Effect of pH, Temperature and Glucose Addition on ... - Science Direct

PII:

JournulofFood Engineering33(lYY7)239-256 0 lYY7 Elsevier Science Limited All rights reserved. Printed in Great Hritain 0260-8774197 $17.00 +O.OO

SO260-8774(97)00032-O

ELSEVIER

Effect of pH, Temperature and Glucose Addition on Flow Behaviour of Fruit PurCes I. Banana Puree S. N. Guerrero” Departamento

de Industrias,

Facultad

& S. M. Alzamora? de Ciencias Exactas y Naturales,

Universidad

de

Buenos Aires, Ciudad Universitaria, (1428) Buenos Aires, Argentina (Received

25 November

1996; accepted

13 April 1997)

ABSTRACT The influence of pH (3.0 and 5.1) and addition of glucose to attain water activities (a,) ranging from 0.89 to 0.97 on the flow behaviour of banana purees was evaluated in the temperature range IO-55°C. All purees were shearthinning fluids with appreciable yield stress values, the f7ow curves essentially following the Herschel-Bulkley model. Glucose addition generally decreased the apparent viscosities and increased the temperature dependence of the flow properties. There were no patterns with respect to the efject of pH on flow characteristics. The effect of temperature (IO-40°C) and concentration (21.4-50.9”Brix) on the consistency coejjicient was represented by a single equation derived by combining an Arrhenius type relationship for the dependence on temperature and an exponential relationship for the influence of concentration. 0 1997 Elsevier Science Limited. All rights reserved

NOTATION

a, T

CT *, i s M

Water activity Temperature (“C or K) Shear stress (Pa) Shear rate (s- ‘) Shear stress factor (Pa/scale grade) Registered signal of mechanical torque Shear rate factor (s- ‘/scale grade)

*To whom correspondence should be addressed. @indust.di.fcen.uba.ar TMember of Consejo National de Investigaciones tina. 239

Tel./fax: (54-I) 784-0208; e-mail: sguerrero Cientificas

y TCcnicas of Reptiblica

Argen-

240

B’

R2adj x’ X XL XH

S. N. Guerrero, S. M. Alzamora

Rotor speed (rpm) Yield stress (Pa) Fluid consistency coefficient in Herschel-Bulkley model (Pa s”) Flow index in Herschel-Bulkley model Non-Newtonian shear rate (SK ) Radius of external cylinder (mm) Radius of inner cylinder (mm) Arrhenius constant for the dependence of m with T (Pa s”) (eqn 4) Activation energy for flow (kJ/mol) (eqn 4) Gas constant (8.3 J/kmol) Coefficient for the dependence of m with C (Pa sn) ieqn 5) Coefficient for the dependence of m with C (“Brix ) (eqn 5) Soluble solids concentration (“Brix) Coefficient for the combined dependence of m with C and T (Pa s”) (eqn 6) Coefficient for the combined dependence of m with C and T (“Brix-‘) (eqn 6) Adjusted determination coefficient Coded variable Independent variable Lowest level of the independent variable Highest level of the independent variable

INTRODUCTION Banana (Muss cavendishii) is an important world crop grown mainly in tropical and subtropical areas. It is consumed mostly as fresh fruit, only a small quantity being used for processing (Growther, 1979). Production of fruit juices, concentrates and purees may be a good way for the surplus and rejected stocks. Minimal preservation processes based on the ‘hurdle’ concept have been recently developed for obtaining shelf-stable fruit purees (Alzamora et al., 1995; Guerrero et al., 1994). They use in combination, among other preservation factors, the control of pH and the reduction of water activity (uW) through sugar addition. Knowledge of the rheological properties of these fruit derivatives is essential for design of food-processing equipment, quality control, consumer acceptability and better understanding of product structures. Considerable attention has been given to the study of the flow properties of many liquid and semi-liquid fruit products (Holdsworth, 1969 and Holdsworth, 1971; Rao, 1977; Khalil et al., 1989). The most common approach has been the characterization of flow properties irrespective of the fruit structure and limited to viscometric data for pulp, concentrates and purees (Charm, 1960 and Charm, 1962; Rao et al., 1974; Kbalil et al., 1989). Using the phenomenological approach, Holdsworth (1971) and Rao et al. (1974) pointed out the non-Newtonian nature of most of the fruit purees studied. Saravacos (1970) found a rather small effect of temperature on the apparent viscosity of apple, pear the flow and peach purees. Barbosa Canovas and Peleg (1983) characterized behaviour of samples of commercial banana and peach baby foods in a coaxial viscometer and found that the flow curves varied considerably between the different products and within the same products of different manufacturers. Vitali and Rao

Flow behaviour of banana purt?e

241

(1984) also discussed the effect of temperature and concentration in low-pulp concentrated orange juice and the applicability of the different flow models to these samples. With a structural approach, Mizrahi (1979) studied different rheological aspects of some fluid fruit, correlating flow parameters with their physicochemical properties. Duran and Costell (1982) also used structural rheological models for relating flow characteristics to apricot puree microstructure. Despite the various studies on flow properties of fruit purees, very little work has been published about the influence of pH and/or the addition of sugars on their rheological behaviour. A fruit puree is a very complex system consisting basically of a dispersing medium (serum) and suspended particles (pulp). The serum contains low (mainly sugars, salts and organic acids) and high molecular weight solutes, the removal of the latter leaving a Newtonian solution of low molecular weight species. Soluble pectic substances generally impart to the serum a non-Newtonian behaviour, finite yield stresses and thixotropic flow properties depending on their concentration and molecular weight. Concentration, size distribution, particle shape, flexibility, electrical and surface properties, and mode of interaction of the suspended particles considerably affect also the flow behaviour. Moreover, soluble solids play a major role in determining the mode and extent of interparticle interaction and changes in pH markedly affect the zeta potential, influencing the rheological behaviour (Mizrahi, 1979). This investigation studied the influence of pH (‘natural’ or adjusted to 3.0) and glucose addition (to attain a, 0.89, 0.93 or 0.97) on the flow characteristics of banana puree over the temperature range lo-55°C.

MATERIALS

AND METHODS

Preparation of samples Ripe bananas of Giant cavendishii variety (pH 5.1, 21.4”Brix and a, 0.98) were obtained from a local market. Additives used included: Cerelose (glucose tnonohydrate, food grade, Refinerias de Maiz S.A., Argentina) and citric acid (analytical grade, Merck Quimica Argentina, Argentina). The fruits were washed, peeled and pulped using a Sorval Omni-Mixer (Omni Corporation International, Waterbury, USA). Cerelose was added to reduce a, to 0.97, 0.93 or 0.89. The amount of glucose needed was calculated using Ross (1975) and Norrish (1966) equations. For studying the effect of pH, citric acid was added to some samples to adjust pH to 3.OkO.2. Fresh purees with ‘natural’ pH or with pH adjusted to 3.0 but without glucose addition were processed with the mixer and analysed as controls. The addition of glucose did not alter the ‘natural’ pH of the puree (pH = 5.1) by more than 0.01 unit. Analytical method The fruit purees were analysed for total pectin content and fractional pectin components (Robertson, 1979). Determinations were made in quadruplicate and the average is reported. An Atago refractometer (mod. Pr 101, Atago Co. Ltd, Tokyo, Japan) was used to measure the solid soluble content (“Brix) of the samples at 25°C. The pH of the samples was determined with a glass electrode attached to a Meth-

242


rom pHmeter E 632 (Methrom Herisam, Switzerland). The a, was measured with a psychrometric instrument (mod. CX2, Decagon Devices Inc., Washington, USA) operated following the procedure described in detail by Roa and Tapia de Daza (1991). Instrumental tests Rheological measurements were made at 10, 25, 40 and 55°C (&O.l”C) in a concentric cylinder viscometer RV12 Rotovisco and F4-refrigerated bath with circulation (Haake Buchler Instruments, Inc., USA). Rheograms were obtained with MVIP sensor (roughened fixture) and with MVI sensor (smooth fixture) (inner cylinder radius: 20.04 mm; outer cylinder radius: 21.00 mm; cylinder height: 60.00 mm) to evaluate the possibility of slip occurring in the coaxial rheometer. The use of a roughened cylinder did not change the shape of the curve obtained with the MVI sensor system, indicating slip was relatively unimportant. These unimportant slip effects have been also reported by Qiu and Rao (1990) in canned banana baby food. Samples were maintained at rest for 30 min in the sensor cell before measurements in order to reduce immediate effects due to network degradation and for thermostatization (Duran & Costell, 1982). Speeds of the rotating inner cylinder varied from 0 to 128 rpm. Flow curves were automatically recorded. Results were reported as average of two rheograms obtained for each condition, being the error less than 1%. To obtain shear stress-shear rate diagrams, the samples were sheared first in ascending order and then in descending order (9 min per cycle). When a hysteresis loop was observed, the procedure was repeated until the ascending speed curve coincided with the descending one to verify that thixotropic structure breakdown had been effective. The signal S registered as the mechanical torque and the rotor speed (N) were converted to shear stress (Pa) and shear rate (s- ‘) by using the following relations: o=LS

(14

$=MN

(lb)

where the values of the shear stress factor provided by the manufacturer.

L and the shear rate factor

M were

Calculation of flow behaviour parameters Flow curves of banana purees (Herschel & Bulkley, 1926):

were fitted 0 = a,+mj$

to the Herschel

and Bulkley

model (2)

The yield stress (a,), the fluid consistency coefficient (M) and the flow index (n) were calculated by non-linear regression analysis. The non-Newtonian shear rate (jsp) used in this model was calculated from the following expression (Hangen & Tung, 1976):

(3)

243

Flow behaviour of banana purtie

Statistical analysis The statistical program STATGRAPHICS (Statistical Analytical System, 1988) was used for non-linear regression analysis for Herschel-Bulkley and Arrhenius models and other applications.

RESULTS Characterization

AND DISCUSSION

of samples

Water-soluble, oxalate-soluble and residual pectin (protopectin) fractions were determined in fresh banana puree, the values being 0.16 kO.03, 0.30 +O.Ol and 0.08&0.01 g galacturonic acid per 100 g fresh fruit respectively. The low-methoxyl pectin content of banana as well as its high methoxyl-low methoxyl pectin relationship are higher than the ones of the other tropical fruits (i.e. papaya, mango) (Guerrero et al., 1996). Fruit content, soluble solids concentration and a, for the banana purees samples with different glucose content and pH 5.1 or 3.0 are shown in Table 1. Characterization

of flow curves

Typical torque versus rotational speed curves (first cycle) at 10°C for banana purees with different a, and pH 3.0 are presented in Fig. 1 as a mode of example. The flow curves for all glucose-containing banana purees defined hysteresis loops characteristic of a time-dependent flow behaviour. Loop areas were generally larger for samples with higher sugar content. Time dependence was effectively eliminated by shearing the purees between three and seven cycles. The number of loops before overlapping occurred depended on the temperature of application; at low temperatures (1oOC and 25”C), more time was required to destroy the network structure. The difference between the upper and the down curves was significant at 1% significance level at both pHs and at all temperatures. Flow curves tended to a limiting stress value at small shear rates, demonstrating the presence of an apparent yield stress. TABLE 1 Fruit Content, Fruit content (%w/w)

100 74 59 24

Soluble Solids Concentration

and a, of Banana Pukes

a,

Soluble solids concentration (“Rrk)

0.98 0.97 0.93 0.89

21.4 28.1 39.8 50.9

244


80

Signal (S)

70

0

32

84 N

Fig. 1.

88

128

(w-N

Experimental rheograms recorded at 10°C on banana purtes with pH 3.0 and different

a,,,: dashed line, a, 0.97; dotted line, a, 0.93; thin line, a, 0.89; thick line, control.

Rheograms obtained for fresh banana purCes with ‘natural’ or adjusted pH indicated no dependence of shearing with time of application. Figure 2 shows the non-time-dependent flow curves for the different banana purees. Flow curves exhibited at all temperatures a pseudoplastic character after applying a shear stress higher than a critical value (yield stress). The flow behaviour was largely affected by temperature, glucose addition and pH. Glucose additions and increasing temperatures were associated with lower apparent viscosities as indicated by the decreased shear stress values under a given shear rate. At pH 5.1 (Fig. 2b) and 40°C or 55°C the flow curves of banana purees with a, 0.89, 0.93 and 0.97 were rather similar. When temperature decreased (10°C and 25”(Z), the flow properties of the purees with different a, were different, the shear stress falling with increasing glucose content except for a, 0.93. The same behaviour was observed at 25°C for reduced a, banana purees with pH 3.0 (Fig. 2a). However, on the contrary with purees of pH 5.1, at pH 3.0 and 40°C the dependence of the flow properties with a, was important and the shear stress values fell as the glucose content was increased. At pH 3.0 and 10°C the flow curves of purees with reduced a, tended to join and at 55°C flow curves of purees with a, 0.97 and 0.89 were similar. Temperature in the range IO-40°C had a rather small effect on the flow curves of the control purees (~~-0.98) principally at pH 5.1. It became more significant when glucose was added. These results are in agreement with those reported by other authors about the limited temperature dependence of flow properties of fruit purCes, except in concentrated purees or when sugar had been added (Saravacos, 1970; Saravacos & Kostaropoulos, 1995). The effect of glucose on flow properties of banana puree could be due, among others, to a specific effect on the a,. As is well known, the low-methoxyl pectin Gackbone’ adopts a worm-like conformation in solution (Axelos & Thibault, 1991). High sugar concentration can reduce the intrinsic viscosity (parameter related to the hydrodynamic volume of the molecule) of pectin by inhibiting molecular hydration (Mizrahi, 1979).

banana par&

245

(a) 180

1e-3

40

2a 0

*-a

---A/

M

IW

150

4

200

25a

arm

0

(1’)

XI

loo

)2-‘)

2bO

250

ma

180 56 F

1w i

Fig. 2. Effect of a,, pH and temperature on shear stress versus shear rate curves of banana purbes. Experimental data: I, a, 0.89; A, a, 0+93: +, a, 0.97: l , control. Prcdictcd by Herschel-Bulkley model: (a) pH 3.0; (b) ‘natural’ PH.

246

S. N Guerrero, S. M. Alzamora

Model for flow behaviour of banana purkes

As can be seen in Fig. 2, the Herschel-Bulkley model described well the flow behaviour of the purees for each temperature, concentration and pH, the adjusted determination coefficient being in all cases equal or higher than 0.995. This model has already been selected from others (power law, Casson, Bingham) for fruit purees because of its inherent compatibility as far as yield stress was concerned (Duran & Costell, 1982; Guerrero, 1993). Data on Herschel and Bulkley parameters (yield stresses, consistency coefficients and flow behaviour indices) of the banana puree samples are summarized in Table 2. The flow index values ranged from 0.41 to 0.64 under different conditions of pH, a, and temperature, demonstrating the pseudoplastic character of banana purees. For the purpose of comparison, the values of the consistency coefficient and flow index obtained by other authors for other fruit purees of similar soluble solid content and the models used for their calculation are given in Table 3. The values of m and it obtained in this paper are in agreement with those published in the literature for different fruit purees at various temperatures. The differences may be attributed to the biological variability among fruits and varieties, to the different treatments applied to the fruits, and to the different composition of the samples. In the information about flow parameters of banana, the value of flow index obtained by Rao et al. (1974) is lower than the others reported at similar temperature. This fact may be due that these authors employed the power law model to characterize banana puree. In this model, the yield stress is not considered; therefore, the initial energy necessary to destroy the network structure is included as the total fluid response to the applied shear rate. This results in higher m values and lower n values being obtained (Duran & Costell, 1982; Guerrero, 1993). Consistency coefficient Effect of temperature

An Arrhenius type equation was attempted for modelling the variation of the consistency coefficient with the temperature in the range of shear rates of O-300 s- ‘: m = mr exp(Ea,lRT)

(4)

Figure 3 shows the effect of temperature on the consistency coefficient of the samples with different a, and pH. Table 4 lists the rheological constants for the Arrhenius model and the temperature range of applicability. Except for the control at pH 5.1, the experimental data at 55°C did not follow this model, probably due to some interactions promoted by high temperatures which could affect the temperature dependence of the consistency coefficient (Van Buren, 1991). At pH 3.0, the effect of temperature in decreasing the consistency coefficient of banana purees was more pronounced at higher sugar concentrations (aW 0.89 and 0.93). The Ea, values for these purees were approximately five- to six-fold higher than those obtained for the purees with a, 0.97 and a, 0.98 (control). However, at pH 5.1, the effect of temperature was similar for the control and for the purees with different amounts of glucose. As can be seen, the pH did not affect the temperature dependence of the purees with a, 0.93 or 0.89. Saravacos (1970), Harper and

247

Flow hehaviour of banana puke

Lebermann (1962) and Harper and El Sharigi (1965) obtained Ea, values of fresh apple and peach purees similar to the Eu, values obtained for the control and the a, 0.97 purees, both at pH 3.0 (Eaapplelsaucr = 7.94 kJ/mol; Eapeach puree = 7.10 kJ/ mol).

TABLE 2 Herschel-Bulkley Parameters for Banana PurCe Samples T (“C)

@I (pa)

m (Pas”)

n

10

32.5 + 0.8 31+1 2452 21.1*0.5

6.65 *0.18 6.08 f0.15 4.6 + 0.3 2.76 + 0.08

0.46 +O.Ol 0.45 + 0.01 0.46 + 0.02 0.48 + 0.01

!

30+1 30+1 28+3 9.8kO.5

9.2kO.3 8.6kO.3 5.lkO.4 2.22 * 0.07

0.44+0.01 0.41+0.01 0.48 +0.02 0.49 + 0.01

3.0

10 25 40 55

28+1 19.6 $0.9 14*2 15*1

6.2kO.2 5.4kO.2 4.OkO.3 3.8 +0.2

0.43 + 0.01 0.41+0.01 0.42 +_0.02 0.41 fO.O1

5.1

10 25 40 55

31f2 19.3 +0.8 8.9 + 0.5 5.3+0.3

5.2*0.2 2.6+0.1 0.78 + 0.04 1.05 +0.05

0.52f0.01 0.48+0.01 0.61* 0.02 0.50 +0.01

:z 40 55

20+1 14+1 5.9 +0.9 4.8kO.9

6.0 + 0.2 2.48 rt 0.05 0.70 f 0.02 0.60 + 0.03

0.44 +0.01 0.45 * 0.01 0.61+ 0.01 0.58 + 0.01

5.1

10 25 40 55

22fl 14+1 6.9 +_0.3 5.8 +0.2

6.0 +0.2 2.5 k 0.2 0.64 to.02 0.66 * 0.02

0.46-~0.01 0.47 +0.01 0.63 +O.Ol 0.60+0.01

3.0

10 25 40 55

19k2 12.9_+0.3 5.6kO.3 5.5 kO.3

3.4 _+0.2 1.06 + 0.03 0.40 _+0.02 0.30 +0.02

0.54 _+0.02 0.57 _t0.01 0.64 _+0.02 0.60+0.01

;: 40 55

22*2 1051 6.3 + 0.4 4.0 f 0.3

5.5 kO.3 2.5 + 0.2 0.90 + 0.04 0.74 * 0.03

0.50 + 0.02 0.52kO.02 0.58+0.01 0.57 + 0.01

a,.

PH

0.98 (control)

3.0

25 40 55 5.1

10 25 f

0.97

0.93

0.89

3.0

5.1


248

Effect

of soluble solids concentration

The solids concentration strongly influenced the m value. The variation of consistency coefficient with the concentration was described by an exponential relationship: m = m’, exp(B”C)

(5) where magnitudes of the constants m’, and B’ were obtained by regression analysis of the experimental data. Exponential and power law relationships between concentration and viscosity and/or consistency coefficient have been proposed for pureed fruits, juices and vegetables products and non-food concentrated suspensions (Jinescu, 1974; Vitali & Rao, 1984; Khalil et al., 1989; Wang & Meisen, 1994). In the present work, it was found that the exponential model yielded a better fit. Figure 4 shows the exponential plots of In consistency coejjicient vs. soluble solids concentration at 10, 2.5, 40 and 55°C. The consistency coefficient was a decreasing function of solid concentration. Table 5 lists the rheological constants derived from the exponential plot, together with the correlation coefficient in each case. Again, at 55°C the data did not fit very well.

TABLE 3 Literature Values of the Consistency Coefficient and the Flow Index for Fruit Purees Fruit pm-&e

Apricot (canned) Apple (canned sauce) Banana (canned) Banana (canned, brand A and B) Banana (baby food with tapioca, brand A and B) Banana (pH 3.0; a, 0.97 and thermal treated) Banana Guava (fresh, with 2000 ppm SMB) Mango (blanched and freeze-thawed) Papaya (fresh, with 2000 ppm SMB)

Soluble solids (“Brix)

T (“C)

111 (Pa s”)

n

Model”

Reference

12.4-15.3

22.5

0.8-16.2

0.24-0.39

HB

11.0

25.0

6.4-6.8

0.42

HB

17.7 nr

22.0 24.0

10.7 0.28 6.5, 10.7 0.33, 0.46

PL PL

Duran & Costell (1982) Dervisoglu & Kokini (1986) Rao et al. (1974) Charm (1960; 1962)

1.5.0-15.1

25.0

5.4, 6.9

0.59, 0.62

HB

5.0

25.0

5.0 25.0

15.8 1.9

0.51 0.52

21.4-50.9

I o-55

0.3-9.2

0.41-0.64

HB

This paper

10.3

23.4

3.9

0.49

PL

Rao et a[. (1974).

9.3

24.2

2.1

0.33

PL

Rao ef al. (1974)

7.3

26

0.9

0.53

PL

Rao et al. (1974)

“HB: Herschel and Bulkley model; PL: power law model. SMB: Sodium metabisulphite. nr: non reported.

Barbosa Canovas and Peleg (1983)

HB Guerrero HB 1-

(1993)

Flow hehaviour of banana purke

33

32

34

249

30

10

l/T. (lo3)( K’) Fig. 3. Applicability of Arrhenius model for the consistency coefficient Experimental data: +, a, 0.89; A, a, 0.93; n, a, 0.97; x, control. Arrhenius (b) ‘natural’ pH.

Parameters

of the Arrhenius

TABLE 4 Model for the Effect of Temperature Coefficient of Banana Pukes

of banana pukes. model: (a) pH 3.0;

on the Consistency

Temperature range (“Cj

0.89 0.89 0.93 0.93 0.97 0.97 0.98 (control) 0.98 (control)

10-40 10-40 10-40 I O-40 10-40 10-40 IO-40 25-55

3.0 5.1 3.0 5.1 3.0 5.1 3.0 5.1

54& I 42_+ 1 SO_+1 s4*1 Xt_l 46+1 8f2 36.8 +_O.S

6.8 x IO 4.8x 10 1.9x 10 7.2 x 10 8.0 x 10 2.1 x 10 1.5x10 3.6 x 10

I” ri ‘j I” 2 r: ’ ”

0.99

0.99 0.98 0.9X 0.94 0.97 0.91 0.97


250

-2.00

1 10

20

30

40

w

so

C I’ Bdx)

10

J

iQ

30

10

C

M

w

(’ Bdx)

Fig. 4. Effect of soluble solids concentration (OBrix) on the consistency coefficient of banana purees. Experimental data: +, 10°C; n, 25°C; A, 40°C; x, 55°C. Exponential model: (a) pH 3.0; (b) ‘natural’ pH.

TABLE 5

Exponential

Constants

for the Effect of Soluble Solids (“Brix) on the Consistency of Banana Purees

Coefficient

PH

T (“(7

(Pa snj

B” (“Bti-‘)

Rldj

3.0

10 :;

10.1 k1.6 23.2 19+1 + 1.6

-0.018 + 0.009 - - 0.050 0.09 +_ + 0.01 0.007

0.90 0.95

55

11_+3

;:

10.1+ 1.5 37.9 * 1.4 78+1 65.2* 1.4

5.1

z:

-0.09+0.02 -0.012+5x 10-j -0.075 + 1 x lo-” -0.13+0.01 - 0.055 + 0.002

0.97 0.80 0.85 0.97 0.83

251

Flow behaviour of banana pur&e

TABLE 6 of eqn 6 Describing the Effect of Temperature and Soluble Solids Content the Consistency Index (m) of Banana Puke

Parameters

rnTc‘ (Pa sn)

PH

3.0 5.1

0.008 (+ 1 x lo-‘) 0.0030 (+ 1 x 10-y

Combined

Ea,,, Wlgmol) 18.4f 1.1 26.8-t 1.5

BC‘ (“Bri- ‘) -0.0055 (+ 1 x 10~._3) -0.0046(+1 ~10~~)

on RL, 0.88 0.83

eflect of temperature and concentration

The overall dependency of the consistency coefficient on temperature and soluble solids concentration was assessed in terms of a single equation derived by combining the Arrhenius and the exponential relationships: m = mTCexp(Ea,lRT)exp(B’C)

(6)

Such equations have been used by many authors such as Harper & El Sharigi (I 965) in tomato concentrates, Vitali and Rao (1984) in concentrated orange juices and Wang and Meisen (1994) in canolapastes. The constants mTc, Ea, and B’ were obtained by regression analysis of the experimental data (except data obtained at 55°C) and are exhibited in Table 6. The adjusted determination coefficient (Rzdj) between the experimental data and the predicted values was 83% for pH 5.1 and 88% for pH 3.0. These results show that temperature and solid concentration have a strong influence on the consistency coefficient of banana puree. Figure 5 shows the response surface obtained with eqn 6 as well as the fitting of the data. The suggested equation will be useful in the prediction of the m values at various temperatures between 10 and 40°C and for soluble solids concentrations within the range of 21.4-50.9”Brix. Flow index As can be seen in Table 2, there was no regular trend of this parameter with either temperature, concentration or pH. As Harper and El Sharigi (1965) pointed out, a large part of the variation in n can be attributed to experimental limitations when food materials are assayed and a relatively small change in the recorded experimental curve in the region of high shear rate has an important effect on n. Yield stress Banana purees had a finite yield stress that generally decreased with soluble solid concentration and temperature (Table 2). The global variation in yield stress with temperature and a, was found by fitting the experimental data to a polynomial equation over the temperature range lo-40°C for both pHs. These equations and the respective analysis of variance are displayed on Table 7. The F-value attributed to the model was highly significant at both pHs. The response surfaces generated by the polynomial equations are plotted in Fig. 6. They are rather similar for both pHs, but at ‘natural’ pH the temperature dependence increased when a, decreased. This fact was deduced from the signiticance of the cross-linear term in the polynomial equation.

252

S. N Guerrero, S. M. Afzamora

Temperature

(K)

soli$ pH. content Fig. 5 Response surface representing the effect of tenfperature (a) pHand 3.0;soluble (b) ‘natural (&ix) on the consistency coefficient of banana purees.

253

Flow hehaviour of banana p&e

TABLE7 Parameters

and Analysis of Variance of the Polynomial Equations for Describing of Temperature and a, on Yield Stress Values of Banana Purees

Parameter

PH

3.0

Equation” F-ratio Significance R,,,

o. = 13.3-7T’+4a,‘+3.3aw” 234 level 0.00001 0.989

“The equations are expressed as a function independent variables (aw, 7’) by:

x’ =

the Effect --

5.1

co = 13.2-8.9T’+3.Saw’+2.1a,+“32 0.008 0.959 of coded variables

(x’: a,‘,

1.7T’a,,’

T’) related

to the

2x - (x,_+xtl)

WH-x1 ) Temperature a, range:

range: lo-40°C

0.89-0.97.

CONCLUSIONS Glucose addition, pH value and temperature significantly affected the flow characteristics of the banana purees. The reduction of a, generally decreased shear stress values under a given shear rate. As pH decreases, the dependence of flow curves on temperature and a, was more pronounced The flow behaviour of banana puree was well described by the Herschel-Bulkley model. Combined exponential equations were found to be suitable to describe the dependence of the consistency coefficient with the running temperature and the soluble solids content of the puree at pH 3.0 and at natural pH. No pattern was observed with respect to the effect of pH, a, and temperature on the flow index. In general, yield stress decreased with temperature and soluble solids concentration. As many of the combined technologies developed for banana puree preservation involve a reduction of a, through sugar addition, the control of pH and a hot-filling stage, these results will be a powerful tool in the prediction of the flow behaviour of banana purees, allowing the calculation of the pumping requirements and the heat transfer parameters.

ACKNOWLEDGEMENTS The authors acknowledge financial support from the CYTED Union, CONICET and Universidad de Buenos Aires.

Program,

European


254

P

I

a

la

LO ..:.

. _

.

.

a

Fig. 6. Response surface representing the effect of temperature and a,,, on the yield stress of banana p&es over the temperature range lo-40°C. (a) pH 3.0; (b) ‘natural’ pH.

Flow behaviour of banana puke

255

REFERENCES Alzamora, S. M., Cerrutti, P., Guerrero, S. & upez-Malo, A. (1995). Minimally processed fruits by combined methods. Part III. In Food Preservation by Moisture Control: Fundamentak and Applications, eds G. Barbosa-CBnovas & J. Welti Chanes. Technomic Publishing Co., Lancaster, pp. 463-492. Axelos, M. A. V. & Thibault, J. F. (1991). The chemistry of low-methoxil pectin gelation. In 7Ii;_~~;rnisty and Technology of Pectin, ed. R. H. Walter. Academic Press, New York, pp. Barbosa Cgnovas, G. & Peleg, M. (1983). Flow parameters of selected commercial semiliquid food products. J. TeJcture Stud., 14, 213-234. Charm, S. E. (1960). Viscometry of non-Newtonian food materials. Food Res., 25, 351-362. Charm, S. E. (1962). The nature and role of fluid consistency in food engineering applications. Adv. Food Res., 11,355-435. Dervisoglu, M. & Kokini, J. L. (1986). Steady shear rheology and fluid mechanics of four semisolid foods. J. Food Sci., 51,541-546. Durdn, L. & Costell, E. (1982). Rheology of apricot purie: characterization of flow. J. Texture Stud., 13, 43-58.

Growther, P.C. (1979). The processing of banana products for food use. Report, Tropical Product Institute, G12, London. Guerrero, S. N. (1993). Desarrollo de una tecnologia de factores combinados para preservar un purC de banana de alta humedad. Ph.D. thesis, Universidad de Buenos Aires, Argentina. Guerrero, S., Alzamora, S. M. & Gerschenson, L. N. (1994). Development of a shelf-stable banana puree by combined factors: microbial stability. J. Food Protection, 57, 902-907. Guerrero, S., Alzamora, S. M. & Gerschenson, L. N.(1996). Rheology of tropical fruit purCes: effect of water activity, pH, temperature and glucose content. Paper 40-B6, presented at IFT Annual Meeting, New Orleans, USA. Hangen, P. & Tung, M. A. (1976). Rheograms for power-law fluids using coaxial cylinder viscometers and a template method. Can. Inst. Food Sci. Technof., 9, 98-104. Harper, J. C. & El Sharigi, A. F. (1965). Viscometric behavior of tomato concentrates. J. Food Sci., 30, 470-476.

Harper J. C., Lebermann, K. W. (1962). Rheological behaviour of pea purees. 1st International Congress of Food Science and Technology, London. Herschel, W. H. & Bulkley, R. (1926). Konziztenz Messungen von gummibensollosungen. Kolloid-Zetischr, 39, 291; Proc ASTM, 26, 621-625

Holdsworth, S. D. (1969). Processing of non-newtonian fluids. Process Biochem., 4, 10 35-21. Holdsworth, S. D. (1971). Applicability of rheological models to the interpretation of flow and processing behaviour of fluid food products. J. Texture Stud., 2, 393-418. Jinescu, V. V. (1974). The rheology of suspensions. Znt. Chem. Eng., 14,397-420. Khalil, K. E., Ramakrishna, P., Nanjundaswamy, A. M. & Patwardhan, M. V. (1989). Rheological behaviour of clarified banana juice: effect of temperature and concentration. /. Food Eng., 10,231-240. Mizrahi, S. (1979). A review of the physicochemical approach to the analysis of the structural viscosity of fluid fruit products. J. Texture Stud., 10,67-82. Norrish, R. S. (1966). An equation for the activity coefficients and equilibrium relative humidities of water in confectionary syrups. J. Food Technol., 1, 25-28. Qiu, C. G. & Rao, M. A. (1990). Quantitative estimates of slip of food suspensions in a concentric cylinder viscometer. In Engineering and Food, Vol. 1, eds W. E. L. Spiess & H. Schubert. Elsevier Applied Science, London, pp. 41-48. Rao, M. A, (1977). Rheology of fluid foods: a review. J. Texture Stud., 8, 135-167. Rao, M. A., Palomino, L. N. 0 & Bernhardt, L. W. (1974). Flow properties of tropical fruit purees. J. Food Sci., 39, 160-161.

256


Roa, V. & Tapia de Daza, M. S. (1991). Evaluation of water activity measurements with a dew point electronic humidity meter. Lebensm. Wiss. Technol., 24, 208-213. Robertson, G. L. (1979). The fractional extraction and quantitative determination of pectic substances in grapes and musts. Am. J. Enol. Viticulture, 30, 182-186. Ross, K. D. (1975). Estimation of water activity in intermediate moisture foods. Food Technol., 29, 26-32. Saravacos, G. D. (1970). Effect of temperature on viscosity of fruit juices and purees. J. Food Sci., 35, 123-125. Saravacos, G. D. & Kostaropoulos, A. E. (1995). Transport properties in processing of fruits and vegetables. Food Technol., 49, 99-107. Van Buren, J. P. (1991). Function of pectin in plant tissue structure and firmness. In: The f!w$>try and Technology of Pectin, ed. R. H. Walter. Academic Press, New York, pp. Vitali, A. L. & Rao, M. A. (1984). Flow properties of low-pulp concentrated orange juice: serum viscosity and effect of pulp content. J. Food Sci., 49, 876-881. Wang, F. & Meisen, A. (1994). Flow properties of Canola seed pastes. J. Food Sci., 59, 1091-1095.