Vacuum impregnation for development of new dehydrated products

Pedro Fito

*

, Amparo Chiralt, Jose M. Barat, Ana Andres, Javier Martõnez-Monzo,

Nuria Martõnez-Navarrete

Food Technology Department, Universidad Politecnica de Valencia, P.O. Box 22012, 46071 Valencia, Spain

Abstract

Vacuum impregnation (VI) of structured foods implies the partial release of gas from pores and its substitution by an external

liquid. Therefore, important changes in physicochemical and structural properties take place in the food and these a€ect its behavior

in drying operations (air-drying (AD) and/or osmotic dehydration (OD)). The adequate control of VI prior to dehydration may be

used as a tool both to improve mass transfer and to develop engineered products. In order to evaluate this alternative, the e€ec-

tiveness of VI as a tool in porous matrix formulation is analyzed. Likewise, its in¯uence on some physical and transport properties

of the plant tissue and the relevant changes induced in osmotic and convective drying processes are discussed, since these are

probably the most interesting alternative processes to lengthen the impregnated product shelf-life. Improved yield of some dehy-

dration processes, such as fruit candyin, when VI is applied at the beginning, is also discussed in terms of the cell network relaxation

mechanism, responsible for hydrodynamic tissue impregnation. Ó 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Dehydrated fruits; Vacuum impregnation; Osmotic processes; Food matrix engineering

1. Introduction

Vacuum impregnation (VI) of a porous product

consists of exchanging the internal gas or liquid oc-

cluded in open pores for an external liquid phase, due to

the action of hydrodynamic mechanisms (HDM) pro-

moted by pressure changes (Fito, 1994; Fito & Pastor,

1994). The operation is carried out in two steps after the

product immersion in the tank containing the liquid

phase. In the ®rst step, vacuum pressure (p

1

50±100

mbar) is imposed on the system for a short time …t

1

† in

the closed tank, thus promoting the expansion and

out¯ow of the product internal gas. The releasing of the

gas takes the product pore native liquid with it. In the

second step the atmospheric pressure …p

2

† is restored in

the tank for a time …t

2

† and compression leads to a great

volume reduction of the remaining gas in the pores and

so to the subsequent in ¯ow of the external liquid in the

porous structure. Compression can also reduce the pore

size depending on the mechanical resistance of the solid

matrix.

The volume fraction of the initial sample (X) im-

pregnated by the external liquid when mechanical

equilibrium was achieved in the sample has been mod-

eled (Eq. (1)) as a function of the compression ratio r

(Eq. (2)), sample e€ective porosity …e

e

†, and sample

volume deformations at the end of the process …c† and

the vacuum step …c

1

†, as described in Eq. (2) (Fito,

Andres, Chiralt, & Pardo, 1996). If c ˆ c

1

ˆ 0, Eq. (1)

gives the relationship for VI of non-deformed products

(Fito, 1994). In practical terms, sample deformations in

VI are seen to be negligible for a great number of fruits

(Salvatori, Andres, Chiralt, & Fito, 1998; Chiralt et al.,

1999).

r ˆ

p

2

‡ p

c

p

1

;

…1†

e

e

…r 1† ˆ …X

c†r ‡ c

1

:

…2†

The aim of this paper is to discuss how VI may modify

composition of porous food, and therefore its e€ective-

ness as a tool in porous matrix formulation, its in¯uence

on some physical and transport properties of the plant

tissue and the relevant changes induced in osmotic and

convective drying processes, which are probably the

most interesting alternative processes to lengthen the

impregnated product shelf-life. Improved yield of some

dehydration processes, such as fruit candy, when VI is

applied at the beginning is also discussed. Special em-

phasis is placed on the behavior of apple samples be-

cause of their high porosity and so their suitability for

VI processes.

Journal of Food Engineering 49 (2001) 297±302

www.elsevier.com/locate/jfoodeng

*

Corresponding author. Tel.: [phone]; fax: [phone].

E-mail address: p®[e-mail] (P. Fito).

0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 2 2 6 - 0

2. VI process for the formulation of porous matrices

VI operation can allow us to incorporate any ingre-

dients in a porous product in order to adapt its com-

position to certain stability or quality requirements, in a

quick and simple way. Structured foods such as fruit

and vegetables have a great amount of pores (intercel-

lular spaces) which are occupied by gas, or native liquid

to quite an extent, and which o€er the possibility of

being impregnated by a determined solution thereby

improving composition by adding speci®c/selected sol-

utes: incorporation of acids, preservatives, sugars or

other water activity depressors, special nutrients, etc. In

this sense, VI can be considered as a tool in the devel-

opment of fruit or vegetable products without disrupt-

ing their cellular structure, while conveniently modifying

their original composition (Chiralt et al., 1999). VI also

modi®es some physical properties of the product and

this will have advantages for other processes (Martõnez-

Monzo, Martõnez-Navarrete, Fito, & Chiralt, 1998a,

Martõnez-Monzo, Barat, Gonzalez-Martõnez, Chiralt, &

Fito, 2000).

These aspects may be considered as food matrix

(porous solid) engineering since the original product is

adapted to new requirements. The mass fraction of any

component (i: water or solutes) reached in the impreg-

nated product …x

I

i

† can be estimated by Eq. (3) deduced

from a mass balance in the system, in terms of the

sample initial composition …x

0

i

†, the mass ratio of the

impregnating solution in the initial product …x

HDM

† and

the solution composition …y

i

†. The value of x

HDM

is easily

deduced from the sample porosity level by applying Eqs.

(1), (2) and (4), if sample deformation is assumed neg-

ligible throughout VI process. From these Equations,

the required solution concentration of a determined

component (water, sugar, acid, additive, etc.) can be

calculated in order to achieve a desired level in the ®nal

product. In Fig. 1, the predicted values of mass fraction

of any ingredients i reached in a porous sample in a VI

operation was plotted as a function of their mass frac-

tion in the external solution and the sample porosity.

The following assumptions were made to obtain pre-

dicted values: pressure at vacuum and atmospheric

conditions were 50 and 1030 mbar, respectively, the

initial i level in sample was zero and no changes in initial

sample porosity occurred during VI. The limiting factors

for VI are the solubility of component i in the external

liquid and the sample e€ective porosity.

x

I

i

ˆ

x

0

i

‡ x

HDM

y

i

1 ‡ x

HDM

:

…3†

x

HDM

ˆ X

q

IS

q

0

:

…4†

3. VI e€ects on physical properties of fruit

The physical property which is most clearly a€ected

by VI is the product density. Since air in the pores is

replaced by liquid, density increases, thus a€ecting other

related properties such as thermal conductivity which is

Notation

p

c

capillary pressure

p

1

vacuum pressure in the ®rst VI process step

p

2

atmospheric pressure in the second VI process step

e

e

sample e€ective porosity

r

compression ratio in the VI process

X

sample volume fraction impregnated by the solution at the

end of the VI process

c

1

relative volume deformation of the sample due to pressure

change at the end of the ®rst VI step

c

relative volume deformation of the sample due to pressure

change at the end of the VI process

q

0

density of the initial product

q

IS

density of the impregnating solution

q

0

r

initial real sample density

x

HDM

mass ratio of the impregnated solution in the initial product

y

i

mass fraction of the component i in the impregnating

solution

x

I

i

mass fraction of component i in the impregnated product

x

0

i

mass fraction of component i in the initial product

K

thermal conductivity

a

thermal di€usivity

e

0

; e

00

real and imaginary components of the complex dielectric

permitivity

Fig. 1. Mass fraction of impregnated component i in a porous product

as a function of its mass fraction in the external solution and the

product porosity. Values predicted applying Eqs. (1)±(4), assuming:

c ˆ c

1

ˆ 0, x

0

i

ˆ 0, r ˆ p

2

=p

1

, p

1

ˆ 50 mbar, p

2

ˆ 1030 mbar,

q

0

r

…kg=m

3

† ˆ 1056,

q

0

…kg=m

3

† ˆ q

0

r

…1 e

e

†,

q

IS

…kg=m

3

† ˆ 1=

‰……1 y

i

†=1000† ‡ …y

i

=1500†Š.

298

P. Fito et al. / Journal of Food Engineering 49 (2001) 297±302

greatly a€ected, especially when highly porous. Changes

are dependent on total porosity and pore distribution in

relation with the direction of heat ¯ow and impregnat-

ing solution composition (Martõnez-Monzo et al., 2000;

Barat, Martõnez-Monzo, Alvarruiz, Chiralt, & Fito,

1994). Fig. 2 shows the values of thermal conductivity

and di€usivity of apple (var. Granny Smith ) with and

without VI (p

1

ˆ 50 mbar) with an isotonic solution as a

function of temperature. Determination method of these

properties has been previously described (Martõnez-

Monzo et al., 2000). Lesser modi®cations were observed

in thermal di€usivity than in conductivity because of the

simultaneous sample density increase during VI (at

25°C: q

0

…NI† ˆ 813 kg=m

3

; q

0

…I† ˆ 986 kg=m

3

†. Spe-

ci®c heat …c

p

† does not change if no changes in com-

position are induced in VI, nevertheless the product c

p

q

0

may increase throughout the process. In apple samples

c

p

q

0

varies from 3:12 10

6

to 3:79 10

6

J=m

3

K, in the

range 30±60°C, due to VI with an isotonic solution. An

increase in the impregnating solution concentration will

promote the expected reduction in all thermal properties

as compared with impregnation without compositional

changes (Fito, Chiralt, Barat, & Martõnez-Monzo,

2000).

In microwave drying (MWD) dielectric properties of

a material greatly a€ect heat generation and drying rate.

Both real …e

0

† and imaginary …e

00

† components of the

complex permitivity decrease when product porosity

increases (Kent & Kress-Rogers, 1987). Gas±liquid ex-

change in the product pores promoted by VI could im-

prove microwave-product interactions and so be

advantageous in MWD or combined AD±MWD.

Dielectric properties of apple (initial e

e

ˆ 0:23) I and

NI with an isotonic solution have been reported for

samples dried at di€erent moisture levels (Martõn, Fito,

Martõnez-Navarrete, & Chiralt, 1999). The reported

values of e

0

and e

00

re¯ected the in¯uence of changes in

water content and structure on the capability of the

sample to store and dissipate energy, respectively. Both

components evolve as a function of the moisture content

in a di€erent way for NI and I samples. Loss tangent

…e

00

=e

0

† increases in line with sample drying level, this

being faster for impregnated samples. In¯uence of im-

pregnation of apple on the complex permittivity mod-

ulus (



e

02

‡ e

002

p

) does not seem noticeable, this decreases

when moisture content decreases.

4. Mass transport properties in osmotic and convective

dehydration

Mass transport properties, such as the e€ective dif-

fusion coecient of the plant tissue in osmotic dehy-

dration (OD) processes are also modi®ed by previous VI

pretreatment. This modi®cation has been analyzed in

apple tissue VI with isotonic solutions in order to avoid

any change in the value of the process driven force

(Martõnez-Monzo, Martõnez-Navarrete, Chiralt, & Fito,

1998b). The values of the e€ective di€usion coecient

…D

e

† in the product liquid phase (PLP), sugar gain …DM

s

†

and water loss …DM

w

† for NI and I cylindrical samples

(1 cm characteristic dimensions) treated for 3 h, at 30°C,

with concentrated grape must …a

w

ˆ 0:789† are given in

Table 1. Impregnation was carried out with low mo-

lecular weight and high molecular weight solutes (HM

pectin) in order to observe the e€ect of the viscosity of

the liquid in the intercellular spaces. It can be seen that

VI promotes e€ective di€usion in the fruit liquid phase

when impregnated solution has low viscosity, even

considering the corrected D

e

value of NI sample with its

porosity value. Obtained results are in agreement with

the coupling of di€usional and osmotic (water selective

di€usion through cell membrane) mechanisms to a dif-

ferent extent in each case. Sugar gain occurs through

di€usion in the pores, but this mechanism contributes

little to water loss which mainly occurs cell to cell by

osmotic mechanisms. When intercellular spaces are oc-

cupied by gas (NI sample) or by a very viscous solution,

solute di€usion through intercellular spaces seems to be

inhibited and the sample reaches the chemical equilib-

rium with the osmotic solution after a greater osmotic

water loss. This aspect also implies greater PLP and

volume losses of the samples.

In practical OD processes of porous fruits, VI pre-

treatments were carried out with the osmotic solution at

the beginning of the process. This is called pulsed vac-

uum OD (PVOD) and has been observed to be very

e€ective in promoting mass transfer kinetics (Fito &

Chiralt, 1995, 1997; Shi & Fito, 1993; Shi, Fito, &

Chiralt, 1995). VI implies transporting the external sol-

ute and water into the tissue while it modi®es the ef-

fective di€usion coecient in the PLP. The gas liquid

exchange implies a fast change of the sample overall

Fig. 2. Thermal conductivity (circles) and di€usivity (rhombus) values

of apple samples impregnated (closed symbols) and non-impregnated

(open symbols) with an isotonic solution (adapted from Martõnez-

Monzo et al., 2000).

P. Fito et al. / Journal of Food Engineering 49 (2001) 297±302

299

composition that modi®es the process driven force at the

very beginning of the process, while pores remain full of

liquid. Fig. 3 shows the comparison of D

e

values in the

PLP, obtained for di€erent fruits in mass transfer kinetic

analysis of parallel processes at atmospheric pressure

OD and PVOD. Compositional change promoted by

vacuum pulse was corrected in the analysis to make the

D

e

values comparable, according to a model previously

published (Fito & Chiralt, 1997). Higher values of D

e

in

PVOD processes can be observed in almost all the cases,

the higher the fruit porosity, the greater the D

e

promo-

tion by PVOD.

In addition to promoting di€usional mechanism in

the pores, which was commented on the above, VI

brings with it a di€erent structural development of the

tissue throughout the osmotic process, thus a€ecting the

tissue response to mass transport. Di€erences in the

structural features observed in vacuum-impregnated and

non-impregnated (NI) samples have been explained in

terms of the di€erent pressure drop of ¯uid in the in-

tercellular spaces ¯owing towards the volume generated

by cell water loss, which is very di€erent for gas or liquid

phases in the intercellular space (Fito et al., 2000). When

there is a liquid in the pores, the force balance on the

double layer plasmalemma-cell wall leads to layer sep-

aration while plasmalemma shrinks in line with water

loss with scarce deformation of cell wall. In OD pro-

cesses, where a gas phase occupies the intercellular

space, plasmalemma shrinks together with cell wall that

deforms greatly when osmotic the process progresses

(Barat, Chiralt, Albors, & Fito, 1999). Fig. 4 schemati-

cally shows the structure development of the cellular

tissue in OD and PVOD treatments at di€erent steps of

the process (Fito et al., 2000).

Air-drying processes (AD) of fruit and vegetables

involve heat and mass transport phenomena in the plant

tissue. Transport rate is greatly a€ected by the tissue

structure and composition, both de®ning the e€ective

values of its transport properties (thermal properties and

water e€ective di€usion coecient). Therefore, VI pre-

treatments may lead to a di€erent drying behavior of

fruit and vegetable, as well as to di€erent ®nal properties

of the product. Fig. 5 shows the e€ect of VI with an

isotonic solution on drying curves of apple slices (7 mm

initial thick). In this case, VI slows down the drying rate

of the product. The increase of the variable c

p

q

0

of the

product as well as the limitation of vapor di€usion in the

sample pores are the main factors that made the process

rate slower in I samples. However, when combined AD±

MWD is applied, drying rate of I samples overtakes that

of NI samples, starting from the application of a de-

termined MW power, probably due to the induced

changes in dielectric properties (Martõn et al., 1998).

5. Impregnation in long-term osmotic processes. Fruit

candying

Impregnation of the fruit pores due to HDMs has

been seen to occur without external pressure changes

when the cellular tissue remains immersed in a liquid

phase for a long time (e.g., syrup canned and candied

fruits). This was explained in terms of the capillary

forces, pressure and temperature ¯uctuations in the

system and relaxation phenomena of the shrunk cellular

matrix when hypertonic solutions were used in the

treatments (Fito et al., 2000; Barat, Chiralt, & Fito,

1998, 1999). It has been observed that HDM contributes

Table 1

E€ective di€usion coecient, sugar gain and water loss of apple (Granny Smith) osmosed with recti®ed grape must (a

w

ˆ 0:789) for 3 h at 30°C

(adapted from Martõnez-Monzo et al., 1998b)

Sample

D

e

10

10

…m

2

=s†

DM

s

a

DM

w

a

NI

3.96 (5.13)

b

9.0

)40.1

I

c

6.51

16.2

)34.7

I

d

3.07

7.3

)34.7

a

Expressed as g of water or solute gain per 100 g of initial product, for 3 h of treatment.

b

In brackets corrected value taking into account porosity value of apple (D

e

=…1 e

e

††.

c

VI with an isotonic solution containing sugar and 2% (w/w) HM pectin.

d

VI with sugar isotonic solution.

Fig. 3. Comparison of D

e

values obtained in parallel osmotic dehy-

dration processes of di€erent fruit at atmospheric pressure (OD) and

pulsed vacuum OD (PVOD). Temperature range: 30±50°C. Osmotic

solution: sugar solution between 50 and 70 Brix.

300

P. Fito et al. / Journal of Food Engineering 49 (2001) 297±302

to the total mass transfer throughout long-term osmotic

processes due to the internal pressure gradients in the

tissue associated to relaxation of deformed cell network

resulting from cell water losses (Fito, Chiralt, Barat,

Salvatori, & Andres, 1998; Barat et al., 1998). Table 2

summarizes the di€erent steps involved in a long-term

osmotic process on the way to equilibrium as well as the

phenomena responsible for the changes in the system

free energy, which occur in each step. Fig. 4 also shows,

in a schematic way, the development of a cellular tissue

throughout the equilibrium pathway in this kind of

process for samples initially vacuum impregnated (I)

with the osmotic solution and for those non-impreg-

nated (NI).

After the chemical equilibrium was reached in the

system, the cell network will have stored a great deal of

mechanical energy in the deformed cell walls and cell

bondings. The amount of stored energy will be depen-

dent on the sti€ness of the cellular arrangement in the

tissue. Weak cell bonds and soft/thin cell walls will imply

a great dissipation of energy during water loss and de-

formation. On the contrary, strong cell bonds and hard/

thick cell walls will lead to a notable store of free energy

as mechanical tension in the structure. Relaxation of

mechanical energy leads to sample volume recovery and,

if the sample is immersed in a liquid, the subsequent

mass gain due to hydrodynamic ¯ow of the external

liquid.

In apple samples, it has been observed that the vol-

ume was recovered until practically the initial value in a

wide range of process conditions (treatments between

30°C and 50°C with osmotic solutions from 15 to

65°Brix). Volume and mass recovery in PVOD-treated

samples was faster than in those OD treated and the

recovery level was in all cases higher (Barat et al., 1998,

Table 2

Process steps in osmotic dehydration, driven forces and phenomena responsible for changes in the system free energy

Process step

Time range

Driven force

Dissipative free energy

phenomena

Storage free energy

phenomena

Impregnation (capillary, VI)

Minutes

Pressure gradients

Flow of external liquid

Volume reduction (gas

phase)

Osmotic dehydration

Hours

Chemical potential of

components (mainly

water)

Water loss

Volume loss of cells

Solute gain

Plasmolysis

a

Deformation of cellular

matrix

Liquid ¯ow through cell

wall

a

Impregnation

Days

Pressure gradients

Relaxation of cellular

network

EQUILIBRIUM

Flow of external ¯uid

a

In osmotic processes where no VI with osmotic solution has been carried out these phenomena occur coupled with the relaxation of shrunk cellular

network in the next impregnation step (Fito et al., 1999a).

Fig. 4. Pathway of the structure changes in the cellular tissue throughout osmotic treatments OD and PVOD. …J

HDM

: hydrodynamic ¯ux of external

liquid, J

w

: water ¯ux, is: intercellular space, ic: intracellular content, isgv: intercellular space generated volume, A and B: cell bonding points).

Fig. 5. Drying curves of apple slices (7 mm thick) impregnated (I) and

non-impregnated (NI) with an isotonic solution (air conditions: 2 m/s,

40°C) (adapted from Martõn et al., 1998).

P. Fito et al. / Journal of Food Engineering 49 (2001) 297±302

301

1999). This has been explained in terms of a greater

irreversibility of deformations induced in the cell net-

work by OD, associated with the greater cell wall de-

formation (Fig. 4).

Mass and volume gains in long-term osmotic pro-

cesses have great practical interest in fruit candy pro-

cesses. These are improved in PVOD processes as shown

in Table 3 for some candied fruits with 6°Brix in their

liquid phase.

Acknowledgements

We thank the Comision Interministerial de Ciencia y

Tecnologia, the UE (STD3 programme) and the CY-

TED Program for their ®nancial support.

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treatment on mass transfer during osmotic dehydration of fruits.

Food Research International, 28, 445±454.

Shi, X. Q., & Fito, P. (1993). Vacuum osmotic dehydration of fruits.

Drying Technology, 11, 1429±1442.

Table 3

Mass …DM† and volume …DV † change (referred to the sample initial values) of di€erent 65°Brix candied fruits obtained by OD or PVOD (at 40°C)

with osmotic solutions of increasing concentration (24 h in 45 Brix OS and 24 h in 65 Brix OS)

Treatment

OD

PVOD

Candied fruit

DM (%)

DV (%)

DM (%)

DV (%)

Pineapple

)28

)45.8

2

)21.4

Apple

)55

)71.3

)20

)38.3

302

P. Fito et al. / Journal of Food Engineering 49 (2001) 297±302