shrinkage density porosity shape changes drying pumpkin

7/24/2019 Shrinkage Density Porosity Shape Changes Drying Pumpkin

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Shrinkage, density, porosity and shape changes during dehydration

of pumpkin (Cucurbita pepo L.) fruits

L. Mayor a,, R. Moreira b, A.M. Sereno c

a Instituto Universitario de Ingeniera de Alimentos para el Desarrollo, Universidad Politcnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spainb Departamento de Enxeara Qumica, Universidade de Santiago de Compostela, Ra Lope Gmez de Marzoa s/n, E-15782 Santiago de Compostela, Spainc REQUIMTE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

a r t i c l e i n f o

Article history:

Received 2 March 2010

Received in revised form 27 August 2010

Accepted 29 August 2010

Available online 8 October 2010

Keywords:

Air drying

Bulk density

Modelling

Osmotic dehydration

Particle density

Shape factors

a b s t r a c t

The aim of this work was to study the changes in volume, density, porosity and shape factors of pumpkin

tissue during osmotic dehydration (OD) and air drying (AD). Pumpkin cylinders with length/diameter

ratio of 5/3 were used. OD experiments were carried out with solutions of sucrose, sodium chloride

and mixtures of both solutes at different temperatures. AD experiments were conducted at 70 C. Volume

of samples decreased linearly with weight reduction (WR). Bulk density varied in a restricted range (5

13%) during dehydration and for all the methods maximum values were found. Particle density increased

during both processes. Porosity increased at advanced degrees of dehydration, showing a minimum value

at the beginning of OD and AD. The proposed models to evaluate shrinkage, bulk and particle densities

and porosity from WR were satisfactorily applied. Image analysis showed that shrinkage of samples dur-

ing OD was isotropic. Pumpkin cylinders increased elongation and decreased roundness and compactness

during osmotic dehydration.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

The knowledge of physicochemical properties of food materials

is important for an adequate design of food operations as well as

for the control and improvement of the quality of the final product

(Rahman, 2005).

Particularly in dehydration processes, the heat and mass trans-

fer flows can modify physicochemical properties of the material

such as chemical composition (McLaughlin and Magee 1998),

mechanical properties (Lewicki and Lukaszuk, 2000) and volume

and porosity. The quality of the dehydrated product depends on

the extension of these changes. Regarding to the changes in vol-

ume and porosity, high shrinkage and low porosity lead to prod-

ucts with poor rehydration capability (McMinn and Magee,

1997). Furthermore, the changes in volume and dimensions must

be considered for mass transfer modelling during dehydration

(Simal et al., 1998; Khalloufi et al., 2009). In fact, experimental

shrinkage data during air drying (AD) of foods is relatively abun-

dant (Mayor and Sereno, 2004).

Knowledge of the bulk density of food materials is an important

parameter in storage, transport, mixing and packaging operations

(Rahman, 2005). Heat and mass transfer in solids depend on den-

sity and porosity values (Rahman, 2001). Textural and sensorial

properties of foods are also related to density and porosity

(Rahman, 2001). Porosity is related to the chemical stability of

dried products; degradation of sugars (White and Bell, 1999) and

lipid oxidation (Shimada et al., 1991) depended on this property.

Food shape is one of the main quality attributes perceived by

the consumer (Fernandez et al., 2005). Drying not only causes vol-

ume changes but also may cause changes in shape. In this sense,

product deformation is not fully described by the evaluation of

volumetric shrinkage (Panyawong and Devahastin, 2007).

Osmotic dehydration (OD) is a non-thermal processthat consists

in the immersion of a food material in a hypertonic solution. The

difference of the chemical potential between the material and the

solution promotes two main fluxes: the outcome of water from

the material to the osmotic solution, and the income of soluble sol-

ids from the osmotic solutionto the material. As osmotic agents are

often used sugars (sucrose or glucose) and salts (sodium chloride).

Volume changes during OD are mainly due to compositional

changes and mechanical stresses associated to mass fluxes. These

changes have been analyzed as variations in the volumes of solid,

liquidand gas phases of the food material during the process (Barat

et al., 2001), and have been correlated with changes in moisture

content and WR (Moreira and Sereno, 2003), or with WL (Nieto

et al., 2004). These three aforementioned works studied shrinkage

phenomena during OD of apples. Volumetric shrinkage during OD

of other food products has also been reported, as the case of

strawberries (Viberg et al., 1998), mangos (Giraldo et al., 2003)

and tomatoes (Souza et al., 2007; Bui et al., 2009). In spite of these

0260-8774/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.jfoodeng.2010.08.031

Corresponding author.

E-mail address: [email protected](L. Mayor).

Journal of Food Engineering 103 (2011) 2937

Contents lists available at ScienceDirect

Journal of Food Engineering

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j f o o d e n g
http://dx.doi.org/10.1016/j.jfoodeng.2010.08.031mailto:[email protected]://dx.doi.org/10.1016/j.jfoodeng.2010.08.031http://www.sciencedirect.com/science/journal/02608774http://www.elsevier.com/locate/jfoodenghttp://www.elsevier.com/locate/jfoodenghttp://www.sciencedirect.com/science/journal/02608774http://dx.doi.org/10.1016/j.jfoodeng.2010.08.031mailto:[email protected]://dx.doi.org/10.1016/j.jfoodeng.2010.08.031


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works, few experimental data on shrinkage during OD of fruits and

vegetables are found in the literature, and the variety of food mate-

rials studied is restricted.

Pumpkin (Cucurbita pepo L.) is a seasonal crop, which has been

used for human and animal feed. OD of pumpkin, as a single or

combined process, can be an alternative way to develop new food

products (Kowalska et al., 2008; Konopacka et al., 2010). OD kinet-

ics of pumpkin were already determined under several experimen-

tal conditions by using binary solutions of sucrose and glucose

(Kowalska et al., 2008) or sodium chloride (Mayor et al., 2006)and ternary solutions of sucrose and sodium chloride (Mayor

et al., 2007).

The aim of this work was to present experimental data on

changes in volume, bulk and particle densities and porosity during

OD of pumpkin fruits. The obtained results were compared with

the changes observed during air drying (the most conventional

method of dehydration) of this food material. Models to estimate

these physical properties during dehydration were proposed.

Dimensional and shape changes were also studied by means of im-

age analysis.

2. Materials and methods

2.1. Sample preparation

Pumpkin fruits ( C. pepo L.) were purchased from a local pro-

ducer, and stored at 1520 C in a chamber until processing. Pump-

kins with similar initial moisture content (9597 kg water/100 kg

product) and soluble solids (24 Brix) were selected for the exper-

iments. Cylinders (25 mm length, 15 mm diameter) from the

parenchyma tissue were obtained employing a metallic cork borer

and a cutter. In order to obtain a good structural and compositional

homogeneity in the samples, the cylinders were taken from the

middle zone of the mesocarp, parallel to the major axis of the fruit.

2.2. Dehydration experiments

Pumpkin cylinders were dehydrated with sucrose solutions atdifferent concentrations and temperatures selected using a uni-

form shell design (Doehlert, 1970), as observed inTable 1. These

solutions were prepared with distilled water and commercial

sucrose. The cylinders were put in baskets, which were fully

immersed into stirred glass vessels (diameter 15 cm, height

25 cm) containing the osmotic solution. Then, the vessels were

hermetically closed to avoid water evaporation during the osmotic

treatments. Agitation was conducted using a magnetic stirrer; the

speed was chosen according to the kinematic viscosity of the

osmotic solution to obtain a constant Reynolds number (ca.

3000) (Mayor et al., 2006). Reynolds number was calculatedaccording Eq.(1)(Perry and Green, 1999)

ReNd

2

m1

where kinematic viscosities of the osmotic solutions were obtained

from other authors (Chenlo et al., 2002), and ranged from

0.94 106 up to 37.5 106 m2/s.

The weight ratio of osmotic solution to pumpkin cylinders was

20:1 to maintain a constant concentration of the osmotic solution

during OD. Thermoregulation was obtained by means of a thermo-

static bath (0.2 C). Some dehydrated samples were removed

from the vessels at different process times, immediately they were

gently blotted with paper to remove the excess of osmotic solution

and kept in plastic boxes till experimental determinations.The same experimental procedure was followed in the OD

experiments with sodium chloride solutions at concentrations of

Nomenclature

a parameter of Eqs.(11) and (12)AD air dryingARD average relative deviationb parameter of Eq.(11)c parameter of Eq.(11)

C compactnessd diameter of the stirrer (m)D diameter (m)E elongationL length (m)Lm length of major axis (m)m sample mass (kg)N number of revolutions per second (s1)NMC normalized moisture contentOD osmotic dehydrationp perimeter (m)R roundnessR2 coefficient of determinationRe Reynolds numbers solid mass (kg)

S surface area (m2)SG solids gain (kg/kg)

suc sucroseT temperature (C)V volume (m3)WL water loss (kg/kg)WR weight reduction (kg/kg)

X variable of Eq.(11)Y coded variable

Greek symbolse porositym kinematic viscosity (m2/s)q density (kg/m3)

Subscriptsb bulkis insoluble solidso initialp particless initial soluble solidssuc sucrosew water

Table 1

Experimental design for dehydration of pumpkin with sucrose solutions.

Coded experimental plan Actual experimental plan

Y1 Y2 Sucrose (kg/100 kg) T(C)

1 0 30 25

0 0 45 25

1 0 60 25

0.5 0.866 37.5 12

0.5 0.866 37.5 38

0.5 0.866 52.5 12

0.5 0.866 52.5 38

30 L. Mayor et al./ Journal of Food Engineering 103 (2011) 2937


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5, 10 and 20 kg/100 kg at 25, 38 and 12 C, respectively; and with

ternary solutions 3.75% NaCl58% sucrose and 7.5% NaCl45% su-

crose at 25 C.

AD of fresh samples was performed in an oven at 70 C under

natural convection and an air relative humidity of 6 2% (deter-

mined by wet bulb thermometer). A vent in the oven avoided

humidity build-up during drying. Pumpkin cylinders were put on

a plastic wire net to allow heat and mass transfer through thewhole surface of the samples. At different process times some cyl-

inders were removed from the oven and kept in plastic boxes till

experimental determinations.

2.3. Experimental determinations

At each selected process time, four samples were used for gravi-

metric determinations. The bulk volume (V) of each cylinder was

calculated from the resultant buoyant force of the sample when

immersed inn-heptane (Lozano et al., 1980). After this measure-

ment, particle volume (Vp) was obtained with a gas pycnometer

specially designed to measure this property in moist products

(Sereno et al., 2007). Sample porosity (e) was calculated by Eq.(2)

e VVpV

1 VpV

2

After that, the same samples used for volume measurements

were weighed to determine its weight reduction (WR), Eq. (3),

and the solids gain (SG), Eq. (4), was evaluated after vacuum drying

at less than 104 Pa at 70 C till constant weight (AOAC, 1984)

according to:

WRmom

mo3

SGs somo

4

Water loss (WL) and normalized moisture content (NMC) were

determined by Eqs.(5) and (6), respectively:

WL SG WR 5

NMC m smomosom

6

Particle (qp) and bulk (qb) densities were calculated from Eqs.

(7) and (8), respectively:

qp m

Vp7

qb m

V 8

Since the soluble solids of pumpkin flesh are mostly sugars, sol-

uble solids of fresh samples were determined by refractometry

(Abbe-3L refractometer, Bausch and Lomb, Rochester, NY, USA) at

20

C. The clear juice was extracted by manually pressing rawpumpkin flesh (ca. 2 g) between to plastic discs (diameter 3 cm)

and analyzed directly in the refractometer. Insoluble solids were

obtained from a mass balance with the values of soluble solids

and total solids in fresh material.

Image analysis was performed in samples osmodehydrated

with 60% sucrose solutions at 25 C. Six samples were removed

at different process times for the analysis of size and shape param-

eters. From each sample, one rectangular slab of ca. 0.51 mm of

thickness was gently cut parallel to the height of the cylinders at

the maximum section area with a razor blade. One face of the slab

was stained with a solution of methylene blue 0.1% (Mayor et al.,

2005) during 15 s. After that, samples were ready for observationunder the stereomicroscope (Olympus SZ-11, Tokyo, Japan) work-

ing in transmitted light mode. A digital video camera (Sony SSC-

DC50AP, Tokyo, Japan) was attached to the microscope and con-

nected to a computer. Image acquisition was carried out with an

interface (PCTV videocard, Pinneacle Systems GmbH, Munich,

Germany). Images were calibrated with a stage micrometer of

2 mm length and divisions of 0.01 mm intervals (Leitz Wetzlar,

Germany). Image analysis of the isolated sample contour was

performed using free software UTHSCSA Image Tool v.2.0 (Health

Science Centre, University of Texas, San Antonio, TX). Several geo-

metrical parameters of the samples were analyzed (Lewicki and

Pawlak, 2003; Mayor et al., 2005): surface area (S); perimeter of

the contour (p), length of the major axis (Lm), defined as the length

of the longest line that can be drawn through the object; length of

the minor axis, defined as the length of the longest line that can be

drawn through the object perpendicular to the major axis; elonga-

tion (E), defined as the ratio of Lm to the length of the minor axis;

roundness (R) and compactness (C) defined by the Eqs. (9) and (10),

respectively:

R4 pS

p2 9

C

ffiffiffiffiffi4 Sp

q

Lm 10

Rand Cparameters give an idea of the circularity of the object. Both

shape factors range from 0 to 1; when the value is one, the object is

a perfect circle, when their value decrease the object becomes less

circular and less round.

In order to determine the linear dimensions, average values of

five measurements at different zones of the cylinders were per-

formed for diameter (D) and length (L).Table 2shows a summary

of the experimental determinations performed in this work, as well

as the sampling times selected for each experiment.

3. Results and discussion

3.1. Shrinkage during dehydration

Fig. 1shows experimental shrinkage data of pumpkin cylinders

versus WL, WR and NMC, respectively. Plots on the left correspond

to OD with sucrose solutions at different conditions, whereas plotson the right correspond to OD with different osmotic agents and

AD. Osmodehydrated samples shrank up to 27% of their initial vol-

ume, depending on the process conditions used. Air dried samples

shrank at the end of drying up to 5% of the initial volume.

Table 2

Summary of experimental determinations.

Dehydration treatment Process conditions Experimental determinations Sampling times (h)

Osmotic dehydration Sucrose,Table 1 a, b, c 0, 0.08, 0.25, 0.5, 1, 1.5, 2, 2.5, 3, 4, 6 and 9

60% Sucrose, 25 C a, d 0, 0.5, 1, 3, 6 and 9

NaCl a, b 0, 0.08, 0.5, 1, 2, 2.5, 3, 4, 5, 6 and 8

NaCl/sucrose a, b 0, 0.08, 0.25, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5 and 6

Air drying Natural convection (oven drying) at 70 C a, b, c 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6 and 8

(a) Kinetic parameters (water loss, solids gain, weight reduction, normalized moisture content); (b) bulk volume; (c) particle volume; and (d) image analysis.

L. Mayor et al. / Journal of Food Engineering 103 (2011) 2937 31
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InFig. 1a can be observed a linear decrease of volume with WL

during OD. The decrease is more accentuated in the case of NaCl

solutions, followed by sucrose solutions and NaCl/sucrose solu-

tions. No effect of process conditions concentration and tempera-

ture is observed for the same osmotic agent, as shown in Fig. 1a

for sucrose solutions. Nieto et al. (2004), observed a linear decrease

of sample volume with WL during OD of apple; the decrease was

more accentuated at the same WL for the samples osmodehydrat-

ed with sucrose solutions compared with glucose solutions due to

the corresponding lower SG value.Mavroudis et al. (1998) during

OD of apples (var. Granny Smith) with sucrose solutions also ob-

served a linear decrease of volume with the decrease of water in

the material; they observed no effect of process temperature onshrinkage.

Air dried samples also showed a linear volume decrease with

WL, but in this case the decrease is higher than the shrinkage of

OD samples. At the same WL, the SG during OD decreases the vol-

ume reduction promoted by the water removal. Volume decreases

linearly with increasing WR (Fig. 1b), independently on the process

conditions and dehydration methods. Moreira and Sereno (2003)

during OD of apple with sucrose solutions found linear relation-

ships between WR and shrinkage independently on concentration,

temperature and hydrodynamic conditions of the osmotic solu-

tions. During AD, this linear behaviour of shrinkage against WR

(and consequently against WL) is often reported (Lozano et al.,

1983; Zogzas et al., 1994). Shrinkage of osmodehydrated and air

dried samples shows a non-linear decrease with moisture content(Fig. 1c). This decrease is faster for pumpkins submitted to AD, due

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

WL (kg/kg)

30% suc, 25C45% suc, 25C

60% suc, 25C37.5% suc, 12C

37.5% suc, 38C

52.5% suc, 12C52.5% suc, 38C

Eq. (11)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/V

o

WR (kg/kg)

30% suc, 25C45% suc, 25C

60% suc, 25C

37.5% suc, 12C37.5% suc, 38C

52.5% suc, 12C

52.5% suc, 38C

Eq. (11)

1.0 0.9 0.8 0.7 0.6 0.50.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

NMC

30% suc, 25C

45% suc, 25C

60% suc, 25C37.5% suc, 12C

37.5% suc, 38C

52.5% suc, 12C

52.5% suc, 38C

Eq. (11)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

WR (kg/kg)

OD sucrose solutions

OD NaCl/sucrose solutions

OD NaCl solutions

Air drying

Eq. (11), all treatments

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

WL (kg/kg)

OD, sucrose solutions

OD, NaCl/sucrose solutionsOD, NaCl solutions

Air drying

(a)

(b)

(c)

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

NMC

OD, sucrose solutionsOD, NaCl/suc solutions

OD, NaCl solutionsAir drying

Fig. 1. Shrinkage during dehydration of pumpkin cylinders versus (a) water loss, (b) weight reduction, and (c) normalized moisture content. Left figures correspond to

osmotic dehydration with sucrose solutions, whereas right figures correspond to osmotic dehydration with different solutions and air drying.



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to the aforementioned explanations given for the analysis of

shrinkage against WL.

Variation in the volume of the gas phase inside the vegetable

tissue also can contribute to increase or decrease the theoretical

shrinkage produced by the removal of water. An ideal shrinkage

could be defined as the shrinkage due to mass fluxes (WL and

SG), assuming that the volume of the initial solids of the material

remains constant. Considering volume additivity, the changes involume were calculated from experimental data of WL and solute

gain. The density values of water, sucrose and NaCl at 25 C, are

shown in Table 3. The assumption that volumes are additive is

acceptable for the calculations, since excess volume of binary solu-

tions of sucrose and NaCl and ternary NaCl/sucrose solutions ob-

tained comparing experimental data from other authors is near

to 1%, rarely exceeding 2% (Chenlo et al., 2002; Lide 2005).

Fig. 2shows the ideal shrinkage of pumpkin cylinders dehy-

drated with binary solutions of sucrose and NaCl and air dried ver-

sus the experimental shrinkage data. It is observed that, for each

dehydration method, experimental shrinkage is higher than the

ideal shrinkage given by the mass fluxes during dehydration.

These differences increase at high degrees of shrinkage, and conse-

quently with the volume of removed water, and are related to the

decrease of the air volume and collapse of the material ( Khalloufi

et al., 2009).Barat et al. (2001), during OD of apples with sucrose

solutions, observed that the decrease of total volume was higher

than the decrease of the liquid phase volume in the samples. Sev-

eral phenomena can promote the collapse, such as capillary forces

caused by the water removal and loss of turgor pressure in the cells

(Prothon et al., 2003).

Shrinkage of pumpkin during dehydration can be correlated

with the kinetic parameters, X, by means of empirical polynomial

equations:

V

Vo1 aXbX

2cX3 11

whereXcan be WL, WR, or NMC. Eq. (11) was fitted to experimental

shrinkage data obtained in this work. The corresponding fits were

carried out considering each osmotic agent alone, considering all

the osmotic treatments together, AD alone, and all the dehydration

treatments together (AD and OD). Table 4shows the values of the

parameters after fitting (only were considered acceptable the fit-

tings with ARD < 10% andR2 > 0.9).Fig. 1shows some of these fit-

tings, as examples. For WL and WR the linear fit is satisfactory in

the case of OD treatments and AD, separately. For all the dehydra-

tion methods together only WR fit (linear) is acceptable. For NMC,

cubic models are acceptable (except for AD and for the global anal-

ysis of all treatments). These equations are useful because allow the

prediction of shrinkage data independently on the process condi-

tions (concentration and temperature) used.

3.2. Bulk density, particle density and porosity

Table 5 shows the values of some physicochemical properties of

raw pumpkin parenchymatic tissue obtained in this work. Particle

and bulk densities and porosity present the typical variability ob-

served in other vegetables; for this reason, the changes in these

properties during dehydration are presented as reduced values.

The change of bulk density during OD with sucrose or NaCl

solutions was restricted (ca. 5%). Higher change was obtained for

ternary NaCl/sucrose solutions (ca. 10%) and AD (ca. 13%). Similar

trend is observed for all the treatments during dehydration; bulk

density initially increases, reaching a maximum value and then de-creases or fluctuates till the end of the process (data not shown).

Table 3

Density,q , for fresh and dehydrated pumpkin components at 25 C.

Component q (kg/m3) Reference

Water 997 Lide (2005)

Fructose 1665 Lide (2005)

Glucose 1562 Lide (2005)

NaCl 2170 Lide (2005)

Sucrose 1581 Lide (2005)

Cellulose 1550 Lozano et al. (1980)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Idealvolume

loss/Vo

(Vo-V)/Vo

O.D. Sucrose solutions

O.D. NaCl solutions

Air drying

Diagonal

Fig. 2. Ideal volume loss versus actual volume loss during osmotic dehydration of

pumpkin fruits with binary sucrose and NaCl solutions and air drying.

Table 4

Parameters of Eq.(11)for osmotic dehydration (OD) and air drying.

X a b c R2 ARD (%)


WL (linear) 0.88 0.99 1.65

WR (linear) 1.01 0.99 1.44

NMC (cubic) 1.74 0.18 1.94 0.97 5.26

OD NaCl solutions

WL (linear) 0.94 0.96 2.40

WR (linear) 1.09 0.99 1.19

NMC (cubic) 16.04 37.82 21.74 0.91 6.38

OD NaCl/sucrose solutions

WL (linear) 0.82 0.99 1.18

WR (linear) 1.02 0.99 1.96

NMC (cubic) 2.17 1.50 0.66 0.99 2.67

All OD treatments

WL(linear) 0.87 0.99 3.13

WR(linear) 1.02 0.99 1.81

NMC (cubic) 1.74 0.22 1.49 0.91 8.09

Air drying

WL, WR (linear) 1.00 0.99 2.86

All dehydration methods

WR (linear) 1.02 0.99 2.43

WL= water loss, WR= weight reduction, NMC = normalized moisture content,

R2 = coefficient of determination, ARD= average relative deviation.

Table 5

Some physicochemical properties of raw pumpkin parenchymatic tissue.

Property Average value Range

Moisture content (%) 95.57 [94.4496.92]

Soluble solids (%) 3.22 [2.143.63]

Insoluble solids (%) 1.21 [0.781.97]

Bulk density (kg/m3) 890 [860920]

Particle density (kg/m3) 1040 [10031070]

Porosity (%) 14.79 [10.2218.18]



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Nieto et al. (2004), during OD of apples with sucrose and glu-

cose solutions, observed an increase of bulk density of apple sam-

ples in the beginning of the treatments (around the first hour of

treatment) for both osmotic agents, and then bulk density fluctu-

ated until the end of the processes. During AD of different fruits

and vegetables, Lozano et al. (1983) and Krokida and Maroulis

(1997)observed that the change in bulk density was different for

each food material tested. In some cases bulk density increasedduring dehydration (banana, carrot), in other cases decreased

(apple), and still in other cases initially increased, reached a max-

imum value and then decreased (sweet potato, garlic). This differ-

ent behaviour could be associated with different physicochemical

characteristics of the raw material, such as chemical composition,

initial porosity or the presence of soft/rigid structures, which can

lead to different type of stresses during processing (Rahman,

2001). It is reasonable to think that for OD the characteristics of

the raw material also can influence in the change in bulk density

as observed for AD of vegetables. The type and amount of infused

solids are other important factors in the change of bulk density

during OD.

Bulk density changes can be predicted by means of Eq. (12):

qb

qbo

1 WR

V=Vo

1 WR

1 aWR 12

where a is the corresponding regression coefficient of Eq. (11)

(Table 4), for shrinkage and WR correlations for each osmotic agent

and for AD. Average and maximum relative deviations between

experimental and predicted density values were, respectively,

2.4% and 5.6% for sucrose solutions; 1.3% and 4.8% for NaCl solu-

tions; 3.2% and 6.0% for NaCl and sucrose solutions, and 2.9% and

6.1% for AD.

Fig. 3 shows the changes in normalized particle density (qp/qpo)

and normalized porosity (e/eo) against WR. Similar trends of bulk

and particle densities and porosity were observed with WL and

WR of samples. For NMC the observed trend was not as clear as

in the case of WL and WR (data not shown).

Particle density (Fig. 3a) increases slowly at the beginning of ODwith sucrose solutions, but above WR = 0.5 the increase is more

pronounced. No significant differences are observed among the

process conditions tested. Particle density increases by the compo-

sitional change of the wet solid matrix during dehydration. Ini-

tially, water content is high, but during dehydration the content

of more dense substances (sucrose, cellulose) increases leading to

the increase of particle density. For air dried samples, the behav-

iour is similar; initially particle density increases very low and

above WR = 0.6 increases more pronouncedly up to a value around

40% higher at the end of the process. During ODof apple with sugar

solutions, a progressive increase of particle density along the pro-

cess was observed (Nieto et al., 2004). For vegetables, the same

behaviour of particle density along drying was also observed

(Krokida and Maroulis, 1997).

The modelling of particle density, Eq. (7), during dehydration

can be performed from the composition of the material. The parti-

cle volume can be calculated from the masses and densities of each

component. For pumpkin parenchyma, fresh material is composed

by water, insoluble and soluble solids, and gas phase. So the parti-

cle volume (without the gas phase) can be defined as

Vpo VwVisVss 13

In terms of masses of the components and densities, Eq.(13)can be

rewritten as

Vpomwqw

misqis

mssqss

14

For osmodehydrated pumpkin with sucrose and NaCl solutions,

the gained solids must be taken into account, so the particle vol-ume can be defined, as

Vp mwqw

misqis

mssqss

msucqsuc

mNaClqNaCl

15

Lozano et al. (1980)considered the insoluble solids of apple tis-

sue as cellulose. In this work the same assumption was considered.

Soluble solids in pumpkin are mainly fructose and glucose in the

same proportion, so an average value of the densities of fructose

and glucose was taken as the density of initial soluble solids. Den-

sity values used in the calculations are shown inTable 3.Predictedvalues of particle volume for fresh and dehydrated pumpkin fruits

with sucrose solutions and AD were compared with experimental

data obtained with the gas pycnometer. This predictioncan be con-

sidered adequate, leading to a relative deviation of the predicted

values of 2.63% on average.

Finally, normalized particle density can be obtained by means

of Eq.(16)

qp

qpo

1 WR

Vp=Vpo16

where Vpo and Vp are obtained by means of Eqs. (14) and (15),

respectively.

Average relative deviation (ARD) between experimental data

and predicted values obtained with Eq.(15)was 2.02%, indicating

that the model gives a good prediction of experimental data. Fig. 3a

shows predicted values of normalized particle density for OD with

60% sucrose solutions at 25 C and AD at 70 C.

Porosity of dehydrated pumpkin with sucrose solutions (Fig. 3b)

slightly decreases up to intermediate WR values (ca. 0.4); after that

point porosity increases till the end of the process. No effect of the

process conditions on porosity trend with WR or WL is observed.

For AD the behaviour is similar; at the beginning of the process

porosity fluctuates but above WR = 0.5, porosity increases and at

the end of the process almost triplicates (e/eo= 2.8, WR = 0.95) its

initial value.

Mavroudis et al. (1998) observed an increase of porosity in

osmodehydrated apples at the end of the process. Giraldo et al.

(2003) showed that during OD of mango porosity of dehydratedsamples initially decreased and after that increased. The initial de-

crease of sample porosity can be explained by the fast initial

impregnation of the tissue with the osmotic solution, which pene-

trates into the external pores by capillary forces and other mass

transfer mechanisms. The accumulation of sucrose in the external

surface of the material generates a dense layer that hinders the fur-

ther penetration of the osmotic solution and simultaneously min-

imizes the gas flow from the material to the solution. These

combined phenomena increase the food porosity.

Porosity during AD of foodstuffs can follow different behaviours

(Lozano et al., 1983; Krokida and Maroulis, 1997); in some cases it

decreases (sweet potato); in others it initially decreases and then

increases (pear) and still in other cases it increases during the

whole drying process (apple, banana). As commented for bulk den-sity change, this different behaviour can be associated with the ini-

tial structural and compositional characteristics of the raw

material, as well as the process conditions.

Fig. 4shows the changes in total volume, particle volume and

air volume during OD of pumpkin with 60% sucrose solutions

and AD. In the initial stage (up to WR = 0.5) in both treatments

the three volumes decrease during dehydration and the relative

decrease of air volume is the highest. The relative decrease of par-

ticle volume and total volume is practically the same for OD. For

AD, total volume decreases more than particle volume. Above

WR = 0.5, in OD the particle volume starts to decrease in percent-

age more than total volume, and air volume fluctuates and practi-

cally remains constant until the end of the process. In the same

range for dried samples, the air phase volume remains constantup to WR = 0.7 and then decreases until the end of drying; total



7/9

volume decreases less than particle volume like in OD samples. In

this way, it seems that the gas phase is better retained during OD

than in AD. As commented before, the formation of a dense layer of

osmotic agent in the material surface can be the cause of this

phenomenon.

Porosity, Eq. (2), can be predicted by means of Vp calculated

from Eqs.(14) and (15), andVwith Eq.(11). ARD in the predictionof porosity values was 14.5%.Fig. 3b shows experimental and pre-

dicted values of normalized porosity for air dried and osmotic

dehydrated pumpkin with 60% sucrose solutions.

3.3. Sample shape analysis

Fig. 5shows the contour of cylinders dehydrated with sucrose

solutions (60%, 25 C), at different process times. It is observed asize decrease during the process. The shape also changes during

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

WR (kg/kg)

30% suc, 25C 45% suc, 25C

60% suc, 25C 37.5% suc, 12C

37.5% suc, 38C 52.5% suc, 12C

52.5% suc, 38C Predicted 60% suc

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.8

0.9

1.0

1.1

1.2

1.3

WR (kg/kg)

30% suc, 25C 45% suc, 25C

60% suc, 25C 37.5% suc, 12C

37.5% suc, 38C 52.5% suc, 12C

52.5% suc, 38C Predicted, 60% suc

(a)

(b)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

WR (kg/kg)


Air drying

Predicted, air drying

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.5

1.0

1.5

2.0

2.5

3.0

WR (kg/kg)


Air drying

Predicted, air drying

Fig. 3. Changes in particle density (a) and porosity (b) during dehydration of pumpkincylinders versus weight reduction. Left figures correspond to osmotic dehydration with

sucrose solutions at different process conditions, whereas figures on the right correspond to osmotic dehydration with sucrose solutions and air drying.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

WR (kg/kg)

Total volume

Particle volume

Air volume

(a) (b)

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

V/Vo

WR (kg/kg)

Total volume

Particle volume

Air volume

Fig. 4. Relative volume changes for total volume, particle volume and air volume during dehydration of pumpkin cylinders in (a) 60% sucrose solutions at 25 C and (b) air

drying at 70 C.



8/9

dehydration; shrinkage is more accentuated at mid-length and

mid-thickness of the cylinder, whereas at the edges shrinkage is

less pronounced; this corner effect is clearly observed in the

most dehydrated cylinder (Fig. 5f).Del Valle et al. (1998)also re-

ported this effect during OD of apple cylinders, and Mulet et al.(2000)during AD of potato cubes.

As observed inFig. 6, no significant differences were found in

the decrease of length and diameter during the process, indicating

an isotropic shrinkage. Trujillo et al. (2007) found similar results

during the AD of beef meat discs (L/D 0.25). However, Mulet

et al. (2000)showed an anisotropic shrinkage in the AD of potato

(L/D 4.6) and cauliflower stem (L/D 1.7) cylinders, where the

diameter decrease was higher than the length decrease for bothproducts. In the case of potato cylinders, the authors suggested

that the high L/D ratio favoured the formation of an inner core

along the axis length maintaining the shape along this axis. In

the case of cauliflower stems, the presence of oriented fibres made

the product stiffer in a preferential orientation and the shrinkage

was more pronounced in the diameter. Based on those results,

two important factors can affect the shrinkage isotropicity: the

existence of preferential pathways of mass transfer (due to geo-

metric and structural features) and the homogeneity of the mate-

rial structure (due to structural features).

Fig. 7shows the relative changes in the shape factors as a func-

tion of WR. The average initial values of the shape factors were

1.657, 0.670 and 0.749 for elongation, roundness and compactness,

respectively. Elongation slightly increases, whereas roundness andcompactness decrease during dehydration. It is often reported a

decrease of roundness during dehydration of foods, as in the case

of AD of apple discs (Mayor et al., 2005; Fernandez et al., 2005)

or apricot cubes (Riva et al., 2005). The tissue suffers deformations

as a consequence of the water removed in the material; in this way

roundness and compactness decrease during dehydration. Elonga-

tion increases mainly due to the corner effect that reduces the

minor axis length (minimum value of the diameter of the cylinder)

but maintains the major axis length (distance between two oppo-

site corners of the cylinder).

4. Conclusions

Shrinkage of samples was observed for all the dehydrationmethods studied. Independently on the process conditions (con-

centration and temperature of the osmotic solution), volume de-

creased linearly with WL and WR during OD. Air dried samples

showed the same behaviour but shrinkage was more accentuated

for the same WL.

Bulk density varied in a restricted range during dehydration;

differences between maximum and minimum values were around

5% for OD with binary sucrose and NaCl solutions, 10% for ternary

NaCl/sucrose solutions and 13% for AD. For all the dehydration

methods, bulk density initially increased, then reached a maximum

value and after that decreased or fluctuated till the end of the

process.

For all the treatments, particle density increased slowly at the

beginning of the process, and at certain WR value (0.5 for OD withsucrosesolutionsand 0.6for AD)the increasewas morepronounced.

Fig. 5. Changes in shape and size during osmotic dehydration of pumpkin cylinders

in 60% sucrose solutions at 25 C, at different process times (h). (a) 0, (b) 0.5, (c) 1,

(d) 3, (e) 6, and (f) 9. The horizontal line at the bottom of images corresponds to

2 mm.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Normalizeddimension

WR (kg/kg)

Diameter

Length

Fig. 6. Relative changes in dimensions of osmodehydrated pumpkin cylinders (60%sucrose, 25 C) versus weight reduction.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Normalizedsh

apefactor

WR (kg/kg)

Elongation

Roundness

Compactness

Fig. 7. Changes in shape factors during osmotic dehydration (60% sucrose, 25C) of

pumpkin cylinders versus weight reduction.



9/9

The change in porosity also followed the same behaviour during

OD and AD. Initially, porosity decreased (up to WR = 0.4 and 0.5 for

OD and AD, respectively) and then porosity started to increase till

the end of the treatment, increasing twofold and threefold the

initial value for OD with sucrose solutions and AD, respectively.

The proposed models to evaluate shrinkage, bulk and particle

densities and porosity from WR were satisfactorily applied.

Experimental results indicated that shrinkage of pumpkin cylin-ders was isotropic. During dehydration, elongation increased and

roundness and compactness decreased.

Acknowledgement

The author Luis Mayor wishes to acknowledge SFRH/BD/3414/

2000 PhD Grant to Fundaao para a Cincia e a Tecnologia,

Portugal.

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