1-s2.0-0260877489900204-main.pdf

Journal of

Food Engineering10 (1989) 51-63

Research Note Rheological Properties of Clarified Pear Juice Concentrates

ABSTRACT The rheological behaviour of clartfied and depectinized pear juice is reported. Pear juices free of pectin and puIp behave as Newtonian liquids. The effect of temperature and concentration on the viscosityof thesejuices is examined. Juices of different concentration were obtained by diluting 71”Brix juice. The effect of temperature was studied on ten different temperatures between 5 and 60°C. Finally, an expression for the combined #ect of temperature and concentration on the viscosityis given.

NOTATION A B c

r rl 7.

Constant in eqn (4) (“Brix’) Constant in eqn (4) (“Brix-*) Concentration in soluble solids (“Brix) Activation energy of the flow (kcal grnoll Consistency coefficient (Pa s”) Constant in eqn (3) (Pa s) Constant in eqn (4) (Pa s) Constant in eqn (5) (Pa s) Constantineqn(5)(0Brix-1) Constant in eqn (5) (“Brix-*) Flow behaviour index (dimensionless) Correlation coefficient Gas constant (kcal gmol- l K- ‘) Temperature (“C or K) Shear rate (s-l) Viscosity (Pa s) Shear stress (Pa)

Journul

of Food

E, K K0 K, K* K, K4 n L T

)

57 Engineering

0260-8774/89/$03.50

Publishers Ltd, England. Printed in Great Britain

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0

1989

Elsevier

Science

A. Zban et al.

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INTRODUCTION Juices are the most important liquid derivatives of fruits in the food industry. They can be classified according to their pulp content into two groups: purees and juices, and can be obtained by crushing or by squeezing (Costell & Dur&n, 1982~). Most of the previously published information relates to purees and juices obtained by crushing, which are characterized by high suspendedsolids contents (Foda & McCollum, 1970; Costell & Duran, 19823; Costell et al, 1982). The rheological behaviour of pear juices is reported here. Pear juices are mainly marketed in frozen and concentrated form and, to obtain them, any suspended solid particles normally present in juices obtained by squeezing must be completely removed by filtering and clarifying. Generally, the rheological behaviour of a clarified juice can be described by a power law relationship (Ibarz & Pagan, 1987): r=K(j)”

(1)

where t is the shear stress, i is the shear rate, K is the consistency coefficient and n is the flow behaviour index. When the clarified juice is depectinized, its behaviour is Newtonian (Ibarz et al., 1987): r=qj

(2)

where q is the coefficient of dynamic viscosity.

EXPERIMENTAL Preparation of the samples The samples were prepared from a commercial juice manufactured from the most prevalent pear varieties grown in the Lleida region of Spain (Jules Guyot, William, Blanca de Aranjuez, Conference, Coscia, etc). A commercial juice of 71”Brix was obtained by squeezing, and the pectin was removed by enzymic treatment. The resulting juice was clarified by sedimentation and filtration, and was finally concentrated by evaporation. Samples with lower soluble-solids contents were obtained by diluting the 7 1“Brix juice with distilled water. The soluble-solids content of each juice was determined by means of an Abbe-Zeiss refractometer at 20°C the concentration being expressed in “Brix.

Rheological properties of clatafiedpear juice concentrates

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Rheological measurements The rheological measurements were carried out using a Rotovisco RV 12 (Haake) viscometer, with a M 500-type attachment which can measure a maximum torque of 4.90 N cm, an NV-type pair of coaxial cylinders and a thermostatic bath to control the working temperature within the range 560°C (Ibarz et d, 1987). Rotor speeds were variable in the range l-5 12 rpm, which enabled rheograms (shear stress, t, vs shear rate, i) to be constructed. Readings were taken at increasing rotor speeds until the maximum speed was reached, after which it was gradually reduced. In some cases, it was not possible to measure the shear stress at low rotational speeds, whereas in others difficulty occurred at high speeds when the readings were off the scale of the instrument. Values for the coefficient of viscosity were obtained from the experimental values for the corresponding shear stress and shear rate according to eqn (2).

RESULTS The experimental results were fitted to eqn (2) by the least-squares method and the coefficient of viscosity of the different samples obtained. These results are given in Table 1; all correlation coefficients were greater than O-995. The degree of fit and the estimates of the viscosity TABLE 1 Relationship between Viscosity and Soluble-Solids Temperatures

5 10 15

20 25 30 35 4.5 55 60

Content of Pear Juice at Different

4o”Brix

45”Brix

5o”Brix

55”Brix

6O”Brix

11 k 0.6 9 f 0.6 7 f 0.4 6 f 0.5 5fO-2 4.2 f O-2 3.8+0-4 3 * 0.3 2.3 f 0.2 2 f 0.2

17kO.4 13*0.4 11 f 0.4 9 sf:0.4 7kO.6 6fO.4 5 f 0.6 4f0.2 3.2 + 0.3 3 * 0.2

29 z!z0.3 22 zk0.5 171kO.6 14 f 0.5 llzko.2 9f0.5 8 f O-5 6 f 0.6 4.5 & 0.4 4 f 0.2

52 f 0.5 36 + 0.5 27 k 0.6 21 zko.3 17&O-6 14Iko.7 11 zk0.2 8 + 0.3 6 f 0.2 5.5 * 0.5

112kO.7 79 f 0.6 59 -e 0.4 42 k 0.4 33+06 25fO.3 20 + 0.4 13f0.3 9 k 0.5 8-5 + 1.0

6S’Brix

71“Brix

335 f 3.0 198 f 5.0 135 + 1.0 104 f 1.0 70 f o-5 52+0-4 40 + 0.3 25 k 0.5 17fO-1 14f0.2

1816 a 14.0 1014+9*0 601 f 3.0 386 + 2.0 254 -f-2.0 176 + 1.0 123 kO.5 67 + 1.0 43 f 0.5 35f@5

A.Zbarz etal.

60

are significant at the 95% probability level. It can be seen from the table that the higher the temperature the lower the viscosity, and the higher the soluble-solids content the higher the viscosity. The effect of temperature The change in viscosity with temeprature can be described Arrhenius-type equation (Saravacos, 1970; Rao et al., 1984):

by an

(3) where K, is a constant, E, is the activation energy of flow in kcal (g mol)- ‘, R is the gas constant, and T is the temperature in K. The log of the viscosity coefficient at a particular concentration is plotted against l/T to obtain the values of the constants K0 and E,. In Fig. 1, the resulting straight line is shown for each concentration. Activation energies of flow are obtained from the slope of each straight line and increase with the soluble-solids content. The effect of concentration Two types of equation describing the change in viscosity coefficient with the soluble-solids content can be found in the literature (Vitali & Rao, 10:

2, : l-

-1

O.l-

O.Ol_

1 0.001 3

Fig. 1.

Effect of temperature

3.2

3.4

‘s(?/Tixl03

(K“)

on the juice viscosity. l,40; A, 45;n,50;0,55;A, 60; v,65;q,71"Brix.

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1982; l3ao ec al., 1984; Ibarz et al., 1987). In one of these, the change in viscosity coefficient with concentration follows a power law relationship, and in the other, it is exponential. In the present work, an exponential-type equation is used: 7 = K,exp(AC+

BC2)

(4)

K,, A and B are constants, and C is the concentration in degrees Brix. This equation gives the best fit at all the temperatures studied. To evaluate the constants, the viscosity measurements at a particular temperature and different concentrations are fitted to the linear form of eqn (4) by the least-squares method. The results of these correlations are given in Table 2. The fittings and the estimates of the constants are significant at the probability level of 95%. The values of the constants decrease as the temperature increases. where

Effect of Concentration

TABLE 2 on the Viscosity of Pear Juice at Different Temperatures

Exponential model: 7 = K,exp(AC+ A (“Brix- ‘)

B (“Brix- ‘)

rz

Pa 3) 1.242 0.871 0.269 0.196 0.091 0.057 0.047 0.025 0.015 0.005

- 0.272 - 0.259 -0.217 - 0.207 -0.181 - 0.165 -0.159 -0-135 - 0.122 - 0.085

0.0039 0.0037 0.0032 0*0030 0.0027 0.002 5 0.0024 0.002 1 0.0019 0.0016

0.998 0.997 @997 0.998 0.998 0.998 0.997 0.997 0.995 0.994

K, 5

10 15 20 25 30 35 45 55 60

BC2)

The combined effect of temperature and concentration A single equation describing the combined effect of temperature concentration on the juice viscosity would be useful. The viscosity results shown in Table 1 were fitted to the equation

and

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TABLE 3 Combined Effect of Temperature and Concentration on the Viscosity of Pear Juice Exponentialmodel: II= K,exp(E,,/RT+

K, K,

K‘l

1.009 x 10-7 - 0.180 0.002 7

4 r?

8.21 0.969

K,C+ KJC’jO

Pa s “Brix ’ “B,.k - ? kcal (g mol)- ’

“Units: viscosity in Pa s; temperature in degrees Kelvin.

by multiple linear regression. The values of the constants obtained from this process are shown in Table 3. The fitting and the estimates of the constants are significant at a 95% probability level. It should be emphasized that the constants of eqn (5) are applicable only over the ranges of temperatures and concentrations studied. The value of the activation energy of flow obtained from this fit is the arithmetic mean of the activation energies for the different concentration samples. The values of the constants K, and K, are the arithmetic means of the constants A and B, respectively, and are given in Table 2.

REFERENCES Costell, E. & Durban, L. (1982a). Reologia fisico-quimica de 10s zumos y put-es de frutas. Rev. Agroquim. Tecnol. Aliment., 22(l), 80-94. Costell, E. & DurQn, L. (1982b). Reologia fisico-quimica de1 pure de albaricoque I. Determination de las caracteristicas quimicas, fisicas y estructurales. Rev. Agroquim. Tecnol, Aliment., 22( 3), 38 l-94. Costell, E., Clemente, G. & Dunin, L. (1982). Reologia fisico-quimica de1 pure de albaricoque II. Caracterizacion de1 flujo y relation entre 10s partimetros reologicos y las caracteristicas quimicas y fisicas de1 producto. Rev. Agroquim. Tecnol. Aliment., 22(4), 539-50.

Foda, Y. H. & McCollum, J. P. ( 1970). Viscosity as affected by various constituents of tomato juice. J. Food Sci., 35(4), 333-8. Ibarz, A. & Pagin, J. (1987). Rheology of raspberry juices. J. Food Eng., 6, 269-89.

Ibarz, A., Vicente, M. & Graell, J. (1987). Rheological behaviour of apple and pear juices and the concentrates. J. Food Eng., 6,257-67. Rao, M. A., Cooley, H. J. & Vitali, A. A. (1984). Flow properties of concentrated juices at low temperature. Food Technol., 38( 3), 113-19. Saravacos, G. D. ( 1970). Effect of temperature on viscosity of fruit juices and purees. J. FoodSci., 35,122-5.

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Vitali, A. A. & Rao, M. A. ( 1982). Flow behaviour of guava as a function of temperature and concentration. J. TextureStudies, 13,275-89. A. Ibarz, J. Pagh Departament de Tecnologia dilliments i Quimica Agricola, Escola Tknica Superior d’Enginyers Agrdnoms de Lleida, UniversitatPolitknica de Catalunya, Avgda. Aicalde Rovira Rowe, 1772.5(X%-Lleida,Spain J. GutiCrrez & M. Vicente Departament d’Enginye& Quimica, Facultat de Quimica, Universitatde Barcelona, Barcelona, Spain (Received 7 October 1988; revised version received 23 March 1989; accepted 29 March 1989)