Microwave Vacuum Drying of Cranberries P
Microwave Vacuum Drying of Cranberries P
Microwave Vacuum Drying of Cranberries P
J. YONGSAWATDIGUL
Seafood Laboratory
Oregon State University
Astoria, OR 97103
and
S. GUNASEKARAN'
ABSTRACT
Journal of Food Processing and Preservation 20 (1996) 121-143. All Rights Reserved.
0 Copyright 1996 by Food & Nutrition Press, Inc., Trumbull, CT 06611 121
J. YONGSAWATDIGUL and S. GUNASEKARAN
INTRODUCTION
Drying
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 S~trikes 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.
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 (kg/h-g dry matter). The effects of variable factors on drying
parameters were analyzed using ANOVA (Box et 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.
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 kJ/kg C, which is
very close to the value reported in the literature, 3.77 kJ/kg C (Hayes 1987).
The specific heat of cranberries (Cb) pretreated with 30*B and 60*B fructose
solution determined at different temperatures (T, Q and moisture contents (M,
% wet basis) were fitted to the following model:
C,b = 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 Cb.
Total Power-On, Drying Time. Total power-on time and drying time for
both CD and Pl) 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 Pl) 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 I'D 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 th e 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 PI) was affected by pressure level,
initial moisture content, power-on time and power-off time as shown in Table 7
TABLE 4. MEAN (± STANDARD DEVIATION) VALUES OF TOTAL POWER-ON TIME (MIN) OF BOTH CD AND PD
PD
Initial moisture Pressure CD
content (kPa) Power-on 30 s at 250 W - Power-on 60 s at 250 W
(*/o wet
basis)
250 W 500 W Off 60 s Off 90 S Off 150 s Off 60 s Off 90 S Off 150 s
5.33 15.5±0.5 11.510.5 10.3±0.3 9.8±0.3 8.5±0.5 12.0±0.0 12.0*0.5 10.5±0.5
62
10.67 17.5±0.5 12.510.5 11.510.5 10.5±0.5 9.0±0.5 14.5*0.5 14.0±0.0 13.0±0.5
5.33 19.0±1.0 13.8+-0.3 12.5±0.5 12.0.+0.3 9.5±0.2 14.5.+0.5 13.50.5 12.0±0.3
76
10.67 20.0.+0.3 15.0-+0.5 14.5±0.5 13.3-+0.3 10.5-+0.3 17.0±0.5-1 16.5±0.5 13.0±0.5
TABLE 7.
EFFECTS OF OPERATING CONDITIONS ON THE DRYING PARAMETERS OF PD
0
Denotes a statistically significant effect (P :5 0.05). The same letter within a row indicates no statistically significant
difference at P & 0.05 (Least square multiple range test).
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 1:1 (60s:60s), 1.5:1 (90s:60s),
2:1 (60s:30s), 2.5:1 (150s:60s), 3:1 (90s:30s) to 5:1 (150s: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 10.
COMPARISON OF DRYING PARAMETERS BETWEEN CD AND PD*
PD
Drying parameter CD on 30 s on 30 s on 30 s on 60 a on 60 s on 60 s
off 60 s off 90 s off 150 3 off 60 s off 90 s off 150 s
rying time (min) 17.8:kl.32a 36.611.32c 45.5:hl.32' 56.311.320 29.011.32~ 35.0-+1.32c 42.411.32' Z
Q
W
otal power-on time (min) 17.8*0.36d 12.2+0.36" 11.4+0.366 9.4±0.36a 14.510.36" 14.00.36c 12.110.36" >
:14,
nergy absorbed x 103 >
J/kg dry matter) 5.070.ir 5.420. 1 r 5.44+0.17a 5.47*0.ir 5.44*0. 17" 5.2010.ir 5.50+0.178
nergy input x 103 W/kg 10.0310.25d 7.3410.25b 6.93+0.25b 5.9310.25a 8.7010.25c 8.11+0.250 7.24+0.25b
y matter)
nergy utilization +000 +0 ad
0.50+0.0021 0.74+0.002c 0.8W.002d 0.9010.0020 0.62*0.002b 0.63 . 2b 0.75 .002
efficient
rying efficiency (Ml/kg 4.90+0.10d 3.27+0.16b 3.01+0.10b 2.66*0.10a 3.88*0.100 3.86-0.10c 3.2610. 1 ob
ater)
rying rate (keAg dry 7.03*0.lr 1 3.670. 19C 2.95*0.10 2.4010.19a 4.65*0. 19d 3 70.+0.19c 3.24+0.19be
atte~ I I,. I * I
Mean values ± standard error of all drying conditions at 250 W. The same letter within a row indicates no statistically significant
difference between means at P :5 0.05 (least squares multiple range test). -
MICROWAVE-VACUUM DRYING PART 1 139
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.
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.
REF7,RENCES