MASS TRANSFER KINETICS OF OSMODEHYDRATED GUAVAS: EVALUATION OF VACUUM
PULSE
J. L.G. CORRÊA1*, L. M. PEREIRA2, G.S. VIEIRA2 and M.D. HUBINGER2
1*
Depart. Food Science, Federal University of Lavras, 37200-000, Lavras, MG, Brazil,
[email protected]
2
Depart. Food Engineering, Faculty of Food Engineering, State University of Campinas,
13083-862, Campinas, SP, Brazil, [email protected]
Abstract: The influence of vacuum pulse and solution concentration on mass transfer of
osmotically dehydrated guavas (Psidium guajava L) were studied. Kinetics of water loss
(WL) and solid gain (SG) were obtained using sucrose solutions at 40; 50 and 60 ºBrix
and vacuum pulse of 100 mbar for 0; 10 and 15 min at the beginning of the process.
Kinetic data were obtained until 300 min for all process condition studied. The
experimental data were fit with Fito’s hydrodynamics model in order to obtain diffusivity
coefficients. An effective influence of concentration and vacuum pulse on the WL and
SG was observed. Fito’s model carried out to good agreements.
Keywords:.PVOD, sucrose concentration, dehydrated fruits, HDM
1. INTRODUCTION
Osmotic dehydration of fruits is a process that carries out to small differences of sensorial properties with respect
to the product in natura. It can be performed at atmospheric pressure (OD) or with vacuum pulse (PVOD).
PVOD process consists on immersion of the product in a hypertonic osmotic solution (OS) with the application
of sub-atmospheric pressure for a small period at the beginning of the process. After this, the osmotic process is
developed at atmospheric pressure. Several works were published considering PVOD of fruits (Fito, 1994;
Giraldo et al., 2003; Chafer et al., 2003; Panadés et al., 2006; Ito et al., 2007; Deng et al., 2008; Moraga et al.,
2009) and most of them reported the improvement on product quality in a PVOD with respect to OD products.
Among the available mathematical models to describe osmotic dehydration, the most used approach is based on
the Fick diffusion law (Shi and Maguer, 2002). In PVOD process, it was demonstrated that the consideration of
hydrodynamics process coupled with Fick’s diffusion law results in best agreements (Fito, 1994; Fito and
Chiralt, 1997).
In the present work, kinetics of WL and SG on mass transfer of guavas were studied at OD and PVOD
conditions. Different sugar concentrations (40; 50 and 60 °Brix) and pulse vacuum time (0; 10 and 15 min) were
analyzed and the experimental data obtained were fit with Fito’s hydrodynamics model.
2. MATERIAL AND METHODS
2.1 Material and samples preparation
Red guava (Psidium guava L.) fruits were purchased in a local market (CEASA-Campinas, SP, Brazil) and
selected based on a similar ripening grade (80% of skin yellowness) and soluble solids content of fruits (around
8 ºBrix) to minimize raw material differences.
The fruits were washed with tap water and peeled manually. They were cut into halves and had the seeds
removed. From each half, two slices of 0.05 m x 0.025 m was obtained, preserving the original guavas thickness
(around 0.005 m).
2.2 Osmotic dehydration
Osmotic dehydration tests were carried out on a jacketed stainless steel chamber designed to work at
atmospheric pressure and/or under vacuum (Ito et al., 2007). The bath temperature was maintained at 40 ºC with
a controlled thermostatic bath. The OS was stirred by a controlled flow recirculation system, performed by a
turbine-type measurement device. Vacuum was obtained with a vacuum pump. The equipment (Figure 1) used
was a pilot scale device with a minimum solution volume of 22 L. The ratio of weight product-to-weight
solution was about 1:45 (w:w).
Fig. 1. Pulsed vacuum osmotic dehydration equipment.
For OD and PVOD treatments, guava slices were placed in a single layer on perforated metallic trays and
immersed in the sucrose solutions at 40; 50 and 60°Brix, at 40°C. For treatments under vacuum, pressure of 100
mbar was applied to the system for the first 10 or 15 min of the osmotic process, afterwards restoring the
atmospheric pressure. At predetermined times (15; 30; 60; 120; 180; and 300 min), samples were removed,
rinsed with water, and placed on absorbent paper to remove excess solution. The samples were then weighed and
analyzed in terms of water loss (WL) and solids gain (SG), according to Equations 1 and 2. The samples
moisture content was determined according to AOAC (2007).
WL(%)
x 0w Mo0
x fw Mof
Mo0
(1)
100
SG(%)
o
xST
f Mf
o
xST
0 M0
Mo0
(2)
100
where Mo0 = initial sample weight (kg), Mof = final sample weight (kg), x 0w = initial moisture content (%), x fw =
ST
final moisture content (%), x ST
0 = initial solids content (%), and x f = final solids content (%).
3. MATHEMATICAL MODELING—DIFFUSION COEFFICIENTS
The diffusion coefficients were estimated using the Fito Hidrodynamics model (Fito and Chiralt, 1997). This
mathematical model considers an equilibrium approach (Equation 3):
zSS
ySS
(3)
SS
where z is the mass fraction of soluble solids in the food and ySS is the mass fraction of soluble solids in the
OS, both at equilibrium state.
The variation on the Food Liquid Phase (FLP) composition is related to the hydrodynamic mechanism (HDM) at
the very beginning of the process (t=0) and the dependency of the activity gradients, correspondent to the
pseudo-diffusion mechanism (PDM) are modeled with Fick’s equation for semi-infinite slab and short time
(Crank, 1975). These two effects were coupled by Fito and Chiralt (1997) to consider the effect of HDM at t = 0
(Equation 4), where Ytw
PD, t 0
, the reduced driving force, is defined by equation 5, where Deff is the effective
diffusivity, k would reflect the effect of mechanisms that are neither diffusional nor hydrodynamic, PD is
pseudo-diffusion.and t is the time.
1 Ytw
PD, t 0
2
Deff t
t2
0.5
0.5
k
(4)
Ytw
Ytw
PD, t 0
Ytw
HDM, t 0
zw
t
zw
t
yw zw
t
HDM
yw
y w / z0w
yw
(5)
The effect of HDM on the FLP concentration was calculated by equation 6, where z w
t
is the calculated
HDM, t 0
initial FLP concentration when the pseudo-diffusion mechanism (PDM) acts.
zw
t
M o0 x 0w
HDM, t 0
M o0 x 0w
(1
)VX
M o0 x ST
0
(1
It is supposed that z w
t
os y
w
)VX
(6)
os
≈ z 0w in the case of OD. In the case of PVOD, z w
t
HDM, t 0
HDM, t 0
≠ z 0w due to the
massive flux of OS because of the HDM action (Fito, 1994). V is the sample volume, X is the food volume
fraction occupied by impregnating solution, γ is the relative volume deformation, os is the OS density, yw is the
mass fraction of water in the OS, Mo0 is the overall mass sample, x 0w is the initial mass fraction of water, x ST
0 is
the initial mass fraction of total solutes and The Deff and K parameters were obtained for each experiment from
the linear fitting of the experimental 1 Ytw
versus t0.5
PD, t 0
4. RESULTS AND DISCUSSION
Figures 2 to 4 show the kinetics of WL with respect to solution concentration and pressure. It is possible to
evaluate from such Figures that WL increased with solution concentration and vacuum pulse application.
However, the solution concentration was more influent than vacuum pulse on guavas water loss.
60,0
Water loss [%]
50,0
40,0
30,0
20,0
10,0
0,0
0
100
200
300
Time [min]
Atm
10 min vacuum
15 min vacuum
Atm
10 min vacuum
15 min vacuum
Fig. 2. Kinetics of water loss of guava slices osmotically dehydrated in sucrose solution at 40ºBrix. Atm:
osmotic process at atmospheric pressure; 10 min and 15 mim vaccum: osmotic process with vacuum pulse
application during 10 and 15 minutes, respectivelly.
Water loss [%]
60,0
50,0
40,0
30,0
20,0
10,0
0,0
0
100
200
300
Time [min]
Atm
Atm
10 min vacuum
10 min vacuum
15 min vacuum
15 min vacuum
Fig. 3. Kinetics of water loss of guava slices osmotically dehydrated in sucrose solution at 50ºBrix. Atm:
osmotic process at atmospheric pressure; 10 min and 15 mim vaccum: osmotic process with vacuum pulse
application during 10 and 15 minutes, respectivelly.
60,0
Water loss [%]
50,0
40,0
30,0
20,0
10,0
0,0
0
100
Time [min]
200
300
Atm
10 min vacuum
15 min vacuum
Atm
10 min vacuum
15 min vacuum
Fig. 4. Kinetics of water loss of guava slices osmotically dehydrated in sucrose solution at 60ºBrix. Atm:
osmotic process at atmospheric pressure; 10 min and 15 mim vaccum: osmotic process with vacuum pulse
application during 10 and 15 minutes, respectivelly.
WL was favored by higher solution concentrations, due to the increase in the osmotic gradient. These results
corroborates with the ones obtained by Ito et al. (2007) on PVOD of mango slices; Madamba and Lopez (2002)
in OD of mango and Mastrantonio et al. (2005) in OD of guavas.
WL was positively influenced by vacuum pulse application. This effect of vacuum pulse is due to the greater
occupation of the internal spaces of the pores with the OS that occurs with application of vacuum. The use of
vacuum pulse creates a larger interface surface available to mass transfer (Fito, 1994). Such trends were also
observed in other works (Fito, 1994, Panadés et al., 2006; Deng and Zhao, 2008). Moreover, the influence of
vacuum pulse was more evident in the higher solution concentrations, suggesting an interaction between these
two variables.
In most treatments, SG was negatively affected by OS concentration (Figures 5 to 7). The negative influence of
solution concentration on SG is a consequence of a dense layer of sucrose at the surface, which acts as a barrier
against solutes penetration and makes mass transfer more difficult (Mastrantonio et al., 2005). The results
obtained are also similar to the ones of Barat et al. (2001), Madamba and Lopez (2002), Mastrantonio et al.
(2005) and Ito et al. (2007). It is important to note that a negative influence on SG is highly desirable in an
osmotic dehydration process. It results in a smaller change on the fruit composition.
With respect to the influence of vacuum pulse, it was directly proportional to solid gain (Panadés et al., 2006; Ito
et al., 2007; Deng and Zhao, 2008) and was more evident at lower solution concentrations, showing again a
possible interaction between these two variables. This behavior can be explained by two different factors: the
occupation of the internal spaces of the guavas pores with the osmotic solution due to the application of vacuum
favoring the SG and the formation of a dense layer of sucrose at the surface with the increase of solution
concentration, acting as a barrier against solutes penetration.
Solid gain [%]
25,0
20,0
15,0
10,0
5,0
0,0
0
100
200
300
Time [min]
Atm
Atm
10 min vacuum
10 min vacuum
15 min vacuum
15 min vacuum
Fig. 5. Kinetics of solid gain of guava slices osmotically dehydrated in sucrose solution at 40ºBrix. Atm: osmotic
process at atmospheric pressure; 10 min and 15 mim vaccum: osmotic process with vacuum pulse application
during 10 and 15 minutes, respectivelly.
Solid gain [%]
25,0
20,0
15,0
10,0
5,0
0,0
0
100
200
300
Time [min]
Atm
Atm
10 min vacuum
10 min vacuum
15 min vacuum
15 min vacuum
Fig. 6. Kinetics of solid gain of guava slices osmotically dehydrated in sucrose solution at 50ºBrix. Atm: osmotic
process at atmospheric pressure; 10 min and 15 mim vaccum: osmotic process with vacuum pulse application
during 10 and 15 minutes, respectivelly.
Solid gain [%]
25,0
20,0
15,0
10,0
5,0
0,0
0
100
200
300
Time [min]
Atm
10 min vacuum
15 min vacuum
Atm
10 min vacuum
15 min vacuum
Fig. 7. Kinetics of solid gain of guava slices osmotically dehydrated in sucrose solution at 60ºBrix. Atm: osmotic
process at atmospheric pressure; 10 min and 15 mim vaccum: osmotic process with vacuum pulse application
during 10 and 15 minutes, respectivelly.
In a general way (Figures 2 to 7), could be observed that the behavior of WL and SG of guavas osmotically
dehydrated with the application of vacuum pulse during 10 minutes at the beginning of the process were very
similar to the ones processed at atmospheric pressure conditions. Moreover, the main differences between OD
and PVOD treatments occurred only after 120 min of treatment, as also observed by Ito et al. (2007). For the
process conditions studied, it was also observed that the sucrose solution concentration is a more influent
variable on kinetics of osmotically dehydrated guavas than vacuum pulse.
Table 1 shows that the values obtained with Fito’s hidrodynamics model for Deff ranged from 0.64 to 2.20 x10-10
m2s-1. An increase on effective diffusivity could be observed with the decrease of OS concentration and the
increase of vacuum pulse time, although of some divergences observed with treatments at 50 °Brix. This
tendency was also observed by Fito et al., 2001; Barat et a., 2001; Chafer et al., 2003 and Giraldo et al., 2003.
Table 1. Effective diffusivities for water and solids
Condition
Deff x1010 [m2s-1]
r2
40 ºBrix, atm
1.18
0.9414
40 ºBrix, 10min vacuum pulse
2.20
0.9526
40 ºBrix, 15min vacuum pulse
2.20
0.9354
50 ºBrix, atm
1.23
0.9459
50 ºBrix, 10min vacuum pulse
1.08
0.9937
50 ºBrix, 15min vacuum pulse
1.54
0.9662
60 ºBrix, atm
0.64
0.9637
60 ºBrix, 10min vacuum pulse
0.71
0.9538
60 ºBrix, 15min vacuum pulse
1.38
0.9894
Higher Deff with vacuum pulse application is promoted due to the substitution of gas volume in the intercellular
spaces by liquid phase, where solutes and water can be transferred (Giraldo et al., 2003). The reduction of Deff
with increase of OS concentration may be explained by the different induced viscosity in the free liquid phase
occupying intercellular spaces after the vacuum pulse (Giraldo et al., 2003) and due to the formation of a dense
layer of sucrose at the surface, which will affect the diffusion.
5. CONCLUSIONS
An effective influence of OS concentration and vacuum pulse on WL and SG of osmotically dehydrated guavas
was observed.
Diffusivity values, obtained by Fito’s hydrodynamics model, varied from 0.64x10-10 m2s-1 to 2.20 x10-10 m2s-1,
and good agreements was obtained using the mathematical model with r2 higher than 0.93 for all the studied
treatments.
A tendency to increase the effective diffusivity was observed with the decrease of OS concentration and increase
of vacuum pulse time.
6. ACKNOWLEDGEMENTS
The authors are grateful to FAPEMIG (Process 548/08), FAPESP (Process 2001/13809-5 and Process 06/598901) and CNPq for the financial support.
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