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Viscosity changes in orange juice after ultrafiltration and evaporation
JoumalofFoodEngineering 25 (1995) 387-396 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0260-X774/95/$9.50 ELSEVIER
0260-8774(94)00013-l
Viscosity Changes in Orange Juice After Ultrafiltration and Evaporation E. Hernandez, * USDA Citrus and Subtropical
Products Laboratory, FL 33881, USA
600 Ave. S NW, Winter Haven.
C. S. Chen, J. Johnson University of Florida, IFAS/CREC,
700 Exp. Station Road, Lake Alfred, Florida, USA &
R. D. Carter Florida Department
of Citrus, 700 Exp. Station Road, Lake Alfred, Florida, USA (Accepted 3 March 1994) ABSTRACT
Suspended solids, including cloud and pectin, were removedfrom orange juice using a hollow fiber ultrafiltration membrane (500 kDa). Orange juice serum had a clear amber color with no measurable turbidity Regular and clarified orange juices were concentrated in a four stage, three effect TASTE (thermally accelerated short time evaporator) evaporator. The viscosity of the clarified juice was appreciably lower than that of regular juice, i.e. 1555 mPa for serum and 164 mPa (at 100 s-‘) for regular orange juice concentrate (6.X 66.5 “Brix). Flow behavior of orange juice serum tended to be Newtonian. Deviations from Arrhenius behavior were found for changes in viscosity of orange juice with temperature. Reduction in viscosity of orange juice increased heat transfer coeficients as reflected by the higher evaporation rates obtained for serum concentration. NOTATION
CP
D
Heat capacity (J/kg K) Tube diameter (m)
*Present address: Texas A & M University, Food Protein R & D Center, Cater-Mattil Hall, College Station, TX 77843, USA. 387
E. Hernandez et al.
388 Ea
hc
h*
k”
4
&
F ;lr Re R T V W 11
va 7
Y 17
Energy of activation (kcal/gmol) Heat transfer coefficient (W/m2 K) Dimensionless heat transfer coefficient @D/k) Consistency coefficient Heat conductivity (W/mK) Consistency coefficient of solvent Consistency coefficient of bulk fluid Consistency coefficient at wall temperature Tube length (m) Flow behavior index Prandtl number Reynolds number Gas constant Temperature (“C) Volume rate (l/s) Flow rate (kg/s) Viscosity Apparent viscosity Shear stress Shear rate Surface tension INTRODUCTION
Viscosity of fruit juices in general changes with content of soluble and suspended solids. In orange juice for example, pectin and sugar concentration are the main factors in changes of viscosity (Rouse et al., 1974). Viscosity changes, particularly in increases of viscosity, are determinant factors in operations such as concentration by evaporation and reverse osmosis, pumping, homogenization and blending (Rao & Anantheswaran, 1982). Therefore, the removal of suspended solids from fruit facilitates handling of the juice and increases heat transfer coefficients during evaporation (Hernandez et al., 1992a). Orange juice concentrate, as is the case of many fluid foods, is nonNewtonian pseudoplastic and, in general, obeys the power law model shown in eqn ( 1). For high pulp juice concentrates, flow behavior shows a yield stress and obeys a modified Casson equation (eqn (2)) (Saravacos, 1970; Vitali & Rao, 1984a, b; de Dios-Alvarado & Romero, 1991). r= Ky” ro.5 _ K, zz KY”
(1) (2)
Juice concentrates with pulp content higher than 20% show an appreciable yield stress (Mizrahi & Berk, 1972). However, dilute juices show Newtonian flow behavior in general. The effect of temperature on apparent viscosity is usually described by an Arrhenius model, particularly at high temperatures va = 7, exp(EalRT
)
(3)
Viscosity changes in orange juice
389
The presence of suspended solids, such as pulp and pectin, tends to increase the apparent viscosity of the juice and also tends to decrease the activation energy. The effect of viscosity of fruit juice concentrates on heat transfer and on transfer phenomena in general has not been widely studied. For heat transfer in laminar flow, energy balance equations have been solved and useful equations have been derived for Newtonian fluids, as summarized by Skelland ( 1967) (4) and for power law fluids, h,D/k=
2[wCP/kl]“3[(K,/K,)(3n+
1)/2(3n-
1)]“‘14
(5)
In the case of evaporation, heat transfer coefficients will depend on the type of flow and on the type of evaporator. Several empirical expressions for local heat transfer coefficients (Table 1) have been proposed for different types of evaporators or heat exchangers. In general a reduction in fluid viscosity will increase heat transfer coefficients and allow a more efficient handling of product, i.e. pumping and flow through pipes. Ultrafiltration of fruit juices was first introduced as a means to remove haze in juices such as apple, pear and grape (Heatherbell et al., 1977; Hernandez & Schwartzberg, 1984). Now ultrafiltration is commercially applied to orange and grapefruit juices with the purpose of removing pulp from the juice to facilitate juice flow through debittering resin columns (Hernandez et al., 1992b). The pulp removed from these juices is usually added back to the debittered serum right after the debittering stage. Ultrafiltration also has been used to facilitate evaporation and spray drying of clarified passion fruit juice (Yu & Chiang, 1986; Chen et al., 1991). The rheological properties of both the clear serum (permeate) and concentrated pulp (retentate) differ from the properties of the original juice. The clear serum is composed mostly of sugars and acids and its flow behavior is generally Newtonian (Crandall et al., 1982). The main objective of this work was to study the changes in viscosity of orange juice and permeate at concentrations and temperatures typically encountered in juice evaporators and to see how a decrease in viscosity affected the evaporation rates during the evaporation process.
TABLE 1
Examples of Local Heat Transfer Coefficients h* = 0.0086 Refl.XPpb (a,/~) h* = 0.023 Re0’8 P#“4
h* = 0.042
Re0’17
Ptil.53
h* = 8.5 Rew2 Pr1/3 ( VJ V,)
Natural circulation evaporators (Piret & Isbin, 1954) Forced circulation evaporators (Coulson & Richardson, 1978) Falling film evaporators (Re > 104) (Muddawar & El-Masri, 1986) Climbing film evaporators (Burgois & Le Maguer, 1984’1
E. Hernandez et al.
390
EXPERIMENTAL Ultrafiltration
Samples of freshly squeezed Valencia orange juices were extracted and finished with a FMC extractor and finisher (Lakeland, FL) using the same settings used for commercial operations for a ‘soft squeeze’ type of extraction operation (3 in cup, 0.04 in strainer hole, 0.020 in finisher strainer openings). A pilot ultrafiltration system for fruit juices (Koch, Inc., Willmington, MA) was used to remove suspended solids in the juice. It consisted of three hollow fiber cartridges (Polysulfone, 5 x lo5 MW cut off, 4.68 m*, 0.75 mm i.d.) arranged in parallel. A 15 hp pump was used to sustain the pressure in the system. The system was operated at 15 psig transmembrane pressure. Juice samples were processed at a 8 : 1 concentration ratio in a continuous mode. A shell and tube heat exchanger, placed before the UF cartridges, was used to maintain constant temperature (25°C). Samples of feed, clarified serum, and retentate were collected at different time intervals for later analyses. Evaporation
Evaporation of fresh orange juice and juice serum was conducted in a threeeffect, four-stage TASTE pilot evaporator (Gulf Machinery, Co., Clearwater, FL) and concentrated to 65” Brix or higher. The juice was fed at 4.99 kg/min, and 25°C inlet temperature. Evaporation rates were measured by monitoring changes in soluble solids of concentrate and by condensate collection at different time intervals. A more detailed operation of the evaporator is described by Bates and Carter (1984). Experimental overall heat transfer coefficients (U) were calculated from temperature changes and evaporation rates at each effect, tube area and temperature difference ( U= Q/A T ). Rheological measurements
The viscosity of concentrated (45-65” Brix) juice and permeate was measured using a Haake Rotovisco RV-12 viscometer (Haake Instruments, Paramus, NJ). The MVI sensor system was used along with the M 500 measuring drive unit. A Haake PG-142 programmer was used to control the applied shear rate. A constant temperature bath with circulating glycol, was used to control the temperature. Viscosities of permeate and juice of less than 45” Brix were measured with a glass capillary Cannon-Fenske viscometer in a constant temperature bath. Soluble and suspended
solids measurements
Orange juice, permeate and retentate were analyzed for pH, titratable acidity (titrated with standard alkali solution), soluble solids (measured as degrees Brix with a bench top refractometer) and pectin content (as galacturonic acid by the calorimetric assay of Blumenkrantz & Asboe-Hansen, 1973). Suspended solids were determined by the sinking pulp method (Ting & Rouseff, 1986) by centrifuging a 50 ml sample of juice in a conical test tube at 367.2 x g for 10 min
Viscositychanges in orange juice
391
and measuring the volume of the settled pulp. Turbidity was measured in a Hach Ratio X/R Nephelometer (Hach Co., Loveland, CO). All measurements were done in triplicate. RESULTS AND DISCUSSION Ultrafiltration of orange juice removed all the pulp and most of the pectin and the resulting serum had negligible turbidity in the nephelometer. Removal of suspended solids from the juice produced a significant decrease in Brix in the permeate. As reported by Scott et al. (1960), the presence of suspended and soluble pectin in orange juice increases refractometric readings. The evaporation rates and heat transfer coefficients were at least 20% higher for permeate than for conventional orange juice concentrate (see Table 2). Concentrations of up to 82” Brix were achieved for permeate versus 65” Brix for regular orange juice at similar conditions of evaporation temperature and pressure. Flow regimes and heat transfer in TASTE evaporators have not been studied in depth so there are no available equations to predict local or overall heat transfer coefficients. The removal of pulp, however, is likely to improve heat transfer characteristics of the fluid by allowing higher Reynolds numbers of juice and therefore higher turbulency. This was reflected by higher Brix in the final product and higher evaporation rates (Table 2). Increases in evaporation rates due to lowering of orange juice viscosity by homogenization (Crandall et al.. 1988) and for centrifuged orange juice serum (Peleg & Mannheim, 1970) have also been reported. Figure 1 shows changes in apparent viscosities of orange juice concentrate at different temperatures. As reported by other workers (Vitali & Rao, 1984a, b; Crandall et al., 1982) orange juice concentrate is non-Newtonian and generally fits the power law model. As expected the viscosity of orange juice increased sharply at concentrations above 60” Brix. At this concentration the solubility of pectin decreases and a gel network starts to build up. The presence of high methoxyl pectin, sugar, acids, and calcium ions makes orange juice concentrate very susceptible to gel formation, hence the sharp increase in viscosity at concentrations above 65” Brix. No yield stress was detected in the temperaure range studied (575°C). In the higher temperature range the flow behavior of TABLE 2
Changes in Soluble, Suspended Solids and Evaporation Rates in Orange Juice and Permeate
Fresh juice Permeate
Soluble solids “Brix
pH
10-8 9.8
3.21 3.20
*As polygalacturonic acid. hAt 100 s-l, 65”C, 66.5 Brix. (At 65”C, 66.5 Brix.
Pectin” (mgpGA Iti 1.42 00057
Evaporation rate (kglh) 192.0 240.8
Viscosity (mPa s) 163h 15.5’
(& 601% 719.7
K)
392
E. Hemandez et al. Br1x-e
66.5
*
*
59.6
54.0
+
50.7
+
44.9
1600 1600 1400 1300 1200 1100 1000 900 600 700 600 500 400 300 200 100
J
0 0
10
20
30
40
TEMPERATURE
50
60
70
60
C
Fig. 1. Changes in apparent viscosity in orange juice concentrate (100 s-l).
orange juice concentrate approached that of a Newtonian fluid. Flow behavior indices for conventional orange juice concentrate were in the range of 0.7 and 0.8. The viscosity of orange juice concentrate was still appreciably higher than that of serum in the higher temperature range. The contribution of suspended solids to viscosity of orange juice seemed to be more accentuated at lower temperatures. The consistency index (K) decreased as temperature decreased in the range studied, i.e. 4-27 at 5°C and 0.46 at 75°C (665” Brix) (See Table 3). This is in agreement with results reported by Vitali and Rao ( 1984~). Changes in viscosity for orange juice concentrate showed some deviation from Arrhenius behavior, particularly at high degrees Brix (Fig. 2). Energy of activation tended to be the same at concentrations studied between 45 and 75”C, ranging between 2.9 and 3.5 kcal/gmol; Ea between 5 and 45°C was also the same at all the concentrations studied, ranging between 8.4 and 7.8. A decrease in Ea at lower temperatures has been reported previously (Vitali & Rao, 1984a, 6) for viscosity of orange juice concentrate measured between - 18 and 30°C and has been associated with increased particle-particle interaction. Apparently the presence of pulp and pectin contributes to this deviation especially at higher concentrations. Removal of pulp lowered the viscosity of orange juice appreciably. This effect was more apparent at higher temperatures. Samples of permeate at low and high concentrations and low and high temperatures showed Newtonian behavior. Figure 3 shows viscosity changes of orange juice serum with temperature at different concentrations. The apparent viscosities are in fact somewhat lower than sugar solutions. This is due to the presence of other dissolved solids such as acids. Viscosities of serum concentrate reported by others (Vitali & Rao, 1984~) are higher than conventional sucrose solutions. This is due to the fact
Viscosity changes in orange juice
393
TABLE 3
Power Law Parameters for Orange Juice Concentrate Temperature (“C)
Concentration 66.5
5
15
25
35
45
55
65
75
k
4.27 0.786
0.771 2.52
0.711 2.16
0.675 1.62
0.773 0.648
0.767 0.539
0.762 0.487
0.742 0.460
k
0.739 1.84
0.803 0.721
0.813 0.425
0.802 0.314
0.792 0.259
0.778 0.238
0.77 1 0.226
0.75 1 0.258
:
0.544 0.819
0.819 0,325
0.818 0.225
0.815 0.166
0.807 0.135
0.794 0.125
0.782 0.132
0,784 0.135
k
0.852 0.238
0.849 0.160
0.828 0.129
0.814 0.104
0.813 0.086
0.800 0.081
0.797 0.079
k
0.161 0.842
0.842 0.095
0.821 0.079
0.805 0.066
0.780 0.064
(Brix)
59.8
54.8
50.7
44.9
0
Brix -8-
65.5
B
+
59.8
B
-9-
54.6
6
+
50.7
B
-
44.9
B
‘OOO; ; 100
4
10”“““““““““““““““““““~ 0.26
0.29
0.30
0.31
0.32
0.33
0.34
0.35
0.36
l/T (l/K)
Fig. 2.
Arrhenius
curves for orange juice concentrate.
that these workers obtained juice serum by centrifugation and some remained in the serum. The effect of temperature on viscosity of orange juice serum Arrhenius equation in the lower Brix range ( 12.2-41.2” Brix) (see Fig. the Arrhenius plots of these samples R*-values were greater than
pectin obeyed 4). For 0.994.
E. Hernandez et al.
394 +
O Brix
*
66.5
-%-
51.4
41.2
+
30.9
+
12.2
200 160 160 d
140
E _
120
z 5
100
g
60
5
60 40 20
0
30
20
10
TEMPERATURE
Fig. 3. 0
Brix
60
70
60
C
Viscosity changes in orange juice permeate with temperature.
-8-
66.5
--t
30.6
0.26
50
40
0.29
B
0.30
*
55.5
--c
20.5
0.31
B
0.32
51.4
+
12.2
0.33 lfr
Fig. 4.
*
0.34
+
41.2
B
0.35
0.36
0.37
0.36
WI
Arrhenius curves for orange juice serum.
However at concentrations higher than 5 1.4“ Brix the permeate deviated from an Arrhenius type of behavior over the studied temperature range (5-75°C). This effect was less pronounced for regular orange juice concentrate. Activation energies relating viscosity to temperature ranged from 5.8 kcal/ mole at 40” Brix to 4.5 kcal/mole at 12” Brix. Vitali and Rao (1984a, b) also reported a slight decrease in activation energy at lower temperatures.
Viscosity changes in orange juice
395
Concentration of orange juice serum obtained by ultrafiltration presents a further advantage over concentration of regular orange juice with respect to preservation of aromas. Many aroma compounds, particularly hydrophobic ones remain in the retentate during the ultrafiltration step (Hernandez er al., 1992~). Therefore, many aroma compounds are not subjected to the longer heating conditions found in the evaporator. CONCLUSIONS Removal of suspended solids from orange juice greatly reduced its viscosity and allowed for higher concentration in the final product. Flow behavior of orange juice concentrate agreed with the power law equation and no yield stress was detected. Changes in viscosity with temperature did not strictly follow the Arrhenius equation. Calculation of parameters such as Re and Pr and therefore prediction of local h* is difficult because the viscosity changes of the product in the evaporator are not known, particularly at high degrees Brix. Actual calculation of apparent viscosity at conditions found in the evaporator is difficult because the flow regimes found in the evaporator have not been well studied and these should be the subject of further research. REFERENCES Bates, R. P. & Carter, R. D. (1984). The suitability of TASTE evaporator for muscadine grape juice concentrate production. Proc. Fla. State Hort. Sot.. 97,84-9. Blumenkrantz, N. & Absoe-Hansen, G. (1973). New method for quantitative determination of uranic acids. Anal. Biochem., 54484-9. Burgois, J. & LeMaguer, M. ( 1984). Modelling of heat transfer in climbing film evaporator III: Application to an industrial evaporator. J. Food Engng, 3, 39-50. Chen, C. S., Carter, R. D., Barros, S. M., Nagy, S. & Hernandez, E. (1991). Evaluation of citrus processing system for passion fruit juice concentration. Proc. Fla. State Hort. sot., 104,51-4.
J. F. (1978). Chemical Engineering Vol. 2. Pergamon Press, New York. Crandall, P. G., Chen, C. S. & Carter, R. D. (1982). Models for predicting viscosity of orange juice concentrate. Food Technol., 5,245-52.
Coulson, J. M. & Richardson,
Crandall, P. G., Davis, K. C., Carter, R. D. 8~ Sadler, G. D. ( 1988). Viscosity reduction by homogenization of orange juice concentrate in a pilot plant TASTE evaporator. J. FoodSci., 53 (5), 1477-81. de Dios-Alvarado, J. & Romero, C. H. (1991). Physical properties of fruits - density and viscosity of juices as functions of soluble solids content and temperature. Fruit Processing, 1,5-8. Heatherbell, D. A., Short, J. & Struebi, P. (1977). Apple juice clarification by ultrafiltration. Confructa, 22,157-69. Hernandez, E. & Schwartzberg, H. G. (1984). Improved clarification of apple juice by ultrafiltration. Paper presented at the 1984 AIChE National Meeting. San Francisco, CA. Paper 59A. Hernandez, E., Chen, P. E., Shaw, C. S., Carter, R. D. & Barros, S. (1992a). Ultrafiltration of orange juice: Effect of soluble solids, suspended solids and aroma. J. Agr. Food Chem., 40 (6), 986-8.
Hernandez, E., Couture, R., Chen, C. S., Rouseff, R. L. & Barros, S. (1992 b). Evaluation of ultrafiltration and adsorption to debitter grapefruit juice and grapefruit pulp wash. J. Food Sci., 57 (3), 664-6,670.
396
E. Hernandez et al.
Mizrahi, S. & Berk, Z. (1972). Flow behavior of concentrated orange juice: Mathematical treatment. J. Texture Studies, 3,69-79. Muddawar, I. A. & El-Masri, M. A. (1986). Momentum and heat transfer across freely falling turbulent liquid films. Znt. J. Multiphase Flow, 12 (5), 77 l-90. Peleg, M. & Mannheim, C. H. (1970). Production of orange-juice concentrate from centrifugally separated serum and pulp. J. Food Sci., 35,649-5 1. Piret, E. L. & Isbin, H. S. (1954). Natural circulation evaporation. Chem. Engng Progr., 50 (6), 305-7.
Rao, M. A. & Anantheswaran,
R. C. (1982). Rheology of fluids in food processing. Food
Technol., 2, 116-26.
Rouse, A. H., Albrigo, L. G., Huggart, R. L. & Moore, E. L. (1974). Viscometric measurements and pectic content of frozen concentrated orange juices for citrus futures. Proc. Flu. State Hot-t. Sot., 293-6. Scott, W. C., Morgan, D. A. & Veldhuis, M. K. (1960). The determination of soluble solids in citrus juices. I. The effect of non-sucrose components on refractometer values. Food Technol., 14 (9), 423-8. Skelland, A. H. P. (1967). Non-Newtonian Flow and Heat Transfer. John Wiley and Sons, New York. Saravacos, G. D. (1970). Effect of temperature on fruit juices and purees. J. Food Sci., 35,122-5.
Ting, S. V. & Rouseff, R. L. (1986). Citrus Fruits and their Products. Analysis and Technology Marcel Dekker, New York. Vitali, A. A. & Rao, M. A. (1984a). Flow properties of low-pulp concentrated orange juice: Serum viscosity and effect of pulp content. J. Food Sci., 49,876-81. Vitali, A. A. & Rao, M. A. (19846). Flow properties of low-pulp concentrated orange juice: Effect of temperature and concentration. J. Food Sci., 49,882-g. Yu, Z. R. & Chiang, B. H. (1986). Passion fruit juice concentration by evaporation and ultrafiltration. J. Food Sci., 5 1,150 1-5.
0260-8774(94)00013-l
Viscosity Changes in Orange Juice After Ultrafiltration and Evaporation E. Hernandez, * USDA Citrus and Subtropical
Products Laboratory, FL 33881, USA
600 Ave. S NW, Winter Haven.
C. S. Chen, J. Johnson University of Florida, IFAS/CREC,
700 Exp. Station Road, Lake Alfred, Florida, USA &
R. D. Carter Florida Department
of Citrus, 700 Exp. Station Road, Lake Alfred, Florida, USA (Accepted 3 March 1994) ABSTRACT
Suspended solids, including cloud and pectin, were removedfrom orange juice using a hollow fiber ultrafiltration membrane (500 kDa). Orange juice serum had a clear amber color with no measurable turbidity Regular and clarified orange juices were concentrated in a four stage, three effect TASTE (thermally accelerated short time evaporator) evaporator. The viscosity of the clarified juice was appreciably lower than that of regular juice, i.e. 1555 mPa for serum and 164 mPa (at 100 s-‘) for regular orange juice concentrate (6.X 66.5 “Brix). Flow behavior of orange juice serum tended to be Newtonian. Deviations from Arrhenius behavior were found for changes in viscosity of orange juice with temperature. Reduction in viscosity of orange juice increased heat transfer coeficients as reflected by the higher evaporation rates obtained for serum concentration. NOTATION
CP
D
Heat capacity (J/kg K) Tube diameter (m)
*Present address: Texas A & M University, Food Protein R & D Center, Cater-Mattil Hall, College Station, TX 77843, USA. 387
E. Hernandez et al.
388 Ea
hc
h*
k”
4
&
F ;lr Re R T V W 11
va 7
Y 17
Energy of activation (kcal/gmol) Heat transfer coefficient (W/m2 K) Dimensionless heat transfer coefficient @D/k) Consistency coefficient Heat conductivity (W/mK) Consistency coefficient of solvent Consistency coefficient of bulk fluid Consistency coefficient at wall temperature Tube length (m) Flow behavior index Prandtl number Reynolds number Gas constant Temperature (“C) Volume rate (l/s) Flow rate (kg/s) Viscosity Apparent viscosity Shear stress Shear rate Surface tension INTRODUCTION
Viscosity of fruit juices in general changes with content of soluble and suspended solids. In orange juice for example, pectin and sugar concentration are the main factors in changes of viscosity (Rouse et al., 1974). Viscosity changes, particularly in increases of viscosity, are determinant factors in operations such as concentration by evaporation and reverse osmosis, pumping, homogenization and blending (Rao & Anantheswaran, 1982). Therefore, the removal of suspended solids from fruit facilitates handling of the juice and increases heat transfer coefficients during evaporation (Hernandez et al., 1992a). Orange juice concentrate, as is the case of many fluid foods, is nonNewtonian pseudoplastic and, in general, obeys the power law model shown in eqn ( 1). For high pulp juice concentrates, flow behavior shows a yield stress and obeys a modified Casson equation (eqn (2)) (Saravacos, 1970; Vitali & Rao, 1984a, b; de Dios-Alvarado & Romero, 1991). r= Ky” ro.5 _ K, zz KY”
(1) (2)
Juice concentrates with pulp content higher than 20% show an appreciable yield stress (Mizrahi & Berk, 1972). However, dilute juices show Newtonian flow behavior in general. The effect of temperature on apparent viscosity is usually described by an Arrhenius model, particularly at high temperatures va = 7, exp(EalRT
)
(3)
Viscosity changes in orange juice
389
The presence of suspended solids, such as pulp and pectin, tends to increase the apparent viscosity of the juice and also tends to decrease the activation energy. The effect of viscosity of fruit juice concentrates on heat transfer and on transfer phenomena in general has not been widely studied. For heat transfer in laminar flow, energy balance equations have been solved and useful equations have been derived for Newtonian fluids, as summarized by Skelland ( 1967) (4) and for power law fluids, h,D/k=
2[wCP/kl]“3[(K,/K,)(3n+
1)/2(3n-
1)]“‘14
(5)
In the case of evaporation, heat transfer coefficients will depend on the type of flow and on the type of evaporator. Several empirical expressions for local heat transfer coefficients (Table 1) have been proposed for different types of evaporators or heat exchangers. In general a reduction in fluid viscosity will increase heat transfer coefficients and allow a more efficient handling of product, i.e. pumping and flow through pipes. Ultrafiltration of fruit juices was first introduced as a means to remove haze in juices such as apple, pear and grape (Heatherbell et al., 1977; Hernandez & Schwartzberg, 1984). Now ultrafiltration is commercially applied to orange and grapefruit juices with the purpose of removing pulp from the juice to facilitate juice flow through debittering resin columns (Hernandez et al., 1992b). The pulp removed from these juices is usually added back to the debittered serum right after the debittering stage. Ultrafiltration also has been used to facilitate evaporation and spray drying of clarified passion fruit juice (Yu & Chiang, 1986; Chen et al., 1991). The rheological properties of both the clear serum (permeate) and concentrated pulp (retentate) differ from the properties of the original juice. The clear serum is composed mostly of sugars and acids and its flow behavior is generally Newtonian (Crandall et al., 1982). The main objective of this work was to study the changes in viscosity of orange juice and permeate at concentrations and temperatures typically encountered in juice evaporators and to see how a decrease in viscosity affected the evaporation rates during the evaporation process.
TABLE 1
Examples of Local Heat Transfer Coefficients h* = 0.0086 Refl.XPpb (a,/~) h* = 0.023 Re0’8 P#“4
h* = 0.042
Re0’17
Ptil.53
h* = 8.5 Rew2 Pr1/3 ( VJ V,)
Natural circulation evaporators (Piret & Isbin, 1954) Forced circulation evaporators (Coulson & Richardson, 1978) Falling film evaporators (Re > 104) (Muddawar & El-Masri, 1986) Climbing film evaporators (Burgois & Le Maguer, 1984’1
E. Hernandez et al.
390
EXPERIMENTAL Ultrafiltration
Samples of freshly squeezed Valencia orange juices were extracted and finished with a FMC extractor and finisher (Lakeland, FL) using the same settings used for commercial operations for a ‘soft squeeze’ type of extraction operation (3 in cup, 0.04 in strainer hole, 0.020 in finisher strainer openings). A pilot ultrafiltration system for fruit juices (Koch, Inc., Willmington, MA) was used to remove suspended solids in the juice. It consisted of three hollow fiber cartridges (Polysulfone, 5 x lo5 MW cut off, 4.68 m*, 0.75 mm i.d.) arranged in parallel. A 15 hp pump was used to sustain the pressure in the system. The system was operated at 15 psig transmembrane pressure. Juice samples were processed at a 8 : 1 concentration ratio in a continuous mode. A shell and tube heat exchanger, placed before the UF cartridges, was used to maintain constant temperature (25°C). Samples of feed, clarified serum, and retentate were collected at different time intervals for later analyses. Evaporation
Evaporation of fresh orange juice and juice serum was conducted in a threeeffect, four-stage TASTE pilot evaporator (Gulf Machinery, Co., Clearwater, FL) and concentrated to 65” Brix or higher. The juice was fed at 4.99 kg/min, and 25°C inlet temperature. Evaporation rates were measured by monitoring changes in soluble solids of concentrate and by condensate collection at different time intervals. A more detailed operation of the evaporator is described by Bates and Carter (1984). Experimental overall heat transfer coefficients (U) were calculated from temperature changes and evaporation rates at each effect, tube area and temperature difference ( U= Q/A T ). Rheological measurements
The viscosity of concentrated (45-65” Brix) juice and permeate was measured using a Haake Rotovisco RV-12 viscometer (Haake Instruments, Paramus, NJ). The MVI sensor system was used along with the M 500 measuring drive unit. A Haake PG-142 programmer was used to control the applied shear rate. A constant temperature bath with circulating glycol, was used to control the temperature. Viscosities of permeate and juice of less than 45” Brix were measured with a glass capillary Cannon-Fenske viscometer in a constant temperature bath. Soluble and suspended
solids measurements
Orange juice, permeate and retentate were analyzed for pH, titratable acidity (titrated with standard alkali solution), soluble solids (measured as degrees Brix with a bench top refractometer) and pectin content (as galacturonic acid by the calorimetric assay of Blumenkrantz & Asboe-Hansen, 1973). Suspended solids were determined by the sinking pulp method (Ting & Rouseff, 1986) by centrifuging a 50 ml sample of juice in a conical test tube at 367.2 x g for 10 min
Viscositychanges in orange juice
391
and measuring the volume of the settled pulp. Turbidity was measured in a Hach Ratio X/R Nephelometer (Hach Co., Loveland, CO). All measurements were done in triplicate. RESULTS AND DISCUSSION Ultrafiltration of orange juice removed all the pulp and most of the pectin and the resulting serum had negligible turbidity in the nephelometer. Removal of suspended solids from the juice produced a significant decrease in Brix in the permeate. As reported by Scott et al. (1960), the presence of suspended and soluble pectin in orange juice increases refractometric readings. The evaporation rates and heat transfer coefficients were at least 20% higher for permeate than for conventional orange juice concentrate (see Table 2). Concentrations of up to 82” Brix were achieved for permeate versus 65” Brix for regular orange juice at similar conditions of evaporation temperature and pressure. Flow regimes and heat transfer in TASTE evaporators have not been studied in depth so there are no available equations to predict local or overall heat transfer coefficients. The removal of pulp, however, is likely to improve heat transfer characteristics of the fluid by allowing higher Reynolds numbers of juice and therefore higher turbulency. This was reflected by higher Brix in the final product and higher evaporation rates (Table 2). Increases in evaporation rates due to lowering of orange juice viscosity by homogenization (Crandall et al.. 1988) and for centrifuged orange juice serum (Peleg & Mannheim, 1970) have also been reported. Figure 1 shows changes in apparent viscosities of orange juice concentrate at different temperatures. As reported by other workers (Vitali & Rao, 1984a, b; Crandall et al., 1982) orange juice concentrate is non-Newtonian and generally fits the power law model. As expected the viscosity of orange juice increased sharply at concentrations above 60” Brix. At this concentration the solubility of pectin decreases and a gel network starts to build up. The presence of high methoxyl pectin, sugar, acids, and calcium ions makes orange juice concentrate very susceptible to gel formation, hence the sharp increase in viscosity at concentrations above 65” Brix. No yield stress was detected in the temperaure range studied (575°C). In the higher temperature range the flow behavior of TABLE 2
Changes in Soluble, Suspended Solids and Evaporation Rates in Orange Juice and Permeate
Fresh juice Permeate
Soluble solids “Brix
pH
10-8 9.8
3.21 3.20
*As polygalacturonic acid. hAt 100 s-l, 65”C, 66.5 Brix. (At 65”C, 66.5 Brix.
Pectin” (mgpGA Iti 1.42 00057
Evaporation rate (kglh) 192.0 240.8
Viscosity (mPa s) 163h 15.5’
(& 601% 719.7
K)
392
E. Hemandez et al. Br1x-e
66.5
*
*
59.6
54.0
+
50.7
+
44.9
1600 1600 1400 1300 1200 1100 1000 900 600 700 600 500 400 300 200 100
J
0 0
10
20
30
40
TEMPERATURE
50
60
70
60
C
Fig. 1. Changes in apparent viscosity in orange juice concentrate (100 s-l).
orange juice concentrate approached that of a Newtonian fluid. Flow behavior indices for conventional orange juice concentrate were in the range of 0.7 and 0.8. The viscosity of orange juice concentrate was still appreciably higher than that of serum in the higher temperature range. The contribution of suspended solids to viscosity of orange juice seemed to be more accentuated at lower temperatures. The consistency index (K) decreased as temperature decreased in the range studied, i.e. 4-27 at 5°C and 0.46 at 75°C (665” Brix) (See Table 3). This is in agreement with results reported by Vitali and Rao ( 1984~). Changes in viscosity for orange juice concentrate showed some deviation from Arrhenius behavior, particularly at high degrees Brix (Fig. 2). Energy of activation tended to be the same at concentrations studied between 45 and 75”C, ranging between 2.9 and 3.5 kcal/gmol; Ea between 5 and 45°C was also the same at all the concentrations studied, ranging between 8.4 and 7.8. A decrease in Ea at lower temperatures has been reported previously (Vitali & Rao, 1984a, 6) for viscosity of orange juice concentrate measured between - 18 and 30°C and has been associated with increased particle-particle interaction. Apparently the presence of pulp and pectin contributes to this deviation especially at higher concentrations. Removal of pulp lowered the viscosity of orange juice appreciably. This effect was more apparent at higher temperatures. Samples of permeate at low and high concentrations and low and high temperatures showed Newtonian behavior. Figure 3 shows viscosity changes of orange juice serum with temperature at different concentrations. The apparent viscosities are in fact somewhat lower than sugar solutions. This is due to the presence of other dissolved solids such as acids. Viscosities of serum concentrate reported by others (Vitali & Rao, 1984~) are higher than conventional sucrose solutions. This is due to the fact
Viscosity changes in orange juice
393
TABLE 3
Power Law Parameters for Orange Juice Concentrate Temperature (“C)
Concentration 66.5
5
15
25
35
45
55
65
75
k
4.27 0.786
0.771 2.52
0.711 2.16
0.675 1.62
0.773 0.648
0.767 0.539
0.762 0.487
0.742 0.460
k
0.739 1.84
0.803 0.721
0.813 0.425
0.802 0.314
0.792 0.259
0.778 0.238
0.77 1 0.226
0.75 1 0.258
:
0.544 0.819
0.819 0,325
0.818 0.225
0.815 0.166
0.807 0.135
0.794 0.125
0.782 0.132
0,784 0.135
k
0.852 0.238
0.849 0.160
0.828 0.129
0.814 0.104
0.813 0.086
0.800 0.081
0.797 0.079
k
0.161 0.842
0.842 0.095
0.821 0.079
0.805 0.066
0.780 0.064
(Brix)
59.8
54.8
50.7
44.9
0
Brix -8-
65.5
B
+
59.8
B
-9-
54.6
6
+
50.7
B
-
44.9
B
‘OOO; ; 100
4
10”“““““““““““““““““““~ 0.26
0.29
0.30
0.31
0.32
0.33
0.34
0.35
0.36
l/T (l/K)
Fig. 2.
Arrhenius
curves for orange juice concentrate.
that these workers obtained juice serum by centrifugation and some remained in the serum. The effect of temperature on viscosity of orange juice serum Arrhenius equation in the lower Brix range ( 12.2-41.2” Brix) (see Fig. the Arrhenius plots of these samples R*-values were greater than
pectin obeyed 4). For 0.994.
E. Hernandez et al.
394 +
O Brix
*
66.5
-%-
51.4
41.2
+
30.9
+
12.2
200 160 160 d
140
E _
120
z 5
100
g
60
5
60 40 20
0
30
20
10
TEMPERATURE
Fig. 3. 0
Brix
60
70
60
C
Viscosity changes in orange juice permeate with temperature.
-8-
66.5
--t
30.6
0.26
50
40
0.29
B
0.30
*
55.5
--c
20.5
0.31
B
0.32
51.4
+
12.2
0.33 lfr
Fig. 4.
*
0.34
+
41.2
B
0.35
0.36
0.37
0.36
WI
Arrhenius curves for orange juice serum.
However at concentrations higher than 5 1.4“ Brix the permeate deviated from an Arrhenius type of behavior over the studied temperature range (5-75°C). This effect was less pronounced for regular orange juice concentrate. Activation energies relating viscosity to temperature ranged from 5.8 kcal/ mole at 40” Brix to 4.5 kcal/mole at 12” Brix. Vitali and Rao (1984a, b) also reported a slight decrease in activation energy at lower temperatures.
Viscosity changes in orange juice
395
Concentration of orange juice serum obtained by ultrafiltration presents a further advantage over concentration of regular orange juice with respect to preservation of aromas. Many aroma compounds, particularly hydrophobic ones remain in the retentate during the ultrafiltration step (Hernandez er al., 1992~). Therefore, many aroma compounds are not subjected to the longer heating conditions found in the evaporator. CONCLUSIONS Removal of suspended solids from orange juice greatly reduced its viscosity and allowed for higher concentration in the final product. Flow behavior of orange juice concentrate agreed with the power law equation and no yield stress was detected. Changes in viscosity with temperature did not strictly follow the Arrhenius equation. Calculation of parameters such as Re and Pr and therefore prediction of local h* is difficult because the viscosity changes of the product in the evaporator are not known, particularly at high degrees Brix. Actual calculation of apparent viscosity at conditions found in the evaporator is difficult because the flow regimes found in the evaporator have not been well studied and these should be the subject of further research. REFERENCES Bates, R. P. & Carter, R. D. (1984). The suitability of TASTE evaporator for muscadine grape juice concentrate production. Proc. Fla. State Hort. Sot.. 97,84-9. Blumenkrantz, N. & Absoe-Hansen, G. (1973). New method for quantitative determination of uranic acids. Anal. Biochem., 54484-9. Burgois, J. & LeMaguer, M. ( 1984). Modelling of heat transfer in climbing film evaporator III: Application to an industrial evaporator. J. Food Engng, 3, 39-50. Chen, C. S., Carter, R. D., Barros, S. M., Nagy, S. & Hernandez, E. (1991). Evaluation of citrus processing system for passion fruit juice concentration. Proc. Fla. State Hort. sot., 104,51-4.
J. F. (1978). Chemical Engineering Vol. 2. Pergamon Press, New York. Crandall, P. G., Chen, C. S. & Carter, R. D. (1982). Models for predicting viscosity of orange juice concentrate. Food Technol., 5,245-52.
Coulson, J. M. & Richardson,
Crandall, P. G., Davis, K. C., Carter, R. D. 8~ Sadler, G. D. ( 1988). Viscosity reduction by homogenization of orange juice concentrate in a pilot plant TASTE evaporator. J. FoodSci., 53 (5), 1477-81. de Dios-Alvarado, J. & Romero, C. H. (1991). Physical properties of fruits - density and viscosity of juices as functions of soluble solids content and temperature. Fruit Processing, 1,5-8. Heatherbell, D. A., Short, J. & Struebi, P. (1977). Apple juice clarification by ultrafiltration. Confructa, 22,157-69. Hernandez, E. & Schwartzberg, H. G. (1984). Improved clarification of apple juice by ultrafiltration. Paper presented at the 1984 AIChE National Meeting. San Francisco, CA. Paper 59A. Hernandez, E., Chen, P. E., Shaw, C. S., Carter, R. D. & Barros, S. (1992a). Ultrafiltration of orange juice: Effect of soluble solids, suspended solids and aroma. J. Agr. Food Chem., 40 (6), 986-8.
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Rao, M. A. & Anantheswaran,
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Rouse, A. H., Albrigo, L. G., Huggart, R. L. & Moore, E. L. (1974). Viscometric measurements and pectic content of frozen concentrated orange juices for citrus futures. Proc. Flu. State Hot-t. Sot., 293-6. Scott, W. C., Morgan, D. A. & Veldhuis, M. K. (1960). The determination of soluble solids in citrus juices. I. The effect of non-sucrose components on refractometer values. Food Technol., 14 (9), 423-8. Skelland, A. H. P. (1967). Non-Newtonian Flow and Heat Transfer. John Wiley and Sons, New York. Saravacos, G. D. (1970). Effect of temperature on fruit juices and purees. J. Food Sci., 35,122-5.
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