Microwave Convective and Microw Nieznany

DRYING TECHNOLOGY

Vol. 22, No. 5, pp. 1217–1231, 2004

Microwave-Convective and

Microwave-Vacuum Drying of

Cranberries: A Comparative Study

P. S. Sunjka,

T. J. Rennie,

C. Beaudry,

and G. S. V. Raghavan

Department of Bioresource Engineering, Macdonald Campus

of McGill University, Quebec, Canada

BioEnvelop Agro Inc., Quebec, Canada

ABSTRACT

Two drying methods of cranberries (microwave-vacuum and micro-
wave-convective) are reviewed, and their advantages and disadvan-
tages regarding the quality of dried product and the process
performance are presented. Mechanically and osmotically pretreated
berries were subjected to drying and quality evaluation. Quality
parameters are color (in Hunter L

coordinates), textural

characteristics (toughness and Young’s modulus), and organoleptic

*Correspondence: G. S. V. Raghavan, Department of Bioresource Engineering,
Macdonald Campus of McGill University, 21 111 Lakeshore Road, Ste-Anne-de-
Bellevue, QC, Canada H9X 3V9; E-mail: raghavan@macdonald.mcgill.ca.

1217

DOI: 10.1081/DRT-120038588

0737-3937 (Print); 1532-2300 (Online)

www.dekker.com

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properties (color, texture, taste, and overall appearance). Special
emphasis was given to the energy performance of the process,
monitoring of the real-time temperature profile, and the total
microwave power-on time. Two microwave power densities are
assessed, as well as different microwave power-on/power-off cycling
periods. In almost all observed parameters, microwave-vacuum
drying exhibited enhanced characteristics when compared to micro-
wave-convective drying. Drying performance results (defined as mass
of evaporated water per unit of supplied energy) showed that
microwave-vacuum drying is more energy-efficient than microwave-
convective. Tasting panel results exhibited slight preference in all
parameters for microwave-convective dried samples.

Key Words:

Energy efficiency; Dehydration; Fruit; Hybrid drying;

Quality analysis.

INTRODUCTION

The modern food industry sets strict conditions on each process

performed today. Increasing awareness of the importance in energy
savings can result in fundamental changes in traditional processes.
Drying is one of the most energy demanding processes (together with
distillation), and there is no exception. In the last decade, many studies
have focused on the improvement of convective drying, combining it with
other processes, or total replacement by another method.

Cranberry fruit and its products could be a healthy and beneficial

addition to every-day diet. High vitamin C content and proven health
benefits make this fruit a highly recommended dietary constituent,
especially for people with urinary tract diseases. Dehydration of
cranberries can make it available year-round, and guarantee a long
shelf-life; this is also possible by freezing cranberries, the traditional way
of long term storage for these berries. High moisture content of fresh
cranberries (88%, wet basis) must be reduced to between 15 and 20%
(wet basis), with water activity (a

) of less than 0.7. This level of water

activity was determined as safe regarding microbial growth, but some
enzymatic and nonenzymatic reactions still may occur.

[1]

The objective of this study is to compare available data on cranberry

drying, particularly on two drying methods: microwave-convective and
microwave-vacuum. Special importance will be given to energy aspects
for each drying method, as well as sensory evaluation of obtained
products.

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Sunjka et al.

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DRYING METHODS

Microwave-Convective Drying

The use of microwaves (MW) overcomes the usual problem of poor

heat transfer in conventional hot-air drying. In microwave-assisted
drying, heat is not transferred to, but generated in material being dried.
The energy transfer rates are much higher than in conventional drying
operations, especially during the falling drying rate period.

[2,3]

Micro-

waves can be used as an additional energy source, and therefore lower
temperatures of hot air can be applied. Microwave heating is
characterized with better penetrating effect than most of the other
methods, with ‘‘targeted’’ heating (i.e., MW energy is absorbed mostly by
water in the product), and it is easier to control than heating using hot-air
if the dryer is designed properly. MW technique is suitable to combine
with other methods, such as hot-air drying, vacuum drying, freeze drying,
heat pump drying.

[4]

Final product quality can be improved using MW,

and the drying time can be reduced up to eight times. Schiffmann

[5]

reported that these drying systems are capable of handling 300 lb (136 kg)
of product per hour with 60 kW of microwave energy at 915 MHz. One
of the problems related to MW drying equipment is certainly high
investment cost that can go up to $3500 per kW of power.

[5]

Beaudry et al.

[3]

dried cranberries using MW and hot air, and showed

that product quality is equivalent to freeze-dried cranberries. It is suitable
to combine MW with hot air because it improves both drying efficiency
and economics of the process, as shown by Tulasidas et al.,

[6,7]

in study

on drying grapes.

Microwave-Vacuum Drying

Further drying improvements can be obtained by using subatmo-

spheric pressures. Water evaporation takes place at lower temperatures
under vacuum, and hence the product processing temperature can be
significantly lower, offering higher product quality. Many comparisons
have been made between MW-vacuum drying and other systems, mainly
focusing on hot air and freeze drying.

The MW-vacuum dehydration was first used for concentration of

citrus juice.

[8]

In the food industry, MW-vacuum drying is used for drying

of pastas, powders, and many porous solids. McDonnell Company has
built a MW-vacuum drying system (MIVAC

) to dry grains; absolute

pressure ranging from 3.4 to 6.6 kPa offers moisture evaporation at

Drying Methods of Cranberries

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temperatures from 26 to 52

C. However, it was not commercially suc-

cessful due to economics. Evaporated water from the product in this
MW-vacuum drying system is removed usually by condensing using a
cooling system, fed with water.

[4]

Drying Performance

One way to evaluate the feasibility of the process is to calculate the

drying performance with the following equation, modified from
Yongsawatdigul and Gunasekaran

[9]

DE ¼

Pð

1 m

1Þ

where:

Drying performance (kg of evaporated water/J of supplied
energy)

Total time of MW power-on (s)

MW power input (W)

, m

Initial and final moisture contents (ratios, wet basis)

Initial sample mass (kg)

Equation 1 only considers the efficiency of MW systems; it does not

consider energy required to heat air, or energy required for vacuum
pump.

Pressure and power level must be correctly chosen to maximize the

efficiency. Drouzas et al.

[10]

showed that the drying rate was significantly

raised with increase of the pressure or the MW power level, but the final
quality of dried banana slices was lower. The same trend was observed in
Wadsworth et al.

[11]

drying efficiency (defined as the amount of water

evaporated from the sample divided by the amount of MW energy
entering the drying cavity) of parboiled rice was significantly influenced
by both MW power level and dryer operating pressure.

One way to counter disadvantages of MW drying such as non-

uniform heating is to operate in a pulsed mode, by alternating between
MW power-on and power-off. This permits better redistribution of the
temperature and the moisture profile within the product during power-off
times. For a given product, the MW power-on time and the pulsing ratio
should be optimized. Pulsed application of MW energy combined with
vacuum to dry cranberries has been found more efficient than continuous

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Sunjka et al.

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application.

[9]

In pulsed mode, shorter power-on time and longer power-

off time provided a higher drying efficiency, where the energy utilization
coefficient (defined as a ratio of energy absorbed by the sample and
energy absorbed by the same mass of distilled water) pulsed mode ranged
from 0.53 to 0.95, and for continuous MW application it was significantly
lower, ranging from 0.43 to 0.67. Concerning the quality properties,
continuously dried samples had a higher redness and undesirable tougher
texture than the samples dried with pulsed mode.

MATERIALS AND METHODS

The same equipment was used for both MW-convective and MW-

vacuum drying methods, shown in Fig. 1. This equipment consisted of
air blower, air heaters, magnetron device, part that monitored and
controlled reflected MW power, wave guides, vacuum pump, pressure
and MW power measuring instruments, MW chamber with balance,
temperature measuring devices such as thermocouples and optical fibres,
and data analysis system.

Air blower

Air heaters

Vacuum chamber

MW chamber

Vacuum pump

Desiccator

Vacuum meter

Balance

Data collector

Mesh

MW Generator

Figure 1.

Equipment used for MW-convective and MW-vacuum drying

experiments.

Drying Methods of Cranberries

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Microwave-convective Drying

Cranberries (Vaccinium macrocarpon) of the Stevens cultivar were

harvested by hand in Quebec and frozen. After thawing, they were cut in
halves and subjected to osmotic dehydration using high fructose corn
syrup (76

Brix) for 24 h at room temperature and 1:1 syrup-to-fruit mass

ratio. This resulted in osmotically dehydrated cranberries with 57 1%
moisture content (wet basis). Samples of 125 5 g were then subjected to
MW drying using hot air. MW power densities were 1.00 and 1.25 W/g of
initial sample mass, and MW modes were 30 s on/30 s off and 30 s on/60 s
off. All combinations were replicated three times. Samples were placed in
one layer on the sample holder (mesh) attached to a balance. Air was
heated by three 2 kW heaters and continuously blown throughout the
mesh by a 0.2 kW blower placed below the drying cavity. Air temperature
was kept at 62 2

C and superficial velocity at 1 0.1 m/s (average

values throughout the process). Temperatures of the air inlet, air outlet,
and the sample were monitored using type T thermocouples throughout
the experiment. Cranberry mass was recorded with balance, temperature
recorded using optical fibre (Fisher Scientific, Nepean, ON) inserted at
one berry placed close to the mesh center, and they were dried until
moisture content reached 15% (wet basis).

Microwave-vacuum Drying

Cranberries from the same source as above were used for MW-

vacuum drying, but they were cut in quarters and subjected to osmotic
dehydration with 2:1 syrup-to-fruit mass ratio. Nevertheless, their initial
properties were almost the same as for cranberries used for MW-
convective drying, with moisture content of 55 1% (wet basis). MW
power densities were 1.00 and 1.25 W/g per total sample mass, and
MW modes similar to MW-convective; namely 30 s on/30 s off and 30 s
on/45 s off. Vacuum was maintained at 3.4 kPa of absolute pressure by
means of vacuum pump (John Scientific Inc., Canada) whose power was
0.7 kW (Fig. 1). In this part of experiment a sample holder (mesh) was
replaced with thick glass jar which served as a vacuum chamber, and it
was hanged on a balance by means of plastic holder (bag) made of
netting with a hook. Cranberries were placed at the bottom of the jar
in a 3-cm layer. Temperature of the sample was recorded using optical
fibre thermometer inserted at one berry piece placed on the top of layer
during experiment.

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Quality Evaluation

The quality of dried cranberries depends on the initial sample

quality, drying method, and the drying conditions. Tested quality
parameters were color and textural properties (toughness and Young’s
modulus). Color was measured in L

coordinates, where L

is the

lightness (0 for black, 100 for white), a

for the red-purple (positive

values) to the bluish-green (negative values) and b

indicates the

yellowness (positive values) and blueness (negative values),

[12]

using a

Minolta Chromameter Model CR-300X (Minolta camera Co. Ltd.,
Japan). Three derived color parameters—hue angle h

, Chroma value C

and color difference E were calculated using the following equations:

arctan

2Þ

¼ ð

þ ð

1=2

3Þ

E ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ð

þ ð

4Þ

where:

5Þ

6Þ

7Þ

where subscript st represents L

, a

, and b

values of a standard

cranberry (fresh cranberry of Stevens cultivar).

The textural properties (toughness and Young’s modulus in MPa) of

cranberry samples were determined by using the Instron Universal
Testing Machine (Series IX, Automated Materials Testing System 1.16).

Drying performance DE was calculated using Eq. (1).
Sensory analysis was performed using a tasting panel of six untrained

judges that compared color, texture, taste, and overall appearance. They
assigned marks to three samples (MW-convective dried, MW-vacuum
dried, and hot-air dried) ranging from 1 to 5, where 1—unacceptable
quality, 2—poor quality, 3—medium, 4—good, and 5—excellent quality.
Judges were asked to give their remarks about each of the samples.

Drying Methods of Cranberries

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RESULTS AND DISCUSSION

Color and Textural Properties

Color parameters are presented in Table 1. It is clear that cranberries

dried with longer power-off time (45 or 60 s) had lower color differences
(E ) in three of four times than those dried with shorter power-off times
(30 s). They also showed lighter color in three of four times (L

). Higher

power level exhibited lowering of the L

value, giving darker cranberries,

and E increased with increasing of MW power density. MW-vacuum
cranberries were redder (higher a

value) and yellower (higher b

value).

Textural properties are presented in Table 2.

Table 1.

Comparison of color values of MW-convective and MW-vacuum dried

cranberries.

Drying
method

MW density

(W/g)

MW mode

(s on/s off)

MW-convective

1.00

30/30

31.0

27.4

11.9

23.3

29.8

5.9

1.00

30/60

33.2

32.0

13.8

23.3

34.9

2.4

1.25

30/30

32.5

25.0

12.5

26.5

27.9

6.4

1.25

30/60

29.6

26.8

10.8

21.9

28.9

6.6

MW-vacuum

1.00

30/30

32.4

35.2

14.9

23.1

38.2

6.0

1.00

30/45

37.0

39.6

14.7

20.3

42.3

5.9

1.25

30/30

31.6

31.3

14.4

24.9

34.5

9.0

1.25

30/45

33.9

35.3

15.6

23.8

38.6

4.0

Table

Toughness

and

Young’s

modulus

for

MW-convective

and

MW-vacuum dried cranberries.

Drying
method

MW density

(W/g)

MW mode

(s on/s off)

Toughness

(MPa)

Young’s modulus

(MPa)

MW-convective

1.00

30/30

0.0198

10.9

1.00

30/60

0.0208

12.1

1.25

30/30

0.0240

11.6

1.25

30/60

0.0214

11.4

MW-vacuum

1.00

30/30

0.0166

8.14

1.00

30/45

0.0158

7.91

1.25

30/30

0.0176

6.36

1.25

30/45

0.0171

6.30

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As can be seen from Table 2, MW-vacuum drying offers cranberries

with softer texture (lower toughness) and with lower Young’s modulus,
representing chewier samples.

Temperature During Drying

Temperature history for MW-convective, MW-vacuum, and con-

vective dried cranberries can be seen in Fig. 2. Convective drying was
performed only for comparison purpose and temperature of the heated
air was 62

C, with velocity of 1.0 m/s. Convective drying lasted for almost

4 h, however only the temperature profile during first 30 min is presented
here because the temperature remained constant thereafter. Oscillations
during both MW processes are expected, because temperature increases
during MW power-on time, and decreases during MW power-off time.
Average temperature during MW-vacuum process is slightly lower than
during MW-convective one, which is expected, because during the MW-
convective drying additional thermal energy was brought with hot-air.
The expected cranberry temperature of approximately 27

C, which is

temperature of water evaporation at 3.4 kPa

[13]

was not noticeable,

probably because of internal heat build-up and increased temperature of
the sample, indicating that the supplied MW power was too high. This

120

Time (min)

Temperature (

°C)

MW-convective

MW-vacuum

Convective

Figure 2.

Typical evolution of temperature for MW-convective, MW-vacuum,

and convective dried cranberries.

Drying Methods of Cranberries

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temperature of 27

C can be obtained with precise control of MW

power input and vacuum regulation. Heat build-up is evident because
cranberries, as most of complex systems, have poor heat diffusivity and
have nonuniform temperature profiles.

Peak of 80

C in MW-vacuum process temperature profile can be

explained with high initial moisture of cranberries, where higher water
content is responsible of absorbing more MW energy. As moisture
content decreases, the temperature of sample stabilizes.

Energy Aspects

Drying performances of two drying methods were calculated using

Eq. (1). This equation is not precise, because the total power input is
not only by MW, but also includes air blower and air heater in
MW-convective process, and the vacuum pump in MW-vacuum process.
However, for these calculations only MW power input is used, in
order to compare these results with those from Yongsawatdigul and
Gunasekaran.

[9]

Similar approach was used in Grabowski et al.

[14]

where they adapted

the following equations proposed by Kudra

[15]

for instantaneous energy

efficiency "

(ratio)

energy used for evaporation at time t

input energy at time t

8Þ

and cumulative energy efficiency " (ratio) over a given time interval:

" ¼

"ðtÞ dt

9Þ

Cumulative energy efficiencies (calculated as averaged energy consump-
tion for water evaporation over supplied energy in time t) for both MW-
convective and MW-vacuum drying are presented in Table 3. Mass and
energy balances for MW-convective and MW-vacuum drying systems
should be set in that way to incorporate all energy inputs in the system,
meaning that it should incorporate power added with hot air and air
blower in case of MW-convective drying, and the power of vacuum pump
in case of MW-vacuum drying.

In Table 3 drying performance (DE ) is presented for two compared

drying methods. MW-vacuum drying is apparently more energy-efficient
than MW-convective because DE values are higher. It is interesting to

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Sunjka et al.

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notice that drying efficiency for MW-convective increases in MW mode
with longer power-off time, and decreases in corresponding MW modes
in MW-vacuum drying. These values for drying performance are compar-
able to those obtained by Yongsawatdigul and Gunasekaran,

[9]

which

ranged from 0.2 kg/MJ for continuous MW mode to the 0.38 kg/MJ for
MW mode with the longest power-off time in MW-vacuum drying
process. The same conclusions can be drawn when comparing the
respective values of the cumulative energy efficiency (Table 3).

Sensory Evaluation

Statistical analysis for sensory evaluation was carried out using

Kruskal-Wallis’ test for nonparametric statistics.

[16]

Significance level was

0.05, experimental design was completely randomized design, and testing
of hypotheses was done using

distribution.

Drying method does not have significant influence on any of the

parameters tested (Table 4). Some differences can be observed, especially
in the overall appearance where convective drying ranked the best, and
this can be explained with small number of judges or too narrow ranking
system (only five units). Judges commented MW-vacuum dried sample as
with tough texture, with noticeable ‘‘caramelized’’ odour and burnt taste,
probably because of the nonuniform dried sample. Nonuniformity in all
parameters (above all color) was the main remark for MW dried samples,
especially for MW-vacuum dried samples.

Table 3.

MW power-on times and drying efficiencies for cranberries dried with

MW-convective and MW-vacuum method.

Drying
method

density

(W/g)

mode

(s on/s off)

power-on

time (min)

Drying

performance

(kg

water

/J) 10

Cumulative

energy

efficiency

MW-convective

1.00

30/30

75.3

0.11

0.2470

1.00

30/60

71.8

0.12

0.2590

1.25

30/30

65.4

0.10

0.2293

1.25

30/60

58.6

0.11

0.2559

MW-vacuum

1.00

30/30

44.3

0.18

0.4307

1.00

30/45

61.1

0.13

0.3123

1.25

30/30

18.3

0.35

0.8342

1.25

30/45

21.3

0.30

0.7167

Drying Methods of Cranberries

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CONCLUSIONS

Several differences were detected between MW-convective and MW-

vacuum drying and they can be summarized as follows:

Color parameters for both methods are quite similar. It can be
seen that MW power level and MW mode have more influence on
the color than application of vacuum or hot air.

Textural properties can depend on drying method, and MW-
vacuum dried cranberries showed softer texture and were less
tough than MW-convective.

Total power input was much higher during MW-convective
drying, confirming better energy consumption characteristics of
MW-vacuum process, confirmed with higher drying performance,
and cumulative energy efficiency values.

Organoleptic analysis showed that although no significant
difference was detected in all tested parameters (color, texture,
taste, overall appearance) MW-convective dried cranberries were
more appreciated by judges than MW-vacuum dried, but both of
MW drying methods were beaten by ordinary hot-air dried
cranberries. Nonuniformity of MW dried samples is very serious
problem that needs to be addressed.

NOMENCLATURE

Redness (dimensionless)

Redness of a standard (dimensionless)

Yellowness (dimensionless)

Yellowness of a standard (dimensionless)

Table 4.

Organoleptic analysis of three cranberry samples dried using three

different drying methods.

Drying method

Color

Texture

Taste

Overall appearance

MW-vacuum

3.0

2.8

3.3

2.8

MW-convective

3.2

3.3

3.7

3.0

Convective

4.0

3.7

4.0

4.2

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Sunjka et al.

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Chroma value (dimensionless)

Drying performance (kg/J)

Hue angle (degrees)

Lightness (dimensionless)

Lightness of a standard (dimensionless)

, m

Initial and final moisture contents (ratios, wet basis)

Initial sample mass (kg)

Microwave power level (W)

Total time of MW power-on (s)

Redness difference (dimensionless)

Yellowness difference (dimensionless)

Color difference (dimensionless)

Lightness difference (dimensionless)

Cumulative energy efficiency (ratio)

Instantaneous energy efficiency (ratio)

ACKNOWLEDGMENTS

The authors of this research article would like to express their
appreciation towards Natural Sciences and Engineering Research
Council of Canada (NSERC) for their financial support and to Mr.
Yvan Garie´py for his constructive recommendations during course of
this work.

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