepuram, India; (3) a colloid mill (pilot scale unit) – vertical typefitted with a pair of corundum stones of 150 mm diameter, modelMMS/O-Eco, 3750 W, 2800 RPM; M/s Fryma, Rheinfelden,Switzerland.

2.3. Grinding techniques

2.3.1. Optimization of rice to water ratioRice samples were soaked for 4 h in tap water at room temper-

ature before grinding. The moisture content of raw and parboiledrice increased from �10–11% to 37.4% and 62.9% (d.b.), respec-tively after soaking. Preliminary runs were conducted in all thethree wet grinding systems to optimize the rice to water ratio toget the desired consistency and particle size. Accordingly, rice (asis basis) to water (addition including water absorbed during soak-ing) ratio was maintained as 1:1.10 and 1:1.75 (by weight), for rawand parboiled rice, respectively in all the systems. For every exper-imental run, 500 g of rice was used in stone grinder and colloidmill, while 200 g of rice was used in mixer grinder.

2.3.2. Stone grinderA pre-determined quantity of water based on preliminary runs

was added gradually during grinding. The water added transportsthe solid particles through the grinding zone in the form of freeflowing slurry and facilitate smooth grinding. The temperaturedid not show any significant rise during the grinding operation ofboth raw and parboiled rice and remained at �28 �C (roomtemperature).

2.3.3. Mixer grinderA pre-determined quantity of water was added intermittently

(for smooth operation of the grinder) by stopping the grinder.The corresponding final temperatures of the batter were 32–33 �C and 34–35 �C for raw and parboiled rice, respectively.

2.3.4. Colloid millA pre-determined quantity of water was continuously added

during grinding. Required particle size was obtained by adjustingthe gap between the rotor and stationary member in the grindingzone. In the case of raw rice attempts were also made using ice as apartial replacement of water that also served as a coolant.

2.4. Grinding energy

Size reduction is quantified by comparing the new surface areagenerated to the energy consumed for generating that area. Math-ematically, it is expressed as

oEod¼ KðdÞn ð1Þ

where oE is the differential energy required to produce a change, od,in a particle of typical size dimension, d, and K and n are constants(Earle, 1996). Kick, Rittinger and Bond assumed the value of n as �1,�2, and �3/2, respectively, and derived the following expressions:

E ¼ Kk lnd1

d2ð2Þ

E ¼ KR1d1� 1

d2

� �ð3Þ

E ¼W i

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi100d2

� �s�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi100d1

� �s" #ð4Þ

These laws of grinding were analyzed for their applicability tobatch wet grinders.

2.5. Energy assessment

The energy consumption during grinding in mixer and stonegrinders was measured with an energy meter. In the case of colloidmill, it was measured using powerline supervisor instrument(model SPVR–96, M/s Maharashtra Electric Corporation, Mumbai,India) driven by Powermaster software.

The energy consumed during wet grinding operation was ex-pressed as

E ¼ Input electrical energy ðkJÞWeight of rice ðkgÞ ð5Þ

2.6. Particle size analysis

Particle size analysis of wet ground batter was carried out bysuccessive sieving (Solanki et al., 2005) starting from large to smallsieves (18-24-44-60-80-100-150-240 B.S.S) with 850, 710, 355,250, 180, 150, 105 and 63 lm openings, respectively. The weightfractions were obtained from the ratio of individual fractions tothe total sum of the fractions. The final average particle size was

Table 2Grinding characteristics of parboiled rice in batch grinders

Description E d2 KK KR Wi

Grinding time (s) (kJ/kg) (lm) (kJ/kg) (kJ/kg m) (kJ/kg)

Stone grinder360 165.6 710 118.3 156,046 27.7480 221.0 625 144.7 176,440 32.7600 275.8 525 162.0 177,049 34.9720 331.2 490 187.0 195,560 39.5CV of work indices 18.9 9.2 14.5

Mixer grinder120 155.0 900 133.2 202,883 33.3135 194.9 645 130.3 162,023 29.7150 220.0 635 145.5 179,182 33.0165 239.9 600 153.0 181,849 34.2CV of work indices 7.6 9.2 6.0

Table 1Grinding characteristics of raw rice in batch grinders

Description E d2 KK KR Wi

Grinding time (s) (kJ/kg) (lm) (kJ/kg) (kJ/kg m) (kJ/kg)

Stone grinder240 107.3 660 72.8 91,855 16.7360 160.6 590 101.3 119,137 22.5480 214.6 470 118.4 120,510 24.7600 267.8 290 116.7 86,371 21.1CV of work indices 20.6 17.1 15.8

Mixer grinder60 74.9 610 48.2 57,951 10.875 99.9 600 63.7 75,714 14.290 115.0 550 69.5 78,194 15.2105 140.0 475 77.7 79,657 16.3CV of work indices 19.2 13.8 16.6

P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506 501

determined graphically by plotting cumulative weight fractionsversus sieve size and selecting a notional sieve size, which allows80% of the ground material to pass through it (McCabe and Smith,1976).

2.7. Determination of starch damage

The damaged starch granules undergo amylase hydrolysis rap-idly compared to intact starch granules that resist the amylasehydrolysis. Starch damage in raw and parboiled rice batter wasestimated from their freeze dried samples generally according toAACC method 76-30A (AACC, 1999). This method is recommendedfor milled products of wheat (flour and semolina), and was adoptedwith slight modifications for determination of starch damage inrice samples (Solanki et al., 2005). Freeze dried samples were usedin the analyses after ascertaining that the differences in starchdamage values between the freeze dried and fresh samples werewithin the acceptable range.

2.8. Microscopic structure of ground particles

Photomicrographs of the freeze dried wet ground samples wereobtained using a scanning electron microscope (model: Leo 435 VP,M/s Leo Electron Microscopy, UK) for microscopic structureanalysis.

2.9. Viscosity

Measurements were carried out at room temperature (�28 �C)using disc spindle measuring system in a synchro-electric viscom-eter (model DV-II+RV viscometer, series 21,723, M/s BrookfieldEngineering Laboratories Inc., Stoughton, USA) within 2 h of grind-ing to avoid the influence of fermentation in batter samples. Theinstrument measures torque and speed which are then convertedin to viscosity based on the geometry of the measuring deviceusing the built-in software. Measurements were carried outgenerally with spindle No. 3 and spindle No. 4 was used whenthe viscosity exceeded the measurement range of spindle No. 3.The measurement ranges of spindle 3 and 4 were 1000–10,000and 2000–20,000 cP, respectively for a corresponding speed rangeof 10–100 RPM. Viscosity at different shear rates were obtained byvarying the spindle speed (10, 20, 50 and 100 RPM) while singleviscosity values reported were measured at 50 RPM. The measure-ments were carried out for whole batter of raw and parboiled riceas well as for a finer fraction (through of 100 mesh fraction) of par-boiled rice batter.

Rice slurries at various required concentrations from raw andparboiled rice were prepared from their whole batter along withaqueous dispersion of CMC as a suspending medium. The concen-tration of CMC was adjusted uniformly to 1% in all the samples ex-cept for higher slurry concentrations (30% and 35%) of parboiledrice samples which did not require the suspending medium. Theslurry concentration was expressed in terms of percent dry solidsafter taking in to account the moisture content of the batter. Inmeasurements with CMC, the influence of suspending medium iscompensated as follows (Jagtap et al., 2008):

Slurry viscosity ¼Measured viscosity

� Viscosity of 1% CMC solutionð190 mPasÞð6Þ

All the experimental runs were carried out in duplicate and thevalues were within ±3%. The mean values are reported. In the caseof batch grinders, coefficient of variation (CV, standard deviationexpressed as a percentage of the mean) was determined for bothraw and parboiled rice to assess the applicability of different grind-ing laws.

3. Results and discussion

3.1. Size reduction

Average particle size (620 lm) of a market sample (ready-to-use idli/dosa batter) determined after successive sieve analysiswas taken as the reference particle size for assessing the perfor-mance of various wet grinders.

3.1.1. Stone grinderThe duration of grinding had a direct influence on the size

reduction during the batch operation in stone grinder; longer theduration, finer was the particle size. Raw rice took 360 s to achievea particle size closer to the reference size while parboiled rice re-quired a longer duration of 480 s to achieve similar reduction(590 and 625 lm, respectively) (Fig. 1; Tables 1 and 2). Viscosityof the slurry increased as the grinding progressed and this increasewas much higher in parboiled rice compared to raw rice (Jagtapet al., 2008). Flow of batter in the batch grinding systems affectsthe effective exposure of particles to the grinding zone and therebythe duration of grinding. Longer grinding duration required for par-boiled rice could be attributed to its greater slurry viscosity affect-ing its flow in the grinding system.

3.1.2. Mixer grinderParticle size reduction in mixer grinder showed a similar trend

as that of stone grinder. However, mixer grinder took much shorterduration compared to the stone grinder, only 60 and 150 s toachieve similar reduction in raw and parboiled rice (610 and635 lm, respectively) closer to the reference particle size (Fig. 1;Tables 1 and 2). The differences in grinding time to achieve the ref-erence particle size between the two different grinders could be

0

200

400

600

800

1000

0 120 240 360 480 600 720 840

Grinding time (s)

0

10

20

30

40

Sta

rch

dam

age

(%)

Par

ticl

e si

ze (

μm)

Reference particle

Fig. 1. Influence of grinding time on average particle size and starch damage in batch grinders. stone grinder – raw rice (particle size); mixer grinder –raw rice (particle size); stone grinder – parboiled rice (particle size); mixer grinder – parboiled rice (particle size); stone grinder – raw rice(starch damage); mixer grinder – raw rice (starch damage); stone grinder – parboiled rice (starch damage); mixer grinder – parboiled rice(starch damage).

Table 3Grinding characteristics of raw and parboiled rice in different wet grinding systems atoptimized grinding conditions

Description Grindingtime (s)

E(kJ/kg)

d2

(lm)KK

(kJ/kg)KR

(kJ/kgm)Wi

(kJ/kg)Starchdamage (%)

Raw riceStone grinder 360 160.6 590 101.3 119,137 22.5 5.9Mixer grinder 60 74.9 610 48.2 57,951 10.8 4.0Colloid mill 21 100.8 495 57.2 60,252 12.1 2.6

Parboiled riceStone grinder 480 221.0 625 144.7 176,440 32.7 30.6Mixer grinder 150 220.0 635 145.5 179,182 33.0 27.1Colloid mill 51 223.2 730 162.6 218,259 38.4 22.6

502 P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506

attributed to the actual grinding forces involved in the mixer grin-der (shearing and cutting) and stone grinder (predominantly com-pression and less of shear) and probably the higher speed of themixer grinder and the loading ratio (actual load to the rated load)employed during grinding.

3.1.3. Colloid millParticle size of the ground material in the colloid mill depends

on the gap between the rotating and stationary members in thegrinding zone as well as the number of passes employed. Raw ricewas susceptible to gelatinization because of the enormous heatgenerated in the grinding zone during grinding with narrow gapsetting while parboiled rice did not pose such a problem owingto their higher gelatinization temperature (Ali and Bhattacharya,1980). Accordingly, the gap in the grinding chamber was increasedwhile grinding raw rice. The results revealed that raw rice could beground to a greater extent (495 lm) compared to parboiled rice(730 lm). Besides, raw rice took lesser duration (21 s) comparedto parboiled rice (51 s) which could be attributed to their influenceon the flow within the system.

In the subsequent run, an attempt was made to minimize thegelatinization while grinding raw rice, by partially replacing thewater required for grinding with ice. There was not much differ-ence in the extent of size reduction (495 and 520 lm, respectively)between water and ice-water as grinding medium. It was also no-ticed that water addition assisted the flow in the system in a bettermanner resulting in reduced grinding duration (21 s) compared toice-water addition (41 s). As expected ice addition dissipated thelocalized heat generated in the grinding zone owing to its latentheat. The batter temperature measured at the grinder outlet wasonly 18 �C while it was 30 �C with mere water addition. The resultsindicated that the required fineness in batter could be achieved byadjusting the gap in the grinding chamber and the temperature risecould be controlled by employing ice-water as the grinding med-ium, which might however increase the grinding duration owingto the hindered flow of batter within the system.

3.2. Energy assessment and suitability of grinding laws

All the three classical laws proposed for dry grinding gavereasonably good results based on the type of grinding: Kick’s law– coarse grinding; Rittinger’s law – fine grinding; and Bond’s law

– intermediate grinding (Fellows, 2000). In the present study, allthe three laws were in general found to be applicable to wet grind-ing. However, Rittinger’s law showed better suitability than theother two, followed by Bond’s law. The variations among the coef-ficients of all the three laws of grinding were lower for mixer grin-der compared to stone grinder indicating their better applicabilityfor the mixer grinder for both raw as well as parboiled rice. Simi-larly, the analysis showed better applicability of these laws to par-boiled rice than raw rice in both the systems (Tables 1 and 2). Theequations for Rittinger’s and Bond’s law were originally developedfrom the studies on dry grinding of hard materials such as coal andlimestone (Fellows, 2000). It can be inferred from the results thatthere is a scope to develop a unified model for wet grinding ofcereals.

Specific energy consumption for grinding raw rice in the stonegrinder (160.6 kJ/kg) was nearly double compared to the mixergrinder (74.9 kJ/kg) for achieving the same extent of size reduction.In the case of parboiled rice, there was not much difference in spe-cific energy consumption for achieving nearly the same extent ofsize reduction in the stone (221.0 kJ/kg) and mixer (220.0 kJ/kg)grinders (Table 3). In the case of colloid mill, specific energy con-sumption was sensitive to grinding medium besides the gap set-ting in the grinding chamber having a consequential effect on thesize reduction. The specific energy consumption increased by�1.57 fold with partial replacement of water with ice as a grindingmedium owing to the fact that the flow of batter within the systemwas not as easier as with water leading to longer duration to

P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506 503

achieve a similar extent of size reduction. Generally parboiled ricerequired greater energy compared to raw rice (Jagtap et al., 2008)in all the three systems (Tables 1–3). The specific energy consump-tion for grinding parboiled rice increased by �1.38, 2.94 and 2.21fold compared to raw rice in stone grinder, mixer grinder and col-loid mill, respectively.

3.3. Starch damage

The starch damage in dry grinding is generally higher as in thecase of raw rice and black gram; however, it was lower in the caseof parboiled rice compared to wet grinding (Solanki et al., 2005). Inraw rice batter, the starch damage was lower in the range of 2.6–5.9% whereas it was much higher in the parboiled rice batter inthe range of 22.6–30.6% (Table 3). These starch damage values ob-tained with IR-64 variety were higher compared to MTU-1000variety used in our earlier work (Solanki et al., 2005).

In the case of parboiled rice, starch damage measured could bean artifact since it may not be only due to grinding effect but alsodue to parboiling. However, the extent of starch damage owing tothe hydrothermal treatment during parboiling would not be great-er than the minimum value (6.6%) reported for dry grinding of par-boiled rice (Solanki et al., 2005) and any further increase observedcould be reasonably attributed to the effect of wet grinding. Earlier,it was believed that the hardness of parboiled rice is responsiblefor the longer duration of wet grinding, eventually leading to itsgreater starch damage (Solanki et al., 2005). Subsequent studiesshowed the probable role of slurry viscosity on the duration ofgrinding (Jagtap et al., 2008). The greater starch damage in par-boiled rice during wet grinding could be attributed to its greatersusceptibility to undergo damage owing to its softness (lowerhardness) after soaking as well as to the longer duration of grind-ing due to its greater slurry viscosity.

In batch grinders, starch damage and extent of size reductionwere complementary to each other; greater the size reduction,greater was the starch damage in both raw as well as parboiled rice(Fig. 1; Tables 1 and 2). In our earlier investigation, analysis of var-ious size fractions obtained after sieving revealed that finer theparticle size the greater was the starch damage both in dry andwet grinding operations (Solanki et al., 2005). The starch damagewas higher in stone grinder followed by mixer grinder and colloidmill for both raw as well as parboiled rice under standardized con-ditions (Fig. 1 and Table 3). These results indicated the probablerole of the type of forces involved in the individual wet grindingsystems. In stone grinder, the grinding action is due to more ofcompression and less of shearing while mixer grinder and colloid

Fig. 2. Photomicrographs of raw and

mill are mainly shear imparting systems. Compression along withshear forces imparted greater damage as compared to shear forcesalone. Probably, the highest starch damage in raw as well as par-boiled rice obtained in the stone grinder could be the reason forits preference for the preparation of idli and dosa.

3.4. Scanning electron microscopy (SEM)

Micrographs revealed that both physical as well as chemicalchanges had taken place during wet grinding of raw and parboiledrice (Figs. 2 and 3). In the case of raw rice, the starch granules ap-peared as separate discrete bundles and semi-crystalline in natureat a lower resolution (Fig. 2) and the protein-starch matrices weresurrounded by the fibrous cellulose cell walls. However, in par-boiled rice, the semi-crystalline nature of starch granules was lar-gely lost, appearing merely as small and big lumps (Fig. 2), owingto gelatinization and retrogradation and probably the effects ofsubsequent wet grinding operation.

The presence of fibrous material in all the samples examined ata higher resolution showed that they are not starch alone butwhole batters containing starch, protein and other cellulosic cellwall materials (Fig. 3). The micrographs of batter prepared fromdifferent wet grinding systems indicated the effects of forces in-volved (Fig. 3). In the case of stone ground batter, larger disruptionin the protein matrix was observed signifying greater damage toprotein-starch network (Fig. 3A) owing to the predominant com-pressive forces involved in grinding, eventually resulting in higherstarch damage (Table 3 and Fig. 1). However, in shear impartingsystems starch granules of raw rice were well separated from eachother (Fig. 3B and C) owing to multiform disruption caused byshearing and cutting forces unlike compressive forces acting on asingle plane as in the case of stone grinder. Similar effect was notclearly seen in parboiled rice as the starch granules were clumpedowing to the pretreatment received during parboiling.

3.5. Viscosity

Viscosity of batter during wet grinding has an influence on thepower required for the movement/circulation of batter within thebatch grinders (Jagtap et al., 2008). The viscosity of batter is likelyto change at various levels of processing (Fellows, 2000) duringsnack food preparation such as fermentation, mixing and spread-ing as well as cooking. The apparent viscosity of wet ground rawand parboiled rice batter obtained from different wet grinding sys-tems under standardized conditions at various solids concentra-tions are presented in Fig. 4. The viscosity of parboiled rice slurry

parboiled rice (mixer grinder).

Fig. 3. Photomicrographs of raw and parboiled rice obtained from different wet grinding systems.

504 P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506

was higher at all concentrations than raw rice and the difference intheir viscosity increased with increase in concentration irrespec-tive of the grinding system employed. These results are in agree-

ment with our earlier study carried out with raw and parboiledrice batter (through of 140 mesh) obtained from the mixer grinder.The starch granules that had undergone gelatinization due to the

0

200

400

600

800

1000

0 5 10 15 20 25 30

Concentration (%)

Vis

cosi

ty (

mP

as)

Fig. 4. Apparent viscosity of raw and parboiled rice batter obtained from different wet grinding systems under standardized conditions (measurement speed 50 RPM).stone grinder – raw rice; mixer grinder – raw rice; colloid mill – raw rice; stone grinder – parboiled rice; mixer grinder –

parboiled rice; colloid mill – parboiled rice.

P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506 505

hydrothermal treatment of parboiling and retrogradation duringsubsequent drying contributed to the greater viscosity in parboiledrice slurry and also to its progressive increase at higher concentra-tions (Jagtap et al., 2008). Although the average particle size wascloser to the reference particle size in all the batters used in thisstudy, the actual viscosity values varied with various wet grindingsystems (Fig. 4). This suggested the type of forces involved in a par-ticular grinding system as well as the temperature rise experiencedby the batter during grinding could play a role in its viscosity.However, a detailed study may be necessary for betterunderstanding.

Considering the importance of flow behaviour of batter, viscos-ity measurements were made at various shear rates (10, 20, 50 and100 RPM). The whole batter of raw as well as parboiled rice pre-

0

400

800

1200

1600

2000

0 20 40

Spindle

Vis

cosi

ty (

mP

as)

Fig. 5. Pseudoplastic behaviour of raw and parboiled rice batter (mixer grinder).raw rice – concentration 30%; parboiled rice – concentration 10%;

pared in the mixer grinder displayed shear thinning behaviour(decreasing viscosity with an increasing shear rate) at all concen-trations measured between 10% and 30% (Fig. 5). This type of flowis also known as non-Newtonian pseudoplastic behaviour. The par-ticle size of flour strongly influenced the viscosity, probably thefiner flours, due to their greater surface area per unit weight,underwent easier and greater swelling in water compared to coar-ser flours and hence showed greater viscosity (Sandhya Rani andBhattacharya, 1989). Therefore, studies were also carried out witha specific size fraction of parboiled rice batter for appropriate char-acterization. The minus 100 mesh fraction (�150 lm) of parboiledrice showed a similar non-Newtonian pseudoplastic behaviour(Fig. 6) as that of the whole batter. But the viscosity values weremuch higher than the corresponding whole batter of parboiled rice

60 80 100

speed (RPM)

raw rice – concentration 10%; raw rice – concentration 20%;parboiled rice – concentration 20%; parboiled rice – concentration 30%.

0

1000

2000

3000

0 20 40 60 80 100

Spindle speed (RPM)

Vis

cosi

ty (

mP

as)

Fig. 6. Pseudoplastic behaviour of parboiled rice batter (mixer grinder; �100 mesh fraction). 5% dry solids; 10% dry solids; 15% dry solids;20% dry solids; 25% dry solids.

506 P. Sharma et al. / Journal of Food Engineering 88 (2008) 499–506

which could be attributed to the size difference; finer the particlegreater is the viscosity, owing to its greater swelling in water.

4. Conclusions

Evaluation of grinding characteristics and batter quality with dif-ferent wet grinding systems revealed the influence of type of forcesinvolved in grinding. In batch grinders, the duration of grinding hadan inverse effect on the average particle size and a direct impact onthe starch damage and energy consumption. Shorter duration ofgrinding in mixer grinder (shearing and cutting forces) to achievethe reference particle size compared to stone grinder (predomi-nantly compressive) could be attributed to the actual grindingforces involved as well as the speed of the machine. Predominantcompressive forces involved in stone grinder imparted greater dam-age to starch granules as compared to the shear and cutting forcesinvolved in other grinders. All the three laws proposed for dry grind-ing (Kick’s, Rittinger’s and Bond’s) seemed to be applicable for wetgrinding. However, Rittinger’s law showed better suitability thanthe other two especially for parboiled rice and mixer grinder. Thespecific energy consumption for parboiled rice was�1.38–2.94 foldgreater compared to raw rice. Higher specific energy consumptionand longer duration required for parboiled rice could be attributedto its greater slurry viscosity during grinding affecting its flow inthe batch grinding systems. The higher viscosity of parboiled riceslurry compared to raw rice could be attributed to the presence ofpregelatinized and retrograded starch. The batter of raw as well asparboiled rice displayed non-Newtonian pseudoplastic behaviourat various concentrations (5–30%). A detailed study may be neces-sary to understand the rheological behaviour of batter vis-à-visthe type of forces involved in a particular wet grinding system aswell as the influence of temperature rise in batter during grinding.

Acknowledgments

S.G. Jayaprakashan helped in conducting the experiments incolloid mill, K. Anbalagan helped in obtaining the photomicro-graphs in SEM and B. Manohar helped in viscosity measurements.

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