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The effect of homogenization on the stability of PineapplePulp
Journal: International Journal of Food Science and Technology
Manuscript ID: IJFST-2010-06128.R1
Manuscript Type: Original Manuscript
Keywords: Pineapple, Viscosity, Food Quality
Institute of Food Science and Technology
International Journal of Food Science & Technology
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The effect of homogenization on the stability of1
Pineapple Pulp2
Homogenization of Pineapple Pulp3
Silva, V. M.; Sato, A. C. K.; Barbosa, G.; Dacanal, G. .; Velsquez, H. J. C.; Cunha, R. L.*4
Department of Food Engineering - School of Food Engineering5
UNICAMP (University of Campinas), P.O. Box 6121, Post Code: 13083-862, Campinas, So6
Paulo, Brazil.7
*rosiane@fea.unicamp.br; phone: +55 19 3521.4047; fax: +55 19 3521.40278
Keywords: pineapple pulp, stability, homogenization, sedimentation, viscosity9
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ABSTRACT10
Pineapple pulp was homogenized at different pressures and its stability investigated by way of11
flow curves, particle size distribution, morphology, cloudiness and sedimentation. The12
particle size of the homogenized pulp ranged from 400 to 100 m for homogenization13
pressures of between 0 and 700 bar. The pineapple pulp showed shear thinning behaviour14
with increasing flow index (n) after processing at higher pressures. In addition, the pulps with15
smaller particles showed less serum cloudiness, even though the sedimentation tests showed16
the highest stability for pulp homogenized between 200 and 300 bar. Above 400 bar the pulp17
showed phase separation and higher sedimentation indexes, similar to that observed for the18
untreated samples, which was attributed to the formation of aggregates due to inter-particle19
attraction.20
21
INTRODUCTION22
Pineapple, Ananas comusus (L.) Merr., is an important fruit crop in many tropical and23
subtropical countries. It may be consumed fresh or in a number of processed forms such as24
juices, desserts or pulps to be added as ingredients in foods such as yoghurts and ice creams.25
Fruit pulps are complex multicomponent systems, opalescent or turbid due to the presence of26
insoluble solids in suspension (Benitez et al., 2007a). Pineapple pulps or juices are unstable27
suspensions that settle quickly after extraction, and such phase separation depreciates the28
visual appearance of the product. However, it is believed that a better definition of the29
manufacturing conditions would contribute to an increased stability of the juices (Beveridge,30
2002). Suspensions can be diluted with no particle-particle interactions, sterically stabilized,31
flocculated with a fully formed structure, partially stable with some structure formation or32
sedimentation.The formation of a structure depends on the chemical structure of both phases,33
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the particle size and shape, the surface effects and/or the presence of any additives (Ferguson34
& Kemblowski, 1991).35
Many investigations have been carried out on the stability of cloud suspensions in apple,36
pineapple, passion fruit and carrot pulps (Beveridge, 2002; Okoth et al., 2000; Liang et al.,37
2006; Mollov et al., 2006). The cloud components of fruit pulps can be classified into four38
categories: coarsely dispersed particles (> 1 mm) such as fibres, pulp particles and stone cells,39
which sediment rather quickly; finely dispersed particles (1~100 m) such as pulp fragments,40
cell aggregates, whole cells, cell wall fragments and starch particles, that sediment a little41
slower during storage due to their positively charged protein-carbohydrate-complexes42
surrounded by negatively charged pectin coatings; colloidal substances (0.1~0.001 m) such43
as pectin, hemicellulose, proteins and dissolved starch, in which sedimentation can only be44
caused by enzymatic activity; and emulsified substances (
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relationship. In general, higher pressure processing produces smaller particles, up to a certain58
limit of micronization.59
Rheological measurements have been considered as an analytical tool to provide a60
fundamental insight into the structural organization of foods (Rao, 1999). Various factors61
affect the rheological behaviour of fruit pulps, including temperature, total and soluble62
solids/concentration, size, shape, size distribution and the arrangement of the particles that63
compose the dispersed phase (Pelegrine et al., 2002).64
The stability of a suspension is very important for process control and as a quality65
parameter, and is affected by a number of factors, such as: the total solid content, particle size,66
particle shape and particle size distribution, that are closely related to the maximum packing67
fraction, which indicates the maximum amount of particles that can be packed into a given68
volume. Such stability may be closely associated with the rheological properties. Thus, the69
aim of this work was to evaluate the effects of different homogenization pressures on the70
rheological properties and stability of pineapple pulp.71
MATERIAL AND METHODS72
Material73
Frozen pasteurized pineapple pulp was acquired on the local market (DeMarchi,74
Jundia, So Paulo, Brazil), and characterized with 13.4 % of solids, 11.2 Brix, pH of 3.7675
and a density of 1.034 g/cm3. The pulp was centrifuged at 1000g for 30 minutes for serum76
separation, and the pulp content characterized as 45.5 %.77
Homogenization process78
The pineapple pulp was homogenized at 25C in a Panda 2K NS1001L (Niro Inc.,79
Hudson, Ohio, USA) homogenizer, using pressures varying from 50 to 700 bar (5 to 70 MPa).80
Potassium sorbate was added to the pineapple pulps at 0.1% w/w to avoid fungal growth, and81
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the pulps were analysed for particle size distribution, rheological measurements, optical82
microscopy, cloudiness and stability to sedimentation.83
Particle size distribution84
The particle size distribution and mean volumetric diameter D [4,3] (Equation 1) of85
the raw and homogenized pineapple pulps were obtained using a Mastersizer Laser Light86
Scattering Spectrometer (model MAM 5005, Malvern Instruments Ltd., Worcester,87
Worcestershire, U.K) by dispersing the samples into deionized water. The measurements were88
taken in quintuplicate and the difference between the D [4,3] evaluated using the Tukey test.89
[ ]
=
==n
i
i
n
i
i
dn
dn
1
3
1
4
.
.
4,3D
((1)
Where diis the diameter of the particles in the sample and n is the number of particles.90
Rheological measurements91
The flow property measurements were performed using a controlled stress rheometer (Carri-92
Med CSL2 500, TA Instruments, New Castle, Delaware, USA) at 25C. Parallel stainless93
steel plates with a rough surface (4 cm diameter and 1.25 mm gap) were used for the94
measurements. Prior to analysis, the product was gently mixed to homogenize the sample and95
all experiments were performed at least in triplicate. Flow curve measurements were carried96
out with a shear rate ranging from 0 to 300 s -1 in three sweeps (up, down and up-cycles), to97
evaluate thixotropy. The data obtained from the third sweep were fitted to the power law98
model (n
k &= ), where is the shear stress (Pa), & the shear rate (s-1
), k the consistency99
index (Pa sn) and n is the flow behaviour index (dimensionless). Flow curves were adjusted by100
a non-linear regression analysis using the Statistica 5.0 software (StatSoft, Tulsa, Oklahoma,101
USA).102
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Optical microscopy103
The particles of the raw and homogenized pineapple pulp were diluted and coloured104
with a diluted violet crystal solution. The product was then deposited and dispersed on a glass105
slide to be observed under a stereomicroscope (Citoval 2, Zeiss, Jena, Thringen, Germany)106
equipped with a Kodak zoom digital camera model EasyShare DX4530 (Eastman Kodak107
Company, Rochester, Minnesota, USA). The images were captured at least in quintuplicate108
for each sample.109
Stability110
Sedimentation test111
Raw and homogenized pineapple pulps were transferred to graduated 100 ml tubes,112
stored at 25C and observed for 12 days. The volume of sediment was measured (total volume113
minus the serum phase) and the sedimentation index calculated as follows:114
totalV
VIS inf=
(2)
where Vinf is the sediment volume (ml) and Vtotal is the total volume of the sample115
(ml).116
Cloudiness117
118
Twenty five millilitres of pulp were added to 75 mL of water and centrifuged at 320 g119
for 10 min (Novatecnica NT810, Atibaia, So Paulo, Brazil). The optical density (absorbance)120
of the supernatant (serum phase) was determined at 660 nm using a spectrophotometer121
(Beckman DU640, Corona, California, USA) with distilled water as the reference. Turbidity122
measurements allow for the determination of the amount of light absorbed by the suspended123
particles in the beverage, thus higher absorbance readings correspond to greater cloudiness124
(Okoth et al., 2000).125
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RESULTS AND DISCUSSION126
Optical microscopy127
The micrographs showed that the homogenization of the pineapple pulp considerably128
reduced the size of the suspended particles, as compared to the non-homogenized pulp (Fig.129
1).130
It can be seen that the non-homogenized pulp (0 bar) consisted mainly of large131
deformable particles with irregular shapes and a number of small particles (Fig. 1a). In this132
case, the larger particles were associated with whole cells or aggregates, while the latter were133
related to other cell materials, such as cells from the parenchyma tissue (Ouden and Vliet,134
1997). During homogenization, these suspended particles were broken down resulting in a135
pulp with a large number of small particles such as fibrous particles, cells, cell wall fragments136
and polymers, amongst others (Figs. 1b and 1c), forming a new structure. In this case, the137
smaller particles formed a network different from that observed in the non-homogenized pulp,138
even though their shape/surface contour remained irregular. The same behaviour was139
observed by Bayod et al. (2007) for tomato suspensions before and after the process of140
homogenization.141
Particle size distribution142
Fig. 2 shows that the pineapple pulp presented a monomodal particle size distribution,143
independent of the pressure evaluated. The mean particle size decreased with increase in the144
homogenization pressure, with significant differences (p
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m for 0, 50 and 100 bar, respectively, whilst higher homogenization pressures only150
promoted smaller reductions in the mean particle size (D[4,3]).151
In addition to the evolution of the distribution to smaller sizes, the increase in152
homogenization pressure led to a narrower distribution, indicating greater uniformity, even153
though the mean diameter did not vary much for pressures above 200 bar. The154
homogenization process can reduce the size of the coarser particles leading to an increase in155
the concentration of fine particles. A similar behaviour was observed by Bayod et al. (2007)156
for tomato suspensions.157
Rheological measurements158
None of the pulps evaluated showed differences between the first and third shear159
cycles (data not shown), indicating that the samples were not dependent on the time of shear,160
i.e., they are not thixotropic or rheopetic.161
The power law model was adequate to fit the flow behaviour of the pineapple pulp,162
showing high determination coefficients (R > 0.99). The pineapple pulp showed shear163
thinning behaviour for all working pressures, without significant yield stress (Fig. 3). Similar164
behaviour was observed in previous studies for other fruit products, such as jaboticaba (Sato165
& Cunha, 2009), guava (Ferreira et al., 2002), pineapple (Pelegrine et al., 2000), araa166
(Haminiuk et al. 2006) and aai (Tonon et. al., 2009) pulps, without a homogenization167
process. The consistency index (k) decreased from 3.48 to 0.24 Pa.sn, whereas the flow168
behaviour index (n) increased from 0.29 to 0.55 as the working pressure increased from 0 to169
700 bar (Inset Table on Fig. 3), i.e. the decrease in particle size promoted an increase in170
flowability as observed for green chilli puree (Ahmed et al., 2000). The serum pulp was also171
evaluated and showed a consistency index (k) of 0.005 Pa.sn
and a flow behaviour index (n)172
of 0.94.173
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The viscosity was evaluated at different shear rates of 1, 10 and 100 s-1
(Fig. 4).174
Increasing the working pressures up to 200 bar led to a more pronounced drop in viscosity as175
in the case of the particle size reduction (Fig. 2), while only a slight reduction in viscosity was176
observed from 200 bar to 700 bar. A similar result was observed by Schijvens et al. (1998) for177
applesauce. However, the opposite behaviour was observed for tomato pulp (Kalamaki et al.,178
2003; Ouden & Vliet, 2002), coconut milk (Chiewchan et al., 2006) and carrot juice179
(Sichaipanit & Kerr, 2007), when an increase in the apparent viscosity was observed with180
decreasing particle size. Such a difference can be attributed to the fact that the breakup of181
some tissues such as tomato cells during homogenization is much more intense than for182
applesauce cells (Ouden & Vliet, 2002). According to Guyot et al. (2002), it is still not183
possible to predict a priori how a complex particle size distribution will affect the viscosity of184
a dispersion of small particles. However one can speculate that the pineapple pulp might185
behave in a manner more similar to that of the applesauce, in which the cells show186
considerable brightness under polarized microscopy, indicating the presence of semi-187
crystalline structures, probably mainly cellulose present in the relatively thick cell walls.188
Stability189
Cloudiness190
The cloud is a result of dispersed insoluble particles such as pectin, protein, lipids,191
cellulose and hemicelluloses (Scott et al. 1965). Fig. 5 shows the relationship between192
cloudiness and homogenization pressure/mean particle size of pineapple pulp. It can be seen193
that an increase in homogenization pressure from 0 to 700 bar decreased pulp cloudiness due194
to the decrease in particle size, with a greater reduction up to 200 bar. Decreased cloudiness195
with increasing homogenization pressure can be attributed to the decrease in particle size,196
which allows more light to go through the juice (Okoth et al., 2000).197
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Sedimentation test198
Fig. 6 shows the sedimentation index as a function of storage time for the several199
homogenization pressure treatments. In the first 24 hours, only the sample that had not been200
homogenized showed phase separation, which is common behaviour for some fruit pulps201
(Beveridge, 2002, Neidhart et al., 2002, Okoth et al., 2000). After the first day of storage,202
samples treated with pressures equal or higher than 400 bar started to show particle203
sedimentation, while the pulps homogenized at 50 bar only showed particle sedimentation as204
from the 5th
day. In this case, pressures higher than 400 bar did not prevent phase separation205
due to the low viscosities of these samples, insufficient to maintain the stability of the system.206
On the other hand, pulps subjected to pressures of 100, 200, 250 and 300 bar were stable207
throughout the observation period (10 days), showing that these pressure treatments could be208
used to stabilize pineapple pulp. It should be emphasized that, even though the particle sizes209
obtained after the homogenization treatments in the present work did not approach the values210
of 0.5 to 2 m required to stabilize some fruit juices (Beveridge, 2002), stabilization was211
obtained for particles with D[4,3] ranging from 125 175 m.212
Stokes law relates the sedimentation velocity to the properties of the particles and213
suspending medium. In general, according to Stokes law, the particle size and/or the214
difference between the densities of the particles and the medium are crucial factors affecting215
the stability of a suspension. Since the continuous medium is the same for all the samples216
(pulp serum), it would be expected that more stable pulps would be obtained with smaller217
particles. However, it should be considered that surface forces, such as van der Waals218
attraction, are more significant for smaller particles, while larger particles are basically219
influenced by hydrodynamic forces (Genovese et al., 2007). Thus it could be expected that the220
smaller particles produced by higher homogenization pressures, would form aggregates due to221
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forces of attraction, even though the particle sizes are not in the colloidal domain. The smaller222
the particles are, the stronger the attractive forces, leading to larger aggregates and,223
consequently, faster sedimentation.224
CONCLUSIONS225
A stable pineapple pulp with negligible phase separation within the period evaluated226
was obtained for dispersions processed within the pressure range from 100 to 300. Under227
these conditions, the coarse particles were reduced in size to the range between 175 and 125228
m, respectively. Even though higher homogenization pressures reduced the particle size even229
more, sedimentation was observed when pressures above 400 bar were applied, and the230
highest sedimentation index was observed for samples treated at 700 bar. Such results indicate231
that Stokes law did not explain the stability of pineapple pulp in the present study,232
emphasizing the importance of inter-particle forces. Thus it was suggested that apart from233
hydrodynamic forces, surface forces, such as van der Waals attraction, should be considered234
even for particles of about 100m, classified as non-colloidal.235
ACKNOWLEDGEMENTS236
The authors gratefully acknowledge Conselho Nacional de Desenvolvimento Cientfico e237
Tecnolgico (CNPq, process number 301869/2006-5) for the financial support.238
239
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Tonon, R.V., Alexandre, D., Hubinger, M.D. & Cunha, R.L. (2009). Steady and dynamic311
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Legends to Figures314
Figure 1 Microscopical images of pineapple pulp after homogenization at (a) 0 (non-315
homogenized), (b) 300 and (c) 600 bar of pressure.316
Figure 2. Particle size distribution of pineapple pulp homogenized at the different pressures.317
Inset Table shows the volume mean diameter of these samples.318
Figure 3 Flow curves for pineapple pulp homogenized at different pressures. Inset Table319
shows the rheological parameters of the power law model fitted to the flow behavior of the320
pineapple pulp homogenized at different pressures. Different letters indicate significant321
difference (p
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Microscopical images of pineapple pulp after homogenization at (a) 0 (non- homogenized), (b) 300and (c) 600 bar of pressure.23x10mm (600 x 600 DPI)
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Particle size distribution of pineapple pulp homogenized at the different pressures. Inset Tableshows the volume mean diameter of these samples
25x17mm (600 x 600 DPI)
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Flow curves for pineapple pulp homogenized at different pressures. Inset Table shows therheological parameters of the power law model fitted to the flow behavior of the pineapple pulphomogenized at different pressures. Different letters indicate significant difference (p
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Viscosity of the pineapple pulp at different shear rates as a function of homogenization pressure andparticle size (inset Figure).40x26mm (600 x 600 DPI)
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Correlation between cloudiness and homogenization pressure/volume mean diameter (inset Figure)24x16mm (600 x 600 DPI)
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Sedimentation index as a function of storage time for the several homogenization pressuretreatments
40x26mm (600 x 600 DPI)
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