Microwaves phenomena during drying of apple cylinders

Cristina Bilbao-Sainz *, Ana Andres, Amparo Chiralt, Pedro Fito

Department of Food Technology, Polytechnic University of Valencia, Camino de Vera s/n, 46022 Valencia, Spain

Abstract

Dehydration of apple cylinders applying microwaves (3 and 10 W/g initial incident microwave power) combined with forced air

(40 �C air temperature) was performed. Drying rate and sample temperature and volume were recorded in order to analyse the cou-

pling of heat and mass transfer mechanisms and deformation–relaxation phenomena in line with the sample dehydration. Experi-

mental results showed the effect of temperature raise on internal evaporation phenomena producing both plasticization of sample

matrix and an increase of internal pressure. As a consequence changes in sample temperature and volume, as well as the drying rate

and dissipated power showed a common pathway. A general description has been developed to explain the behaviour of apple tissue

under microwave drying.

Keywords: Microwaves; Drying; Vacuum impregnation; Apple

1. Introduction

Microwave drying is one of the emerging food-pro-

cessing methods incorporating microwave radiation in

a conventional air drying. Microwave drying utilizes

very fast nearly instantaneous volumetric heating due

to the microwave energy coupling with foods, while con-

ventional drying relies on slow transfer of heat from the

surface to the interior of the food, the presence of air at

a certain flow in addition to improve the product qualityis used to carry away the moisture from the surface of

the product.

The basic physical phenomena that is responsible for

the heating of food materials at microwave frequen-

cies is dipole rotation (Schiffmann, 1995). The dipole

rotation mechanism relies on the fact that water mole-

cules are subject to a microwave field that rapidly

change direction, the dipoles try to align with the direc-

tion of electrical field. The electrical field thus providesenergy for water molecules to rotate into alignment.

The energy is converted into kinetic energy of water

molecules and then into heat, when the water molecules

realign in the changing electrical field and interact with

the surrounding molecules (friction) (Khraisheh,

Cooper, & Magee, 1997). On the other hand, patterns

of power absorption in a food heated in a microwave

oven depend on oven and load factors, according withthese heating patterns a volumetric heat source is dissi-

pated thought the material due to dielectric losses as well

as direct conductive effect. Then the process itself is

essentially governed by two mechanisms: heat transfer

and mass transfer.

The objective of this work was to investigate the

combined effect of the application of microwave

energy and air on the drying of apple cylinders ana-lyzing heat and mass transfer equations throughout

the whole process of dehydration to describe the

physical mechanisms occurring during microwave

drying.

Nomenclature

VI vacuum impregnation

Q source heat of microwave power (J)M sample weight (g)

Cp specific heat at constant pressure (J/g K)

hV latent heat of vaporizaion (J/g)

A sample surface (m2)

h convective heat transfer coefficient (W/m2 K)

T product temperature (K)

Ts temperature at the sample surface (K)

Ta air dying temperature (K)t process time

q density (kg/m3)

xti mass fraction in the product of component i

at time t of the drying process

zi mass fraction of component i in the food

liquid phase at time t of the drying process

V volume

DV volume change referred to the total initialvolume

DVGP volume change of GP referred to the total

initial volumeDVLP volume change of LP referred to the total

initial volume

Subscripts

w water

s solids

v vapor water

LP liquid phaseGP gas phase

SM solid matrix

Superscripts

0 initial values (t = 0)

t values at time t

2. Experimental procedure

Granny Smith apples were used as test material. This

selection was based on the fact that the cells of the

parenchyma tissue are homogeneous and structurally

less complex than other vegetables which facilitates the

understanding of the interaction between material and

microwaves. It is also an appropriate fruit for vacuumimpregnation treatments since raw apple exhibits poro-

sity between 18% and 24%.

Samples were obtained by cutting the apples into

cylindrical pieces with a diameter of 20 and 20 mm

height. The axis of each cylinder was parallel to the main

axis of the fruit. Apple cylinders were vacuum impreg-

nated with a commercial apple juice for 10 min at

50 mbar and 10 min more at atmospheric pressure. Dur-ing the vacuum step internal gas in the pores of the

product partially flows out of the material and when

the atmospheric pressure is restored the isotonic solu-

tion is introduced into the product throughout its pores

promoting an effective exchange of the product internal

gas for the external liquid due to the hydrodynamic

mechanism (Fito et al., 2001).

Pretreated and non-pretreated samples were dried ina combined air–microwave oven. The drier consisted

essentially of an air-flow section and a microwave sec-

tion, both of them assembled to a microwave oven

cavity functioning as a drying chamber. The air condi-

tions were set at 40 �C air temperature and 1 m/s air

velocity. The microwave incident power was fixed at 3

and 10 W/g of initial sample weight.

During drying sample weigh and temperature were

continuously monitored. Fiber optic probes (model

790, Luxtron Corp.) were used to measure the tempera-

tures at the sample center and surface. Initial moisture

content was determinate with an A&D Model AE100

Infra Red balance. The moisture content of the sample

at intermediate drying stages was calculated from the

weigh lost during drying. Another set of samples wereperiodically removed from the drier to measure volume

changes during the dehydration process. The volumes

were estimated using the ADOBE PHOTOSHOP pro-

gramme (Microsoft Corporation, 1997). For this pur-

pose, a tool included in the programme, which is able

to identify and count the pixels of the image with a cer-

tain colour, was used.

2.1. Cryo-SEM observations

Some samples were processed to observe microstruc-

ture changes. After treatment, apple cylinder was cut

parallel to the longitudinal axis using a scalpel. This type

of cut allowed viewing of the specimen by sweeping

over the treated cylinder, from the interface to the

central plane. Prior to cryo-SEM observations, the sam-ple was cryo-fixed in order to fix and stabilise the struc-

ture and composition of the biological system. The

sample was placed in the SEM sample holder and

plunged into subcooled nitrogen (nitrogen slush) close

to the freezing point of nitrogen (�210 �C). Slush nitro-gen was used for its efficient cooling properties (Jeffree &

Read, 1991, chap. 8). The frozen sample was transferred

to the cryo-stage and then freeze fractured, etched and

gold coated. The samples were viewed in a cryo-scan-

ning electron microscope JSM-5410 (JEOL, Kyoto,

Japan).

3. Results and discussion

To analyse the different situations involved during the

dehydration with microwaves, product temperature and

mass loss were registered along the drying process, in the

same way, the dissipated power was calculated from the

next energy conservation equation, proposed and solved

throughout the whole process:

Q ¼ M s � Cps � ðT t2 � T t1Þ þ Mt2w � T t2 �Mt1

w � T t1� �

� Cpw þMv � hv � h � A � ðT a � T sÞ � ðt2 � t1Þ ð1Þ

The heat transfer is based on the energy conservation

of the sensible heat, latent heat, convection heat and

source heat of microwave power (Q).

Dissipated Power (No VI)

-5

0

5

10

15

20

25

0 50 100 150t (min)

P (W

)

Temperature Registration (No VI)

0

20

40

60

80

100

120

0 50 100t (min)

T (°

C)

centre

interface

Drying Rate Curve (No VI)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150t (min)

dXw/d

t

Fig. 1. Temperature registration, drying rate curve and dissipated power alon

of microwave incident power.

Figs. 1 and 2 show the temperature registration, dry-

ing rate curve and dissipated power during drying of ap-

ple cylinders with air at 40 �C and two different levels of

microwave incident power. It can be observed that the

curves follow the same pathway which is coherent with

the fact that the three magnitudes measure the samephenomena.

In these figures four periods can be distinguished

(Andres, Bilbao, & Fito, 2004):

(0) At the beginning of the process moisture content of

the sample was so high, around 86% (wb), this high

content of water was the main responsible of an effi-

cient internal heat generation causing an increase inproduct temperature which in turn produces higher

diffusion resulting a first maximum in the drying

rate curve, this effect can be higher for vacuum

impregnated samples since the lost of gas phase dur-

ing the vacuum impregnation results in a higher

product density and as density increases so do the

Temperature Registration (VI)

0

20

40

60

80

100

120

0 50 100 150t (min)

T (°

C)

centre

interface

Drying Rate Curve (VI)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100t (min)

dXw/d

t

Dissipated Power (VI)

-5

0

5

10

15

20

25

0 50 100 150t (min)

P (W

)

g the drying process of apple cylinders dried with air at 40 �C and 3 W/g

Temperature Registration (No VI)

0

20

40

60

80

100

120

0 50 100 150t (min)

T (°

C)

centre

interface

Drying Rate Curve (No VI)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150t (min)

dXw/d

t

Dissipated Power (No VI)

-5

0

5

10

15

20

25

0 50 100 150

t (min)

P (W

)

Temperature Registration (VI)

0

20

40

60

80

100

120

0 50 100 150t (min)

T (°

C)

centre

interface

Drying Rate Curve (VI)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150t (min)

dXw/d

t

Dissipated Power (VI)

0

5

10

15

20

25

0 50 100 150

t (min)

P (W

)

Fig. 2. Temperature registration, drying rate curve and dissipated power along the drying process of apple cylinders dried with air at 40 �C and

10 W/g of microwave incident power.

dielectric properties, and heating is increased. For

samples dried at 3 W/g microwave power as well

as for non-impregnated samples dried at 10 W/g

microwave power, the drying proceeds by evapora-

tion as opposed to vaporization that occurs whenthe temperature of the medium exceeds the boiling

point, in this case the total gaseous pressure gradi-

ent acting on both gas and liquid phases becomes

the major driving force (Constant, Moyne, & Perre,

1996). For impregnated samples dried at higher

microwave power, in only 3 min of process the boil-

ing point was reached (Fig. 2), liquid droplets

appeared on the surface of the apple cylinder. Thiscould be due to an inadequate air flow to carry away

the excess of liquid but the capacity of the air to dry

was about 11 g of water/min, therefore the existence

ofwater droplets at the surfacewas probably caused

by the condensation of a great amount of vapor

arriving from inside to a cooler surface or because

of a liquid movement period that usually occurs

for high power density and media which contain

large amounts of liquid (Constant et al., 1996).

(I) At this point, both, the drying rate and the temper-

ature decreased, free water flowed from the innerto the cylinder surface where was removed by the

air. This period is much more longer for impreg-

nated samples due to the higher amount of free

water in the intercellular spaces. Considering that

this amount of water is less than the evaporated

water during this period it is obvious to assume

that some of the cells were destroyed contributing

to an increase in the free water content. The eva-poration of free water involves the refrigeration

of the sample and consequently the reduction of

the drying rate.

(II) Once free water is removed, product temperature

and drying rate increased again until a second

maximum. The samples which were previously

impregnated with an isotonic solution, reached a

second maximum after 20 min of dehydration at

higher microwave power, these samples having a

moisture content of about 80% (wb) absorbed

microwave power leading to an increase in the

product temperature up to the boiling point ofthe liquid phase. In this case, once again the pres-

sure gradient acting on both gas and liquid phases,

drove the water from the interior of the cylinder to

the surface under a type of pumping action, as a

consequence, 30% of the total water content was

removed in only 8 min of drying process.

(III) In the last falling rate period, the drying rate

decreased, power absorption largely depends onmoisture content and as the product lost moisture

the microwave absorption decreased progressively.

The absorbed microwave energy was mainly used to

evaporate the water during the dehydration process,

however, in the final period, as the moisture content de-

creases a larger part of the energy was used by convec-

tive heat losses or to heat the sample.Focusing on the product�s temperature graphs, it is

observed that the temperatures at the center of the sam-

ples were during the whole drying process higher than

the temperatures at the surfaces, this is due to the size

and cylindrical shape of the samples that promote inter-

nal heating concentration, this focusing effect refers to a

much large value of electric field inside the sample near

its center as compared to the value at its surface, suchfocusing is expected for cylinders of radius not much

bigger than the penetration depth (Zhang & Datta,

2001).

3.1. Changes in sample volume

The volume changes were also measured along the

process (Fig. 3). Vacuum impregnation treatment isassumed to cause a negligible deformation of the solid

matrix when drying apple samples var. Granny Smith

(Salvatori, Da silva, Andres, Chiralt, & Fito, 1996).

However, during the dehydration process samples lost

water which caused an important reduction of its vol-

ume size but it can be observed in this figure that when

the product was heated up to the two maximums, the

sample volume slightly increased since the absorbedmicrowave power caused the expansion of the vapor cre-

ated inside the product.

On the other hand, the apple sample can be considered

as being made up of three phases (Lazarides, Fito, Chi-

ralt, Gekas, & Lenart, 1999): the liquid phase (LP), the

gas phase occupying pores (GP) and the insoluble solid

matrix (MP). Volume or mass loss during a drying pro-

cess may be explained in terms of the partial losses of eachone of the phases. Eq. (2) reflects the total volume varia-

tion in terms of the changes of each phase, all these

referred to the sample initial volume. In practical terms,

it is possible to assume that DVMP = 0, due to its very

small initial value, so only changes in the liquid and gas

phase will describe the volume development. The values

of DVLP can be calculated by applying Eq. (4), where

the density of the LP (qLP) can be estimated from itsempirical relationship with the LP solute mass fraction

(zs).

DV ¼ DV LP þ DV GP þ DV MP ð2Þ

DV ¼ V t � V 0

V 0ð3Þ

DV LP ¼1

V 0

� �Mt xtw þ xts

� �qtLP

�M0 x0w þ x0s

� �q0LP

� �ð4Þ

qLP ¼1

zw1þ zs1.59

ð5Þ

In the drying process, water loss and as a conse-

quence solute concentration define the total volume of

product liquid phase (LP) lost throughout the process.

The difference between the total volume loss and the

LP volume losses are due to the gas phase volume

changes (expansion or compression). The relationship

between sample volume change (DV0) and its LP volumechanges (DVLP) is observed in Fig. 4 for an impregnated

sample dried at higher microwave power. According

with Eq. (2), in this figure the points placed above the

diagonal means an increase in the gas phase and the

points located beneath the diagonal involve a loss in

the product�s pores. Despite that impregnated samples

showed very close values of DV and DVLP, since the

gas volume phase was reduced during the vacuumimpregnation pretreatment (Salvatori, Andres, Albors,

Chiralt, & Fito, 1998), it was observed a little increase

in the gas phase during the warming up period due to

the vapor expansion that dilate the structure thanks to

the plastification of the solid matrix when the polymeric

constituents are warmed up. After this period the vol-

ume decreased as the drying proceed but once again

there was an increase in the third period when it wasreached for the second time the boiling point, from this

moment onward the gas phase volume changes related

to the initial volume decreased anew.

3.2. Cryo-SEM observations

For a better understanding of overall results, apple

samples were observed under electron microscopy. InAccordance with Bomben and King (1982), observa-

tions of fresh apple tissue usually show the turgid cell

walls of parenchyma tissue as bright regions, and the

small intercellular spaces between cells as dark regions

(Fig. 5a), however, when the samples are impregnated

No VI; 10 W/g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0

Vt/Vo dXw/dt

VI; 10 W/g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0

Vt/Vo dXw/dt

VI; 3 W/g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0

Xw / Xw Vt/Vo dXw/dt

No VI; 3 W/g

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0

Vt/Vo dXw/dto

Xw / Xwo

Xw / Xwo

Xw / Xwo

Fig. 3. Relative volume changes (d) and drying rate (�) along the drying process of apple cylinders dried with air at 40 �C combined with different

levels of microwave energy.

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

∆ VLP

∆ V

IV III II I

Fig. 4. Relationship between sample volume change (DV) and its LPvolume change (DVLP) of an impregnated apple cylinder, during air

drying at 40 �C combined with 10 W/g of microwave power.

with an isotonic solution intercellular spaces appear

filled with the solution but not significant changes seem

to occur to the cells due to the vacuum treatment (Fig.

5b) (Martınez-Monzo, Martınez-Navarrete, Fito, &Chiralt, 1998).

In Fig. 6 a micro structural profile of an apple cylin-

der dried until 77% humidity content in wet basis (sec-

ond maximum in the drying rate curve) with 10 W/g of

microwave incident power can be observed.

It is shown that the tissue in the inner part of the cy-linder appear more destroyed than in the zone closer to

the interface, in this area it was still possible to distin-

guish some turgid cells and intercellular spaces, on the

contrary, in the inner zone the intercellular spaces disap-

peared due to the flow of the vacuolar sap from the cells

outwards.

The same effect was observed when drying impreg-

nated samples, in Fig. 7 two micrographs can be ob-served, both of them corresponding to an impregnated

sample dried at 3 W/g until 68% humidity content in

wet basis (second maximum in the drying rate curve).

The first image was taken from the zone next to the

interface, large cell separation was found and solute

concentration was higher in the intercellular spaces than

inside the cells, since water diffusivity value is higher for

free water than for compartmented water inside the cellmembranes. But in any case, cells and intercellular

spaces were easily recognized, however, in the second

image which belongs to the inner zone, the higher tem-

perature caused a higher water loss, then water and

solutes glassed together showing the continuous aspect

of the tissue observed in this micrograph.

Fig. 5. Cryo-SEM micrograph of (a) fresh apple tissue and (b)

impregnated apple tissue.

Fig. 6. Microstructural profile partially dried combining air at 40 �C with microwaves at 10 W/g incident power.

Fig. 7. Impregnated apple tissue partially dried combining air at 40 �Cwith microwaves at 3 W/g incident power: (a) interface zone and

(b) inner zone.

4. Conclusions

Mass and heat transfer equations were solved along

the drying process of apple cylinders to construct the

drying rate and dissipated power absorption curves,

both showing the same pathway which reveal that these

two transfer mechanisms are fully coupled. The process

was divided in four periods which have been describedaccording with the occurrence of different phenomena.

It was also observed that the deformation–relaxation

phenomena is also coupled with heat and mass transfer.

Due to the shape and size of the sample the focusing

effect was observed, which could be confirmed by the

micro structural analysis of the apple tissue.

Acknowledgements

The authors want to thank the microscopy service of

the Polytechnic University of Valencia for the helpful

work and comments in relation with the cryo-SEM

micrographs.

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