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16/04/2013
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Creating microstructure of dehydrated fruits and vegetables with a multi-flash drying process
João B. LaurindoBarbara Porciuncula (PhD student) Marta F. Zotarelli (PhD student) Ricardo Monteiro (MS student)
ESPCA/São Paulo School of Advanced ScienceAdvances in Molecular Structuring of Food Materials
Creating microstructure of dehydrated fruits and vegetables with a multi-flash drying process
Federal University of Santa Catarina, Florianópolis-SC, BrazilDepartment of Chemical and Food EngineeringCampus Universitário – Trindade – 88040-900 [email protected]
João B. LaurindoBarbara Porciuncula (PhD student) Marta F. Zotarelli (PhD student) Ricardo Monteiro (MS student)
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What is the (old) problem?
Tropical fruits as bananas and mangoes are perishable products with significant economic importance in many countries.
The development of solutions for their preservation and processing is necessary to reduce losses, obtain better products and add value to them
Dehydration is one of the oldest techniques of preservationHow can we choose the drying process and process variables in order to control
the microstructure of dehydrated fruits and vegetables?
Drying
Convective drying
Solar drying
Vacuum drying
Visual, color, textureSoluble solids (oBrix)Texture (penetration)
Wash, cut
Selection
Raw fruits
Convective drying: allows controlling drying rates, thermal degradation (T, t) , fruits shrinkage leads to undesirable texture properties
Solar drying: cheap and traditional, drying rate depends on weather conditions
Freeze-drying
Freeze-drying: Best dehydration process for het sensitive products, minimal shrinkage, time-consuming, energy-consuming, high cost
Processes for fruits dehydration
•CMFD•KMFD
Microwave vacuum drying
Convective-multiflash drying Conductive-multiflash drying
Creating microstructure
Vacuum drying: Reduces problems of convective drying, heat sensitive foods, does not avoid shrinkage
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What is the Convective-multi-flash drying process (CMFD)?
CMFD is a dehydration process developed for fruits and vegetables dehydration and texturization
Patent application 017110000045 - Brazil
Selection and preparation of fruit samples on a stainless steel grid Insertion of the grid+samples into a jacketed container
banana ( Musa sapientum L.) : slices with thickness of 5 mm , 26 oBrixmango (Mangifera indica L.) : square slices (30 mm), 8 mm thick, 12.5 oBrix
Heating the samples up to 60oC, at Patm, inside the container,
using hot air (70 oC) convection, conduction, radiation
60oC Application of vacuum pulse (Pabs =15 mbar) for 3-5 min flash drying-and-cooling
Recovery of the atmospheric pressure in the jacketed container
CM
FD c
ycle
CMFD – Experimental device
The CMFD process doesn’t use pressures higher than the
atmospheric pressure
Schematic representation of the CMFD process
P
T
15 mbars
350 m3/h
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P
T
KMFD – Experimental device
The KMFD process doesn’t use pressures higher than the
atmospheric pressure
350 m3/h
PID control for the plates temperature
Vacuum ovenHeated plates
60 0CFruit samples
T
Temperature (0C)
Lo
g p
ressu
re, kP
a
Heating + flash evaporation cycles
101.3kPaPat kJ/kg2250 H
Moist sample
Container/flash evaporationPatmVacuum
Psat (T)
^
v
p
w
H
Tcmm
Water evaporation at each cycle, considering flash evaporation as an adiabatic process
P=1013 mbar V=1.6 m3/kg
P=15 mbar V=70 m3/kg
texturization
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The puff-drying is performed by submitting the product partially dehydrated to
pressures of 6-8 atm and high temperatures, followed by a sudden
decompression to the atmospheric pressure, which creates a product with
an open structure
It was developed in order to dehydrate fruits and vegetables at large scale,
which could be quickly reconstituted, with operating costs comparable to
convective drying
Pre-dehydration of the product to be submitted to the puff-drying is
essential to avoid product disintegration during the sudden decompression,
(THAKUR e THAKUR, 2000; LOUKA and ALLAF, 2002; IGUEDJTAL et al.,
2007; MUJUMDAR, 2007).
It is used also for processes intensification (oil extraction)
LOUKA and ALLAF, 2002
CMFD, KMFD X SIMILAR PROCESSES
Puff drying
SOME RESULTS OF BANANA AND MANGO DRYING
USING THE CMFDCONVECTIVE MULTIFLASH DRYING PROCESS
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Convective-multi-flash drying process – CMFDMovie banana – flash drying
Convective-multi-flash drying process – CMFDMovie mango – flash drying + texturization
P=15-20 mbarWater boiling at approximately 15 oC
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Convective-multi-flash drying process – CMFD (banana)
Evolution of the pressure in the jacketed container and fruit temperature during the CMFD process applied to banana samples
Evolution of experimental moisture content of banana during the CMFD process and the estimated contribution of flash-evaporation to the whole drying process
Evolution of banana samples water activity during the processing.
Water activity
Drying curve
Flash-drying contribution
Convective-multi-flash drying process - CMFD
After flash evaporation (= flash cooling) heat transfer from hot air (70 0C) to the
fruits is improved by the high temperature difference between hot air and cooled fruits
During flash-evaporation part of the internal moisture is drained to the fruit surface
and improve evaporation during the heating step (convective drying)
CMFD is a very efficient
dehydration process
P=15-20 mbar aprox. 15 oC P=15-20 mbar aprox.15 oC
Drying of banana
60 oC
15 h6-7 h3 h
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Pictures of mango fruits after every heating-vacuum pulse cycle
Pictures of mango fruits after 12
cycles of heating-vacuum pulses
CMFD cycles
Some pictures of mango during dehydration by CMFD
SEM of freeze-dried mango - 20x
SEM of mango dehydrated by CMFD – 20x
SEM of oven-dried mango – 20x
DEHYDRATED MANGO
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Puncture tests of dehydrated mango
0
5
10
15
20
25
30
35
0 10 20 30 40 50 60 70
Fo
rce (
N)
Strain (%)
Dried by hot air
Dried by CMFD
Freeze-dried
Jagged curves crispy foodsNumber of peaks, fractal dimension
SEM of freeze-dried banana- 20x
SEM of banana dehydrated by CMFD – 20x
SEM of oven-dried banana – 20x
DEHYDRATED BANANA
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0
2
4
6
8
10
12
14
0 10 20 30 40 50
Deformação Relativa (%)
Fo
rça
(N
)
Banana "in natura" seca por 12 CAPV
Banana Liofilizada Comercial
Dried by CMFD
Freeze-dried Strain (%)
Forc
e (
N)
Puncture tests of dehydrated banana
Jagged curves crispy foodsNumber of peaks, fractal dimension
REMARKS
It is possible to produce dried-and-crisp fruits using the
convective-multi-flash drying process (CMFD) with texture
properties that are similar to those obtained with freeze-drying
Product color is preserved due to the use of moderate process
temperatures ( Do you want it? )
Process time of CMFD and KMFD are shorter (about 2-3 hours
at laboratory scale) than the characteristic times of freeze-drying
Equipment and process are simple and use low pressures and
temperatures smaller investment and energy requirements
than freeze-drying
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Can we control the microstructure of dehydrated fruits and vegetables?
How can we follow the microstructure evolution during drying?
YES, WE CAN!
Bulk or apparent volume
Vb= apparent volume (cm³)mb= sample weight in air (g)ma= support mass (g)mb1= “sample weight” immersed n-heptane (g)ma1= support weight immersed in n-heptane (g)
1 1( ) ( )b a b a
b
solvent
m m m mV
d
Becker with n-heptane
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True volume of a porous material using a gas picnometer (Sereno et al., 2007)
Vp= true volume (cm³)Moist material = volume of the solids forming the sample structure + volume of the liquid Dry material = volume of the solids forming the fruit structure
V1 V2
Material porosity,
Vb= bulk volume (cm³)Vp= true volume (cm³)
1001%
b
p
V
V
10010
b
bb
V
VS
Accessible porosity during drying
Accessible porosity increases at each cycle of CMFD or KMFD during drying
Fruits shrinking during drying
Vb0
Vb
FROM THE BULK VOLUME AND THE TRUE VOLUME
WE CAN DETERMINE
shrinking patterns
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SEM- Scanning electronic microscopy
Samples preparation fruit slices were removed from thedryer, frozen by liquid nitrogen and freeze-dried(It was considered that ultra rapid freezing and freeze-dryingdid not change significantly the fruit structure)
Convective drying
T = 60 °C
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0
1
2
3
4
5
0 250 500 750 1000 1250 1500
Tru
evo
lum
e (
cm³)
Time (min)
Strong shrinking
True volumeof dried fruits
True volumes (solid + liquid)as determined with the gas porosimeter
1001%
b
p
V
V
0
10
20
30
40
50
60
70
0 250 500 750 1000 1250 1500
Vo
idsp
ace
s(%
)
Time (min)
Porosity of dried fruits(57%)
Evolution of the void spaces during drying using convective drying (moist samples)
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0
10
20
30
40
50
60
70
80
0 250 500 750 1000 1250 1500
Shri
nki
ng
(%)
Time (min)
Shrinking (determined by immersion into heptane)
Vb = bulk volume at t (cm³)Vb0= bulk volume at t=0 (cm³) Vb0= average value= 5 cm3
10010
b
bb
V
VS
dried fruits(Sb=65-70%)
Initial sample 4 hours
8 hours 12 hours20x
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Vacuum drying
T = 60 °C and P = 15mbar
Vacuum Drying
Vacuum drying of bananas was performed using a vacuum oven (Ethick Technology, 440-DE model-SP, Brazil). Banana slices (5mm) were placed evenly in a thin single layer on the drying tray and placed inside the vacuum oven.
The vacuum pressure and the drying temperature were kept constant at 15mbar and 600C.
The fruits were dried for 6h to a final moisture content of 0,0760 g/g
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0
1
2
3
4
5
0 100 200 300 400
Tru
evo
lum
e (
cm³)
Time (min)
True volumes (solid + liquid)as determined with the gas porosimeter
Vacuum Drying
1001%
b
p
V
V
0
10
20
30
40
50
60
70
80
90
0 100 200 300 400
Vo
idsp
ace
s(%
)
Time (min)
Porosity of dried fruits (70%)
Evolution of the void spaces during drying using vacuum drying (moist samples)
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10010
b
bb
V
VS
0
10
20
30
40
50
60
70
0 100 200 300 400
Shri
nki
ng
(%)
Time (min)
dried fruits (Sb=68%)
Shrinking - Vacuum Drying(determined by immersion into heptane)
Vb = bulk volume at t (cm³)Vb0= bulk volume at t=0 (cm³) Vb0= average value= 5 cm3
Initial sample 2 hours
4 hours 6 hours20x
Vacuum Drying
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Initial sample 2 hours
4 hours 6 hours
Vacuum Drying
50x
Microwave Vacuum Drying
Vacuum helps to reduce drying temperature by reducing the boiling point of water during microwave-vacuum drying.
Drying rates of carrot slices (Durance, 1999)- hot air at 70 C
- Freeze drying
- microwave-vacuum drying
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Microwave vacuum dryingExperimental device
Trapping
Vacuum pump
Digital vacuum meter
Vacuum
chamber
Microwave oven/dryer
12:00
Turntable plate
SEM - Microwave vacuum drying
Microwave vacuum drying
Drying time 19 min
Moisture 0,03 g/g
Bulk volume 2,46 cm³
Porosity 66%
Shrinking 51%
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Structural changes in banana samples during drying by KMFD
KMFD-Conductive multi-flash drying, T = 80°C
0
0,2
0,4
0,6
0,8
1
0 25 50 75 100
Wat
er a
ctiv
ity
(Aw
)
Time (min)
0,00
0,50
1,00
1,50
2,00
2,50
0 25 50 75 100 125
Mo
istu
re(g
/g)
Time (min)
KMFD
Drying curvesreproducibility
Triplicate
0,00
0,50
1,00
1,50
2,00
2,50
0 25 50 75 100 125
Mo
istu
re (
g/g
)
Time (min)
Before flash drying
After flash drying
Free water
Conductive and flash evaporation contributions
Water activity
flash evaporation is important
12 3
4
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0
5
10
15
20
25
30
35
40
0% 10% 20% 30% 40% 50% 60% 70%
Forc
e (
N)
Strain(%)
Pulse 5
Pulse 6
Pulse 7
Pulse 8
Pulse 9
Pulse 10
Pulse 11
Pulse 12
Puncture tests of fruits dried by KMFDPulse 10
aw = 0,401
Pulse 12aw = 0,240 Pulse 11
aw= 0,353
1 2 3 4 5 6 7 8 9 10 11 12
Pictures of banana slices after each KMFD cycle
0
10
20
30
40
50
60
70
80
90
0% 10% 20% 30% 40% 50% 60% 70%
Forc
e (N
)
Strain(%)
0
10
20
30
40
50
60
70
80
90
0% 10% 20% 30% 40% 50% 60% 70%
Forc
e (N
)
Strain(%)
0
10
20
30
40
50
60
70
80
90
0% 10% 20% 30% 40% 50% 60% 70%
Forc
e (N
)
Strain(%)
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70
Forc
e (N
)
Strain(%)
Pulse 9
Pulse 12Pulse 11
Pulse 10
Puncture tests of fruits during drying by KMFD
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0
1
2
3
4
5
0 25 50 75 100 125
Tru
evo
lum
e (
cm³)
Time (min)
2
True volumes (solid + liquid)as determined with the gas porosimeter
KMFD PROCESS
1
3 45
True volumeof dried fruits
True volumeSolid matrix + liquid
0
10
20
30
40
50
60
70
80
90
0 25 50 75 100 125
Vo
idsp
ace
s(%
)
Time (min)
Porosity of dried fruits (75%)
1001%
b
p
V
V
1
3
2
5
4
6
Evolution of the void spaces during drying using KMFD (moist samples)
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0
10
20
30
40
50
0 25 50 75 100 125
Shri
nki
ng
(%)
Time (min)
Shrinking - KMFD PROCESS (determined by immersion into heptane)
10010
b
bb
V
VS
1
2
34
5 dried fruits (Sb=40%)
Vb = bulk volume at t (cm³)Vb0= bulk volume at t=0 (cm³) Vb0= average value= 5 cm3
Structural changes in banana samples during drying – KMFD
Optical microscopies and SEM
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Initial sample 1st pulse 2nd pulse 3rd pulse
4th pulse 5th pulse 6th pulse 7th pulse
8th pulse 9th pulse 10th pulse 11th pulse
KMFD, 20x
12th pulse
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Initial sample 1st pulse 2nd pulse 3rd pulse
4th pulse 5th pulse 6th pulse 7th pulse
8th pulse 9th pulse 10th pulse 11th pulse
KMFD, 50x
12th pulse
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KMFD (4 vacuum pulses) + VD
Conductive multi-flash drying + vacuum dryingT = 80°C
0
0,2
0,4
0,6
0,8
1
0 25 50 75 100 125
Wat
er a
ctiv
ity
(Aw
)
Time (min)
0,00
0,50
1,00
1,50
2,00
2,50
0 25 50 75 100 125
Mo
istu
re (
g/g
)
Time (min)
Before flash drying
After flash drying
Triplicates
0,00
0,50
1,00
1,50
2,00
2,50
0 25 50 75 100 125
Mo
istu
re (
g/g
)
Time (min)
Drying curvesreproducibility
Conductive and flash evaporation contributions
Water activity
12
3
Vacuum drying – 15 mbar
Vacuum drying – 15 mbar
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0
10
20
30
40
50
60
0% 10% 20% 30% 40% 50% 60% 70%
Forc
e (
N)
Strain(%)
Puncture tests of dried samplesKMFD (4 vacuum pulses) + VD
Only dried samples
Structural changes in banana samples during KMFD + vacuum drying
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0
1
2
3
4
0 25 50 75 100 125
Tru
evo
lum
e (
cm³)
Time (min)
True volumes (solid + liquid)as determined with the gas porosimeter
( KMFD + vacuum drying )
0
10
20
30
40
50
60
70
80
90
100
0 25 50 75 100 125
Vo
idsp
ace
s(%
)
Time (min)
1001%
b
p
V
V
Evolution of the void spaces during drying using KMFD + vacuum drying (moist samples)
Evolution of the void spaces during drying using KMFD + vacuum drying (moist samples)
Porosity of dried fruits (87%)
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Shrinking (determined by immersion into heptane)KMFD + vacuum drying
10010
b
bb
V
VS
0
10
20
30
40
0 25 50 75 100 125
Shri
nki
ng
(%)
Time (min)
dried fruits (Sb=25-35%)
Vb = bulk volume at t (cm³)Vb0= bulk volume at t=0 (cm³) Vb0= average value= 5 cm3
KMFD + vacuum drying SEM –20x, after Freeze drying of samples frozen by liquid
nitrogen
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After 4 pulses + 65 min of vacuum drying
KMFD + vacuum drying (dehydrated sample)
20x 50x
SEM of banana samplesdehydrated by different methods
COMPARISON OF GENERAL PATTERNS
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CMFD
Convective multi-flash drying
KMFD
Conductive multi-flash drying
Microwave
Vacuum-drying
Some pictures of dehydrated banana
Air drying Freeze-drying (retail-store)
US$10/kg US$70/kg
Freeze-drying (Laboratory)
Convective Drying
Vacuum drying
KMFD KMFD + VD
50x
Microwave drying
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0
10
20
30
40
50
60
0 10 20 30 40 50 60 70
Forc
e (
N)
Strain (%) KMFD, CMFD, FREEZE-DRYINGCONVECTIVE DRYING
MICROWAVE-VACUUM DRYING
KMFD+VACUUM DRYING
Puncture test results for different drying processes
Convective drying
Vacuum drying
MW KMFDKMFD +
VD
Drying time 20 h 6 h 20 min 2 h 2 h
Moisture (g/g)
0.12 0.08 0.03 0.03 0.03
Bulk volume (cm³) 1.53 1.61 2.46 2.95 3.25
Porosity (%) 57 71 66 75 87
Shrinkage (%) 70 68 51 40 25-35
Global microstructure parameters of dehydrated banana for the different drying processes
10010
b
bb
V
VS Vb0 5 cm3
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CONCLUDING REMARKS
It is possible to control the whole pattern of dehydrated fruits
(e.g., banana, mango) in order to produce dried-and-crisp fruits.
CMFD and KMFD and their combination with vacuum drying are
appropriate processes to reach this goal
Texture properties can be similar to those obtained with freeze-
drying (depending on the operation conditions). Shorter drying
times
Product color is preserved due to the use of moderate process
temperatures. Vitamins retention and sensorial properties need to
be investigated for each fruit
Equipment and process are simple and use low pressures and
temperatures smaller investment and energy requirements
than freeze-drying
Future studies
i) What the influence of fruits ripening on the microstructure formation?
ii) How the steepness of pressure drop (vacuum pulses) influences the
texture formation during drying of a given fruit?
iii) How to correlate sensorial texture and puncture tests of dehydrated fruits?
iv) How can we combine microwave vacuum drying and vacuum pulse to
produce texture during dehydration? How the microwave power (per kg of fruit
mass) influences the fruit texture?
v) What are the more appropriate fruits and vegetables to be dehydrated by
these techniques?
vi) Energy consumption, costs, scale-up.
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THANK YOU FOR YOUR ATENTION!
FEDERAL UNIVERSITY OF SANTA CATARINABRASIL
