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Research Paper: PH—Postharvest Technology

Effect of vacuum-microwave drying on selected mechanical
and rheological properties of carrot

Bogdan St˛epien´

Agriculture University of Wroc!aw, Institute of Agricultural Engineering, 51-630 Wroc!aw, Che!mon´skiego 37/41, Poland

a r t i c l e

i n f o

Article history:

Received 14 November 2006
Accepted 12 October 2007

Available online 11 December 2007

Carrots were dried using the vacuum-microwave method and strength tested using an
Instron 5566 with measuring heads of class 0.5. The values of the cutting and the
compression forces were calculated. The testing was performed on materials that were
initially blanched, osmotically dehydrated, and untreated before drying. As a result of the
vacuum-microwave drying, dried carrots with a moisture content within the range
(3.2–3.8)% were obtained. The blanching operation resulted in an almost two-fold increase
in the dry matter resistance to compression compared to that of the dry matter obtained
from initially untreated carrot. Osmotic dehydration reduced the product resistance to
compression more than two-fold. A five-element Maxwell model adequately described the
reduction in stresses with time. The values of the modulus of elasticity and the coefficients
of dynamic viscosity for the rheological model were calculated. The highest values of the
modulus of elasticity were obtained for the raw material subjected to blanching.

&

2007 IAgrE. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The competition dominating the marketing of food products
makes it necessary for producers to meet the food quality
requirements imposed by consumers. A characteristic feature
of agricultural production is the seasonal nature of produc-
tion. To supply marketable farm produce throughout the year,
it should be preserved with the aim of preventing the
unfavourable changes that alter its appearance, generate loss
of taste and aromatic qualities, cause loss of nutritive
constituents, and produce other undesirable changes (e.g.
drying shrinkage). Convective drying is the most frequent,
industrial method of food preservation. It is the simplest to
perform, but at the same time it is considered to be the most
destructive, resulting in a product of much reduced quality
when compared to the raw material. There is therefore
a need to thoroughly investigate alternative drying methods,
which may allow cheap food of high quality to be produced
throughout the year. Besides sublimation drying, vacuum-

microwave drying appears to be a very promising method.
With the supply of thermal energy directly to the entire
volume of the object, microwaves make it possible to control
the internal temperature of the material, irrespective of the
external conditions. The heat evolves due to the friction
produced as a result of rotation of dipoles in the material
(

Szarycz, 2001

). Pre-drying treatments of the material can also

influence product quality, with blanching and osmotic
dehydration being used the most often. The basic aim of
blanching before drying is to deactivate tissue enzymes via
thermal denaturation of protein carriers. The osmotic dehy-
dration before drying reduces the moisture content of the raw
material and, as a result of an exchange of mass between the
biological material and the osmoactive solution, the chemical
composition of the product changes. The effect of blanching
of the raw material on the properties of carrots obtained by
various methods of drying was investigated by

Mazza (1983)

and

Prakash et al. (2004)

. It was found that blanching of

carrots before convective drying significantly affected the

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1537-5110/$ - see front matter & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved.
doi:

10.1016/j.biosystemseng.2007.10.013

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B I O S Y S T E M S E N G I N E E R I N G

9 9 ( 2 0 0 8 ) 2 3 4 – 2 3 8

transport of moisture and the product quality (

Mazza, 1983

).

Carrot dried in a fountain bed is characterised by better
colour, better rehydration properties, and better retention of
b

-carotene compared to carrots dried by microwaves or by

convection (

Prakash et al., 2004

).

Ghosh et al. (2004)

examined the kinetics of mass exchange

during the osmotic dehydration of carrot slices. They applied
three different salt concentrations: 5%, 10% and 15%. The
ratio of the raw material mass to that of the solution was 1:5.
The decrease in the carrot moisture during dehydration
varied nonlinearly, and was higher in the first stage of the
process for all the solution concentrations than in the
subsequent time intervals. A model was proposed that
predicted the level of the total mass exchange, based on the
experimental data acquired during the short time of osmotic
dehydration.

Kowalska and Lenart (2001)

investigated the effect of

osmotic dehydration time on the moisture content of selected
vegetables. A 61.5% sugar solution was the osmoactive
solution. The process was conducted at 30 1C up to 180 min.
They found a significant dependence of the osmotic dehydra-
tion process on the plant tissue type. The most significant
variations of the moisture content were observed during the
first 30 min of the dehydration process.

The possibility of using combined drying methods for food

products was also examined.

Litvin et al. (1988)

dehydrated

carrot slices by sublimation down to a 40% moisture content.
They also applied microwaves for 50 s, and obtained a final
moisture content of 5% by convective drying. They did not
find significant differences in the colour of the product, the
drying shrinkage, or in the rehydration properties between
the combined method and the sublimation method.

Studies on the kinetics of vacuum-microwave drying of

carrot slices were conducted by

Zheng-Wei Cui et al. (2004)

.

They proposed a theoretical model based on energy con-
servation of the sensible head, latent heat and source head of
microwave power. The model was subjected to verification
based on experimental data. The best fit was obtained at a
dry-basis moisture content of 2%. The lower the water
content, the greater the difference between the experimental
data and the values calculated on the basis of their theoretical
model.

Wang and Xi (2005)

performed investigations that allowed

to determine the drying characteristics and to assess the
quality of a product obtained as a result of a two-stage
microwave drying. With decreasing thickness of the samples
the energy consumption in the process decreased and the

dehydration rate increased. The thickness of the carrot slices
and the energy consumed in the first and second microwave
drying stages significantly affected the content of b-carotene
and the degree of dry mass rehydration.

Differences in the drying rate and in quality of the product

obtained with the convection and microwave methods were
examined by

Prabhanjan et al. (1995)

. Microwave drying

shortened the process duration by 25–90%. However, the
product quality increased with reduced microwave power
during the drying process.

The perceptions that a person consuming food products

receives depend, to a large degree, on the stimuli received
during biting and chewing the food. The sensations con-
nected to biting depend on the food’s resistance to cutting.
During chewing the consumer receives stimuli connected
both with the mechanical strength of the product eaten and
its rheological properties. It is difficult to unequivocally
estimate the effect of changes in the mechanical and
rheological properties on the consumer’s acceptance or
rejection of a product, because it is within the sphere of the
consumer’s taste. However, knowledge on the quality of food
products, judged on the basis of the criteria dealt within the
present paper, is necessary.

The aim of this work was to determine the effect of

vacuum-microwave drying on: resistance to compression,
resistance to cutting and on the stress relaxation process in
dried carrots obtained by blanching and osmotic dehydration.

2.

Materials and methods

The drying of carrots by the vacuum-microwave method was
performed using a prototype laboratory installation described
at length by

Kramkowski et al. (2003)

The following para-

meters of drying were applied: pressure in the installation
within the range (4–10) kPa, power of magnetrons equal to
480 W, corresponding to 40% of their maximum power. The
material was dried until the equilibrium moisture content
was reached. Samples were prepared in the form of cylinders
20 mm in diameter and 5 mm in height. A strength-testing
machine, type Instron 5566, with measuring heads of class 0.5
and a maximum load amounting to 100 N and 1 kN, was used
for the testing. The stress relaxation and resistance to
compression tests were performed with a cylinder with
piston by compressing carrot layers arranged one over the
other in a 30 mm high column. The compression work was
calculated as the energy necessary to deform the sample

ARTICLE IN PRESS

Nomenclature

a, b, c, d, e

relaxation process parameters

E

i

elasticity modulus of the ith element of the
Maxwell model, kPa

P

p

cutting work, mJ cm

2

P

s

compression work, mJ g

1

T

i

time constant of the ith viscous element of the
Maxwell model, s

t

relaxation process time, s

s

stress, kPa

Z

i

dynamic viscosity coefficient of the ith element of
the Maxwell model, kPa s

d

i

stress in the ith elastic element of the Maxwell
model, kPa

e

relative strain

B I O S Y S T E M S E N G I N E E R I N G

9 9 ( 2 0 0 8 ) 2 3 4 – 2 3 8

235

by 20% of its original height. The penetrator moved at
1.8 mm min

1

which was within the range of quasi-static

speeds recommended by ASAE. The initial stress of the
relaxation process corresponded to that needed to compress
the sample by 20% of its original height. The stress relaxation
lasted for 15 min. An attachment of our design with a knife of
the blade and opening angles equal to 601 each was used to
test the carrot resistance to cutting. The cutter speed was
10 mm min

1

. Because the shrinkage of plant tissue on drying

substantially affects the cutting work values, they were
recalculated per 1 cm

2

of the cut surface. Because of this, it

is possible to use cutting strength as a criterion for evaluation
and comparison of different methods of preliminary treat-
ment. The moisture content of dried carrot was determined
by drying the samples in a KC 100/200 drier at a temperature
of 65 1C for 48 h. The values of the cutting and compression
work were calculated using the trapezium method. The tests
consisted of 10 repetitions of compression and cutting, and 5
repetitions of stress relaxation.

The testing was performed on raw material, and blanched

material, osmotically dehydrated and untreated before the
drying process. Carrot was blanched for 3 min in water at
95 1C. The blanching operation resulted in a ca. 11% loss of dry
mass. The osmotic dehydration was conducted in a 5%
solution of NaCl for 24 h. A decline in moisture content from
88% to 79% was obtained. At each repetition of the pre-
liminary treatment the same ratio of the mass of blanching
fluid or that of osmotic dehydration solution to the raw
material mass was maintained.

3.

Results and discussion

As a result of vacuum-microwave drying, dried material with
a moisture content within the range 3.2–3.8% was obtained.
The lowest moisture content was characteristic for the dried
material obtained from carrot untreated before the drying
process, whereas the dried material obtained from osmoti-
cally dehydrated material had the highest moisture content.

Fig. 1

presents the values of compression work (P

s

) a for

vacuum-microwave dried carrot. It was found that the
blanching operation resulted in an almost two-fold increase

in the dry material resistance to compression with respect to
the material obtained from a non-pretreated carrot. Osmotic
dehydration halves the product resistance to compression.
The reverse trend was found when analysing the cutting
strength (P

p

) (

Fig. 2

). The osmotic dehydration resulted in a ca.

40% increased cutting strength, whereas the blanching of raw
material reduced the strength of the dry material to cutting to
almost a third of that of material that was not pre-treated
before microwave drying. The differences were confirmed as
being significant by examination of the standard deviation
values shown in

Figs. 1 and 2

. This can be explained as

follows: the resistance to compression and cutting of raw
biological material arises from the hydrostatic pressure of
cellular fluids and from the resistance imposed by the
structure of the cellular walls. After drying, only the extent
of the original state of cellular walls that is preserved
determines the resistance to the interference of external
forces, both during compression and cutting. During osmotic
dehydration salt crystals penetrate into the material interior.
They cause additional damages to cellular walls, which
substantially reduces the resistance to compression. How-
ever, the cutting of the whole structure involves overcoming
an additional resistance set by particles of the osmoactive
solution. This is why there is an increase in the cutting
strength with respect to the product obtained from raw
material that was not pretreated before the vacuum-micro-
wave drying.

The possibility of describing the stress relaxation process

with the Maxwell model built of seven, five or three
elementary rheological models in succession was studied.
The value of the determination coefficient was the basis for
evaluation of the usefulness of individual versions of the
generalised Maxwell model. It was found that the five-
element Maxwell model adequately describes the drop of
stresses in time. The model, containing two parallel segments
composed of elastic and viscous elements connected in series
and an elastic element operating in parallel, was described as
follows:

d ¼

a þ be

ð

t=cÞ

þ

de

ð

t=eÞ

.

(1)

The free term of the function s ¼ f(t) represents the stress in

the independently operating elastic element of the model.

ARTICLE IN PRESS

without

pretreatment

blanching

osmotically

dehydration

0

50

100

150

200

250

Ps

[mJ g

–1

]

Fig. 1 – Values of the compression work for the vacuum-
microwave-dried carrot.

0

Pp

[mJ cm

–2

]

200

400

600

800

1000

1200

1400

1600

1800

without

pretreatment

blanching

dehydration

osmotically

Fig. 2 – Values of the cross-cutting work for the vacuum-
microwave-dried carrot.

B I O S Y S T E M S E N G I N E E R I N G

9 9 ( 2 0 0 8 ) 2 3 4 – 2 3 8

236

The parameters a, b and d are the stresses in individual elastic
elements of the rheological model. The parameters c and e are
the time constants of the stress relaxation process, char-
acteristic for individual visco-elastic elements.

The results obtained allowed a mathematical model to be

constructed that described the stress variation in time for the
proposed rheological model (

Table 1

). Based on the model, the

moduli of elasticity and the dynamic viscosity coefficients of
the viscous elements in the rheological model were calcu-
lated in the following way:

E

i

¼

d

i

,

(2)

Z

i

¼

E

i

T

i

.

(3)

Using the values listed in

Table 1

and the measurement

points, diagrams of stress relaxation were drawn for the dry
material obtained from carrot (

Fig. 3

). Straining a sample by

20% of its original height resulted in a different state of stress
inside the material that depended on the pretreatment
applied. The highest stresses appeared for carrots blanched
before the drying process (55 kPa), lower stresses were found
for the dry product from raw material untreated before drying
(40 kPa), and the lowest stresses were found for the osmoti-
cally dehydrated product before vacuum-microwave drying
(20 kPa). These stresses determined the level of load at which
the observation of the stress relaxation process started. The

best fits between the experimental data and the mathema-
tical model were obtained for the dry product obtained from
carrots without preliminary treatment and from carrots that
was osmotically dehydrated before the drying process.

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0

10

20

30

40

50

60

0

200

400

600

800

1000

time [

s]

stress [kPa]

pionts-without pretreatment

curve-without pretreatment

points-blanching

curve-blanching

points-osmotically dehydration

curve-osmotically dehydration

Fig. 3 – Stress relaxation curves for the vacuum-microwave-dried carrots.

Table 1 – Mathematical model parameters describing the stress relaxation process for the vacuum-microwave dried
carrots

d ¼

a þ b exp

t

c

þ

d exp

t

e

a

b

c

d

e

R

2

Without pretreatment

8.77

15.36

3.23

12.20

103.68

0.99

Blanching

13.22

20.93

3.03

17.83

114.11

0.97

osmotically dehydration

3.47

8.41

3.30

6.29

95.93

0.99

I

II

III

61.0

104.27

34.75

76.8

122.4

46.46

43.85

77.31

19.17

0

20

40

60

80

100

120

140

modulus of elasticity [kPa]

E

0

E

1

E

2

Fig. 4 – Moduli of elasticity of the five-element Maxwell
model (I—without pre-treatment, II—blanching,
III—osmotic dehydration).

B I O S Y S T E M S E N G I N E E R I N G

9 9 ( 2 0 0 8 ) 2 3 4 – 2 3 8

237

In

Fig. 4

, the values of the elasticity moduli of the dry

product obtained from the raw material that was osmotically
dehydrated, the raw material without preliminary treatment,
and a blanched raw material are given. The highest values of
the modulus of elasticity were obtained for the raw material
subjected to the blanching process. These values are almost
twice as high as those of the modulus of elasticity of the dry
material that was not pretreated, and ca. 3–3.5 times higher
than those of the modulus of elasticity of the dry material
obtained using the osmotic dehydration process. Both the
literature and our investigation (

St ˛epien´, 2007

) indicate that

salt crystals penetrating into the cellular structure of carrot
during osmotic dehydration cause damage to the cell walls,
which constitute a matrix upholding the structure during the
action of external forces. This causes a significant decrease in
tissue elasticity and the carrot becomes more brittle. The high
temperature in operation during blanching made the cell
walls elastic. The resistance to compression increased,
because during loading no sudden ruptures appear within
the structure, which is so characteristic for brittle materials.

Almost identical relationships were observed for the

dynamic viscosity coefficients (

Fig. 5

). The brittle material,

obtained due to the preliminary osmotic dehydration, is
characterised by lower values of the dynamic viscosity
coefficient. It is to be expected that such a product will be
very prone to macro- and micro-ruptures, which may
adversely affect its transport and storage. In this respect,
substantially better properties are obtained from blanched
carrot. Since both dynamic viscosity coefficients of the
Maxwell model are higher, the whole system has higher
viscosity.

4.

Conclusions

The preliminary treatments applied with vacuum-microwave
drying significantly affect the mechanical and rheological

properties of dehydrated carrots. Blanching viewed as a
pretreatment

before

vacuum-microwave

drying

results

in a product characterised by a doubling in compression
strength and a reduction to a third in cutting strength when
compared to the dry product that is not pretreated. The
product is more elastic and, while eating, it requires a longer
chewing. It can also resist greater loads during storage and
transport.

Carrot dehydrated osmotically before vacuum-microwave

drying is hard although brittle. It is characterised by a
reduction to a third in compression strength and ca. 40%
higher cutting strength when compared with the product
obtained from carrots that were not subjected to pretreat-
ment. The product is not elastic and is fragile. It may be
difficult to store and its direct consumption may be limited.

Acknowledgements

This work was supported by a grant from KBN (Polish
Committee of Science) No. 2 P06T 048 30.

R

E

F

E

R

E

N

C

E

S

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I

II

III

1

2

6324.5

11894.2

3333.6

248.1

370.9

153.3

0

2000

4000

6000

8000

10000

12000

absolute viscosity [kPa

*

s]

Fig. 5 – Dynamic viscosity coefficients of the five-element
Maxwell model (I—without pre-treatment, II—blanching,
III—osmotic dehydration).

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9 9 ( 2 0 0 8 ) 2 3 4 – 2 3 8

238