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Ciênc. ecnol. Aliment., Campinas, 28(3): 696-701, jul.-set. 2008696

ISSN 0101-2061 Ciência e ecnologia de Alimentos

Recebido para publicação em 19/7/2007 Aceito para publicação em 14/12/2007 (002693)1 Faculdade de Engenharia Agrícola – FEAGRI, Universidade Estadual de Campinas – UNICAMP, CP 6011, CEP 13083-875, Campinas - SP, Brasil,

E-mail: [email protected] 2 Embrapa Semi-Árido, CEP 56302-970, Petrolina - PE, Brasil 3 Faculdade de Engenharia de Alimentos – FEA, Departamento de Engenharia de Alimentos, Universidade Estadual de Campinas – UNICAMP, CEP 13083-97, Campinas, SP, Brasil *A quem a correspondência deve ser enviada

Osmotic dehydration o sweet potato (Ipomoea batatas) in ternary solutions

Desidratação osmótica de batata-doce (Ipomoea batatas) em soluções ternárias

Graziella Colato ANONIO1*, Patrícia Moreira AZOUBEL2, Fernanda Elizabeth Xidieh MURR 3, Kil Jin PARK1

1 IntroductionSweet potato is one o the five most important ood crops in

developing countries. It is one o the most efficient ood crops interms o caloric value per cultivated area, being relatively easyto grow even on poor and dry soil.

Te dry matter production potential o certain varietieso sweet potato may be as high as 4.3 to 6.0 tons per ha. Sweetpotato is remarkable because o its high yield, palatability andcrude protein content. Sweet potato, which is one o the mostimportant tubers, is widely distributed in tropical areas. It playsa basic role in the diet o the people rom tropical countries.ropical tubers are either home or industrially processed at

various scales. Nowadays, industrial processes must be im-proved, in order to enhance tuber uses and satisy consumptionrequirements.

Osmotic dehydration is a method applied or water removalo resh oods, by immersing pieces o the ood in a solution ohigher osmotic pressure, and hence o lower water activity than

the ood. Tis solution is sometimes reerred to as a hypertonicsolution in the literature. Water passes rom the ood into thesolution under the influence o the osmotic pressure gradient. Inthis process, the walls o the cells in the ood act as semi perme-able membranes. However, the membranes are not completelyselective. Some soluble natural substances, such as organic acids,sugars, salts, and vitamins, may be lost with the water, whilesolutes rom the solution may penetrate into the ood. Tisgain o solutes by the ood can contribute to the reduction o itswater activity, but may affect the taste o the product (LENAR;FLINK, 1984; KAYMAK-EREKIN; SULANOGLU, 2000;

BRENNAN, 2003; EREN; KAYMAK-EREKIN, 2007).In osmotic dehydration, the rate o water loss is ini-

tially high, but significantly decreases afer 60 to 120 minutes.However, it can take days or the process to reach equilibrium.A typical processing time to reduce the weight o the ood by50% is rom 4 to 6 hours. Some researchers have reported that,

Resumo

O objetivo deste trabalho oi avaliar a desidratação osmótica da batata-doce (Ipomoea batatas) utilizando soluções hipertônicas de sacarosee/ou NaCl, em três concentrações dierentes a 40 °C. As perdas de água mais elevadas oram obtidas quando se utilizou a mistura de sacarosee NaCl. A adição do NaCl às soluções osmóticas aumenta a orça motriz do processo. Verifica-se que o processo de desidratação osmóticaé principalmente influenciado pela mudança da concentração de NaCl, mas o eeito positivo da interação do sal com a sacarose tambémdeterminou a diminuição do ganho de sólidos quando os solutos estavam em concentrações máximas. As cinéticas de transerência de massaoram modeladas de acordo com as Equações de Peleg, de Fick e de Page, e apresentaram bons ajustes aos dados experimentais. A Equaçãode Peleg e o modelo da Page apresentaram os melhores ajustes e mostraram uma excelente capacidade de predição para dados de ganho desólidos e de perda de água. A diusividade eetiva oi determinada utilizando-se a Segunda Lei de Fick aplicada à geometria de placa plana,encontrando valores de 3,82 x 10–11 a 7,46 x 10–11 m2/s para a perda de água, e de 1,18 x 10–10 a 3,38 x 10–11 m2/s para o ganho de sólidos.Palavras-chave: sacarose; sal; difusividade efetiva; modelos empíricos.

Abstract

Te aim o this work was to evaluate the osmotic dehydration o sweet potato (Ipomoea batatas) using hypertonic sucrose solutions, with orwithout NaCl, at three different concentrations, at 40 °C. Highest water losses were obtained when the mixture o sucrose and NaCl was used.Te addition o NaCl to osmotic solutions increases the driving orce o the process and it is verified that the osmotic dehydration process ismainly influenced by changes in NaCl concentration, but the positive effect o the salt-sucrose interaction on soluble solids also determinedthe decrease o solid gain when solutes were at maximum concentrations. Mass transer kinetics were modeled according to Peleg, Fick andPage’s equations, which presented good fittings o the experimental data. Peleg’s equation and Page’s model presented the best fitting andshowed excellent predictive capacity or water loss and salt gain data. Te effective diffusivity determined using Fick’s Second Law appliedto slice geometry was ound to be in the range rom 3.82 x 10–11 to 7.46 x 10–11 m2/s or water loss and rom 1.18 x 10–10 to 3.38 x 10–11 m2/sor solid gain.Keywords: sucrose; salt; effective diffusivity; empirical models.



Ciênc. ecnol. Aliment., Campinas, 28(3): 696-701, jul.-set. 2008 697

Antonio et al.

while water loss decreases rapidly in the first 2 hours, the gaino solutes rom the hypertonic solution continues or a longerperiod o time. Te initial concentration o solutes in the solu-tion can affect the rate o water loss. Sugar concentrations in therange o 40 to 70% are used. Sometimes, a higher concentrationmay be achieved by solid gain, which can affect the taste o theproduct by making it too sweet. In the case o vegetables, sodium

chloride concentrations in the range o 5 to 20% have been used.Again, at high concentrations, the taste o the product may beadversely affected (BRENNAN, 2003).

Solute choice and concentration depend on several actors,namely the effect on organoleptic quality properties, solutesolubility and cell membrane permeability, which establisheffect and cost. Te two most common kinds o solutes inosmotic treatments are sugars (mainly with ruits) and salts(with vegetables, meat and cheese), with a predominance othe use o sucrose and sodium chloride, which show advan-tages already described by several authors (LENAR; FLINK,1984; COLLIGNAN; RAOUL-WACK, 1994; SACCHEI;

GIANOI; DALLA ROSA, 2001).Te objectives o this work were to study the osmotic de-

hydration o sweet potato and examine the predictive capacityo Peleg, Fick and Page’s equations.

1.1 Teory

Peleg (1988) proposed an equation to describe sorptioncurves that approach equilibrium asymptotically. Tis equationwas redefined in terms o soluble solids and moisture content.Park et al. (2002) rewrote the same Equation 1 as:

________

01 2

( ) t

MC t MC k k = ± + (1)

in which MC (t ) is the amount o water or solids at time t, g; MC o

is the initial amount o water or solids, g; k1 and k

2 are Peleg’s

constants; and t is the time, h.

A plate o thickness 2L with uniorm initial amount o wa-ter or solids MC

o, submied to osmotic dehydration at constant

conditions, can be described by Fick’s unidirectional diffusionEquation 2 (CRANK, 1975):

( ) ( )ef

MC t MC t D

t z z

∂ ∂ ∂ = ∂ ∂ ∂ (2)

Using the ollowing initial and boundary conditions:

• Uniform initial amount MC (z,0) = MC 0;

• Symmetry of concentration0

( )0

z

MC t

z =

∂=

∂ ; and

• Equilibrium content at surface MC (L,t) = MC eq

.

And applying (Equation 3):

________

0

1( ) ( , )

L

MC t MC z t dz L

= ∫

(3)

It becomes, in one dimension (CRANCK, 1975)(Equation 4):

________2 2

,2 2 2

0

8 1( ) exp (2 1)

(2 1) 4ef W orS

i

t MC t i D

i L

∞

=

= − + π π + ∑ x

(4)

in which De; W or S

is the effective diffusivity o water loss or solidgain, m2/s; i is the number o series terms; L is the characteristiclength, sample hal-thickness, m; t is the time, seconds.

Te calculation o effective diffusivity or solid gain andwater loss can be based on the dimensionless amount o waterloss (W

W ) and solid gain (W

S) (Equation 5):

________

0

( )eq

W o r S

eq

MC t MC W

MC MC

−=

−

(5)

in which W W

is the dimensionless amount o water loss; W S is

the dimensionless amount o solid gain; MC eq

is the equilibriumamount o water loss or solid gain, g.

One o the most useul empirical models is Page’s Equation 6

(PAGE, 1949):

________

0

( )exp( )

eq B

W o r S

eq

MC t MC W At

MC MC

−= = −

−

(6)

in which, A and B are Page’s water loss or solid gain parameters;and t is the time, s.

2 Materials and methods

2.1 Preparation of sweet potato slice samples

Te raw material used was sweet potato rom the Mona Lisa variety acquired at a local market. Samples were hand peeledand cut into slices 0.5 mm thick. Sliced sweet potatoes wereimmersed in osmotic solutions. Commercial sucrose and saltwere used as osmotic agents.

2.2 Osmotic dehydration

Te sweet potato slices were osmotically dehydrated. Teosmotic process was carried out at different NaCl concentrations(0, 5 and 10% w/w) and fixed sucrose concentration (50% w/w),solution temperature (40 °C) and agitation (90 rpm). he

slices were placed in 250 mL beakers containing the osmoticsolution and maintained in a bath with temperature control(ecnal, E 421). Te mass ratio o the osmotic medium to thesample was 10:1, to avoid significant dilution o the mediumand subsequent decrease o the driving orce during the process.Samples were removed rom the solution at 15, 30, 45, 60, 75,90, 120, 150, 180 and 240 minutes afer immersion, drained,and the excess o solution on the surace was removed withabsorbent paper.

Water Loss (WL) was expressed as the net water loss romthe resh sweet potato sample afer osmotic dehydration, basedon initial sample mass; Solid Gain (SG) was defined as the net




ernary solution OD sweet potato

uptake o solids by the sweet potato sample based on initialsample weight, according to the ollowing Equations 7 and 8:

( )i i f f

i

w X w X WL

w

−=

(7)

1 1100 100

f i f i

i

X X w w

SGw

− − − =

(8)

where wi and w

f are the initial and final (time t ) sample weights,

respectively, (g); X i and X

f are the initial and final (time t ) sample

moisture contents, respectively.

Te average moisture and dry matter content o the sampleswere determined by drying at 70 °C or 24 hours in an oven(AOAC, 1984). All the experiments were carried out in triplicateand the average value was used or calculations.

Te criteria or characterizing the fitting to the model were

the multiple determination coefficients (R

2

) and the mean rela-tive deviations (E) (Equation 9):

1

1(%) 100

en

E P

ie E

V V E

n V =

−= ∑

(9)

in which E is the mean relative deviation, ne is the number o

experimental data, V E is the experimental value (water loss or

solid gain) and V P is the calculated value (water loss or solid

gain).

3 Results and discussion

Te sweet potato osmotic dehydration experiments werecarried out under different salt concentrations (0, 5, 10% w/w),sucrose concentration o 50% w/w and temperature o 40 °C.Te osmotic process was studied in terms o water loss and solidgain and is shown in Figure 1.

Water loss and solid gain o sweet potato slices increasedwith time. A high initial rate o water removal and solid gain,ollowed by a slower removal (and gain) in the later stageswas observed. Several research groups have published simi-lar curves or osmotic dehydration o oods (COLLIGNAN;RAOUL-WACK, 1994; KAYMAK-EREKIN; SULANOGLU,2000; PARK et al., 2002).

Results showed that the more concentrated dehydrationsolution produced the highest water loss. Highest water losseswere obtained when a mixture o sucrose-NaCl was used asthe osmotic solution, due to an apparent synergistic effect othe solutes. It was observed that adding NaCl to the solutionresulted in an increase o the osmotic pressure gradients and,hence, higher water loss values throughout the osmosis period.Sodium chloride increases the driving orce or dehydration,lowering water activity and allowing a higher rate o penetrationin the material due to its low molecular weight (ISLAM; FLINK,1982; LENAR; FLINK, 1984). However, its use is limited sincea salty taste is imparted to the ood.

It is noted that the WL levels with 50% sucrose/5% NaCl arenot much lower than with 50% sucrose/10% NaCl. Tis behaviorpermits assessing that there is an antagonistic effect o the twosolutes on water loss, i.e., the use o the salt as osmotic agentincreasesWL at certain levels o concentration. Tese results arein agreement with those obtained by Lenart and Flink (1984),Collignan and Raoult-Wack (1994) and Sacchetti, Gianotti and

Dalla Rosa (2001). Te salt-sucrose antagonistic effect on waterloss is o particular interest i we consider that the chemicalpotential o the ternary solution increases when the concentra-tion o both solutes increases. Tese results could be explainedby a reduction o plasmatic membrane permeability that takesplace during the plasmolysis process. Furthermore, it wouldbe considered that, due to the different molecular dimensions,sucrose remains mainly in the extracellular space while sodiumchloride can penetrate into the cell, thus leading to a reductiono the osmotic pressure gradient (SACCHEI; GIANOI;DALLA ROSA, 2001).

Te higher concentration o salt in the dehydration solutionproduces the lower solid gain. A solid gain rom 1.5 to 1.6 g

and 1.6 to 1.7 g is obtained afer 2 hours o dehydration at 50%sucrose/10% NaCl and 50% sucrose/5% NaCl solution concen-trations, respectively.

In all osmotic solutions tested, WL and SG increased withincreasing concentrations o salt. In a solution concentrationo 50% sucrose/10% NaCl, the SG is lower than in a solutionconcentration o 50% sucrose/5% NaCl, due to a boundary phaseormed on the product surace at higher solution concentra-tions, decreasing the driving orce between the solution and theproduct. Similar results were observed by Kaymak-Ertekin andSultanoglu (2000), Sacchetti, Gianotti and Dalla Rosa (2001)and Eren and Kaymak-Ertekin (2007), working with ternary

solutions.It was expected that the results would be influenced mainly

by the change in NaCl concentration, but the positive effecto the salt-sucrose interaction on soluble solids determined adecrease o the solid gain at maximum NaCl and sucrose con-centrations (50% sucrose/10% NaCl). Tis effect is generallyattributed to the influence o natural tissue membranes, as wellas to the diffusive properties o water and solutes as a unctiono their respective molar masses. An increase o the osmoticpressure gradient determines a loss o unctionality o the cellplasmatic membrane that allows solute entrance by the loss oits selectivity. At low salt (and high sucrose) concentrations, itis possible that the increase in permeability causes greater su-crose penetration into the samples. However, as long as the saltconcentration increases (and the sucrose concentration, conse-quently, decreases), the competition between these solute flowsincreases and, due to the low molecular weight o NaCl, this saltpenetrates much more easily than sucrose, leading to a reductionin SG. Tis behavior can also be explained by the ormation o asolid barrier at the sample surace, which can make solid masstranser more difficult at high concentrations. Tis phenomenonwas also observed by Lenart and Flink (1984), Heng, Guilbertand Cuq (1990) and, Baroni and Hubinger (2007).

he best results were obtained by Azuara, Garcia andBeristain (1996), using solutions containing sucrose and NaCl




Antonio et al.

with cylindrical slices o potato, given the act that lowerconcentrations o the mixed solutes extracted as much wateras highly concentrated sucrose solutions. Similar results werereported by other authors (ISLAM; FLINK, 1982; LENAR;FLINK, 1984).

Peleg, Fick and Page’s equations were used to fit the experi-mental data. In all cases, the correlation coefficient was higherthan 0.95, indicating the adequacy o the predicting model.

Peleg’s equation parameters (1/k 1 and 1/k

2) or water loss

and solid gain were obtained using non-linear regression analysis

rom the statistical package Statistica 5.0 (SAISICA, 1995).

Te results are shown in able 1. Te parameter 1/k 1 describes

the initial rate o mass exchange. An inverse relationship be-

tween 1/k 1 and solution concentration can be observed or water

loss. From the observations made, the lowest value or 1/k 1 was

obtained or samples dehydrated at lower concentrations. For

solid gain, the initial rate o solids (1/k 1) presented a maximum

value or intermediate levels o NaCl concentration.

he parameter 1/k 2 describes the value o the equi-

librium amount o water loss or solid gain. A relationship

0

1

2

3

4

5

6

W

a t e r

l o s s

( g )

0 50 100 150 200 250Time (minutes)

Peleg

0

1

2

3

4

5

6

W

a t e r

l o s s

( g )

Time (minutes)0 50 100 150 200 250

Fick

0

1

2

3

4

5

6

W a t e r

l o s s

( g )

Time (minutes)0 50 100 150 200 250

Page

0.0

0.2

0.40.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

S o l i d s g a i n e d

( g )

Time (minutes)0 50 100 150 200 250

Peleg

0.0

0.2

0.4

0.60.8

1.0

1.2

1.4

1.6

1.8

2.0

S o l i d s g a i n e d

( g )

Time (minutes)0 50 100 150 200 250

Fick

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

S o l i d s g

a i n e d

( g )

Time (minutes)0 50 100 150 200 250

Page

NaCl (0%) - exp NaCl (0%) - cal NaCl (5%) - exp

NaCl (5%) - calNaCl (10%) - exp NaCl (10%) - cal

Figure 1. Sweet potato water loss and solid gain versus time modeled according to Peleg, Fick and Page’s equations.




ernary solution OD sweet potato

4 Conclusions

he rate o water loss during osmotic dehydration osweet potato was directly related to solution concentration.An antagonistic effect between the two solutes (sucrose andNaCl) was observed where the positive salt-sucrose interac-tion effect on soluble solids determined a decrease o solidgain when NaCl and sucrose were at maximum concentrations

(50% sucrose/10% NaCl).

Te experimental values o water loss and solid gain dur-ing the osmotic dehydration o sweet potato slices were fittedto Peleg, Fick and Page’s equations. Peleg’s equation presentedthe best adjustment o the experimental data.

Peleg’s parameters 1/k 1 and 1/k

2 or water loss were rom

0.09 to 0.30 and rom 3.13 to 5.56, respectively, and or solidgain they varied rom 0.01 to 0.18 and 0.67 to 1.67, respectively.Page’s parameters A and B or water loss were rom 0.07 to 0.17and rom 0.53 to 0.62, respectively, and or solid gain they rangedrom 0.02 to 0.41 and 0.36 to 0.73, respectively.

Te effective diffusion coefficients obtained rom Fick’sequation ranged rom 3.82 x 10–10 to 7.46 x 10–10 m2/s or waterloss and rom 1.18 x 10–9 to 3.38 x 10–10 m2/s or solid gain.

Acknowledgments

Te authors wish to acknowledge Fundação de Amparo àPesquisa do Estado de São Paulo (FAPESP) , Conselho Nacionalde Desenvolvimento Científico e Tecnológico (CNPq) andFundação de Coordenação e Aperfeiçoamento de Pessoal de NívelSuperior (CAPES).

Reerences

AOAC - ASSOCIAION OF OFFICIAL ANALYICAL CHEMISS.Official Methods o Analysis. 14 ed. Arlington, 1984.

between 1/k 2 and solution concentration can be observed, i.e.,

an increase in osmotic concentration caused an increase in the value o 1/k

2 or water loss and solids. Park et al. (2002) did not

find any relationship between Peleg’s equation parameters andthe increase o sucrose concentration at the same temperature,during osmotic dehydration o pear. Te average calculatedrelative error ranged rom 1.82 to 7.18% or water loss and rom

3.89 to 10.09% or solid gain.Te values o the effective diffusion coefficients obtained

or the different osmotic solution concentration combinations,calculated using Fick’s model, are presented in able 2.

he average calculated relative error varied rom9.74 to 13.93% or water loss and rom 5.08 to 15.59% or solidgain. Concerning effective diffusivity, it can be observed thatexperimental values showed orders o magnitude between10–10 and 10–11. Similar results were observed by elis, Murari andYamashita (2004), working with tomatoes in ternary solutions(sucrose and NaCl). Kaymak-Ertekin and Sultanoglu (2000)ound values ranging rom 3.32 x 10–11 to 21.33 x 10–11 m2/s or

water loss and rom 3.62 x 10–11

to 10.82 x 10–11

m2

/s or solutediffusivity o apples at different temperatures (20-50 °C) anddifferent concentrations o sucrose and dextrose.

Page’s equation parameters obtained or water loss and solidgain are shown in able 3. Te average calculated relative error varied rom 5.94 to 11.47% or water loss and 5.39 to 13.09%or solid gain. Te equation paramweters do not exhibit a trendeither with the increase o solution concentration at the sametemperature or with the increase o temperature at a constantsolution concentration. Similar conclusions were made byPark et al. (2002).

Te lowest average relative error was obtained using Peleg’s

equation. Te experimental and calculated results or water lossand solid gain are shown in Figure 1.

able 1. Values o Peleg’s equation parameters or water loss and solid gain.

(°C) Sucrose(%w/w)

NaCl(%w/w)

Water loss Solid gain

1/k 1

1/k 2

R 2 E (%) 1/k 1

1/k 2

R 2 E (%)

40 50 0 0.09 3.13 0.99 3.92 0.01 0.67 0.98 10.09

5 0.19 4.76 0.99 1.82 0.18 1.67 0.98 3.89

10 0.30 5.56 0.98 7.18 0.04 1.72 0.98 7.58

able 2. Values o Fick’s equation parameters or water loss and solid gain.

(°C) Sucrose(%w/w)

NaCl(%w/w)


De (m2/s) R 2 E (%) D

e (m2/s) R 2 E (%)

40 50 0 3.82 x 10–11 0.98 9.74 1.16 x 10–11 0.99 5.08

5 7.46 x 10–11 0.98 11.23 1.18 x 10–10 0.95 15.59

10 4.28 x 10–11 0.97 13.93 3.38 x 10–11 0.97 10.71

able 3. Values o Page’s equation parameters or water loss and solid gain.

(°C) Sucrose(% w/w)

NaCl(% w/w)


A B R 2 E (%) A B R 2 E (%)

40 50 0 0.07 0.62 0.99 9.08 0.02 0.73 0.99 5.39

5 0.17 0.53 0.99 5.94 0.41 0.36 0.99 10.98

10 0.10 0.58 0.98 11.47 0.06 0.65 0.98 13.09




Antonio et al.

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