Mass transfer kinetics of pulsed vacuum osmotic dehydration of guavas

Journal of Food Engineering 96 (2010) 498–504

Contents lists available at ScienceDirect

Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Mass transfer kinetics of pulsed vacuum osmotic dehydration of guavas Jefferson L.G. Corrêa a,*, Leila M. Pereira b, Gláucia S. Vieira b, Míriam D. Hubinger b a b

Department of Food Science, Federal University of Lavras, Lavras – MG, Brazil Department of Food Engineering, Faculty of Food Engineering, State University of Campinas, Campinas – SP, Brazil

a r t i c l e

i n f o

Article history: Received 3 April 2009 Received in revised form 21 August 2009 Accepted 29 August 2009 Available online 3 September 2009 Keywords: Psidium guajava L., PVOD Hydrodynamic model Sucrose concentration Dehydrated fruit

a b s t r a c t The effects of vacuum pulse and solution concentration on mass transfer of osmotically dehydrated guava slices were studied. Kinetics of weight reduction (WR), water loss (WL), solid gain (SG) and water activity (aw) were obtained using sucrose solutions at 40, 50 and 60 °Brix and vacuum pulse of 100 mbar for 0, 10 and 15 min at the process beginning. Higher solution concentrations and the vacuum pulse application caused an increase on WL of osmotically dehydrated guavas and reduced the samples water activity. The SG was reduced by the increase on osmotic solution concentration and favored by vacuum application. Two different models of kinetics diffusion were tested to obtain diffusivity and to compare the accuracy of these models. The effective diffusivity estimated by the hydrodynamic model well reproduced the effects of process variables on mass transfer kinetics and showed a better agreement to the experimental data than the diffusional model. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The osmotic dehydration process has often been used for the development of new products of fruit and vegetable due to the small changes in the sensorial and nutritional properties of the fresh product. It can be used as a pre-treatment to drying (Al-Harahsheh et al., 2009; Antonio et al., 2008; Jalali et al., 2008) and freezing (Dermesonlouoglou et al., 2008; Mitrakas et al., 2008; Phoon et al., 2008). Furthermore, it can be used to produce minimally processed products (Panadés et al., 2003; Pereira et al., 2004; Rodrigues et al., 2006; Torres et al., 2008). The osmotic process can be performed at atmospheric pressure (OD) or with vacuum pulse application (PVOD) for a small period at the beginning of the process. The water loss and solid gain are higher in the beginning of the PVOD process, when the hydrodynamics mechanism (HDM) takes place, than in the OD process (Chafer et al., 2003; Giraldo et al., 2003). The HDM is a consequence of the pressure gradients, resulting from the combined action of capillary flow and pressure changes imposed on the porous structures of vegetable tissue. It is controlled by the presence of gas or liquid occluded in the intercellular spaces. By applying vacuum pressure, an outflow of internal gas or liquid from the tissue and the entrance of external solution are established that promotes water loss and the uptake of external solutes (Chiralt et al., 2001; Chiralt and Talens, 2005; Fito and Chiralt, 1997; Rastogi et al.,

* Corresponding author. Tel.: +55 35 3829 1658; fax: +55 35 3829 1401. E-mail address: [email protected]fla.br (J.L.G. Corrêa). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.08.032

2002). Temperature, osmotic solution concentration, vacuum time and total processing time are the most important variables in the osmotic process. Increasing the osmotic solution concentration induces an increase in the mass transfer (Barat et al., 2001; Giraldo et al., 2003; Ito et al., 2007; Madamba and Lopez, 2002; Panadés et al., 2006). The vacuum pulse results in a higher mass transfer, but the influence of the vacuum application time could be function of the product’s biological characteristics (Escriche et al., 2000; Panadés et al., 2006). The mathematical models employed to describe the OD process are usually based on Fick’s diffusion law (Corrêa et al., 2008; Falade and Igbeka, 2007; Rastogi et al., 2002; Rastogi and Raghavarao, 2004). However, according to Fito (1994), the increased mass transfer rate due to the vacuum application cannot be satisfactorily explained using the classical, diffusional and osmotic mechanisms. Thus, the consideration of the hydrodynamics mechanism coupled with Fick’s diffusion law can promote a better representation of the mass transfer phenomenon in the pulsed vacuum osmotically dehydration process. The main goal of this work was to study the effects of the vacuum pulse and solution concentration on the mass transfer of osmotically dehydrated guava slices. This was performed by: (1) studying the kinetics of weight reduction (WR), water loss (WL), solid gain (SG) and water activity (aw) for different concentrations of sucrose solutions and with different periods of vacuum pulse application at the beginning of the process; (2) calculating the effective diffusivity by the use of two different models, a diffusional model and a hydrodynamics model in order to compare the accuracy of the models.

499

J.L.G. Corrêa et al. / Journal of Food Engineering 96 (2010) 498–504

2. Material and methods 2.1. Raw material Red guava (Psidium guava L.) fruits were purchased at a local market (CEASA-Campinas, Brazil). The guavas were selected based on having a similar ripening grade (around 8 °Brix and 80% skin yellowness) to minimize differences in the raw material. The composition of the guavas used in the trials, as determined according to the Association of Official Agricultural Chemists (AOAC, 2002), is shown in Table 1. 2.2. Equipment The equipment used in the osmotic dehydration experiments consisted of a jacketed stainless steel chamber designed to work at atmospheric pressure and/or under vacuum (Ito et al., 2007; Vivanco-Pezantes et al., 2004). The chamber presented internal diameter of 426  103 m and height of 430  103 m with a useful volume of around 60 L. The osmotic solution temperature was controlled using a thermostatic bath connected to the equipment. The solution was stirred by a controlled flow recirculation system using a sanitary pump. Vacuum was obtained by using a vacuum pump. The vacuum pressure, flow and the temperature operational conditions were controlled through control equipments that are connected to the equipment and monitored by a data acquisition system. 2.3. Sample preparation and osmotic process The guavas were washed with tap water and manually peeled; then were cut into halves and the seeds were removed. From each half, two slices of 0.050 m  0.025 m were obtained, preserving the original guava thickness of about 0.005 m. The geometry (form and size) was chosen to make the samples suitable to be treated as a semi-infinite plate in the unidirectional diffusion mathematical model, like the one used here. For the OD and the PVOD treatments, the guava slices were initially weighed and placed in a single layer on perforated stainless steel trays, to allow the solution flow through the samples, and immersed in the sucrose solutions at 40, 50 and 60 °Brix. Twelve trays with three guavas slices each were used in the trials. Each tray and the position of the samples on the trays were codified to allow the identification of the samples. The temperature of the osmotic solution used in the trials was 40 °C and the recirculation level was 2.5 m3 h1, condition established in a previous study in order to neglect external resistance to mass transfer in this equipment (Vivanco-Pezantes, 2006). The temperature used was based on the work of Panadés et al. (2008), where it was verified that the larger diffusivity for PVOD in guavas was obtained at 40 and 50 °C and that there was not a significant difference between the diffusivity

obtained at these two temperatures. The equipment used was a pilot scale device with a minimum solution volume of 22 L. The ratio of the weight of the product to the weight of the solution was about 1:45. For the treatments of the product under vacuum, a pressure of 100 mbar was applied to the system for the first 10 or 15 min of the osmotic process, resulting in two experimental conditions based on the application of the vacuum pulse. Following application of the vacuum pulse the atmospheric pressure was restored. At predetermined times (15, 30, 60, 120, 180 and 300 min) the samples were removed, rinsed with water and placed on absorbent paper to remove excess solution. The overall time used was chosen in accordance to other osmotic dehydration studies (Fermin and Corzo, 2005; Ito et al., 2007; Moraga et al., 2009; Park et al., 2002). Sampling was done in triplicate. The samples were then weighed and analyzed in terms of the water loss, the solids gain and the weight reduction. These parameters were calculated according to the following equations:

WL ð%Þ ¼

SG ð%Þ ¼

o w o xw 0 M 0  xf M f

Mo0

o ST o xST f M f  x0 M 0

WR ð%Þ ¼

Mo0 Mo0  Mof M o0

100

ð1Þ

100

ð2Þ

100

where Mo0 = initial sample weight (kg), M of = final sample weight w (kg), xw 0 = initial moisture content (%), xf = final moisture content = initial solids content (%), and xST (%), xST 0 f = final solids content (%).The samples moisture content was determined according to the AOAC (2002). The water activity of the samples was determined using the Aqualab Series 3 TE (Decagon Devices Inc., Pullman, WA). 2.4. Kinetic models — diffusion coefficients The experimental data were fit to two mathematical models to estimate the water and the solids diffusion coefficients. The models are the unidirectional diffusion model, or Method 1, (Eqs. )((4)–(8)) and the hydrodynamics model, or Method 2 (Eqs. (9)–(16)). Even though it is verified a concentration profile in osmotic dehydration processes (Kaminska et al., 2008; Torres et al., 2007), the models used consider a unidirectional diffusion, with average values of concentration and diffusivity in the thickness direction. This consideration was assumed due to the fact that the other dimensions of the slices were much larger than thickness. 2.4.1. Method 1 The model is based on the unidirectional diffusion equation of Fick (Crank, 1975):

  @MCðtÞ @ @MCðtÞ ¼ Deff @t @z @z

Table 1 Guava composition. Analysis

Mean value (%)a

Moisture content (w.b.) (kg kg1) Ashb Protein Lipids Total sugars Fibers Acidity

84.53 ± 1.17 0.32 ± 0.01 0.54 ± 0.05 0.24 ± 0.05 3.02 ± 0.25 10.65b 0.70 ± 0.03

a All data were obtained by triplicate analyses and expressed as mean ± standard deviation. b Calculated by difference.

ð3Þ

ð4Þ

where MCðtÞ is the amount of water or solids at the instant t, Deff is the effective diffusivity and z is a generic directional coordinate. The solid sample was considered as a plate of thickness 2L. The initial condition is an uniform initial amount of moisture and/or solids, MCðz;0Þ ¼ MC0 . The  boundary conditions are the symmetry of concentration, @MCðtÞ ¼ 0, and the equilibrium content at the material surface, @t  z¼0 MCðL;tÞ ¼ MCeq . With consideration to the initial and the boundary conditions, Fick’s unidirectional diffusion equation (Crank, 1975) becomes:

500

J.L.G. Corrêa et al. / Journal of Food Engineering 96 (2010) 498–504

Ww

or s

1 8 X

¼

p2

i¼1

1 ð2i þ 1Þ

2

 exp ð2i þ 1Þ2 p2 Deff;w

!

t or s

2

4L

ð5Þ where Deff, w or s is the effective diffusivity of the water loss or the solids’ gain, i is the number of series terms, L is the characteristic length (sample half-thickness), t is the time and Ww or s is the dimensionless water or solid content. This is valid for a transient operation. The dimensionless water or solid content is given by the following equation

Ww

or s

MCðtÞ  MCeq MC0  MCeq

¼

ð6Þ

The Deff and k parameters were obtained for each experiment 0.5  . from a linear fitting of the experimental 1  Y w t PD;t>0 versus t The criterion used to evaluate the best fit to the model was the estimative standard error (SE):

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i¼1 ðOBS  PREDÞ SE ¼ n

ð16Þ

where OBS is correspondent to the observed value of water or solid mass and PRED is the predicted value of water or solid mass. The term n corresponds to the number of observations. The evaluations were done by the nonlinear estimation procedure using the software Statistica 5.0Ò (Statsoft, Tulsa, USA).

where according to Peleg’s equation (Peleg, 1988) 3. Results and discussion

t k1 þ k2 t

ð7Þ 3.1. Water loss (WL)

which approaches the equilibrium asymptotically (Palou et al., 1993):

MCeq

 ¼ lim MC0 

t k1 þ k2 t

t!1

 ¼ MC0 

1 k2

ð8Þ

The Peleg’s equation parameters (k1 and k2) and the effective diffusivities were obtained using the nonlinear estimation from software Statistica 5.0Ò (Statsoft, Tulsa, USA). 2.4.2. Method 2 The diffusion coefficients were estimated using the Fito and Chiralt hydrodynamics model (Fito and Chiralt, 1997). This mathematical model considers an equilibrium approach: SS zSS 1 ¼ y1

ð9Þ

where zSS 1 is the mass fraction of the soluble solids in the food and ySS 1 is the mass fraction of the soluble solids in the osmotic solution, both at the equilibrium state. As a result, the effective diffusivity (or pseudo diffusivity) is the same for both water and solids:

Deff w

or s ¼

Deff w ¼ Deff s

ð10Þ

The changes in composition are functions of the reduced drive force, Y, given by: s Y ¼ Yw t ¼ Yt ¼

w zw t  zeq w z0  zw eq

ð11Þ

The variation on the Food Liquid Phase (FLP) composition related to the hydrodynamic mechanism (HDM) occurs at the very beginning of the process (t = 0 to t = tHDM), where this mechanism is predominant and is dependent on the pressure gradients:

t¼tHDM  1  Yw ffik t t¼0

ð12Þ

After this period, the phenomena are modeled with Fick’s equation for semi-infinite slab and short time (Crank, 1975), with the approach suggested by Fito and Chiralt (1997):

t¼t  1  Yw t t¼t

HDM

¼2

  Deff t L

2

p0;5 þ 2

1 X i1

!

iL ierfc pffiffiffiffiffiffiffiffiffiffi Deff t

t

t¼t HDM

¼2

 0;5 Deff t

pL2

1



¼kþ2

 0;5 Deff t

pL2

50.0 40.0 30.0 20.0 10.0 0.0 0

100

ð15Þ

200

300

Time [min]

ð14Þ

These two effects were coupled to consider the effect of the hydrodynamics and the pseudo-Fickian mechanisms:

t¼t Yw t t¼0

60.0

ð13Þ

Eq. (13) can be simplified at the first term, resulting in Eq. (14):

t¼t 1  Y w

The kinetics of water loss from guava slices subjected to different osmotic treatments are shown in Fig. 1–3. The use of osmotic solutions at higher sucrose concentrations caused an increase in the WL of guavas osmotically dehydrated at atmospheric conditions (OD) and with vacuum pulse application (PVOD). The vacuum pulse applied at the beginning of the osmotic process also caused an increase in the WL of guavas, and this result was intensified by the vacuum time. However, for guavas treated at 50 °Brix, a relevant effect of the pressure conditions was verified only with the application of 15 min of the vacuum pulse. Similar behavior of the WL was observed for the weight reduction (WR) (data not shown). The water loss was favored by higher solution concentrations, due to the increase of the osmotic gradient between the food material and the osmotic solution. The presence of a large amount of solute causes a higher osmotic pressure that makes the WL easier. These results corroborate those obtained by Fermin and Corzo (2005) and Ito et al. (2007) in the PVOD of cantaloupe cylinders and mango slices, as well as the results from Madamba and Lopez (2002) and Mastrantonio et al. (2005) in the OD of mango slices and guava halves. The increase of the WL of guavas with the application of a vacuum pulse can be explained by the hydrodynamics mechanism that is verified at the beginning of the osmotic process. With the application of the vacuum conditions, the gas occluded in the intercellular spaces of the vegetable tissues is removed and when the

Water loss [%]

MCt ¼ MC0 

OD OD

PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 1. Kinetics of water loss of guava slices osmotically dehydrated in sucrose solution at 40 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

501

J.L.G. Corrêa et al. / Journal of Food Engineering 96 (2010) 498–504

25.0

50.0

20.0

40.0

Solid gain [%]

Water loss [%]

60.0

30.0 20.0 10.0

15.0 10.0 5.0

0.0 0

100

200

300

Time [min] OD

PVOD 10

PVOD 15

0.0 OD

PVOD 10

0

PVOD 15

Fig. 2. Kinetics of water loss of guava slices osmotically dehydrated in sucrose solution at 50 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic 504 process with vacuum pulse application during 10 and 15 min, respectively.

100

200

300

Time [min] OD OD

PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 4. Kinetics of the solid gain of guava slices osmotically dehydrated in sucrose solution at 40 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

60.0

25.0

40.0

20.0 30.0

Solid gain [%]

Water loss [%]

50.0

20.0 10.0 0.0 0

100

200

300

Time [min] OD OD

PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 3. Kinetics of water loss of guava slices osmotically dehydrated in sucrose solution at 60 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

atmospheric pressure condition is restored, the pores of the food material are filled by osmotic solution. This increases the available mass transfer surface area. Similar trends were also observed in other studies (Deng and Zhao, 2008; Fermin and Corzo, 2005; Fito, 1994; Panadés et al., 2006). The influence of the vacuum pulse was more effective on fruits treated with higher osmotic solution concentrations. This suggests an interactive effect of these two variables on the WL of guavas. 3.2. Solid gain (SG) The use of osmotic solutions at higher sucrose concentrations caused a decrease in the solids gain of guavas osmotically dehydrated at atmospheric conditions (OD) and with the vacuum pulse application (PVOD), as shown in Figs. 4–6. The concentrated solutions may have promoted the formation of a dense layer of solutes at the surface of the osmodehydrated guavas. This layer acts as a barrier against penetration of the solutes into the food and makes solutes mass transfer more difficult, which results in a lower solids uptake in fruits tissue. According to Mújica-Paz et al. (2003), diluted solutions penetrate better into the fruit tissue than concentrated solution. With increased sugar concentration, the osmotic solution becomes more viscous, that makes the solutes penetration more difficult. Similar behavior was also observed by Barat et al.

15.0 10.0 5.0 0.0 0

100

200

300

Time [min] OD OD

PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 5. Kinetics of solid gain of guava slices osmotically dehydrated in sucrose solution at 50 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

(2001), Ito et al. (2007), Madamba and Lopez (2002) and Mastrantonio et al. (2005). The application of the vacuum pulse influenced the kinetics of solid gain of guava slices in a similar way as the changes observed in kinetics of water loss. The use of the vacuum pulse at the beginning of the osmotic process caused an increase in the SG of guavas, and this behavior was intensified with increased vacuum time. However, for guavas treated at 50 °Brix, the effect of the pressure conditions was only noted with 15 min of the vacuum pulse application. Similar to the effect seen in the water loss, with the use of the vacuum pulse the gas that was occluded in the fruit pores was removed and the intercellular spaces of the food material were filled with the osmotic solution causing a greater solid uptake in the fruit tissue. A similar effect of the vacuum pulse on solid incorporation of osmotically dehydrated fruits was also observed by Deng and Zhao (2008), Ito et al. (2007) and Panadés et al. (2006). Despite the results presented here, Shi et al. (1995) reported that the vacuum treatments are effective in increasing water loss but with no influence on the sugar gain of fruits in the osmotic dehydration process. It can be inferred that as the sugar gain is

J.L.G. Corrêa et al. / Journal of Food Engineering 96 (2010) 498–504

25.0

0.995

20.0

0.985 0.975

15.0 aw

Solid gain [%]

502

0.965

10.0

0.955 5.0

0.945

0.0

0.935 0

100

200

300

0

100

Time [min] PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 6. Kinetics of solid gain of guava slices osmotically dehydrated in sucrose solution at 60 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

closely related to the porosity of fruits, the different behaviors among osmotically dehydrated fruits can be attributed to the biological characteristic of vegetable tissues. Ito et al. (2007), working with mangos, reported that the main influences of the vacuum pulse and the osmotic solution concentration are observed only after 300 min of treatment for the water loss and the solid gain, and after the first 120 min for the water activity of the fruits. In the present study, the same influences were observed from the beginning of the osmotic process. The different results observed in these studies can also be explained by the differences in biological structural characteristics of the fruits studied, since the vacuum effect depends on the porosity of the specific fruit. In a general way, it could be observed that the application of a 10 min vacuum pulse at the beginning of the osmotic process had a minor influence on the WL behavior and the SG of osmotically dehydrated guavas with respect to atmospheric pressure conditions. However, the effect of a 15 min vacuum pulse application at the beginning of the process was clearly observed on the mass transfer of osmotically dehydrated guavas. For the studied process conditions, it was also observed that the osmotic solution concentration had a greater influence than the vacuum pulse on the kinetics of osmotically dehydrated guavas.

OD OD

300

PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 7. Kinetics of water activity of guava slices osmotically dehydrated in sucrose solution at 40 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

0.995 0.985 0.975

aw

OD OD

200 Time [min]

0.965 0.955 0.945 0.935 0

100

200

300

Time [min] OD OD

PVOD 10 PVOD 10

PVOD 15 PVOD 15

Fig. 8. Kinetics of water activity of guava slices osmotically dehydrated in sucrose solution at 50 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

0.995

3.3. Water activity (aw)

0.975

aw

A reduction in the guavas water activity was observed with the use of osmotic solutions at higher sucrose concentrations and with the application of the vacuum pulse at the beginning of the osmotic process (Figs. 7–9), caused by the mass transfer enhancement verified in these process conditions, as it was also observed in water loss of fruits. Moreover, the decrease in the aw was intensified with the vacuum time for treatments at higher solution concentrations (50 and 60 °Brix), showing again the interactive effects of these two variables on the mass transfer phenomena. The optimum condition in an osmotic dehydration process is the one that results on the higher water loss and the lowest solid gain and water activity. Based on this fact, it can be inferred that among the conditions studied in this work, the best one is the use of 15 min of vacuum pulse with sucrose solution at 60 °Brix. Although the use of 15 min of vacuum pulse lead to a higher solid gain in a 60 °Brix sucrose solution, the values of water loss were higher and the values of water activities were lower than the ones

0.985

0.965 0.955 0.945 0.935 0

100 OD OD

200 Time [min] PVOD 10 PVOD 10

300 PVOD 15 PVOD 15

Fig. 9. Kinetics of water activity of guava slices osmotically dehydrated in sucrose solution at 60 °Brix. OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively.

J.L.G. Corrêa et al. / Journal of Food Engineering 96 (2010) 498–504

obtained at atmospheric pressure or with 10 min of vacuum pulse application. It is important to notice that the local maximum point observed in some curves (Fig. 1–9) does not have physical meaning. It is just related to the exponential agreement obtained with the tendency curve. 3.4. Effective diffusion The diffusion coefficients obtained for the osmotic processing of guavas, calculated by Method 1 and Method 2 are shown in Tables 2 and 3. The effective diffusivities for the water (Deff w ) and solids (Deff s ) was calculated using Method 1, and ranged from 0.32  1010 to 4.59  1010 m2 s1 and from 0.60  1010 to 9.24  1010 m2 s1, respectively (Table 2). Using Method 2, the effective diffusivity was found to be the same for both the water and solids, and ranged from 0.64  1010 to 2.20  1010 m2 s1 (Table 3). The values obtained with both methods are similar to those obtained in osmotic treatment of papayas (Rodrigues et al., 2003), apples (Barrera et al., 2004) and mangos (Ito et al., 2007). According to Barat et al. (2001), Chafer et al. (2003), Fito et al. (2001) and Giraldo et al. (2003), higher effective diffusivity values are obtained with the application of the vacuum pulse and with a decrease in osmotic solution concentration. Similar effects were observed for osmotically dehydrated guavas when the effective diffusion coefficients were estimated using Method 2. This result reveals the influence of the process variables on mass transfer kinetics. Panadés et al. (2008) in a vacuum-pulsed osmotic dehydration (PVOD) showed the suitability of the hydrodynamic model to cases where vacuum pulse is used. Aguilera et al. (2003) warned that hydrodynamic transport of water and solutes due to pressure gradients in open pores is the predominant mass transfer Table 2 Effective diffusivities for the water and solids obtained using Method 1. Treatment 40 40 40 50 50 50 60 60 60

°Brix, °Brix, °Brix, °Brix, °Brix, °Brix, °Brix, °Brix, °Brix,

OD PVOD PVOD OD PVOD PVOD OD PVOD PVOD

10 15 10 15 10 15

Deff W (m2 s1)

R2

SE

Deff S (m2s1)

R2

SE

2.11  1010 3.27  1010 4.59  1010 0.73  1010 0.95  1010 0.32  1010 2.13  1010 1.60 x1010 1.26  1010

0.93 0.97 0.94 0.91 0.97 0.89 0.96 0.95 0.97

0.23 0.05 0.08 0.05 0.04 0.14 0.06 0.06 0.04

2.40  1010 6.19  1010 9.24  1010 0.61  1010 0.60  1010 0.88  1010 2.00  1010 1.55 x1010 1.09  1010

0.93 0.95 0.87 0.91 0.97 0.95 0.95 0.94 0.96

0.08 0.09 0.15 0.17 0.03 0.04 0.06 0.06 0.04

OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively. The effective diffusivities were calculated based on Fick’s second law (Crank, 1975).

Table 3 Effective diffusivities for the water and solids obtained using Method 2. Deff (m2 s1)

Treatment 40 40 40 50 50 50 60 60 60

°Brix, °Brix, °Brix, °Brix, °Brix, °Brix, °Brix, °Brix, °Brix,

OD PVOD PVOD OD PVOD PVOD OD PVOD PVOD

10

10 15 10 15 10 15

1.18  10 2.20  1010 2.20  1010 1.23  1010 1.08  1010 1.54  1010 0.64  1010 0.71  1010 1.38  1010

R2

SE

0.94 0.95 0.94 0.95 0.99 0.97 0.96 0.95 0.99

0.02 0.02 0.02 0.02 0.02 0.05 0.03 0.04 0.02

OD: osmotic process at atmospheric pressure. PVOD 10 and PVOD 15: osmotic process with vacuum pulse application during 10 and 15 min, respectively. The effective diffusivities were calculated based on Fito and Chiralt (1997) hydrodynamics model.

503

mechanism in PVOD of porous fruits. Those authors also reported that there are also other mechanisms, but less relevant, including intercellular transport in liquid phase through plasmodesma, transmembrane flow and Fickean diffusion within non-compartmentalized zones. Amami et al. (2006) concluded that the use of a model that considers convective and diffusivity mechanisms is more appropriate for an osmotic dehydration process preceded by pulsed electric field than the traditional Fickean model. For the diffusivity data calculated using Method 1, an increase in the osmotic solution concentration from 40 to 60 °Brix also caused a decrease on the diffusion coefficients for water and solids. However, there was not a conclusive trend at 50 °Brix. For osmotic treatment at lower sucrose solution concentration (40 °Brix), an increase in the vacuum time resulted in higher effective diffusivity values for the water and solid. At higher sucrose solution concentration (60 °Brix), increased vacuum time resulted in an opposite behavior, making difficult the evaluation of this process variable effects. It was also verified that Method 2 demonstrated a better agreement to the experimental data, presenting correlation coefficients (R2) values between 0.94 and 0.99 and estimated standard error values lower than 0.05. This is compared to the results obtained with Method 1 that demonstrated R2 values from 0.87 to 0.97 and estimated standard error values until 0.23. The greatest correlation between the behavior of mass transfer and the effective diffusivity values can be attributed to the hydrodynamics mechanism coupled with Fick’s diffusion law considered in Method 2, that promotes a better fitness of the mass transfer phenomena in the pulsed vacuum osmotically dehydration process. Although Fick’s approach, treated in Method 1, only considers the diffusional mechanism, it has already been used in some works of pulsed vacuum dehydration to determine diffusional coefficients (Ito et al., 2007; Matusek et al., 2008). Matusek et al. (2008) reported that PVOD is not a simple diffusion controlled process and vacuum treatment causes modification on the diffusion mechanism that results in significantly difference between predicted and measured values. In the present work, this approach was used only to be compared to the hydrodynamic approach and to evaluate the goodness of fit for both ones.

4. Conclusions Mass transfer kinetics of guava slices osmotically dehydrated in sucrose solutions were greatly affected by sucrose concentration and by vacuum pulse application at the beginning of the process. Higher sucrose solution concentrations and the vacuum pulse application caused an increase on WL of osmotically dehydrated guavas. However, the SG was reduced by the increase on osmotic solution concentration, although solid uptake was favored by vacuum application. The behavior of weight reduction and water activity kinetics was similar to the water loss. In a general way, the effects of pressure conditions on the mass transfer kinetics were clearly observed only with the application of the vacuum pulse during 15 min at the beginning of the process. Furthermore, for the process conditions studied, the osmotic solution concentration seems to have a greater influence on the kinetics of osmotically dehydrated guavas than the vacuum pulse. The hydrodynamic model (Method 2) demonstrated a better agreement to the experimental data than the diffusional model (Method 1), presenting correlation coefficients (R2) values between 0.94 and 0.99 and an estimated standard error lower than 0.05, and the effective diffusion coefficients behavior reflected the influence of process variables on mass transfer kinetics.

504

J.L.G. Corrêa et al. / Journal of Food Engineering 96 (2010) 498–504

Acknowledgements The authors are grateful to FAPEMIG (Process CAG 548/08), FAPESP (Process 2001/13809-5 and Process 06/59890-1) and CNPq for the financial support.

References Aguilera, J.M., Chiralt, A., Fito, P., 2003. Food dehydration and product structure. Trends in Food Science and Technology 14 (10), 432–437. Al-Harahsheh, M., Al-Muhtaseb, A.H., Magee, T.R.A., 2009. Microwave drying kinetics of tomato pomace: effect of osmotic dehydration. Chemical Engineering and Processing 48 (1), 524–531. Amami, E., Vorobiev, E., Kechaou, N., 2006. Modelling of mass transfer during osmotic dehydration of apple tissue pre-treated by pulsed electric field. LWT – Food Science and Technology 39 (9), 1014–1021. Antonio, G.C., Alves, D.G., Azoubel, P.M., Murr, F.E.X., Park, K.J., 2008. Influence of osmotic dehydration and high temperature short time processes on dried sweet potato (Ipomoea batatas Lam.). Journal of Food Engineering 84, 375–382. AOAC (Association of Official Analytical Chemists), 2002. Official Methods of Analysis, 18th ed. AOAC International. Barat, J.M., Chiralt, A., Fito, P., 2001. Effect of osmotic solution concentration, temperature and vacuum impregnation pretreatment on osmotic dehydration kinetics of apple slices. Food Science and Technology International 7, 451–456. Barrera, C., Betoret, N., Fito, P., 2004. Ca+2 and Fe+2 influence on the osmotic dehydration kinetics of apple slices (var. Granny Smith). Journal of Food Engineering 65, 9–14. Chafer, M., Gonzalez-Martinez, C., Fernandez, B., Perez, L., Chiralt, A., 2003. Effect of blanching and vacuum pulse application on osmotic dehydration of pear. Food Science and Technology International 9, 321–328. Chiralt, A., Fito, P., Barat, J.M., Andrés, A., González-Martinez, C., Escriche, I., Camacho, M.M., 2001. Use of vacuum impregnation in food salting process. Journal of Food Engineering 49, 141–151. Chiralt, A., Talens, P., 2005. Physical and chemical changes induced by osmotic dehydration in plant tissues. Journal of Food Engineering 67, 167–177. Corrêa, J.L.G., Cacciatore, F.A., da Silva, Z.E., Arakaki, T., 2008. Osmotic dehydration of West Indian cherry (Malpighia emarginata D.C.) – mass transfer kinetics. Revista Ciência Agronômica 39 (3), 403–409 (in Portuguese). Crank, J., 1975. The Mathematics of Diffusion, second ed. Clarendon Press, Oxford. Deng, Y., Zhao, Y., 2008. Effects of pulsed-vacuum and ultrasound on the osmodehydration kinetics and microstructure of apples (Fuji). Journal of Food Engineering 85 (1), 84–93. Dermesonlouoglou, E.K., Pourgouri, S., Taoukis, P.S., 2008. Kinetic study of the effect of the osmotic dehydration pre-treatment to the shelf life of frozen cucumber. Innovative Food Science and Emerging Technologies 9 (4), 542–549. Escriche, I., Garcia-Pinchi, R., Fito, P., 2000. Osmotic dehydration of kiwifruit (Actznidza chinensis): fluxes and mass transfer kinetics. Journal of Food Process Engineering 23, 191–205. Falade, K.O., Igbeka, J.C., 2007. Osmotic dehydration of tropical fruits and vegetables. Food Reviews International 23, 373–405. Fermin, W.J., Corzo, O., 2005. Optimization of vacuum pulse osmotic dehydration of cantaloupe using response surface methodology. Journal of Food Processing and Preservation 29 (1), 20–32. Fito, P., 1994. Modelling of vacuum osmotic dehydration of food. Journal of Food Engineering 22, 313–328. Fito, P., Chiralt, A., 1997. An approach to the modeling of solid food-liquid operations: application to osmotic dehydration. In: Fito, P., Ortega-Rodriguez, E., Barbosa-Canovas, G. (Eds.), Food Engineering 2000. Chapman and Hall, New York, pp. 231–252. Fito, P., Chiralt, A., Barat, J.M., Andrés, A., Martínez-Monzo, J., Martinéz-Navarrete, N., 2001. Vacuum impregnation for development of new dehydrated products. Journal of Food Engineering 49, 297–302. Giraldo, G., Talens, P., Fito, P., Chiralt, A., 2003. Influence of sucrose solution concentration on kinetics and yield during osmotic dehydration of mango. Journal of Food Engineering 58, 33–43. Ito, A.P., Tonon, R.V., Park, K.J., Hubinger, M.D., 2007. Influence of process conditions on the mass transfer kinetics of pulsed vacuum osmotically dehydrated mango slices. Drying Technology 25 (11), 1769–1777. Jalali, V.R.R., Narain, N., da Silva, G.F., 2008. Effect of osmotic predehydration on drying characteristics of banana fruits. Ciência e Tecnologia de Alimentos 28 (2), 269–273 (in Portuguese).

Kaminska, A., Lewicki, P.P., Malczyk, P., 2008. Mass transfer in osmotically dehydrated apple stored at temperatures above zero. Journal of Food Engineering 86 (1), 140–149. Madamba, P., Lopez, R.I., 2002. Optimization of the osmotic dehydration of mango (Mangifera indica L.) slices. Drying Technology 20, 1227–1242. Mastrantonio, S.D.S., Pereira, L.M., Hubinger, M.D., 2005. Osmotic dehydration kinetics of guavas in maltose solutions with calcium salt. Alimentos e Nutrição 16, 309–314. Matusek, A., Czukor, B., Merész, P., Örsi, F., 2008. Comparison of diffusion of fructooligosaccharide components during vacuum impregnation and osmotic dehydration. European Food Research Technology 227, 417–423. Mitrakas, G.E., Koutsoumanis, K.P., Lazarides, H.N., 2008. Impact of edible coating with or without anti-microbial agent on microbial growth during osmotic dehydration and refrigerated storage of a model plant material Innovative. Food Science and Emerging Technologies 9 (4), 550–555. Moraga, M.J., Moraga, G., Fito, P.J., Martínez-Navarrete, N., 2009. Effect of vacuum impregnation with calcium lactate on the osmotic dehydration kinetics and quality of osmodehydrated grapefruit. Journal of Food Engineering 90, 372–379. Mújica-Paz, H., Valdez-Fragoso, A., López-Malo, A., Palou, E., Welti-Chanes, J., 2003. Impregnation and osmotic dehydration of some fruits: effect of the vacuum pressure and syrup concentration. Journal of Food Engineering 57, 305–314. Palou, E., López-Malo, A., Argaíz, A., Welti, J., 1993. Osmotic dehydration of papaya. Effect of syrup concentration. Revista Espanola de Ciencia y Tecnologia de Alimentos 33 (6), 621–630. Panadés, G., Chiralt, A., Fito, P., Rodrıguez, I., Nuñes, M., Albors, A., Jiménez, R., 2003. Influence of operating conditions on sensory quality of minimally processed osmotically dehydrated guava. Journal of Food Quality 26, 91–103. Panadés, G., Fito, P., Aguiar, Y., Villavicencio, M.N., Acosta, V., 2006. Osmotic dehydration of guava: Influence of operating parameters on process kinetics. Journal of Food Engineering 72 (4), 383–389. Panadés, G., Castro, D., Chiralt, A., Fito, P., Nunez, M., Jimenez, R., 2008. Mass transfer mechanisms occurring in osmotic dehydration of guava. Journal of Food Engineering 87 (3), 386–390. Park, K.J., Bin, A., Brod, F.P.R., Park, H.K.B.P., 2002. Osmotic dehydration kinetics of pear D’anjou (Pyrus communis L.). Journal of Food Engineering 52, 293–298. Peleg, M., 1988. An empirical-model for the description of moisture sorption curves. Journal of Food Science 53, 1216–1219. Pereira, L.M., Rodrigues, A.C.C., Sarantópoulos, C.I.G.L., Junqueira, V.C.A., Cunha, R.L., Hubinger, M.D., 2004. Influence of modified atmosphere packaging and osmotic dehydration on the quality maintenance of minimally processed guavas. Journal of Food Science 69 (4), 172–177. Phoon, P.Y., Galindo, F.G., Vicente, A., Deimek, P., 2008. Pulsed electric field in combination with vacuum impregnation with trehalose improves the freezing tolerance of spinach leaves. Journal of Food Engineering 88 (1), 144–148. Rastogi, N.K., Raghavarao, K.S.M.S., Niranjan, K., Knorr, D., 2002. Recent developments in osmotic dehydration: methods to enhance mass transfer. Trends in Food Science and Technology 13, 48–59. Rastogi, N.K., Raghavarao, K.S.M.S., 2004. Mass transfer during osmotic dehydration. Determination of moisture and solute diffusion coefficients from concentration profiles. Food and Bioproducts Processing 82 (C1), 44–48. Rodrigues, A.C.C., Cunha, R.L., Hubinger, M.D., 2003. Reological properties and color evaluation of papaya during osmotic dehydration processing. Journal of Food Engineering 59, 129–135. Rodrigues, A.C.C., Pereira, L.M., Sarantópoulos, C.I.G.L., Bolini, H.M.A., Cunha, R.L., Junqueira, V.C.A., Hubinger, M.D., 2006. Impact of modified atmosphere packaging on the osmodehydrated papaya stability. Journal of Food Processing and Preservation 30 (5), 563–581. Shi, X.Q., Fito, P., Chiralt, A., 1995. Influence of vacuum treatment on mass transfer during osmotic dehydration of fruits. Food Research International 28, 445–454. Torres, J.D., Talens, P., Carot, J.M., Chiralt, A., Escriche, I., 2007. Volatile profile of mango (Mangifera indica L.), as affected by osmotic dehydration. Food Chemistry 101 (1), 219–228. Torres, J.D., Castello, M.L., Escriche, I., Chiralt, A., 2008. Quality characteristics, respiration rates, and microbial stability of osmotically treated mango tissue (Mangifera indica L.) with or without calcium lactate. Food Science and Technology International 14 (4), 355–365. Vivanco-Pezantes, D., 2006. Study of the combined operations of the vacuum osmotic dehydration, liquid smoking and drying in filets of Atlantic bonito (Sarda sarda). PhD Thesis, State University of Campinas, Campinas – Brazil (in Portuguese). Vivanco-Pezantes, D., Hubinger, M.D., Sobral, P.J.A., 2004. Mass transfer in osmotic dehydration of Atlantic bonito (Sarda sarda) fillets under vacuum and atmospheric pressure. In: Proceedings of the 14th International Drying Symposium, São Paulo, Brazil.