Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods (Figiel).pdf

Journal of Food Engineering 98 (2010) 461–470

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Journal of Food Engineering

journal homepage: www.elsevier.com/locate/jfoodeng

Drying kinetics and quality of beetroots dehydrated by combination

of convective and vacuum-microwave methods

Adam Figiel *

Institute of Agricultural Engineering, Wrocław University of Environmental and Life Sciences, Wrocław, Poland

article info

abstract

Article history:

Received 5 November 2009

Received in revised form 30 December 2009

Accepted 23 January 2010

Available online 28 January 2010

Beetroot cubes were dehydrated by convective drying in hot air at 60 C and by the combination of con-

vective pre-drying (CPD) until moisture content 1.6, 0.6 or 0.27 kg/kg db and vacuum-microwave ﬁnish

drying (VMFD) at 240, 360 or 480 W. The control samples were obtained by freeze-drying (FD). The dry-

ing kinetics of beetroot cubes was described with an exponential function. VMFD signiﬁcantly reduced

the total time of drying and decreased drying shrinkage in comparison with convective method. A critical

moisture content divided the temperature proﬁle of samples during VMFD into increasing and falling

periods. At the falling temperature period a signiﬁcant increase in the colour parameters L * , a * and b *

was found. VM treated samples as well as FD ones exhibited lower compressive strength, better rehydra-

tion potential and higher antioxidant activity than those dehydrated in convection. Increasing the micro-

wave wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the

combined method.

Keywords:

Beetroots

Drying

Vacuum-microwaves

Shrinkage

Compressive strength

Colour

Rehydration

Antioxidant activity

1. Introduction

age, which is a result of tissue collapse caused by volume reduction

due to the loss of moisture as well as the presence of internal forces

( Sjöholm and Gekas, 1995; Mayor and Sereno, 2004 ).

Some novel drying methods are free of those weaknesses typi-

cal for convective drying. Nevertheless, their application in exclu-

sive form involves other problems such as low productivity, high

costs or technical inconveniences. Hence, hybrid techniques com-

posed of complementary drying methods which donate their

advantages are of the highest interest. Convective drying in hot

air is still worth consideration due to the satisfactory efﬁciency

at the initial period of dehydration characterized by relatively high

drying rate and large capacity. Therefore convective drying should

be followed by a method which can ensure adequate drying rate at

the ﬁnal period of dehydration and high quality of the dried prod-

uct. Shrinkage and texture are considered to be quality attributes

of dried product ( Rahman, 1999 ). Colour is another quality factor

of a dried product ( Yongsawatdigul and Gunasekaran, 1996 ), being

not only an indicator of the changes occurring in the material dur-

ing drying ( Maskan, 2001 ), but also an important attribute boost-

ing the attractiveness of a food product ( Soysal et al., 2009 ). The

most suitable method satisfying these requirements is drying with

application of microwaves under vacuum.

Drying with the microwave method under vacuum is a modern,

efﬁcient method of food preservation ( Men’shutina et al., 2005 ).

During vacuum-microwave (VM) drying the energy of microwaves

is absorbed by water located in the whole volume of the material

Beetroots (Beta vulgaris) are rich in valuable, active compounds

such as carotenoids ( Dias et al., 2009 ), glycine betaine, ( de Zwart

et al., 2003 ), saponins ( Atamanova et al., 2005 ), betacyanines ( Pat-

kai et al., 1997 ), folates ( Jastrebova et al., 2003 ), betanin, polyphe-

nols and ﬂavonoids ( Váli et al., 2007 ). Therefore, beetroot ingestion

can be considered a factor in cancer prevention ( Kapadia et al.,

1996 ). However, fresh beetroots are exposed to spoilage due to

their high moisture content. One of the preservation methods

ensuring microbial safety of biological products is drying ( Math-

louthi, 2001 ). Dried beetroots can be consumed directly in the form

of chips as a substitute of traditional snacks, that are rich in trans

fatty acids ( Aro et al., 1998 ), or after easy preparation as a compo-

nent of instant food ( Krejcova et al., 2007 ).

Convective drying in hot air is still the most popular method ap-

plied to reduce the moisture content of fruits and vegetables ( Lew-

icki, 2006 ), including beetroots ( Kami ´ ski et al., 2004; Shynkaryk

et al., 2008 ). However, this method has several disadvantages

and limitations; for instance, it requires relatively long times and

high temperatures, which causes degradation of important nutri-

tional substances ( Marﬁl et al., 2008 ) as well as colour alteration

( Chua et al., 2001 ). Another disadvantage of that method is shrink-

* Tel.: +48 71 3205730; fax: +48 71 3482486.

E-mail address: adam.ﬁgiel@up.wroc.pl

doi: 10.1016/j.jfoodeng.2010.01.029

462

A. Figiel / Journal of Food Engineering 98 (2010) 461–470

Nomenclature

a, b, c function parameters

A, B, C pre-drying levels

AC absorption capacity (kg/kg)

AC R absorption capacity rate (kg/kg/h)

CPD convective pre-drying

CPD–VMFD combination of convective pre-drying and vacuum-

microwave ﬁnish drying

db dry basis

dw dry weight

D R drying rate (min 1 )

FD freeze-drying

F max breaking force (N)

FRAP ferric reducing ability of plasma

FSE ﬁt standard error

k drying constant (min 1 )

L * degree of lightness

a * degree of redness

b * degree of yellowness

m A mass of water absorbed from humid air (kg)

m D mass of water removed during drying (kg)

M moisture content (kg/kg db)

M e equilibrium moisture content (kg/kg db)

M 0 initial moisture content (kg/kg db)

M R moisture ratio

MP microwave power

MRPs Maillard reaction products

P 1 ,P 2 breaking points

R 2 coefﬁcient of determination

S surface of beetroot cube cross-section (mm 2 )

t time (min, h)

T Trolox

TPTZ 2,4,6-tri(2-pyridyl)-1,3,5-triazine

V R relative volume of the dried material (m 3 /m 3 )

V volume after drying (m 3 )

V 0 volume before drying (m 3 )

VM vacuum-microwave

VMFD vacuum-microwave ﬁnish drying

Greek symbols

a signiﬁcance level

r max breaking stress (MPa)

Subscripts

a, b, c indicate signiﬁcant differences

Superscripts

u, x, y, z indicate signiﬁcant differences

being dried. This creates a large vapour pressure in the centre of

the material, allowing rapid transfer of moisture to the surround-

ing vacuum and preventing structural collapse ( Lin et al., 1998 ).

As a consequence, the rate of drying is considerably higher than

in traditional methods of dehydration ( Sharma and Prasad, 2004 ).

A decisive factor enhancing drying rate is the wattage of micro-

waves ( Andres et al., 2004; Figiel, 2006 ). The pufﬁng phenomenon,

that accompanies the rapid process of dehydration, creates a por-

ous texture of the food and facilitates obtaining a crispy and deli-

cate texture ( Sham et al., 2001 ), and in this way it reduces the

product’s density as well as shrinkage.

The VM technique has already been satisfactory applied to re-

duce the moisture content of many plant materials, such as carrots

( Cui et al., 2004 ), cranberries ( Sunjka et al., 2004 ), strawberries

( Krulis et al., 2005 ), peanuts ( Delwiche et al., 1986 ), bananas ( Mou-

sa and Farid, 2002 ), apples ( Sham et al., 2001 ), pumpkin ( Nawirska

et al., 2009 ) and garlic ( Cui et al., 2003 ). However, at the beginning

of VM dehydration the intensive water evaporation from the mate-

rial being dried may exceed the vacuum pump capacity. This

would require a reduction in the raw material subjected to drying

or application of a large vacuum installation. This problem can be

overcome by pre-drying of the material using convective drying in

hot air which is very efﬁcient in the initial period of dehydration.

As a result of pre-drying the mass loads of a VM equipment can

be radically decreased ( Hu et al., 2006 ). Pre-drying of the material

by convective method before VM ﬁnish drying (VMFD) reduced the

total cost of dehydration and improved the quality of dried toma-

toes ( Durance and Wang, 2002 ) and nutritional value of strawber-

ries ( Böhm et al., 2006 ).

No scientiﬁc work has yet been reported on the combined dry-

ing of beetroots. The combined method consisting of CPD and

VMFD (CPD–VMFD) could make a signiﬁcant contribution to the

vegetable processing industry. However, it is not obvious when

convective drying should be replaced with VM method and what

microwave wattage is supposed to be applied to ensure the opti-

mal conditions of beetroots dehydration. Therefore the aim of this

work was to determine the effect of microwave power and the

level of CPD on the drying kinetics as well as on some quality fac-

tors of VMFD beetroot cubes in terms of shrinkage, texture, colour,

rehydration potential and antioxidant activity. The assumption

that these quality factors are in some ways interrelated, due to

the decisive impact of water content on a large majority of biolog-

ical material properties, induces the necessity to explain the phe-

nomena which occur within the material subjected to combined

drying.

2. Materials and methods

2.1. Sample preparation

Beetroots of ‘‘Alto F1” variety were cultivated in a ﬁeld situated

close to Wroclaw (Poland). Roots of similar size were washed and

cut into 10 mm cubes by using of a cutter equipped with a knife

moving perpendicularly to a horizontal base. The base was covered

with thick rubber. To ensure proper size of the samples a vertical

base was ﬁxed at 10 mm distance from the action plain of the

knife. Before drying the cubes were mixed in a plastic container

and then divided into 180 g portions.

2.2. Drying

The beetroot cubes were subjected to drying with three meth-

ods. The ﬁrst method was hot air convective drying, the second

one was a combination CPD–VMFD, while the third one was

freeze-drying (FD). The beetroot cubes obtained by FD were con-

sidered as control samples. Convective drying was performed in a

drier designed and built in the Institute of Agriculture Engineering

(Wroclaw, Poland). The air temperature and velocity were 60 C

and 1.8 m/s, respectively. The portions of 180 g were spread on a

round 100 mm tray. The tray was placed on top of a drying pipe.

The convective dryer, equipped with six pipes, enabled simulta-

neous drying of six portions. In the combined method CPD was car-

ried out until the three levels of moisture content: 1.6, 0.6 and

A. Figiel / Journal of Food Engineering 98 (2010) 461–470

463

0.27 kg/kg db. Each CPD portion was divided into three equal por-

tions. The divided portions were dehydrated by VMFD in SM-200

dryer (Plazmatronika, Wroclaw, Poland) at 360 W microwave

power. Samples pre-dried until 0.6 moisture content were addi-

tionally dried by VMFD at 240 or 480 W. The pressure in the vac-

uum-drum ranged from 4 to 6 kPa and the drum was revolving

at 6 rev/min. During FD (freeze drier OE-950, Hungary) the pres-

sure was reduced to 65 Pa. The temperature in the drying chamber

was 60 C, while the heating plate reached 30 C.

The drying kinetics for convective drying and VMFD was deter-

mined on the basis of mass losses of beetroot cubes when the ini-

tial moisture content M 0 amounted to 10.25 kg/kg db. The

experiment was interrupted whenever the mass of dried cubes

was measured. The moisture ratio M R was determined from the

equation:

M R ¼ M ð t Þ M e

M 0 M e

P 1 (F 1 max )

P 2 (F 2 max )

Deformation

ð 1 Þ

Fig. 1. Typical compressive curves for a very dry beetroot sample with breaking

point P 1 and for a sample of high moisture content with breaking point P 2 .

The equilibrium moisture content M e was determined at the ﬁ-

nal stage of drying as an asymptotic value of the function ﬁtted to

the experimental points using Table Curve 2D Windows v2.03

( Nawirska et al., 2009 ).

The moisture content of dehydrated cubes subjected to quality

estimation was in the range 0.053 to 0.15 kg/kg db. This moisture

content was determined by drying the previously ground samples

in vacuum dryer (SPT-200, ZEAMiL Horyzont, Krakow, Poland) for

24 h at temperature 60 C. The result was the mean value of two

repetitions.

gently smoothed using sand paper in order to avoid local ruptur-

ing. Fig. 1 shows typical compressive curves for a very dry sample

with breaking point P 1 and for a sample of high moisture content

with breaking point P 2 . It was assumed that beyond the breaking

point the compressive force is decreasing or the increase of that

force is going along the straight line indicating the yielding period

of compression. The breaking stress was the ratio of the breaking

force to the cross-section of compressed beetroot cubes:

2.3. Temperature measurement

r max ¼ F max

10 3

ð 3 Þ

During VM drying the vacuum-drum was rotating in order to

avoid the local overheating of beetroot samples. Nevertheless,

the temperature of individual cubes differed despite of the drum

rotation. The temperature of beetroot cubes was measured with

an infrared thermometer immediately after taking them out of

the VM dryer. The external temperature of most heated cubes

was recorded. It was supposed that the temperature measured

with this method reﬂected the course of mean temperature during

drying. A direct internal temperature measurement of the cubes in

the drying chamber under vacuum is practically not possible be-

cause the measuring elements inserted into the dried material

are heated by the microwave emission.

Seven replications were performed on samples with the same

moisture content.

2.6. Rehydration test

The maximally dried samples of beetroot were subjected to a

rehydration test. Before that these samples were placed in an exs-

iccator for 3 months in order to reduce and equalise their moisture

content. The samples of ca. 0.7 g weight were placed in a WK111 340

GmbH (Germany) chamber at 21 C and 95% relative humidity. The

curves of absorption capacity were determined at certain intervals

for 50 h on the basis the weight of samples kept in the chamber

compared to the initial weight, which was almost equal to the

dry mass. Longer duration of rehydration exposed the beetroot

samples to microbial spoilage. These samples were weighed each

time, when out of the chamber, in plastic containers on a balance

with 0.001 g accuracy. Absorption capacity AC, expressing the de-

gree of water restoration in dry material resulting from absorption

of water vapour relative to water content before drying, was calcu-

lated from an equation similar to that proposed by Le Loch-Bonazzi

et al. (1992) and Lewicki (1998) :

AC ¼ m A

m D

2.4. Shrinkage

Shrinkage, which occurred during drying as a result of water

evaporation, was evaluated by determination of the relative vol-

ume of dried material. The relative volume was the ratio of beet-

root cube volume after drying to that before drying:

V R ¼ V

V 0

ð 2 Þ

The volume of beetroot samples was calculated by multiplica-

tion of three basic sizes measured with the use of a slide caliper.

Seven replications were performed on samples with the same

moisture content.

ð 4 Þ

Each experimental point was the result of three replicates.

2.7. Colour measurement

2.5. Compressive test

The compressive strength of dried beetroot cubes was deter-

mined with an Instron 5544 strength-testing machine (Instron,

High Wycombe, UK) equipped with one of two replaceable strain

gauges of 2 kN or 100 N range. The individual cubes were com-

pressed between two parallel plates with a speed of 6 mm/min.

The contact surfaces of the cubes deformed by shrinkage were

Colour of dried samples was evaluated by a Minolta Chroma

Meter CR-200 (Minolta Co. Ltd., Osaka, Japan). Instrumental colour

data were expressed as CIE L * , a * , b * coordinates, which deﬁne the

colour in a three-dimensional space: L * (dark–light), a * (redness–

green) and b * (yellowness–blueness). Samples before measurement

were ground using an electric mill. Colour measurements were

performed in triplicate.

464

A. Figiel / Journal of Food Engineering 98 (2010) 461–470

2.8. Antioxidant activity

In the initial phase of convective drying the water loss is rela-

tively rapid, whereas the successive dynamics of water loss de-

creases and drying with that method begins to be time-

consuming ( Maskan, 2000 ). Application of VMFD enabled consid-

erable shortening of the total time of drying. The time of drying

with the convective method necessary to reach a moisture content

of 0.05 kg/kg db reached ca. 420 min. With the early application of

VMFD (at microwave power 360 W) the time was ca. four times

shorter. The total time of drying was shorter when the VMFD dry-

ing was introduced earlier and the applied power of microwaves

was higher. A similar result was obtained with CPD-VMFD of pears

( Figiel et al., 2008 ).

Differentiation of Eq. (5) enabled drying rate estimation:

The total antioxidant potential of beetroot samples was deter-

mined using the ferric reducing ability of plasma (FRAP) assay as

described by Benzie and Strain (1996) . In this method a potential

antioxidant reduces the ferric ion (Fe 3+ ) to the ferrous ion (Fe 2+ );

the latter forms a blue complex (Fe 2+ /TPTZ), which increases

absorption at 593 nm. FRAP reagent was prepared by mixing ace-

tate buffer (300 l M, pH 3.6), a solution of 10 l M TPTZ in 40 l M

HCl, and 20 l M FeCl 3 at 10:1:1 (v/v/v). The reagent (300 l L) and

sample solutions (10 l L) were mixed thoroughly. The absorbance

was taken at 593 nm after 10 min. Standard curve was prepared

using different concentrations of Trolox. All solutions were pre-

pared daily. The results were expressed in l M Trolox per 100 g

dry weight. All determinations were performed in triplicate.

D R ¼ dM R

ð 7 Þ

2.9. Statistical analysis

Because the complex form of Eq. (5) is exponential,

The results obtained in the study were subjected to statistical

analysis. Standard deviations were estimated by means of Micro-

soft Excel (Microsoft Ofﬁce 2000 SR-1 Professional). Table Curve

2D Windows v2.03 (Jandel Scientiﬁc Software, USA) enabled math-

ematical modelling with the best determination coefﬁcient. The re-

sults obtained were evaluated by statistical analysis with the use of

the Statistica v. 8.0 (StatSoft, Inc., Tulsa, USA). Homogeneous

groups were determined with the Duncan’s multiple range test at

signiﬁcance level a = 0.05. The one-way analysis of variance was

applied in order to ﬁnd out if the differences in the mean values

estimated were signiﬁcant.

dM R

dt ¼ k a e k t

ð 8 Þ

Taking into consideration Eq. (5) again,

dM R

dt ¼ k M R

ð 9 Þ

and ﬁnally according to Eq. (7) :

D R ¼ k M R

ð 10 Þ

The relationship between D R and M R for convective drying and

for VMFD ( Fig. 3 ) is linear according to Eq. (10) . The slop of the

straight lines, which represent this relationship, determines the

values of the drying constant k ( Table 1 ). The linear course of

experimental points displayed in Fig. 3 conﬁrms that Eqs. (5) and

(6) describe adequately the drying kinetics of beetroot cubes.

When drying by convective method the value of D R diminished

from 0.02448 to 0.0 min 1 . However, the introduction of VMFD

at M R equal 0.16, which corresponded to 1.6 kg/kg db, resulted in

the jump of D R from 3.51 10 3 to 19.35 10 3 min 1 . The high-

est increase in D R , from 1.33 10 3 to 23.50 10 3 min 1 , was

found for VMFD at 480 W and the lowest one, from 1.33 10 3

to 10.74 10 3 min 1 , occurred for VMFD at 240 W. On the other

hand, the highest k value 747.0 10 3 min 1 was found for the

sample maximally pre-dried by convective method, while the low-

est one 124.0 10 3 min 1 for the sample minimally pre-dried. It

can be stated that the higher the value of k the higher D R for as-

sumed M R . It was found that both the increase in microwave power

and the level of CPD increased the k value.

The positive effect of microwave power on k value was ex-

pected. This expectation was supported by the results of investiga-

tions made on other biological materials such as mint leafs

( Therdthai and Zhou, 2009 ) and apples ( Figiel, 2007 ). However,

the effect of the level of CPD on D R during VMFD has not yet been

studied and needs more consideration. Namely, during convective

drying the external heating is associated with an inward gradient

of moisture. Under such drying conditions water is evacuated from

the surface of the dried material to ambient as a vapour. The sur-

face is all the time supplied with water diffusing from the deeper

and deeper parts of the material. This creates no uniform distribu-

tion of water within the material. Longer CPD enhance transforma-

tion of microwave energy into heat energy by water dipoles

located deeper and more bound with the cellular system of the

material ( Tang, 2005 ). As a result, at the beginning of VMFD the in-

ner temperature of more pre-dried material is higher than the tem-

perature of less pre-dried one of the same moisture content. This

temperature, associated with the pressure within the material

( Szarycz, 2001 ), affects the current drying rate.

3. Results

3.1. Drying kinetics

Drying kinetics of beetroot cubes dehydrated by the convective

as well as by combined method is shown in Fig. 2 . For both the

methods the decrease in moisture ratio M R in time was described

by an exponential function:

M R ¼ a e k t

ð 5 Þ

For convective drying the a value amounted to 1, and Eq. (5)

might be simpliﬁed to the Lewis’ model:

ð 6 Þ

For VMFD the a value was lower than 1 ( Table 1 ). This value was

the lower the longer was the time of convective pre-drying.

1.00

0.16

Convection

A, 360 W

B, 240 W

B, 360 W

B, 480 W

C, 360 W

0.80

0.12

0.60

0.08

0.04

0.40

0.00

105

120

135

150

0.20

0.00

100

200

300

400

500

Time (min)

Fig. 2. Drying kinetics of beetroot cubes dehydrated by convection and by

combined method (CPD until moisture contents: 1.6 (A), 0.6 (B) or 0.27 (C) kg/

kg db and VMFD at microwave power 240, 360 or 480 W).

M R ¼ e k t

A. Figiel / Journal of Food Engineering 98 (2010) 461–470

465

Table 1

Values of the parameters a, b and k of the functions describing the drying kinetics, relative volume and breaking stress of beetroot samples.

Drying method CPD level

(kg/kg db)

MP (W) Drying kinetics

Relative volume

Breaking stress

M R ¼ a e k t

V R ¼ a M þ b

r max ¼ a e b

R 2

FSE

R 2

FSE

R 2

FSE

CPD–VMFD 1.6

360

0.156 0.124 0.9949 3.440 10 3

0.653 0.432 0.9564 6.875 10 3

7.251 0.0292 0.9959 29.81 10 3

0.6

240

0.055 0.195 0.9999 0.165 10 3

0.471 0.351 0.9343 4.012 10 3

61.37 0.0173 0.9870 72.83 10 3

0.6

360

0.051 0.288 0.9984 0.811 10 3

0.585 0.385 0.9636 4.340 10 3

30.55 0.0197 0.9966 38.31 10 3

0.6

480

0.050 0.471 0.9992 0.610 10 3

0.939 0.436 0.9786 4.537 10 3

11.59 0.0234 0.9966 24.40 10 3

0.27

360

0.020 0.747 0.9982 0.376 10 3

0.720 0.386 0.9635 2.814 10 3

16.20 0.0236 0.9883 58.51 10 3

Convection

1.0 0.024 0.9994 6.455 10 3

0.322 0.215 0.9649 2.520 10 3

28.09 0.0226 0.9903 93.64 10 3

CPD = convective pre-drying, VMFD = vacuum-microwave ﬁnish drying, MP = microwave power, R 2 = coefﬁcient of determination, FSE = ﬁt standard error.

0.025

A, 360 W

B, 240 W

B, 360 W

B, 480 W

C, 360 W

0.02

0.015

0.05

0.1

0.15

0.2

0.01

Convection

A, 360 W

B, 240 W

B, 360 W

B, 480 W

C, 360 W

0.005

0.05

0.25

0.45

0.65

0.85

1.05

0.2

0.4

0.6

0.8

Moisture content (kg/kg db)

Moisture ratio

Fig. 3. Relationship between drying rate and moisture ratio for convective drying

and for combined method (CPD until moisture contents: 1.6 (A), 0.6 (B) or 0.27 (C)

kg/kg db and VMFD at microwave power 240, 360 or 480 W).

Fig. 4. Temperature proﬁle for beetroot cubes during VMFD at microwave power

240, 360 or 480 W, after CPD until moisture contents: 1.6 (A), 0.6 (B) or 0.27 (C) kg/

kg db.

It is hard to measure directly the inner temperature of the

material being VM dried. However, the assumption that external

temperature measured with an infrared thermometer reﬂexes

the inner temperature may support the considerations made

above.

Schubert (1996) clamed that with VM drying the temperature of

dehydrated banana slices was increasing reaching the highest va-

lue at the end of drying when the moisture content ranged from

0.05 to 0.08 kg/kg db. The decreasing in sample temperature at

the ﬁnal stage of drying was not recorded. In this study the peak

temperatures were found for the critical moisture contents in the

range from 0.09 to 0.15 kg/kg db. One can presume, that the course

of temperature versus moisture content depends on two phenom-

ena. The ﬁrst is the generation of heat energy by water dipoles in

microwave ﬁeld ( Tang, 2005 ) while the other one is the absorbing

of that energy by water evaporating from the surface of the mate-

rial. The increase in the material temperature until critical mois-

ture content results from the excess of the energy generated over

the energy necessary for water evaporation. Naturally, the

amounts of water generating the energy and water evaporating

are decreasing with decreasing moisture content. Beyond the crit-

ical moisture content the energy generated by water dipoles is

lower than the sum of the energy necessary for water evaporation

and that transferred from the material to the ambient of lower

temperature. However, this explanation needs further, compre-

hensive investigation.

3.2. Temperature

During VMFD an increase in the beetroot cubes temperature

was observed until a critical moisture content. Beyond that mois-

ture content the samples temperature was decreasing ( Fig. 4 ).

The peak temperature 86 C was recorded for VMFD beetroot cubes

maximally pre-dried and the lowest one, amounting to 79 C, for

VMFD cubes dehydrated with the lowest microwave wattage. It

was found that the increase in microwave wattage to 480 W in-

creased the peak temperature to 83 C. These results conﬁrm the

explanations regarding the effect of microwave power and the le-

vel of pre-drying on the drying rate. However, according to those

explanations the peak temperature of the samples pre-dried to

1.6 kg/kg db should be lower than those determined for the sample

pre-dried to 0.6 kg/kg db, while the data presented in Fig. 4 show a

reverse relationship for this case. This discrepancy follows from the

relatively long time of microwave energy accumulation by the

whole volume of those samples pre-dried to 1.6 kg/kg db. Never-

theless, the temperature gradient, resulting from the temperature

distribution within the material, is enhanced by longer pre-drying.

Another problem which needs clariﬁcation is the presence of

peak temperature. During convective drying the temperature of

material is successively increasing reaching the temperature of

hot air at the end of drying ( Roberts et al., 2002 ). Drouzas and

3.3. Shrinkage

The relative volume (V R ) can be treated as a parameter describ-

ing the ability of a material being drying to shrink – the lower V R

the higher shrinkage of the material. The V R determined for VMFD

beetroot samples was signiﬁcantly higher as compared with sam-

ples dried by convection ( Fig. 5 ), despite the noticeably high stan-

dard errors of the mean values of relative volume. These high

standard errors resulted from the anisotropic nature of biological

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