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Rheological Behavior of
Pineapple and Mango
Pulps: effect of the
measuring systems
Comportamento Reológico das Polpas de
Abacaxi e Manga: efeito do sistema de medidas
CARLOS ALBERTO GASPARETTO
Unicamp
[email protected]
DANIELA HELENA PELEGRINE GUIMARÃES
Unicamp
[email protected]
ABSTRACT – The rheological behavior of whole pulps of Pérola variety pineapple (Ananas comusus L. merr) and Keitt variety mango (mangífera indica L.), both at 30˚C, using the Haake Rotovisco RV-20 rotational rheometer was studied. Two
different measuring systems were empolyed: concentric cylinders (ZA30) and parallel plates. The diameters of parallel
plates were 45 mm (PQ45) and 30 mm (PQ30), both with gaps of 0.5 and 1.0 mm. The PQ45 measuring system with gap
of 0.5 mm produced the highest shear rates, up to 900s-1 and showed better performance in describing the behavior of
both pulps. The resulting rheograms were adjusted with the model of Mizrahi-Berk (M-B) having three constants. Samples
showed pseudoplastic behavior for both measuring systems employed. The geometry of those measuring systems greatly
affected the rheological response of both pineapple and mango pulps thus influencing the quality of rheological model fitting. No thixotropy was detected.
Keywords: PINEAPPLE – MANGO – PULP – RHEOMETRY – PSEUDOPLASTIC.
RESUMO – Neste trabalho experimental foram analisados, em reômetro rotacional Haake Rotovisco RV-20, as polpas
integrais de abacaxi Pérola (Ananas comusus L. merr) e manga Keitt (mangífera indica L.) à temperatura de 30ºC. Os
ensaios foram conduzidos com dois sistemas de medida: cilindros concêntricos (ZA30) e placas paralelas com diâmetros
de 45 mm (PQ45) e de 30 mm (PO), ambas com espaçamento de 0,5 e 1,0 mm. O sistema de medida PQ45 com distância entre placas de 0,5 mm produziu as maiores taxas de deformação, até 900s-1 e foi o que melhor descreveu o comportamento das duas polpas. Aos reogramas obtidos foi ajustado o modelo de Mizrahi-Berk (M-B). Observou-se que as duas
polpas apresentam comportamento pseudoplástico para todos os sistemas ensaiados, e que a geometria dos sistemas de
medidas exerce grande influência nos parâmetros reológicos e também na qualidade do ajuste do modelo reológico. Não
foi detectada tixotropia durante os ensaios.
Palavras-chave: ABACAXI – MANGA – POLPA – REOMETRIA – PSEUDOPLÁSTICO.
REVISTA DE CIÊNCIA & TECNOLOGIA • V. 8, Nº 16 – pp. 91-96
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INTRODUCTION
P
ineapple and mango are known as two
appreciable tropical fruits, due both to its pleasant aroma and flavor, and its nutritional
value, with high caloric, vitamins and mineral salts
contents (Donato & Carraro, 1995). Because of
these factors, pineapple and mango are among tropical fruits of great commercial importance (Cunha
et al., 1994).
One way of placing these products into the
market, increasing its shelf life and avoiding the losses due to its appearance effects, is processing them
into juices and nectars (Queiroz, 1998). To handling
these products in the food industry the pulp is submitted to a complete industrialization process. For
such industrial processes to be technical and economically feasible, it is important the knowledge of
physical-chemical properties from which the rheological behavior is one of the most important (Ibarz
et al., 1996).
Among the many factors influencing the rheological behavior of fruit pulps, the measuring systems’ geometry is one of the most important,
having great influence on rheological parameters
definition that describe the fruit pulps, because these
materials are non-newtonians (Pelegrine, 1999).
Most certainly, the difficulty found in the
reproduction of rheological experimental data from
literature can be explained because different measuring systems are used. Smith & Park (1984) said
that when a non-newtonian fluid is tested using
concentric cylinder viscometres, an error exists,
related to the magnitude of the gap between the cup
and the bob. According to these authors, the error
Fig. 1 - Searle rotational rheometer.
92
value tends to zero, when the ratio between the
radii of two cylinders approaches unity. From this, it
can be concluded the importance of the measuring
system used in any rheological test, allowing future
data comparison and reproduction.
The rheological behavior determined through
rotational rheometer is based on torque determination, necessary to maintain constant rotational
speed of a body that is immersed or in contact with
the fluid. This type of rheometer can present several
configurations, depending on the geometry of the
rotational bodies. In this way, the rotational rheometers can be of concentric cylinders, parallel plates,
cone and plate or others (Schramm, 1981).
The main advantages on rotational rheometer
is that this equipment allows a continuous measuring of the relationship between shear rate and shear
stress, also allowing the analysis of time dependent
fluids. Such equipments maintain a constant rotational speed corresponding to certain shear rate; the
shear stress is obtained through torque measurement (Barnes, 1989). The shear rate and shear stress
depend on the radius of the cylinder. In concentric
cylinders, one of them rotates at a certain angular
speed, while the other remains static. There are two
types of concentric cylinders rheometer: Searle and
Couette. In the fist one, the inner cylinder (cup)
rotate while the bob remains static; in Couette rheometer the bob is the movable cylinder. Setting down
several angular speeds for the rotational cylinder
and detecting the torque in the measuring cylinder,
the rheological curves can be obtained, for certain
fluid. Searle and Couette systems are illustrated in
figures 1 and 2.
Fig. 2 - Couette rotational rheometer.
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The parallel plates measuring system is constituted of two plates, separated by a gap. The gap
between the plates can be adjusted, and therefore
different shear rates can be obtained; larger shear
rates are reached with plates of larger diameter and
smaller gap between them.
Fig. 4. ZA-30 measuring system with view of the cup and
the bob.
MATERIAL AND METHODS
Preparation of Pulps
The pulps were produced in pilot plant from
fruits with similar degree of ripeness, as standardized
with a penetration texturometer. The fruits were
washed, peeled and its stales discarded. Then the fruits
were passed through finishers with screen of 1.6 mm.
Finally, the pulp was plate frozen to -20˚C for storage.
The phyical-chemical characteristics of pineapple and
mango pulps are summarized in table 1.
Fig. 5. PQ-30 measuring system.
Tab. 1. Pysical-chemical characteristics of pulps.
pH
Soluble Solids (ºBrix)
Pectin (w/w %)
Insoluble solids (w/w %)
MANGO
PINEAPPLE
4.47
16.60
0.98
1.08
3.50
13.30
0.082
0.54
Rheological Measurements
The rheological properties of the samples were
determined at 30˚C with the Haake Rotovisco (model
RV-20) rotational rheometer, using the concentric
cylinders (ZA30, fig. 3 and 4) and parallel plates
(PQ30 and PQ45, fig. 5, 6, 7 and 8) systems. The gaps
used for both plate diameters were 0.5 and 1.0 mm.
Fig. 3. ZA-30 measuring system with view of the group
cup/bob.
REVISTA DE CIÊNCIA & TECNOLOGIA • V. 8, Nº 16 – pp. 91-96
Fig. 6. PQ-30 measuring system.
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Fig. 7. PQ-45 measuring system.
τ
1/2
– K 0M = K M ⋅ γ̇
nM
(1)
where:
τ = shear stress;
.
γ = shear rate
KM = consistency index;
nM = behavior index;
K0M = squared root of Mizrahi-Berk yield stress.
For each curve fitting it was analyzed the
determination coefficient (R2) and the qui-square
(χ2) paramethers, for each measuring systems.
RESULTS AND DISCUSSIONS
Fig. 8. PQ-45 measuring system.
Rheograms
Figures from 9 to 11 show the average results
for shear rate versus shear stress data for each experimental situation. Figures 12 and 13 show average
data plus adjusted model to each pulp. The rheogramas obtained for all samples and systems did not
show thyxotropic effects.
Fig. 9. Mean shear stress X shear rate data for PQ - 30
system.
The time lenght of run, for each experiment
was programmed to be accomplished in 6 minutes.
During the two initial minutes the shear rate was
maintained constant at its maximum value, depending on the measuring system and, during the two
subsequent minutes, the shear rate varied in a decreasing way until a minimum value near 0 s-1, and the
last two minutes were for speeding up the shear rate
back to its maximum. During the descending speeds, 20 points of shear rate versus shear stress were
obtained as well as another 20 points for the ascending rates, resulting in a total of 40 points. If the
fluid is not thixoprotic, the value of shear stress considered for a shear rate is taken as the arithmetic
mean between descending and ascending rates.
The experiments were conducted with three
repetitions and the resulting shear stress was the
average of the three experiments. Each set of data
was adjusted to the Mizrahi-Berk (M-B) model,
represented by the following equation:
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Fig. 10. Mean shear stress X shear rate data for PQ - 45
system.
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Fig. 11.Mean shear stress X shear rate data for ZA - 30
system.
Tab. 2. Rheological parameters of the Mizhari-Berk model for
pineapple pulp.
PQ30
gap=
0,5mm
PQ45
gap=
0,5 mm
PQ30
gap=
1,0 mm
PQ45
gap=
1,0 mm
ZA30
K0M -0.24747
KM 1.80389
nM 0.12100
K0M 2.19800
KM 2.16045
nM 0.10367
2.05297
0.60412
0.55712
0.2496
0.49357
0.19918
0.06600
0.0361
0.19712
0.38469
0.53155
0.6545
0.52318
0.24747
0.14083
0.1123
0.42550
0.14189
0.04192
0.0242
0.08961
0.10757
0.09527
0.1092
χ2
0.11917
0.02061
0.03572
0.01877
0.0143
R2
0.75510
0.92903
0.85700
0.92643
0.9365
Tab. 3. Rheological parameters of the Mizhari-Berk model for
mango pulp.
Fig. 12. Mizrahi-Berk adjustment for pineapple pulp.
PQ30
gap=
0,5mm
PQ45
gap=
0,5 mm
PQ30
gap=
1,0 mm
PQ45
gap=
1,0 mm
ZA30
0.89149
0.48144
1.14022
0.86608
0.87008
1.46921
1.59808
1.70527
1.69185
2.04231
0.24246
0.23560
0.22654
0.23396
0.22018
0.17380
0.07963
0.46351
0.09570
0.15453
0.13743
0.05955
0.40334
0.07975
0.13500
0.01142
0.00421
0.03020
0.00587
0.00827
χ2
0.00752
0.00106
0.03473
0.00200
0.00330
R2
0.99841
0.99982
0.99001
0.99958
0.99926
K0M
KM
nM
K0M
KM
nM
CONCLUSIONS
Fig. 13. Mizrahi-Berk adjustment for mango pulp.
Rheological Parameters
Tables 2 and 3 present the pulp’s rheological
parameters obtained by Mizhari-Berk adjustment
model, through the software Origin 3.5.
REVISTA DE CIÊNCIA & TECNOLOGIA • V. 8, Nº 16 – pp. 91-96
Regarding the observations referring to the
measuring systems used, it can be concluded that
all the measuring systems detected the pseudoplasticity of the pulps used. From tables 2 and 3, it
can be observed the behavior index smaller than 1
for all the cases, concluding that both pineapple
and mango pulps present pseudoplastic characteristics. For mango, it can be observed (tab. 3) that
the pseudoplasticity varied very little with the
different measuring systems used. From table 2 it
can be observed that PQ30 system is the less
appropriate because shows very poor statistical
parameters. Although the fittings with ZA30 and
PQ45 presented satisfactory results, the PQ45 system’ results for larger shear rates at 0.5 mm gap,
produced better results, mainly if the fluid has low
viscosity. So the rheological parameters obtained
with this measuring system presented better reliability and it is to be recommended for materials
having the same characteristics as the pineapple
and mango pulps.
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CUNHA, G.A.P. et al. Manga para Exportação: aspectos técnicos da produção. Frupex-Manual de Exportação de Frutas. Brasília, 1994, pp. 8-13.
DONATO, M. & CARRARO, A.F. An illustrated guide for importers. FRUPEX – Manual de Exportação de Frutas. Brasília,
1995, pp. 6-7, 16-17.
IBARZ, A. et al. Rheology of clarified passion fruit juices. Fruit Processing, 6: 330-333, 1996.
PELEGRINE, D.H. Comportamento Reológico das Polpas de Manga e Abacaxi. [Dissertação de Mestrado. Campinas: Unicamp, 1999].
QUEIROZ, A.J. et al. Influência dos Sólidos Suspensos na Reologia do Suco de Abacaxi. Congresso Brasileiro de Sistemas Particulados, 14, Uberlândia, 1996.
SCHRAMM, Gebhard. Introduction to pratical viscometry. Germany, Gebrueder Haake, 1981.
SMITH, R.E. & PARK, A. Effect of gap errors in rotacional concentric cylinder viscometers. Journal of Rheology, 28: 155-160,
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