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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