MICROWAVE-VACUUM DRYING OF CRANBERRIES:
PART I. ENERGY USE AND EFFICIENCY
J. YONGSAWATDIGUL
Seafood Laboratory
Oregon State University
Astoria. OR 97103
and
S. GUNASEKARAN'
Department of Biological Systems Engineering
University of Wisconsin-Madison
460 Henry Mall
Madison, WlS3706
Accepted for Publication October 10, 1995
ABSTRACT
Microwave-vacuum drying was investigated as a potential method for drying
cranbem'es. Cranbem'es were pretreated with either 30"B or 60 "Bhighfncctose
corn syrup solution for 24 h. They were dried using a laboratory-scale
microwave-vacuum oven operating either in continuous or pulsed mode until the
final moisture content reached 15% (wet basis). In the continuous mode, two
levels of microwave power (250, 500 W) and absolute pressure (5.33, 10.67
kPa) were applied. In the pulsed mode, microwave power of 250 W and two
levels ofpressure (5.33, 10.67kPa) were used with two levels ofpower-on time
(30, 60 s) and three levels of power-off time (60, 90, I50 s).
Pulsed application of microwave energy was more efficient than continuous
application. In both cases, drying efJiciency improved when lowerpressure (5.33
kPa) was applied. Shorter power-on time and longer power-off time provided
more favorable drying eflciency in pulsed mode. Power-on time of 30 s and
power-off time of 150 s was the most suitable setting for maximum drying
eficiency.
'Author for correspondence.
Journal of Food Processing and Preservation 20 (1996) 121-143. All Rights Reserved.
Copyright 1996 by Food & Nutrition Press, Inc.. Trumbull, CT 06611
121
122
J. YONGSAWATDIGUL and S. GUNASEKARAN
INTRODUCTION
Cranberries (Vaccinium macrocarpon Ait.) are a native fruit of the United
States. A cranberry consists of water, sugar, acids, pectin, waxy materials,
protein and other ash constituents (Eck 1990). The unique characteristics of
cranberries are high titratable acids and low reducing sugars, causing a very tart,
refreshing taste (Aurand 1989). Cranberries can be dried or processed into
various kinds of products such as cranberry sauce, cranberry juice, frozen
cranberries and other products.
Dehydration is one of the most effective methods of preserving fruit. The
moisture content of finished products ranges from 1-5% . Because of such a low
moisture content, dried fruits are not readily prone to microbial spoilage and
undesirable enzymatic reactions. Raw fruits are usually pretreated with an
antioxidant and preservative before dehydration. Soaking raw fruits in a sugar
solution before drying has been beneficial in retarding enzymatic browning
reaction. Moreover, the moisture content of the fruit significantly drops before
drying because of the osmotic pressure effect, thereby reducing drying time.
Consequently, the flavor and color of dried fruit are improved. Tsai (1977)
reported using sucrose and glycerine at a ratio of 3:2 for pretreating cranberries.
Pretreated fruits can be dried using atmospheric forced-air dryers. Drying
is performed at about 74C with air at a velocity of 2.5 m/s for 6-10 h (van
Arsdel ef al. 1973). The major concern in hot air drying is the tremendous
energy consumption and low drying efficiency. Conventional hot air drying
methods also diminish the quality of dried products. Case-hardening is common
in dried fruits due to rapid drying. As drying progresses, the rate of water
evaporation is faster than the rate of water diffusion to the product’s surface.
The outer skin becomes dry and acts as water barrier, causing a wet interior.
Furthermore, loss of volatile compounds inevitably occurs during drying. Since
the products are exposed to a high temperature for a long period, these volatile
compounds are vaporized and lost with water vapor. This causes a significant
loss of characteristic flavor in dried products. High temperature and long drying
time also degrades the product’s original color. Therefore, it is obvious that the
nature of conventional drying methods does not result in best quality and least
cost.
Vacuum drying is an alternative as it allows water to vaporize at a lower
temperature than at atmospheric conditions. Therefore, fruits can be dried
without exposing them to high temperature. Moreover, the absence of air during
dehydration diminishes oxidation reactions. Because of these advantages, the
color, texture and flavor of dried products are improved. However, vacuum
drying has high installation and operating costs (Woodroof and Luh 1986).
Ismail (1989) patented a process for producing semi-moist cranberries using the
vacuum drying process.
MICROWAVE-VACUUM DRYING PART I
123
Applying microwave energy to dry food materials seems to be an applicable
approach for coping with certain drawbacks of conventional drying (Decareau
and Peterson 1986). When microwave energy is applied to foods, heat is
generated within the product. Therefore, the temperature of the product
increases rapidly. Consequently, the rate of water removal is faster than with
conventional drying. Thus, the major advantages of microwave drying include
saving time and energy. Problems in microwave processing, however, include
product damage caused by excessive heating (Gunasekaran 1990; Ramaswamy
et ul. 1991). Physical damages to product such as scorching, off-color, case
hardening and nonuniform temperature distribution have been noticed (Datta
1990). More importantly, energy utilization is not as efficient as it should be.
These problems are due to poorly controlled heat and mass transfer during
microwave drying. Two strategies have been proposed in order to effectively
apply microwaves for drying (Gunasekaran 1990): (1) create a vacuum in the
dryer to lower the drying temperature; (2) apply microwave energy in a pulsed
manner to maximize drying efficiency since continuous heating does not
accelerate the rate of water removal when critical moisture content is reached.
The microwave-vacuum drying technique has been successfully applied to
numerous food materials such as apples, grapes, peanuts, rice, asparagus,
mushrooms and soybeans (Anon. 1990; Delwiche ef ul. 1986; Wadsworth ef al.
1990; Slater 1975).
The objectives of this investigation were to:
(1)
(2)
Study the feasibility of continuous and pulsed microwave-vacuum
drying of cranberries.
Evaluate the effects of the following factors on energy use and
efficiency of continuous and pulsed microwave-vacuum drying.
concentration of high fructose corn syrup solution (30, 60B);
microwave power level (250, 500 W);
pressure level (5.33, 10.67 P a ) ;
power-on time (30, 60 s);
power-off time (60, 90, 150 s).
MATERIALS AND METHODS
Frozen cranberries (Vuccinium macrocupon Ait.) of the Searles variety
obtained from Ocean Spray Co., Babcock, WI, were used. They were thawed
in warm water (20 f 2C) for one h, then sliced in half. The sliced cranberries
124
J . YONGSAWATDIGUL and S. GUNASEKARAN
were pretreated by soaking in a fructose solution. This is a commercial
procedure to reduce the tartness of cranberries for direct consumption. The
pretreating step took 24 h at room temperature (20 f 2C). Two levels of high
fructose corn syrup solution, 30 and 60"B,were selected from a preliminary
study and used for pretreating. The total solids of the solution were measured
using a refractometer (Abbe 3-L). The ratio of cranberries to sugar was 1:l.
The fructose corn syrup was 2655 Invertase (Corn Products, Inc.) which
contained 77.0% dry substance, 39.3% dextrose and 56.3% fructose. The
pretreated cranberries were rinsed with hot water (75 f 2C) before they were
dried to prevent stickiness due to excessive fructose on the skin.
Moisture contents of fresh (thawed), pretreated and dried samples of
cranberries were determined according to the method of AOAC (1984). The
specific heat of cranberries at various moisture contents ranging from 2-70%
(wet basis) was determined by a differential scanning calorimeter (DSC 7,
Perkin-Elmer). The instrument was set to record the specific heat from 20-56C
with a scanning rate of 4C/min.
Drying
A laboratory-scale microwave-vacuum oven (Zwag, Model Labotron 500)
was used in the experiments. In this oven, microwave power can be selected to
be either 250 or 500 W of continuous output. A thick-walled glass bell ja is
placed at the center of a turntable. The space within the bell jar can be
evacuated to obtain pressures (vacuums) in the range of 0-13.3 kPa absolute.
The oven can be operated either in pulsed or continuous mode. In pulsed mode,
the magnetron is alternately turned on and off, corresponding to a set time
ranging from 0.1-200 s.
The relationship between sample size and microwave energy input to the
sample was determined according to Schiffmann (1987) using different amounts
of distilled water. The 100 g samples of cranberries used in the experiments
were much smaller than the maximum drying capacity of the oven. Therefore,
to analyze and compare energy efficiency more accurately, the maximum
microwave energy input to the sample was assumed to be equivalent to the
energy absorbed by 100 rnL of distilled water. This is based on the fact that
dielectric properties responsible for the absorbtivity of the product are
proportionally dependent on the moisture content (Mudgett 1982). Also the
dielectric properties of distilled water are always greater than those of food
products (Schiffmann 1987; Mudgett 1982). However, we would like to caution
readers that the microwave power absorption capacity of a food material is
extremely difficult to predict. It is influenced by sample mass, geometry,
dielectric properties and position within the microwave cavity. To say that
cranberries will always absorb less than water of equal mass may not be correct.
MICROWAVE-VACUUM DRYING PART I
125
Dielectric properties (dielectric constant and loss factor) of a food are not always
less than for pure water. Padua (1993)presented dielectric data for sucrose
containing materials showing loss factor to be maximum at a moisture content
of about 0.6.Another factor which influences energy absorption is the reflection
of microwave energy at the surface of an object. Only a fraction of the
microwave energy which strikes an object will actually penetrate and be
dissipated as heat. Whether or not a wave will penetrate depends upon the angle
of incidence and the dielectric constant of the object. The lower the angle of
incidence and dielectric constant, the more likely the wave will penetrate and
heat the product. As drying proceeds, the mass, dielectric properties, geometry
and temperature of the cranberries continuously changes, making estimation of
power absorption based on that observed in water possible.
Pulsed Microwave-Vacuum Drying (PD). Factors investigated in pulsed
microwave-vacuum drying were microwave power (250,500 W), pressure
(5.33, 10.67 kPa), initial moisture content which was controlled by the
concentration of high fructose corn syrup solution (30, 60"B), magnetron
power-on times (30,60s) and power-off times (60,90,150 s). To eliminate the
insignificant factors with respect to drying parameters (drying time, energy
input, energy absorbed, drying efficiency and drying rate), a 24 screening
factorial design was initially applied to elucidate the effects of initial moisture
content, microwave power, pressure and power-on time. The power-off time
was fixed at 120 s. The results indicated that the microwave power had no effect
on energy absorption, total input energy and drying efficiency. Furthermore, the
effect of pressure on the drying parameters studied was not significant. Based
on these, experiments were conducted at a fixed microwave power of 250 W
Thus a 3 x 23 factorial design was used. All experiments were replicated twice.
Pretreated cranberry samples (100 g each) were taken in glass petri dishes
(15 cm diameter x 1.5cm deep). Moisture content, mass and temperature of the
samples were recorded. Temperature and mass were determined by a thermocouple digital thermometer (Omega, HH99T2) and an electronic balance
(Mettler, PM 4600), respectively. The sample was placed in the vacuum
chamber of the oven, and the drying was performed according to a preset power
onoff schedule. The mass and temperature of the samples were recorded
periodically during drying at the end of each power-on and power-off time by
removing the sample from the oven. The samples were returned to the oven in
time for the subsequent power-on period. Drying was terminated when the
calculated (based on preliminary tests) moisture content of the dried sample
reached 15% (wet basis). The final moisture content of the dried product was
also determined. Dried cranberries were sealed in Ziplock bags and stored at
room temperature (20 f 2C)for quality evaluation (reported in the companion
paper).
126
J . YONGSAWATDIGUL and S. GUNASEKARAN
Total energy input was a multiplication of the estimated microwave power
input and total power-on time. Energy absorbed (Q)was calculated from
changes in sensible and latent heat. Specific heat values at the initial (Ci) and
final moisture content (Cf) of the cranberries were used to calculate the sensible
heat. Heat of vaporization (h), of water at each operating pressure was used to
calculate latent heat changes. This was based on the assumption that the heat of
vaporization of cranberries was equal to the heat of vaporization of water. The
energy balance equation used is:
Drying efficiency was defined as the energy required to evaporate a unit mass
of water (MJikg). The total energy was estimated as described earlier. The
equation used is:
where
DE
t,
P
Mi
= Drying efficiency (MJkg)
= Total power-on time (s)
m,
= Initial moisture content (fraction)
= Final moisture content (fraction).
m,
= Microwave power input (W)
= Initial mass (kg)
Due to our assumption that maximum energy input to the sample was
equivalent to the energy absorbed by 100 mL of distilled water, the DE values
should be treated with caution and only used for comparison purposes.
Drying rates were calculated as quantity of moisture removed per unit time
per unit dry matter (kgh-g dry matter). The effects of variable factors on drying
parameters were analyzed using ANOVA (Box er al. 1978). Least square
multiple range test was performed to differentiate the significant effect of
power-off time. All statistical analyses were done using the Statgraphics
software.
Continuous MicrowaveVacuum Drying (CD).Parameters studied in the
continuous mode were microwave power (250,500 W), pressure (5.33, 10.67
kPa) and concentration of high fructose corn syrup solution (30, 60"B). The
MICROWAVE-VACUUM DRYING PART I
127
experiments were replicated twice. To obtain temperature and moisture profiles,
a series of samples for each treatment was consecutively dried for 1, 2,3 min,
and so on until the final moisture content was attained. This allowed the sample
to be dried continuously without interruption for mass or temperature determination at each time interval. Energy use and efficiency were calculated as
explained in the pulsed mode.
Conventional Hot-Air Drying. A convective hot-air oven (Blue M) was
used for conventional drying of the cranberries. The air temperature can be set
from 25-300Cf 2C. Pretreated cranberries (500g samples) were arranged as
a single layer in an aluminum tray (33 cm diameter x 4 cm deep). They were
dried in the hot-air oven operating at 50C until their moisture content reached
15% (wet basis).
RESULTS AND DISCUSSION
The average moisture content of the fresh, pretreated and dried cranberries
are reported in Table 1. These moisture content values have been used in
reporting results, data analysis and discussion as appropriate. The specific heat
of fresh cranberries at 20C obtained experimentally was 3.78 kJkg C,which is
very close to the value reported in the literature, 3.77 kJkg C (Hayes 1987).
The specific heat of cranberries (C,) pretreated with 30"B and 60"B fructose
solution determined at different temperatures (T, C) and moisture contents (M,
% wet basis) were fitted to the following model:
C, = c
+ aT + bM
where a, b and c are constants. The values of these constants are presented in
Table 2. The constant b is larger than a, indicating a strong effect of moisture
content on C&.
Continuous and Pulsed Microwave-Vacuum Drying
Product Temperature. The temperature of the product varied up and
down as microwave power was turned on and off, respectively. For both PD
and CD the temperature profiles at both pressures overlapped during later drying
stages. This was because product temperature was no longer controlled by
operating pressure when only a limited amount of water was available in the
cranberries. The sensible heat became a dominant factor influencing product
temperature. Besides the pressure level, power-on and -off times also influenced
product temperature.
J. YONGSAWATDIGUL and S. GUNASEKARAN
128
TABLE 1.
MEAN ( f STANDARD ERROR) VALUES OF MOISTURE CONTENT OF FRESH
AND PRETREATED CRANBERRIES
Moisture content C/.wet basis)
Cranberries
Fresh
86.70 0.34'
Pretreated With 30"B hctose solution
76.27 f 0.63b
Pretreated with 60'B hctose solution
62.33
Dried
14.74 f 0.49
* O.8Ob
L
TABLE 2.
CONSTANTS OF THE REGRESSION MODEL (CCb= c + aT + bM)FOR SPECIFIC HEAT
OF CRANBERRIES AS A FUNCTION OF TEMPERATURE (T,"C) AND MOISTURE
CONTENT (M, 96 WET BASIS)
Fructose concentration
('B) used for
CoefBciCnta
R2
pretreating
a
b
C
30
0.0046
0.0179
2.0673
0.9473
60
0.0054
0.0180
2.1558
0.9391
The final temperatures of the products dried by CD were generally higher
than those dried by PD (Table 3). In the continuous mode, an increase in
microwave power tended to increase the product temperature due to an increase
in energy absorbed by the cranberries. As expected, the temperature of the
product dried at higher pressure (10.67 kPa) was higher than the temperature
obtained from lower pressure (5.33Wa). The boiling point of water at 5.33 kPa
(34C)is lower than at 10.67 kPa (47.2C). Therefore, operating at lower
pressure allows water to evaporate at a lower temperature. However, the
product temperature was relatively higher than the corresponding boiling point,
indicating localized sensible heat build-up as drying progressed.
As would be expected, longer on times increased, and longer off times
decreased the final product temperature. The decrease in temperature with
greater off times is due to evaporative cooling and convective and radiative heat
losses from the cranberries.
TABLE 3.
(* STANDARD DEVIATION) VALUES OF FINAL PRODUCT TEMPERATURE (C) OBTAINED BY CD AND PD*
Average final moisture content of all the products dried was 1596.
MEAN
w
4
3D
5
XI
U
3
C
C
fn
c
F
>
2
0
3
c
(
130
J . YONGSAWATDIGUL and S. GUNASEKARAN
Total Power-On, Drying Time. Total power-on time and drying time for
both CD and PD are shown in Tables 4 and 5 , respectively. Total drying times
of CD were less than those of PD, but total power-on times were greater.
Drying was faster in both CD and PD operating at lower pressure. It can be
explained that a pressure gradient established inside cranberries played a vital
role in moisture removal. Wei et al. (1985) stated that vibration of water
molecules generated positive pressure inside the product being dried during
microwave heating. Hence, a lower operating pressure created a larger pressure
gradient accelerating moisture removal. An increase in microwave power
resulted in a decrease in drying time for CD. It is likely that temperature
gradient is another factor influencing moisture removal. Tables 6 and 7 present
the significant factors affecting drying time and total power-on time of CD and
PD, respectively. All factors, including pressure, microwave power, initial
moisture content of the cranberries, power-on time and power-off time, showed
statistically significant effects on total power-on time and drying time.
For PD, drying was faster when a longer power-on time setting was used.
This is because a greater temperature gradient is normally established in
cranberries exposed to longer power-on time. However, total power-on time of
the runs operating at a longer power-on schedule was greater than those
operating at a shorter power-on schedule. Furthermore, total power-on times
obtained from PD were significantly less than those obtained from CD. These
imply that PD utilizes energy more efficiently than CD, and a shorter power-on
time setting provides more favorable energy utilization. A longer power-off time
setting offered a longer drying time and a shorter total power-on time. This is
because diffusion of water during power-off time accelerates water removal. The
difference in total power-on time between 60 and 90 s of the power-off time
setting was not statistically significant. However a power-off time of 150 s
provided the statistically lowest total power-on time. Power-off times of 60 and
90 s were probably too close to provide a significant difference in moisture
diffusion.
Drying rates at various operating modes and conditions are shown in Table
8. The drying rate in CD mode was generally faster than in PD mode. The
drying rate of cranberries dried by CD essentially depended upon microwave
power and initial moisture content of the cranberries. The overall drying rate
increased as microwave power and initial moisture content increased. Higher
microwave power elevated drying rate by providing more energy for vaporizing
water. Cranberries with higher moisture content dried faster. It is also obvious
that the drying rate at lower pressure is faster than at higher pressure. This
result supports the role of pressure gradient in accelerating water removal as
described previously. The drying rate under PD was affected by pressure level,
initial moisture content, power-on time and power-off time as shown in Table
7.
TABLE 4.
MEAN (f STANDARD DEVIATION) VALUES OF TOTAL. POWER-ON TIME (MIN) OF BOTH CD AND PD
TABLE 5.
MEAN (f STANDARD DEVIATION)VALUES OF TOTAL DRYING TIME (min) OF BOTH CD AND PD
I d (Idkg dry
f
Denotes a statistica~ysiiplificant effect (P s 0.05).
Values reported ate mean i standard error of all experiments. For example, drying time 14.743.18 min reported for 5.33
kPa is the average i standard error of drying times of all experiments at different moisture contents and power-on and -off
times.
Drying rate (kglhlkg dry matter)
Drying efficiency ( M J k g water)
Energy utilization
Energy input x
matter)
Energy absorbed x lo3 (Hkg dry
matter)
Drying time (min)
Dtying parameter
TABLE 6.
EFFECTS OF OPERATING CONDITIONS ON THE DRYING PARAMETERS OF CD
c
W
Denotes a statistically significant effect (P s 0.05). The same letter within a row indicates no statistically significant
difference at P s 0.05 (Least square multiple range test).
Values reported are mean f standard m o r of all experiments. For example, drying time 38.233.82 min reported for 5.33
kPa is the average f standard error of drying times of all experiments at different moisture contents and power-on and -off
times.
TABLE 7.
EFFECTS OF OPERATING CONDITIONS ON THE DRYING PARAMETERS OF PD
?z
i?
&
z
Q
C
v,
n
e,
5
U
-n
24
AP
z
0
-e
L.
P
'd
e
76
10.67
5.33
8.85M.09
5.49M.70 4.2M.37 3.71M.31
3.41M.33 6.19M.19 5.4M.42 4.4739.33
11.83iO.25 4.39M.60 3.35M.44 2.99M.38
9.73iO.14 12.63M.36 4.6W.19 3.8W.25
TABLE 8.
MEAN ( f STANDARD DEVIATION) VALUES OF DRYING RATE OF BOTH CD AND PD
136
J. YONGSAWATDIGUL and S. GUNASEKARAN
Energy Input and Energy Absorbed. Energy input was calculated based
on the total power-on time. Therefore, all the observed trends for the total
power-on time were also true for the energy input. As mentioned earlier,
because of the small sample size used in this study (100 g) compared to the
maximum capacity of the microwave-vacuum oven, total energy input to the
cranberries was assumed to be the energy absorbed by 100 mL of distilled
water, which was equal to 273.7 and 304.5 W at 250 and 500 W of microwave
power settings, respectively. These estimated microwave power inputs were used
to calculate the energy input. As previously mentioned, due to the difficulties in
determining the actual energy input accurately, these calculations should be
treated as an estimate for comparison purposes only.
The total energy input for both CD and PD at various operating conditions
are presented in Table 9.In general, drying at a lower pressure requires shorter
total power-on time. This results in less energy input. The effect of microwave
power in CD shown in Table 6 also suggested that higher microwave power
required less energy input. This is because higher microwave power accelerates
moisture removal by creating higher product temperature. Consequently, drying
time and energy input at a high microwave level are smaller.
Power-on and -off time also affected total energy input as shown in Table
7. Shorter power-on time and longer power-off time settings required less
energy input since water was allowed to redistribute within the cranberries.
Water removal was accelerated by both temperature gradients established during
power-on time and water diffusion during power-off time. Thus, the energy was
used more efficiently. Energy input of CD operating at 250 W and PD operating
at various power-on and -off times were statistically compared, as shown in
Table 10. The effects of initial moisture content and drying methods are also
presented in Table. 5. It is seen that the CD needs larger energy input compared
to the PD. In addition, setting power-on time at 30 s and power-off time at 150
s offered the most efficient energy utilization. It is important to note that
power-off time should be prolonged to match an increased power-on time in
order to optimize the energy input. Interestingly, the ratio of power-off time to
power-on time was more meaningful. The energy input decreased as the ratio
of power-off time to power-on time increased from 1:l (60s:60s), 1.5:l
(90s:60s),2:l (60s:30s),2.5:l (150s:60s),3:l (90~:30s)
to 5:l (150~30s).
Energy absorption was calculated from the heat energy required to dry
cranberries to about 15% moisture content (wet basis) as shown in Eq. 1.
Conduction, convection and radiation heat losses during drying were neglected.
Table 9 shows the energy absorption of CD and PD at various operating
conditions. Only the initial moisture content of cranberries statistically affected
energy absorption.
Energy absorption continuously decreased as drying progressed. At the
beginning, higher moisture cranberries absorbed more microwave energy. As
TABLE 9.
MEAN (+ STANDARD DEVIATION) VALUES OF ENERGY ABSORBED. ENERGY INPUT AND ENERGY UTILIZATION COEFFICIENT
OF VARIOUS TREATMENTS IN BOTH PD AND CD
O.SQto.00zd
6.83M.Zb
I
I
0.90M.002e
5.93M.25'
I
I
PD
0.6Uo.00Zb
8.7W.25'
I
7.24M.ZSb
I
0.63M.002b
0.75M.002*
I-I
8.11M.25'
off 150 s
l
a
Mean values standard error of all dryins conditions at 250 W. The same letter within a row indicates no statistically
significant differencebetween means at P s 0.05 (least squares multiple range test).
*
TABLE 10.
COMPARISON OF DRYING PARAMETE= BETWEEN CD AND PD*
z
5
C
n
v1
s
e
z
U
5
?
MICROWAVE-VACUUM DRYING PART I
139
moisture content decreased, the energy absorption was limited by the residual
moisture present in the cranberries. Dielectric properties of the product being
dried are proportionally related to moisture content (Mudgett 1982). Consequently, the ability of the product to absorb energy decreases with moisture content
during drying. Also, sensible heat and latent heat changes of cranberries
decrease as drying proceeds. This is due to a decrease in specific heat and
amount of water vaporized. Therefore, the energy required to increase the
product’s temperature at lower moisture content is less than at higher moisture
content. From energy absorption characteristics, it can be perceived that
supplying microwave energy continuously is inefficient and can damage the
product by excessive heating during the later states of drying.
Energy Utilization Coefficient and Drying Efficiency. From Table 9, it
is seen that energy utilization coefficients of CD are less than for PD. Factors
affecting this parameter and drying efficiency of CD and PD are shown in
Tables 5 and 6. In CD, energy utilization was greater when lower pressure and
higher microwave power were applied. Wadsworth et al. (1990) stated that the
reflection of microwave power was less at lower pressure and higher microwave
power level. More microwave energy, therefore, was available to be absorbed
by the cranberries. At higher pressure, energy loss was caused by condensation
and revaporization. It was noticed that the interior of the bell jar was hardly wet
at the end of the drying under lower pressure, while it was full of condensing
water when higher pressure was applied. At the higher pressure, the interior
surface did not seem to reach boiling temperature (47.2C). On the other hand,
condensation and revaporization did not occur at 5.33 kPa because the boiling
temperature at this pressure (34C) was low enough for complete vaporization.
Because of this energy loss, energy utilization coefficient and drying efficiency
at higher pressure were less than at lower pressure.
Besides the above factors, power-on time inversely affected energy
utilization coefficient and drying efficiency of PD. As described earlier,
temperature gradient is a predominant driving force for water removal when a
longer power-on time is applied. Thus, energy is used for increasing product
temperature and removing water. On the other hand, a shorter power-on time
gradually increases product temperature and allows water to relocalize within the
cranberries. Therefore, energy input is mainly utilized for removing water rather
than increasing product temperature. Consequently, drying efficiency at a shorter
power-on time (30 s) is higher. Unlike the effect of power-on time, a longer
power-off time offered higher energy utilization coefficient and drying
efficiency. Water diffusion was allowed to take place at a longer power-off time
and water was readily removed. Drying efficiencies of the runs operating at
power-off times of 60 and 90 s were not significantly different, but they were
140
J . YONGSAWATDIGUL and S. GUNASEKARAN
significantly different from those operating at 150 s of power-off time. As
mentioned earlier, power-off times of 60 and 90 s might be too close to provide
statistically significant differences.
Since the quantity of cranberries used in conventional drying was small (500
g) compared to aconventional dryer's maximum load, drying efficiency obtained
from those experiments were unsuitable for comparing to those obtained from
microwave-vacuum drying, For this reason, the drying efficiency of prune
dehydration reported in the literature (Hayes 1987;Thompson er al. 1981)was
used to compare the efficiency of conventional and microwave-vacuum drying.
The conditions and physical properties of prunes and cranberries are shown in
Table 11. The similarity of properties and initial and final moisture contents
allowed use of the drying efficiency reported in the literature for prunes as
representative of the drying efficiency for cranberries in the conventional hot-air
method. Mean drying efficiencies of CD and PD shown in Table 12 (2.51-5.62
MJlkg) were more efficient than the drying efficiency obtained from hot-air
drying of prunes, which ranged from 4.54-6.50MJkg (Thompson er al. 1981).
Drying efficiencies obtained from PD were better than those obtained from CD.
This suggests that allowing water to diffuse through the cranberries during
power-off time is a practical strategy to maximize drying efficiency (Gunasekaran 1990).
FitS
spscifichert
(kJ/kE
'c)
Initial moisture content
(% wet basis)
F i i moiature content
6 wet basis)
Pruner
3.68.
69.4-71. lb
17.2b
Cranberries
3.78
60.0-75.0
15.0
'Data obtained from Hayes (1987).
'Experimental data reported by Thompson ef al. (1981).
The most favorable combination of power-on and power-off times obtained
in this study were 30 and 150 s, respectively. The mean value of drying
efficiency of this condition was 2.66 MJkg (presented in Table lo), which
indicated an improvement of about 40-60% over conventional drying and about
46% over continuous microwave-vacuum drying (4 -90 MJkg). Drying
TABLE 12.
MEAN (f STANDARD DEVIATION) VALUES OF DRYING EFFICIENCY OF BOTH CD AND PD
M
2
9
P
tr
2
d0
5
$
bC
9
ii
z
142
J. YONGSAWATDIGUL and S. GUNASEKARAN
efficiencies reported in this study were higher than those for conventional drying
of food grains, which ranged from 4.5-8.0 MJ/kg (Gunasekaran 1986). It may
be possible to further increase the drying efficiency of PD by optimizing
power-on time and power-off time combinations.
CONCLUSIONS
Based on the results of this study, the following conclusions are drawn.
(1)
(2)
(3)
Microwave-vacuum drying is a feasible alternative for drying cranberries.
In the continuous mode, 5.33 kPa pressure and 500 W microwave
power provide more favorable drying time, energy input, drying
efficiency and drying rate. Drying efficiency and drying rate range
from 3.59-5.02 MJkg of water and 4.52-12.63 kgh-kg dry matter,
respectively.
Pulsed microwave-vacuum drying is more energy-efficient than the
continuous mode. Total drying time and power-on time are shorter
when a lower pressure is used. Energy is utilized more efficiently when
shorter power-on and longer power-off times are applied. The most
suitable power-on and -off time settings of those studied at 250 W are
30 s and 150 s, respectively. Drying efficiency and drying rate of
pulsed mode range from 2.51-4.49 MJ/kg of water and 1.49-6.19
kgh-kg dry matter.
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