Rheological behaviour of sloe (Prunus spinosa) fruit juices

Journal of Food Engineering 27 (1996) 423-430 Copyright 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0260-8774/96 $15.00+0.00 0260-8774(95)00024-O

ELSEVIER

Rheological Behaviour of Sloe (Prunus Spinosa) Fruit Juices A. Ibarz Departament

de Tecnologia d’Aliments, ETSEA, Universitat Rowe, 177, 25198 Lleida, Spain

de Lleida, Rovira

A. Garvin & J. Costa Departament

d’Enginyeria Quimica, Facultat de Quimica, Universitat Marti i FranquCs, 1, 08028 Barcelona, Spain

(Received

8 December

de Barcelona,

1994; revised version received 1 March 1995; accepted 2 April 1995)

ABSTRACT The rheological behaviour of sloe fruit juices with low pulp and pectins contents is studied. These juices, containingpectins and pulp, behave as non-Newtonian with a yield stress, so the Bingham model was used to describe the relationship between the shear stress and the shear rate. The effect of temperature on the plastic viscosity was examined, and it was concluded that an Arrhenius type equation describes this effect. The effect of soluble solids contents can be described by two types of equations, power-law and exponential expressions. Finally, an equation describing the combined effect of temperature and concentration on the plastic viscosity was developed.

NOTATION

b, bz b3 C E, K

Constant in eqn (5) (dimensionless) Constant in eqn (6) (“BrW’) Constant in eqn (7) (“Brix-‘) Concentration (“Brix) Activation energy for flow (kcaVmo1) Consistency index (Pa sn) 423

A. Ibarz, A. Garvin, J. Costa

424

: T

Flow behaviour index (dimensionless) Gas constant (1.987 cal/mol K) Temperature (“C or K) Shear rate (s-l) Plastic viscosity (Pa s or mPa s) Constant in eqn (4) (Pa s or mPa “) Constant in eqn (5) (mPa s “Brix- ‘) Constant in eqn (6) (mPa s) Constant in eqn (7) (mPa s) Shear stress (Pa or mPa s) Yield stress (Pa or mPa s)

1. INTRODUCTION The fruit of sloe (Prunus spinosa) has a light acid and nice flavour. Its most important market is jam manufacture and a very popular liquor, called ‘Pacharan’, made in Northern Spain (La Rioja, Navarra). The sloe liquor is obtained by soaking or macerating the fruit in brandy or liquor before distillation. Its market is increasing, therefore a study of its juice behaviour is important for a better knowledge of the properties of the fruit. Generally, the rheological behaviour of a juice cannot be described by a Newtonian equation. There are different equations which describe the nonNewtonian behaviour (power-law, Bingham equation, Herschel-Bulkley, equations (l), (2), (3) respectively): a=K(j)”

(I)

a=f&+q’j

(2)

o=oo+K,(j)”

(3)

where (T is the shear stress, go is the yield stress, j is the shear rate, K and & are the consistency coefficients, n is the flow behaviour index, and q’ is the plastic viscosity. In this work, the rheological behaviour of sloe fruit juice with low pulp and pectin content was studied, as well as the effect of temperature and soluble solids concentration.

2. EXPERIMENTAL 2.1. Preparation

of samples

The samples were prepared from wild sloe fruits obtained in the La Rioja region (Spain). The juice was prepared by peeling, crushing, sieving, and finally, concentration by evaporation. The concentration of soluble solids in the concentrated sloe fruit juice was 58.5 “Brix, determined by refractometry. Samples with lower soluble solids contents were obtained by dilution with distilled water.

Rheological behaviour of sloe fruit juices

42s

2.2. Analytical methods 2.2.1. Soluble solids contents A digital refractometer (Atago 1000X) was used at a temperature of 20°C. The measured concentration of soluble solids in the concentrated sloe fruit juice was 585 “Brix, and the concentration of the other samples, with lower contents, were measured after diluting with distilled water. 2.2.2. pH A Crison 2000 pH-meter 3.11.

was used. The pH of the concentrated

juice was

2.2.3. Acidity This value was determined by titration with NaOH 0.1 N solution using a phenolphthalein indicator. The acidity of the concentrate was 9.8 x lop4 eq. H+/g of concentrated juice. 2.2.4. Pulp content The pulp content was determined by centrifugation, at a rate of 5000 rpm for 5 mitt, before filtration using a filter paper on a Buchner funnel. The value obtained for the concentrated juice was 3.6 g of pulp/100 g of concentrated juice. 2.2.5. Pectin The pectic substances were precipitated by alcohol. The total residue was used for the determination of total pectin. An alcoholic carbazole solution in acidic medium is added to provide a colour which is measured at 525 nm by a Philips PV 8700 spectrophotometer (IFFJP, 1984). For the concentrated juice the total pectin content was 87.5 mg galacturonic acid/kg juice. 2.3. Rheological measurements The rheological measurements were carried out on a Rotovisco RV 12 (Haake) concentric cylinder viscometer equipped with a M-500-type measurement attachment which can transmit a maximum torque of 4.90 N cm, using an NV-type pair of coaxial cylinders. A thermostatic bath controls the working temperature within the range 5-65°C. Rotor speeds were variable in the range 0.01-512 rpm. Readings were taken at decreasing rotor speeds until a minimum speed was reached, after which it was gradually increased. The rheological behaviour of the sloe juice at different soluble solids contents (58.5, 50, 45, 40, 35 and 25 “Brix) and temperatures (5, 15, 25, 35, 45, 55 and 65°C) was studied. 3. RESULTS

AND DISCUSSION

First of all, thixotropy was not observed. The experimental values of shear stress (0) and shear rate ($) have been fitted by the power-law [eqn (l)], Bingham equation [eqn (2)] and by the

A. Ibarz, A. Garvin, J. Costa

426

TABLE 1 Parameters of Bingham’s Equation at Different Soluble Solids Content and Temperatures for Sloe Fruit Juices

25

3.5

40

45

50

58.5

5 15 25 35 45 55 65 5 15 25 35 45 55 65 5 15 25 35 45 55 65 5 15 25 35 45 55 65 5 15 25 35 45 55 65 5 15

32: 45 55 65

9-l * 0.4 6.8f0.7 5.2kO.6 4.0f0.6 3.6*0.8 2.9*0.8 2.5 f 0.7 21.7*0.6 14.2 f 1.1 10.7 + 1.0 8.1 kO.7 6.8kO.4 6.0f0.6 5.3 * 0.0 34.9 + 2.5 24.4 f 1.0 17.8kO.8 13.6 f 0.4 ll.Of0.3 9.15 0.4 7.9 + 0.7 59.9 f 4.3 41.0f2.9 29.4 f 2.1 22.3 + 1.3 18.0+ 1.1 14.9 + 0.8 13.5 + 0.5 120.2A11.5 79.9 * 7.0 45.2k4.0 34.1 k 2.3 26.3 k 2.0 21.9 + 1.6 18.7 f 1.4 772.4 * 87.8 383,3 f 37.2 201.0 + 23.0 118.9k11.9 84-5 + 10.8 56-7 + 5.9 48.3 f 4.9

1.74 k 0.52 2.06 * 0.86 2.14kO.82 2.25 f 0.80 1.74* 1.40 2.15 + 0.92 2.42 * 0.90 4.30 + 0.80 3.91* 1.48 3-46 * 1.40 3.26 k 0.94 2.72 k 0.62 2.09* 1.11 2.20 * 0.00 4.83 & 2.50 4.57& 1.18 4.1ot_o-99 2.97 _t O-51 3.26 f O-35 3.16*0-43 3.04 If:0.95 8.33 f 4.16 6.26 + 3-10 5.55 + 2.34 4.59 f 1.46 4.14 + 1.37 4.29 f 0.99 4.53 + 0.69 7.57 f 5.80 7.35 * 3.55 8.86 f 4.54 6.43 f 2.48 5.79 f 2.44 5.33 f 1.93 5.15 f 1.67 11.19k5.55 LO-28+ 4-48 1231& 6.20 LO-24k 5-48 11.83 k 6.09 11.13k5.93 959 * 5.00

2.07

3.13

3.70

5.53

6.64

10.94

0.9998 0.9996 0.9984 0.9973 0.9998 0.9962 o-9970 O-9998 O-9983 0.9987 0.9990 0.9998 0.9999 1.oooo 0.995 1 0.9982 0.9984 0.9993 0.9997 0.9989 0.9979 0.9954 0.9968 0.9975 0.9983 0.9985 0.9993 0.9996 0.9932 0.9942 0.9961 0.9971 0.9978 0.9980 0.9979 0.9908 0.9918 0.9916 0.9901 0.9919 0.9920 O-9922

Rheological behaviour of sloe fruit juices

427

model of Herschel-Bulkley [eqn (3)]. A better fit was obtained with the Bingham model in all cases. The fitted values (oO,q’) are given in Table 1. The degree of fit and the estimates of the Bingham’s coefficients are significant at the 95% probability level. It can be seen from the table that the higher the temperature, the lower the plastic viscosity, and the higher the soluble solid contents, the higher the plastic viscosity. 3.1. The effect of temperature The variation Arrhenius-type

in plastic viscosity with temperature can be described by an equation (Saravacos, 1970; Rao et al., 1984; Ibarz et al.,

1992a, b): ~l’=vl~, exp(&IRT)

(4)

where q’ is the plastic viscosity, ye, is a constant, E, is the activation energy of flow, R is the gas constant, and T is the absolute temperature in Kelvin. Figure 1 shows the effect of temperature on plastic viscosity of the samples according to the Arrhenius equation and the best fit lines are drawn. The parameters of this equation are shown in Table 2. The activation energy increases with the soluble solids contents, therefore, temperature has a greater effect on the samples with higher soluble solids contents. This tendency is similar to the other juices (Saravacos, 1970; Ibarz & Pagan, 1987; Ibarz et al., 1992a). The yield stress showed little variation with temperature. The average values for each sample are shown in Table 1. 3.2. The effect of concentration Table 1 clearly shows that soluble solids contents increases the plastic viscosity of sloe fruit juices. Therefore, in this section the effect of soluble Icoo

w-2

,-.

t--

Fig. 1.

Effect of temperature

I

1

on plastic viscosity. The best fit lines are depicted.

A. ibun, A. Garvin, J. Costa

428

Arrhenius-type

Parameters

TABLE 2 Relating the Effect of Temperature of Sloe Fruit Juices

(mPa s)

(kcallmol)

El

r

6-35 x low” 7.70 x lop3

3-99 *o-41 4.32 k 0.85

0.9960 0.9857

11.08 7.32 xx lo-” lop3 2.60 x lop3 8.58 x 1O-5

4.70 *f 0.70 4.64 O-51 5.87 f 1.14 8.76 + 1.37

0.9918 0.9954 O-9860 0.9908

ra

25 35 t: 50 58.5

Effect

of Concentration

on the

TABLE 3 Plastic Viscosity Temperatures”

of Sloe Juices,

Model: q ’ = yI (C) hf T/“C 5 15 25 35 45 55 65 “Units:

x x x x x x x

1o-6 10W6 low 1O-6 1O-6 1O-6 1O-6

at Different

Model: ;rl’= q2 exp (b, C)

b,

r

4.84 4.49 4.03 3.81 3.56 3.40 3.39

0.9361 0.9444 0.9496 0.9618 0.9606 0.9753 0.9776

‘II 0.93 2.27 8.08 13.28 27.76 40-38 36.73

on Plastic Viscosity

?2

O-258 0.254 0.286 0.272 0.290 0.287 0.255

b2

r

0.128 0.118 0.106 0.100 0.093 0.088 0.087

o-9740 0.9802 0.9822 0.9896 0.9890 0.9962 0.9967

r’ in mPa s; C in “Brix.

solids contents on the rheology of sloe fruit juices at different temperatures was studied. In order to quantify the dependence of plastic viscosity on soluble solids contents, two equations were applied: power-law type and exponential type, as in other investigations (Vitali & Rao, 1982; Rao et al., 1984; Ibarz & Pagan, 1987; Ibarz et al., 1992a, b): (5) (6) In both equations, vi and bi are constants and C is the concentration in “Brix. For different samples with different soluble solids contents and at different temperatures, the variation in plastic viscosity was fitted to eqns (5) and (6) by the least-squares method, showing in both cases that the fit and the estimates of the parameters were significant at a probability level of 95%. Table 3 shows the values of the parameters of the power-law and the exponential relations [eqns (5) and (6)]. It seems that the exponential model gives better fit than the power-law model.

Rheological behaviour of sloe fruit juices

429

Figure 2 shows the effect of concentration on the plastic viscosity of the samples according to the exponential eqn (6). 3.3. Combined effect of temperature and concentration It is important to obtain a single expression that includes the effect of temperature and soluble solids contents on the rheological behaviour. For sloe fruit juice it is assumed that plastic viscosity varies with temperature and concentration according to an exponential equation of the type: v’=v~ exp(E,lRT+b3C)

(7) in which q3 and b3 are the parameters to be determined, E, is the activation energy of flow, T is the absolute temperature and C is the soluble solids contents in “Brix. The variation of plastic viscosity with temperature and concentration was fitted to the linear form of eqn (7) by multiple regression. Both the fit and the estimates of the parameters proved to be significant, at a probability level of 95%. The values of the different parameters were: q3=3*40 x lo-”

mPa s

b,=0.103 kO.004 “Brix’ E,=5.38 k 0.80 kcal/mol with a correlation coefficient r=0*9787. Note that the activation energy of flow is the average of the values obtained in Table 2. In a previous section we indicated that the yield stress hardly varied with temperature, and its average value was obtained for each sample. The variation of these average values with the soluble solids contents has been assumed to be exponential, and by a least-squares for the following equation

Fig. 2.

Effect of concentration

on plastic viscosity.

430

A. Ibaq

A. Garvin, J. Costa

is obtained: a,=553

exp(0.0502C)

(mPa)

with a correlation coefficient r=0.9939, where C is the soluble contents expressed in “Brix. Concluding, the sloe fruit juice samples showed rheological behaviour can be described by Bingham’s equation (2) where r]‘=3*40 x lop5 exp(2708/T+O.l03C) a,=553

exp(0.0502C)

(8) solids that

(mPa s)

(mPa)

REFERENCES Ibarz, A. & Pagan, J. (1987). Rheology of raspberry juices. .T.Food Engng, 6, 269-89. Ibarz, A., Gonzalez, C., Esplugas, S. & Vicente, M. (1992a). Rheology of clarified fruit juices. I: Peach juices. J. Food Engng, 15, 49-61. Ibarz, A., Pagan, J. & Miguelsanz, R. (1992b). Rheology of clarified fruit juices. II:

Blackcurrant juices. J. Food Engng, 15,63-73. IFFJP (1984). International Federation of Fruit Juice Producers Methods, AnalysenAnalyses. Fruit Union Suisse Association, Zug, Switzerland. Rao, M. A., Cooley, M. J. & Vitali, A. A. (1984). Flow properties of concentrated juices at low temperatures. Food Technol., 38 (3), 113-19. Saravacos, G. D. (1970). Effect of temperature on viscosity of fruit juices and purees. J. Food Sci., 35, 122-5. Vitali, A. A. & Rao, M. A. (1982). Flow behaviour of guava as a function of temperature and concentration. .I. Texture Studies, 13, 275-89.