This article was downloaded by: [University of Tennessee, Knoxville] On: 12 November 2014, At: 08:31 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK International Journal of Food Properties Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ljfp20 Rheological Characteristics of Quince Nectar During Ohmic Heating Hayriye Bozkurt a & Filiz Icier b a Natural and Applied Sciences, Food Engineering Branch , Ege University , Bornova, Izmir, Turkey b Food Engineering Department, Engineering Faculty , Ege University , Bornova, Izmir, Turkey Published online: 21 Aug 2009. To cite this article: Hayriye Bozkurt & Filiz Icier (2009) Rheological Characteristics of Quince Nectar During Ohmic Heating, International Journal of Food Properties, 12:4, 844-859, DOI: 10.1080/10942910802102962 To link to this article: http://dx.doi.org/10.1080/10942910802102962 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms- and-conditions
Transcript
This article was downloaded by: [University of Tennessee, Knoxville]On: 12 November 2014, At: 08:31Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
International Journal of Food PropertiesPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/ljfp20
Rheological Characteristics of QuinceNectar During Ohmic HeatingHayriye Bozkurt a & Filiz Icier ba Natural and Applied Sciences, Food Engineering Branch , EgeUniversity , Bornova, Izmir, Turkeyb Food Engineering Department, Engineering Faculty , EgeUniversity , Bornova, Izmir, TurkeyPublished online: 21 Aug 2009.
To cite this article: Hayriye Bozkurt & Filiz Icier (2009) Rheological Characteristics of QuinceNectar During Ohmic Heating, International Journal of Food Properties, 12:4, 844-859, DOI:10.1080/10942910802102962
To link to this article: http://dx.doi.org/10.1080/10942910802102962
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.
This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
RHEOLOGICAL CHARACTERISTICS OF QUINCE NECTAR DURING OHMIC HEATING
Hayriye Bozkurt1 and Filiz Icier2
1Natural and Applied Sciences, Food Engineering Branch, Ege University, Bornova,Izmir, Turkey2Food Engineering Department, Engineering Faculty, Ege University, Bornova, Izmir,Turkey
Electrical heating of food products provides rapid and uniform heating, resulting in lessthermal damage to the product. In this study, ohmic heating as an electrical heating methodwas applied to the quince nectar by matching the same heating curve of conventionalmethod by changing voltage gradient (10–40 V/cm) at 50 Hz. The change of rheologicalconstants of quince nectar was determined for different holding times (0, 10, 15, 20, and30 minutes) in the temperature range of 65–75ºC by using concentric type viscosimeter. Shearstress–shear rate data were fitted to the Newtonian, Bingham, Herschel Bulkley, Power lawand Casson Models. It was found that Herschel-Bulkley model was the best model to fit theexperimental rheogram adequately since higher regression coefficients (R2; 0.9997) andlow standard errors (SE; 0.054) were obtained by Herschel-Bulkley equation compared toother model equations. It was concluded that quince nectar showed time independent non-Newtonian pseudoplastic character during heating in the range of 20–75ºC, independent onheating method. The activation energy values were 9.88 ± 3.24 kJ/mol and 10.08 ± 2.53 kJ/molfor ohmic heating and conventional heating respectively. Results showed that there was noelectrical effect rather than thermal effects of ohmic heating since similar rheological con-stants were obtained with both methods statistically (p < 0.05). Ohmic heating could be rec-ommended as an alternative fast heating method for fruit nectars.
Quince (Cydonia oblonga) is an ancient and delicious fruit having pronouncedflavour and distinctive taste.[1] In addition to its consumption as fresh fruit, it can beprocessed to both clarified juices and nectars. The quince has a potential fruit juice source,because of having high amount of phenolic and pectic content.[2] Therefore the fruitprovides a suitable medium to microorganisms growth especially molds. The main aimduring quince nectar process was to apply adequate heat which was necessary to avoid themold growth.
In recent years, the minimal processing of fruit juices is one of the novel subjectsto minimize the effects of thermal treatments. For this purpose, electrical treatments
Received 7 December 2007; accepted 4 April 2008.Address correspondence to Filiz Icier, Food Engineering Department, Engineering Faculty, Ege University,
RHEOLOGY OF QUINCE NECTAR DURING OHMIC HEATING 845
play an important role as alternative methods in food processing. Ohmic heating isbased on the passage of electrical current through a food product having electrical resis-tance. The electrical energy is converted to heat. Instant heating occurs depending onthe current passing through the food material. The uniform heat generation results touniform temperature distribution, especially for liquid foods. Ohmic heating is used in awide range of applications such as preheating, blanching, pasteurization, sterilization,extraction of food products.[3,4,5,6] Its advantages compared to conventional heatinginclude maintaining the color and nutritional value of food, short processing time andhigher yield.[7,8,9,3,4,10,11]
In many instants, rheological measurement is used to investigate changes of foodstructures. Rheology concerns with the flow and deformation of a substance under appliedforces; and attempts to define a relationship between the stress acting on a given materialand the resulting deformation and/or flow that takes place. Knowledge on the rheology offruit juices is essential in quality control, sensory evaluation and engineering design. Itserves significant role in the analysis of flow conditions in many food processing opera-tions such as pasteurization, concentration and dehydration.[12]
Rheological properties are determined by measuring force and deformation as afunction of time. Several methods have been used to describe the flow behaviour offoods, for example linear (Newtonian or Bingham), Power law (Ostwald-de-Waele),Power law with yield stress (Herschel–Bulkey) and Casson models.[13] Power lawmodel is the most widely employed model for non-Newtonian foods and is used exten-sively to describe their flow properties in practical engineering applications. It alsodescribes the effect of concentration on apparent viscosity in foods.[14] Moreover,temperature has an important influence on the flow characteristics in foods. Differenttemperatures are encountered in most of the food processing operations, and their rheo-logical properties are studied as a function of temperature. An Arrhenius-type modelgenerally expresses the effect of temperature on the apparent viscosity at a specifiedshear rate.[15,16]
Rheological properties of fruit juices appear to be dependent of their varieties,state of ripening, and concentration of juice/pulp and temperature variation. Althoughthere were numerous researches on the determination of rheological properties offood materials[17-29] and modelling of ohmic heating systems,[30,31,32] only limitedstudies were conducted about the effects of ohmic heating on rheology of foods.[33]
The electrical effects of ohmic heating on food quality rather than thermal effects arealso still unclear. The purpose of this study is (i) to evaluate the effects of ohmic heat-ing on the rheological characteristics of quince nectars by applying different modelsfor different temperatures; and (ii) to compare them with the results of conventionalheating.
MATERIALS AND METHODS
Materials
The quince nectar used in this study was supplied from a fruit juice processingfirm in Izmir, Turkey. Its composition and properties were given in Table 1. This nectarcontains sweeteners (acesulfame K, aspartame, and neohesperidin) without any sugaradded. It has been prepared from quince puree and water for the prescribed sugar con-centration of 12.2%.
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846 BOZKURT AND ICIER
Heating Methodology
The same thermal history was applied for both heating method to investigate theelectrical effects rather than the thermal effects during ohmic heating.
Conventional heating. The samples in the cylindrical glass container (20 ml)inserted to the water bath, were heated to 65, 70, and 75ºC and held at this temperature for0, 10, 15, 20, and 30 minutes. The electronic sensors inserted to the centre point of theglass container. The container was shaken during heating to provide homogenous heating.The temperature difference at different locations was obtained approximately within 1ºCduring heating. The microprocessor board (Omega Eng. Inc., Stanford, CT) monitoredthe temperatures and transmitted this information to the microprocessor at constant timeintervals (1 s).
Ohmic heating. Ohmic heating experiments were conducted in laboratoryscale ohmic heating system consisting of a power supply, an isolating transformer, avariable transformer and a microprocessor board. The detailed technical informationabout the system used was given in Icier and Ilicali.[9] Teflon coated electronic temper-ature sensors (Omega Eng. Inc., Stanford, CT) with a compression fitting were used tomeasure temperatures at the different sections of the sample in the test cell. The micro-processor board (Omega Eng. Inc., Stanford, CT) monitored the temperatures, currentand voltage applied and transmitted this information to the microcomputer at constanttime intervals (1 s). The time constants of Teflon coated temperature sensors weredetermined by calibrating them in standard calibration solutions (Omega Eng. Inc.,Stanford, CT).
The test cell has rectangular cross section. The dimensions of the electrodeswere 0.025 m × 0.025 m. The distance between two electrodes was 0.04 m. The sam-ple was poured through the temperature measurement port; the electronic temperaturesensors were inserted and fitted. The amount of the sample was 20 ml for each treat-ment. The sample was sandwiched between two electrodes in the test cell. Then it washeated ohmically by adjusting the voltage gradient in the range of 10–40 V/cm at 50Hz a.c. The sample was heated from 20 to 65°C, 70ºC and 75ºC, and kept at constanttemperature for different treatment times (0, 10, 15, 20, and 30 minutes) by adjustingpower on and off during ohmic heating. Temperature of each sample was assumed
Table 1 Composition and some properties of thequince nectar.
RHEOLOGY OF QUINCE NECTAR DURING OHMIC HEATING 847
uniform in the cell, since the maximum difference among the measured temperaturesat different locations was approximately within 0.5°C. The experiments were repli-cated three times.
Analysis
Rheological properties were measured using Brookfield Viscometer (Model DV-II,Brookfield Engineering Laboratories, USA). The temperature of sample was kept atconstant using a circulating water bath and a small sample adapter during measure-ments. The measurement range of Brookfield Viscometer between 0 and 100% full-scale torques was adjusted by selecting the specific spindle (S-31) and its rotationalspeed (0.0–200 rpm) for quince nectar. During the rheological measurement, shearstress (SS), shear rate (SR), and % torque (T) values were taken for each rotationalspeed (rpm).
In order to fit the experimental data, various rheological flow models based on shearstress–shear rate data (Newtonian, Bingham, Casson, Power law, Herschel Bulkley) weretested. The models applied Eqs. (1–5) were represented in Table 2. The relation betweenthe consistency coefficient and temperature was described by Arrhenius type equation(Eq.6)[15];
where reference temperature (T0) was taken as 20°C. pH was determined using a mem-brane pH meter (HI 8314, Hanna Instrument, USA). Buffer solutions of pH 7 and pH 4 areused for calibration of pH-meter. The measurement of soluble solid content was performed byusing hand Refractometer (N-1a, ATAGO, Japan) and given as a percentage (%). Totalsolids, fibre and total carbohydrate contents were determined by methods given in AOAC(1995).[34]
Statistical evaluation and non-linear regression analyses were performed usingSPSS Ver.11.0.1 Statistical package.[35] The statistical criteria applied to discriminateamong the samples were R2 (regression coefficient) and standard errors (std. error) foreach coefficient. The comparison of the results were made to analyse the effects of tem-perature, heating methods and holding time on rheological behaviour by using one-wayAnova, paired t-test and Post Hoc (Duncan, LSD) tests. The confidence levels used todetermine statistical significance were 95% and 99%. All measurements were carried outin triplicate.
Table 2 Mathematical models and constitutive equations studied.
Newtonian model Skelland,[49]; Backhurst & Harker[50] Eq. (1)Power Law model Rao et al.[12] Eq. (2)Bingham model Bingham[51] Eq. (3)Herschel Bulkley model Herschel Bulkley[52,53] Eq. (4)Casson model Casson[54] Eq. (5)
t mg= &
t g= K n&t t g= +0 K &
t t g= +0 K n&
t t g0 50
0 5. .= +c cK &
K K e
E
R T Ta
=−
⎛⎝⎜
⎞⎠⎟
0
1 1
0
− (Eq.6)
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848 BOZKURT AND ICIER
RESULTS AND DISCUSSION
The heating curve of quince nectar was similar for ohmic and conventionalmethod (Fig. 1). To assess the possible electrical effect of ohmic heating on rheolog-ical properties, the matching of the thermal history during heating was aimed.Time required reaching to 65, 70 and 75ºC from 20°C were 90 s, 110 s, and 140 s,respectively.
It was observed that the quince nectar had experimental yield stress values at alltemperatures studied. Yield stress is an important quality control parameter to processindustries, which represents a finite stress required to achieve flow.[36] A true value of theyield stress could be beneficial for the optimal design of food-processing systems such asthose required during thermal processing.[37] Significantly higher magnitude of yield wasobserved for nonheated nectar as compared to heated samples (p < 0.05). The yield valueobtained from quince nectar decreased as the temperature increased (p < 0.05) (Table 3).
Figure 1 Heating curve of quince nectar.♦ OH WB
0
10
20
30
40
50
60
70
80
0 25 50 75 100 125 150
Time (seconds)
Tem
per
atu
re (
ºC)
Table 3 Experimental yield stress values.
t0 (Pa)
Temperature (ºC) Time (min) Ohmic heating Conventional heating
RHEOLOGY OF QUINCE NECTAR DURING OHMIC HEATING 849
Since the stress level in the fluid increased, the structure responsible for the yield stressdestroyed. The literature on concentrated suspensions and materials, which exhibit a yieldstress, was voluminous.[37,38] The differences of the results in literature could be due tovariation in soluble solids content, type of instruments used, variation in shear rates andthe processing conditions.
Shear stress–shear rate data of quince nectar samples were tested for variousrheological models (Newtonian, Casson, Bingham, Power law, and Herschel Bulkley)(Tables 4–7). Newtonian and Power law models fitted the data without takingaccount of the yield stress value. Rheological behaviour of Newtonian fluids repre-sents linearity between shear stress and applied shear rate. However, this linearitywas not obtained experimentally in this study. Therefore, the agreement of Newto-nian model to the experimental data was poor (Figs. 3–5). Regression coefficients ofPower law model were high enough due to low yield stress values obtained experi-mentally (Tables 4–7). Furthermore, there was good agreement between Power lawmodel results and experimental data at all temperatures, by neglecting initial yieldstresses (Figs. 3–5). For Bingham model, the regression coefficients were lowest dueto neglecting the flow behaviour index and this model could not fitted the experimen-tal shear stress-shear rate data (Figs. 3–5). Although high regression coefficientswere obtained by Casson model, theoretical yield stress values were significantlydifferent from experimental yield stress values (p < 0.01). Therefore, this modelcould not be used to fit the rheological data adequately. On the other hand, the regres-sion coefficients obtained from Herschel Bulkley Model were higher than othermodels (Tables 4–7) and it was best model fitting the experimental data adequately(Figs. 3–5).
Average n values estimated by Herschel Bulkley Model for 65, 70 and 75ºC were0.34 ± 0.01, 0.34 ± 0.03 and 0.37 ± 0.03, respectively, during ohmic heating, whilethey were 0.33 ± 0.01, 0.34 ± 0.01 and 0.36 ± 0.02, respectively, during conventionalheating. There was no significant difference in flow behaviour indexes of both heatingmethods (p < 0.05). Since n values less than unity indicated the shear-thinning natureof quince nectars, it was obtained that the rheological behaviour of nectar did notchange during holding periods. It is well known that the consistency of juices is signif-icantly affected by pectin and sugar concentration,[39] and could have non-Newtonianbehaviour. Similarly, Krokida et al.[40] performed a study comparing the available datain literature of fruit pulps, including guava, raspberry, pineapple, apricot, apple, mango,tamarind, black currant, and had found that these fruit pulps showed shear-thinning
Table 4 Predicted rheological constants of raw quince nectar at 20ºC.
ModelK (Pasn)*
Kc (Pasn)0.5,**
n (dimensionless)* τ0c (Pa0.5 )** m (Pa)*** R2 SE
Herschel Bulkley model 0.643 0.101 0.999 0.054Power Law model 0.417 0.375 0.994 0.011Bingham model 0.025 – 0.935 0.025Casson model 0.686 0.093 0.992 0.028Newtonian model – 0.020 0.976 0.111
*For Herschel Bulkley, Power Law and Bingham models; **for Casson model; and ***for Newtonian model.
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Tab
le 5
Pred
icte
d rh
eolo
gica
l con
stan
ts o
f qu
ince
nec
tar
heat
ed to
65º
C.
Ohm
ic h
eati
ngC
onve
ntio
nal h
eati
ng
Mod
elT
ime
(min
)K
(Pa
sn )*
Kc
(Pas
n )0.5,
**n
(dim
ensi
onle
ss)*
τ 0
c (P
a0.5 )*
* m
(Pa)
***
R2
SEK
(Pa
sn )*
Kc
(Pas
n )0.5,
**n
(dim
ensi
onle
ss)*
τ 0
c (P
a0.5 )*
*, m
(Pa
)***
R2
SE
Her
sche
l Bul
kley
mod
el0
0.34
70.
342
0.99
80.
022
0.41
50.
343
0.99
90.
251
100.
439
0.34
00.
998
0.02
40.
428
0.32
40.
998
0.02
115
0.36
70.
342
0.99
90.
015
0.38
00.
345
0.99
90.
017
200.
467
0.36
80.
999
0.02
20.
386
0.33
90.
999
0.01
930
0.50
60.
351
1.00
00.
017
0.42
20.
337
0.99
90.
017
Pow
er L
aw m
odel
00.
427
0.31
50.
995
0.55
30.
587
0.29
30.
992
0.82
510
0.57
00.
337
0.99
30.
790
0.50
70.
305
0.99
50.
580
150.
466
0.31
00.
995
0.57
80.
552
0.29
00.
991
0.79
520
0.56
10.
344
0.99
60.
689
0.55
40.
286
0.99
10.
787
300.
698
0.30
40.
993
0.93
30.
567
0.29
80.
992
0.79
0B
ingh
am m
odel
00.
027
—0.
528
0.37
00.
032
—0.
511
0.44
610
0.03
4—
0.54
30.
469
0.03
1—
0.70
60.
444
150.
028
—0.
523
0.39
30.
030
—0.
493
0.41
020
0.04
0—
0.52
30.
358
0.02
9—
0.56
00.
411
300.
040
—0.
551
0.43
10.
032
—0.
583
0.44
9C
asso
n m
odel
00.
728
0.06
80.
998
0.01
10.
842
0.07
30.
997
0.01
410
0.84
20.
075
0.99
60.
015
0.79
30.
070
0.99
70.
013
150.
756
0.07
00.
996
0.01
40.
815
0.07
00.
996
0.01
520
0.83
50.
089
0.99
40.
023
0.81
70.
068
0.99
70.
012
300.
918
0.08
40.
995
0.02
00.
837
0.07
20.
996
0.01
6N
ewto
nian
mod
el0
—0.
019
0.85
80.
961
—0.
023
0.85
90.
982
10—
0.01
70.
964
0.08
3—
0.01
50.
964
0.07
415
—0.
015
0.96
20.
073
—0.
015
0.96
00.
078
20—
0.02
20.
959
0.11
1—
0.01
50.
964
0.07
330
—0.
021
0.95
80.
111
—0.
016
0.95
70.
086
*For
Her
sche
l Bul
kley
, Pow
er L
aw a
nd B
ingh
am m
odel
s; *
*for
Cas
son
mod
el; a
nd *
**fo
r N
ewto
nian
mod
el.
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Tab
le 6
Pred
icte
d rh
eolo
gica
l con
stan
ts o
f qu
ince
nec
tar
heat
ed to
70º
C.
Ohm
ic h
eati
ngC
onve
ntio
nal h
eatin
g
Mod
elT
ime
(min
)K
(Pa
sn )*
Kc
(Pas
n )0.5,
**n
(dim
ensi
onle
ss)*
τ 0
c (P
a0.5 )*
*, m
(P
a)**
*R
2SE
K (
Pasn )*
K
c (P
asn )0.
5,**
n (d
imen
sion
less
)*
τ 0c (
Pa0.
5 )**,
m (
Pa)
***
R2
SE
Her
sche
l Bul
kley
mod
el0
0.40
80.
322
0.99
90.
013
0.36
10.
372
0.99
80.
031
100.
453
0.34
30.
998
0.02
60.
363
0.35
40.
997
0.02
915
0.58
80.
339
0.99
80.
032
0.36
30.
347
0.99
80.
022
200.
489
0.33
30.
999
0.02
30.
391
0.34
40.
999
0.01
930
0.35
10.
392
0.99
80.
033
0.35
00.
353
0.99
90.
019
Pow
er L
aw m
odel
00.
496
0.29
80.
996
0.53
00.
417
0.27
60.
982
0.77
110
0.58
40.
310
0.99
30.
806
0.45
00.
328
0.99
40.
672
150.
742
0.31
00.
994
1.01
10.
479
0.30
90.
994
0.65
120
0.54
80.
328
0.99
40.
774
0.50
40.
311
0.99
40.
683
300.
547
0.32
40.
989
1.01
20.
435
0.32
50.
995
0.57
1B
ingh
am m
odel
00.
029
—0.
731
0.42
50.
031
—0.
404
0.12
110
0.03
5—
0.51
40.
486
0.02
9—
0.37
80.
395
150.
045
—0.
566
0.62
90.
029
—0.
464
0.39
120
0.03
6—
0.61
80.
518
0.03
0—
0.50
40.
421
300.
033
—0.
398
0.41
20.
028
—0.
402
0.38
2C
asso
n m
odel
00.
778
0.06
80.
994
0.01
90.
679
0.06
10.
998
0.00
810
0.85
40.
077
0.99
70.
015
0.75
80.
073
0.99
70.
015
150.
963
0.08
70.
992
0.02
70.
769
0.07
00.
998
0.01
220
0.84
50.
079
0.99
40.
021
0.79
30.
072
0.99
70.
014
300.
816
0.08
10.
988
0.03
00.
737
0.07
20.
995
0.01
8N
ewto
nian
mod
el0
—0.
014
0.94
90.
083
—0.
012
0.98
60.
034
10—
0.01
80.
966
0.08
5—
0.01
60.
970
0.06
915
—0.
032
0.85
20.
929
—0.
021
0.86
50.
968
20—
0.01
80.
956
0.09
9—
0.01
60.
964
0.07
630
—0.
019
0.94
50.
113
—0.
015
0.96
00.
076
*For
Her
sche
l Bul
kley
, Pow
er L
aw a
nd B
ingh
am m
odel
s; *
*for
Cas
son
mod
el; a
nd *
**fo
r N
ewto
nian
mod
el.
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852
Tab
le 7
Pred
icte
d rh
eolo
gica
l con
stan
ts o
f qu
ince
nec
tar
heat
ed to
75º
C.
Ohm
ic h
eati
ngC
onve
ntio
nal h
eatin
g
Mod
elT
ime
(min
)K
(Pa
sn )*
Kc
(Pas
n )0.5,
**n
(dim
ensi
onle
ss)*
τ 0c (
Pa0.
5 )**,
m (
Pa)
***
R2
SEK
(Pa
sn )*
Kc
(Pas
n )0.5,
**n
(dim
ensi
onle
ss)*
τ 0
c (P
a0.5 )*
*, m
(P
a)**
*R
2SE
Her
sche
l Bul
kley
mod
el0
0.35
60.
381
0.99
80.
028
0.33
50.
356
0.99
80.
023
100.
317
0.37
80.
999
0.01
80.
346
0.35
40.
999
0.02
015
0.34
60.
395
0.99
80.
029
0.31
10.
371
0.99
80.
025
200.
316
0.38
10.
999
0.02
00.
305
0.37
50.
998
0.02
430
0.31
90.
376
0.99
90.
015
0.31
60.
343
1.00
00.
008
Pow
er L
aw m
odel
00.
445
0.34
90.
996
0.59
90.
441
0.31
70.
994
0.61
310
0.40
30.
345
0.99
50.
562
0.40
60.
335
0.99
60.
498
150.
417
0.37
30.
996
0.64
00.
413
0.33
00.
994
0.60
820
0.42
90.
334
0.99
50.
599
0.37
00.
349
0.99
70.
428
300.
388
0.35
00.
996
0.49
40.
430
0.29
70.
994
0.52
7B
ingh
am m
odel
00.
032
—0.
298
0.40
50.
027
—0.
360
0.36
610
0.02
8—
0.24
60.
362
0.02
8—
0.39
00.
378
150.
033
—0.
429
0.40
80.
027
—0.
380
0.34
820
0.02
8—
0.28
70.
360
0.02
7—
0.19
40.
350
300.
028
—0.
218
0.36
10.
024
—0.
525
0.34
1C
asso
n m
odel
00.
741
0.08
20.
995
0.01
90.
738
0.07
00.
997
0.01
410
0.70
80.
076
0.99
50.
019
0.71
70.
072
0.99
60.
016
150.
721
0.08
70.
989
0.03
20.
715
0.07
20.
996
0.01
520
0.72
20.
075
0.99
70.
014
0.66
50.
076
0.98
70.
030
300.
695
0.07
60.
996
0.01
70.
716
0.06
40.
994
0.01
7N
ewto
nian
mod
el0
—0.
018
0.96
60.
083
—0.
015
0.96
70.
067
10—
0.01
60.
959
0.08
0—
0.01
50.
965
0.07
115
—0.
019
0.94
80.
110
—0.
015
0.96
90.
065
20—
0.01
60.
969
0.07
1—
0.01
50.
941
0.09
230
—0.
021
0.88
00.
885
—0.
018
0.86
00.
871
*For
Her
sche
l Bul
kley
, Pow
er L
aw a
nd B
ingh
am m
odel
s; *
*for
Cas
son
mod
el; a
nd *
**fo
r N
ewto
nian
mod
el.
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RHEOLOGY OF QUINCE NECTAR DURING OHMIC HEATING 853
behaviour. Pelegrine et al.[41] determined that pineapple and mango pulps were shear-thinning fluids, as their apparent viscosity decreased with an increase in shear rate,and the flow behaviour index (n) was lower than “1”. Branco and Gasparetto[42]
studied the rheological behaviour of ternary mixtures of mango pulp, and juices ofcarrot and orange, where the ternary mixtures presented the non-Newtonian fluidbehaviour.
Consistency coefficients of quince nectar were determined at pasteurizationtemperatures (Tables 5–7). Temperature had significant effect on the consistencycoefficient for both heating method (p < 0.05). Several authors,[43,44,45,13,46] havereported that consistency coefficients significantly decreased as temperatureincreased. According to Krokida et al.,[40] the heat treatment had a major effect on theconsistency coefficient (K) of the non-Newtonian fluid food. Similar to the flowbehaviour indexes, consistency coefficient values changed at the initial heating upperiod while there were no significant changes observed during holding period (p < 0.05).Moreover, the heating method did not affect the consistency coefficient of quince nec-tars (p > 0.05) (Fig. 2).
The energy of activation for Newtonian fluid foods has been found to increasefrom 14.4 kJ/mol (water) to more than 60 kJ/mol (concentrated clear juices and sugar
Figure 2 Shear stress-shear rate data for quince nectar heated to 70ºC; a) ohmically, b) conventionally.� RM � 0 minutes � 10 minutes� 15 minutes � 20 minutes 30 minutes
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
b)
a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
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854 BOZKURT AND ICIER
solutions). The energy of activation for flow in non-Newtonian fluids is significantlylower than the corresponding value for Newtonian fluids of the same solids concen-tration.[47] In suspensions of fluid foods of high non-soluble solids concentration, likefruit or vegetable pulps, (Ea) may be lower than the activation energy for viscous flowof water (14.4 kJ/mol). Pulpy juices, have lower (Ea) values, meaning that tempera-ture has a smaller effect on (K) than in clarified juices.[40] The activation energy forflow increased with the juice concentration and decreased with the presence of sus-pended particles in the fruit product.[47] There is negligible information about therheological characterisation of quince nectar and its activation energy value in litera-ture. The temperature dependency of consistency coefficient was determined by usingArrhenius type equation. The consistency coefficients at reference temperature(20°C) predicted by this relation (0.655 for ohmic heating and 0.654 for conventionalheating) were also similar those obtained by rheology models using experimental dataat 20°C (Table 4). It was obtained that the activation energy values were 9.88 ± 3.24kJ/mol and 10.08 ± 2.53 kJ/mol for ohmic heating and conventional heating respec-tively. There was no significant difference for activation energies of both heatingmethods (p > 0.01). The similar activation energies for consistency coefficientshowed also similar effects of both heating methods. Because of same thermal historyapplied to both juices, it could be said that ohmic heating had not any further electrical
Figure 3 Experimental and model curves for quince nectar heated to 65ºC; holding time of 0 minutes a) ohmically,b) conventionally.
Experimental Bingham Model� Herschel Bulkey Model � Power Law Model� Casson Model � Newtonian Model
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
b)
a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
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RHEOLOGY OF QUINCE NECTAR DURING OHMIC HEATING 855
effect in addition to thermal effect on rheological characteristics of quince nectar. It isthought that these results will give detailed information in the design of continuousohmic heating systems being applied to fruit juice products showing non-Newtonianfluid character.
CONCLUSIONS
The rheological behaviour of quince nectars was examined by application of differentmathematical models. Herschel-Bulkley model fitted the experimental data best, for alltemperatures. Non-Newtonian shear thinning behaviour was obtained for the quince nectar atthe pasteurization conditions (temperature range of 65–70°C, and holding time of 0–30 min).Temperature affected the consistency coefficient and flow behaviour index (p < 0.05).Activation energies of temperature dependent consistency coefficient for ohmic and con-ventional heating was not statistically different (p > 0.01). It could be said that, ohmicheating did not cause different effect on the rheological properties of quince nectar com-pared to conventional heating. It could be applied as an alternative heating method providingrapid and uniform heating.
Figure 4 Experimental and model curves for quince nectar heated to 70ºC; holding time of 15 minutes; a) ohmically,b) conventionally.
Experimental Bingham Model� Herschel Bulkey Model � Power Law Model� Casson Model � Newtonian Model
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
b)
a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
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856 BOZKURT AND ICIER
NOMENCLATURE
Ea Activation energy (kJ/mol)OH Ohmic heatingRM Raw materialSS Shear stressSR Shear rateR Ideal gas constant (8314 J/mol K)T Temperature (°C)T0 Reference temperature (°C)t Time (min), in Fig. 1 (s)V Voltage (V)WB Water bathn Flow behaviour index (dimensionless)K Consistency coefficient, (Pa.sn)K0 Consistency coefficient at reference temperature (Pa.sn)Kc Casson constant (Pa.s)0.5
t Shear rate (s−1 )
Figure 5 Experimental and model curves for quince nectar heated to 75ºC; holding time of 30 minutes; a) ohmically,b) conventionally.
Experimental Bingham Model� Herschel Bulkey Model � Power Law Model� Casson Model � Newtonian Model
a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80
SR (1/s)
SS
(P
a)
b)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 10 20 30 40 50 60 70 80SR (1/s)
SS
(P
a)
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RHEOLOGY OF QUINCE NECTAR DURING OHMIC HEATING 857
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