microwave assisted hot air ventilation drying of tomato slices...drying kinetics 80 0 90.0 100.0 t...
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Microwave Assisted Hot Air Ventilation Drying of Tomato Slices
T.S. Workneh
School of Bioresources Engineering and Environmental HydrologyUniversity of Kwa-Zulu NatalSouth Africa
G.S.V. Raghavan and Y. Gariepy
McGill University, Dept. of Bioresources EngineeringMontrealMontreal,Canada
September 27-30, 2010Pretoria
IntroductionIntroduction Conventional heating or drying involves:◦ High temperature◦ Long drying times◦ Serious damage to flavour, colour and
nutrientsnutrients◦ Heating occurs by convection followed by
conduction Microwave treatment◦ Reduce drying time◦ Reduce quality degradation: Nutritional value Acceptability Acceptability Safety
Disadvantages of conventional drying◦ Thermal damage ~ proportional to the Thermal damage proportional to the
temperature and time◦ Higher temperature and longer drying time
i d t th lit causes serious damage to the quality attributes Flavour, Colour, Nutrients, , Reduction in bulk density Reduction rehydration capacity of the dried
productproductVolatile compounds are vaporised
Search for an alternative efficient drying Search for an alternative efficient drying method
Increasing interest in microwaves for gfood drying
Principle of microwave heating and dryingy g◦ Electromagnetic field energy into thermal
energy◦ Microwave heating is volumetric heating◦ Volumetric heating means that materials can
absorb microwave energy directly and absorb microwave energy directly and internally and convert it into heat◦ In microwave heating, heat is generated
h h h i l l di f throughout the material, leading to faster heating rates◦ Conventional heating where heat is usually Conventional heating where heat is usually
transferred from the surface to the interior◦ Microwave drying is caused by water vapour
diffpressure differences
Advantages of microwave drying◦ High thermal efficiency◦ High thermal efficiency◦ Shorter drying time◦ Improved product quality◦ Improved product quality◦ No problem of case hardening◦ Fast start-up and shut-downFast start up and shut down◦ Inhibition of high surface temperatures◦ Continuation of product respirationp p◦ Lowered product temperatures when
combined with vacuum drying◦ Reduction in the loss of water-soluble
components◦ Energy savings◦ Energy savings
Materials and MethodsMaterials and MethodsRaw Material and Drying processRaw Material and Drying process Ripe red and firm tomato (Marglobe)
5 hi k li 5 mm thickness tomato slices Average weighing 60 ± 2 g
S 13°C l d Storage at 13°C until drying experiment Before drying trial, the tomatoes were kept
at room temperature Initial moisture content was 94.01% The drying process was finished when the
sample reached the moisture content of 10% 10%
Microwave drying Microwave drying systemsystem
Drying Drying ModelsModels
Result and discussions Result and discussions Drying KineticsDrying KineticsDrying KineticsDrying Kinetics
80 090.0
100.0t
Hot Air-50°C
60.070.080.0
cont
ent
1.13 W/ g + Hot Air 50°C
2.08 W/ g FW + Hot Air 50°C
30 040.050.0
oist
ure
c 3.11 W/ g FW + Hot Air 50°C
10.020.030.0
Mo
0.00.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
Time (hrs)Fig. Moisture content of tomato slices changes with microwave power density and time
90.0100.0
Hot Air-40°C
70.080.090.0
ent
Hot Air 40 CHot Air-50°CHot Air-60°C
40 050.060.0
re c
onte
20.030.040.0
Moi
stur
0.010.0M
0.0 5.0 10.0 15.0 20.0Time (hrs)
F M f l h h Fig. Moisture content of tomato slices changes with drying air temperature and time
Dimensionless Moisture contentDimensionless Moisture content1.0
0.80.9
ture
Hot Air-50°C1.13 W/ g + Hot Air 50°C2 08 W/ g FW + Hot Air 50°C
0 50.60.7
ess m
oist
tent
2.08 W/ g FW + Hot Air 50 C
0.30.40.5
ensi
onle
cont
0.10.20.3
Dim
e
0.00 5 10 15
Time (hr)( )Fig. Dimensionless moisture content of tomato slices changes with microwave power density and time
Dimensionless moisture contentDimensionless moisture content1 0
0.80.91.0
istu
re Hot Air-40°C
Hot Air-50°C
0 50.60.7
less
moi
nten
t Hot Air-60°CHot Air-70°C
0.30.40.5
men
sion co
n Hot Air-80°C
0 00.10.2D
im
0.0
0 5 10 15 20Time (hr)Time (hr)
Fig. Dimensionless moisture content of tomato slices changes with drying temperature and time
Tomato slice temperatureTomato slice temperature77 0
67.0
77.0
)
57.0
ture
(C )
37.0
47.0
empe
rat
50°C + 1.13 W g-150°C + 2.08 W g-1
27.0
o sl
ice
te g50°C + 3.11 W g-1
17.00 50 100To
mat
o
i ( i )Time (min)
80
90
60
70
80
atur
e ©
50
60
tem
pera
30
40
to s
lices
10
20
Tom
at
50C 60C 70C
40C 80C0
0 50 100 150 200 250
40C 80C
Time
Drying rateDrying rate3 50
3.00
3.50Hot Air-40°CHot Air-50°CHot Air-60°C
2.00
2.50
min
)
Hot Air 60 CHot Air-70°CHot Air-80°C1.13 W/ g + Hot Air 50°C2 08 W/ FW + H t Ai 50°C
1 00
1.50
rate
(g /
m 2.08 W/ g FW + Hot Air 50°C3.11 W/ g FW + Hot Air 50°C
0.50
1.00
Dry
ing
r
0.000 20 40 60 80
D
Ti ( i )Time (min)
Fig. Drying rate of tomato slices changes with drying time
Constant rate of drying periodConstant rate of drying period0.4
Ai 40°C Ai 0°C
0 3
0.35Hot Air-40°C Hot Air-50°CHot Air-60°C Hot Air-70°CHot Air-80°C
0.25
0.3e
(g/m
in)
0.15
0.2
Dry
ing
rate
0.05
0.1
D
00 20 40 60 80
Fig. Drying rate of tomato slices changes with drying time
Time (min)
Hot Air Ventilation Drying
T t Ti (h) % d ti i DTTemperature Time (h) % reduction in DT40°C 20.5 050°C 13 3 3550°C 13.3 3560°C 9.70 5370°C 8 13 6070°C 8.13 6080°C 6.93 66
Microwave assisted hot air ventilation drying
Treatment Time (h) % reduction in DT( )50°C + 0 W/ g 20.5 050°C + 1.13 W/ g 3.3 8450 C . 3 W/ g
50°C + 2.08 W/ g 1.4 9350°C + 3.11 W/ g 1.1 95
1 0
1.2
ure
Lewis Model
0.8
1.0m
oist
unt
Lewis Model
Page Model
Henderson and Pabis Model
0 4
0.6
ionl
ess
cont
enHenderson and Pabis Model
Experimental moisture ration
0.2
0.4
imen
si
0.00 5 10 15 20 25
D
0 5 10 15 20 25Time (hr)
Fig. Experimental and calculated dimensionless moisture content of tomato slices (T = 40C)
Drying models parameters:Drying models parameters:Model Drying treatment k, min-1 R2Model Drying treatment k, min R
Lewis 1.13 W g-1 + 50°C 1.592 0.991
12.08 W g-1 + 50°C 1.914 0.993
3.11 W g-1 + 50°C 0.584 0.984g
0 W g-1 + 50°C 0.124 0.991
60°C 0.172 0.99470°C 0.210 0.978
80°C 0.246 0.976
40°C 0.086 0.989
Drying models parameters:Drying models parameters:Model Drying treatment k min-1 n R2Model Drying treatment k, min nPage 1.13 W g-1 + 50°C 0.606 0.930 0.986
2.08 W g-1 + 50°C 1.668 1.140 0.997
3.11 W g-1 + 50°C 2.066 1.122 0.9983.11 W g + 50 C 2.066 1.122 0.998
50°C 0.067 1.321 0.998
60°C 0.106 1.289 0.997
70°C 0 144 1 256 0 99770 C 0.144 1.256 0.997
80°C 0.171 1.275 0.997
40°C 0.059 1.167 0.999
Drying models parameters:Drying models parameters:
Model Drying treatment α (k), R2
Model Drying treatment ( ),min-1
Henderson 1.13 W g-1 + 50°C 0.967 0.560 0.986
& Pabis 2.08 W g-1 + 50°C 1.057 1.692 0.996
3 11 W 1 + 50°C 1 048 2 019 0 9973.11 W g-1 + 50°C 1.048 2.019 0.997
40°C 1.092 2.092 0.993
50°C 1.089 0.140 0.98560°C 0 072 0 187 0 98360 C 0.072 0.187 0.983
70°C 1.066 0.227 0.986
80°C 1.066 0.266 0.984
0.80.9
1Lewis ModelPage Model
0.50.60.70.8
cula
ted Page Model
Henderson and Pabis Model
0.20.30.4
Cal
c
Ideal trend line
00.1
0 0 2 0 4 0 6 0 8 10 0.2 0.4 0.6 0.8 1Experimental
Fig. Comparison of experimental and calculated moisture ratios of tomato slices (T = 40°C)
0 8
1.0tu
re Exmerimental, 3.11 W/ g + 50°C
Lewis ModelPage Model
0.6
0.8
s m
oist
ent
Page ModelHenderson and Pabis Model
0.4sion
les
cont
e
0.2
Dim
en
0.0
0 0 0 5 1 0 1 50.0 0.5 1.0 1.5Time (hr)
1 0
0 8
0.9
1.0
Lewis Model
Page Model
0.6
0.7
0.8
ed
Page Model
Henderson and Pabis Model
y = 0.9896x + 0.0054R² = 0.99660.4
0.5
Cal
cula
t
0.2
0.3
Ideal trend line
0.0
0.1
0 0 0 2 0 4 0 6 0 8 1 00.0 0.2 0.4 0.6 0.8 1.0
Experimental
Colour changesColour changesDrying Treatment L* (lightness or
darkness)a* (redness) b* (yellowness)
darkness)Fresh 52.78a 14.15a 12.68f
3.11 W g-1 + 50°C 43.11e 10.96d 17.80c
2 08 W 1 + 50°C 45 00d 13 01b 13 45edf2.08 W g-1 + 50°C 45.00d 13.01b 13.45edf
1.13 W g-1 + 50°C 47.89b 12.76cb 13.04ef
0 W g-1 + 50°C 48.07b 12.85b 13.96ed
60°C 46.02cd 12.06c 14.18d
70°C 45.06d 10.29ed 20.21b
80°C 39.09f 9.70e 21.71a39.09 9.70 21.7140°C 47.57cb 12.79b 13.95ed
Significance
P < 0.0001 < 0.0001 < 0.0001
R2 0.944 0.938 0.975
CV 2.308 3.559 3.983
RMSE 1.063 0.429 0.624
ConclusionsConclusionsTomato experiences all three rates of
drying y g Faster drying when MW heating is
coupled with hot air ventilation at coupled with hot air ventilation at 50°CTh d f l The drying times were significantly reduced by MW power
Better colour quality was maintained with MW assisted hot air ventilation with MW assisted hot air ventilation drying
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