Transcript Milk fat crystal networks formed under shear

Milk fat crystal networks
formed under shear
Crystallisation of milk
fat/sunflower oil blends: kinetics
and reological
properties
Bert Vanhoutte, Imogen Foubert, André Huyghebaert and Koen Dewettink
Department of food science and nutrition
Ghent University, Belgium
Microstructure
Lipid-composition
Polymorphism, polytypism
processing
Fat crystal habit, size, distribution
Spatial distribution of fat crystals
Macroscopic properties
Source: Marangoni&Hartel, Food Technology, 1998
supersaturating
nucleation
crystal size distribution
aggregation
gelation
strong network forming
post- and recrystallisation
Source: PhD Thesis William Kloek
sintering
fraction solid fat
Van der Waals
forces
structure
crystal growth
Crystallisation under
shear
Agitation rate 50, 100,
200 and 300rpm
Temperature recording
SFC measurements
Crystallisation
interrupted at 75% of
equilibrium
Samples for rheological
tests and microscopic
analysis
85mm
Temperature
recording
20mm
15mm
20mm
70mm
Rheological
measurements
25mm
2,5mm
Polarised microscopy
Microstructure formation in tubs not
under microscopic slides
2D images of microstructure by
cryotomography
Particle size measurements of primary
crystal aggregates with a grid
NO Quantitative analysis of spatial
distribution
Processing conditions
Temperature of the coolant 21 and
26.5°C
Agitation rate 50-100-200-300rpm
Five blends High melting fraction milk
fat (HMF) – Sunflower Oil (SFO) 60/40,
70/30, 80/20, 90/10 and 100/0
Multiple effect of
agitation
Effect on the cooling rate

Convective heat transfer coefficient
Effect on the mass transfer

Shear rate
Convective heat transfer coefficient
(assumption temperature perfectly homogeneous in
vessel)
Tcs
m.c p .
  h .F .(Tcs  Tw )
t
Shear rate
(calculated at the tip of the impeller compared to the
vessel wall)
?
Lipid composition
Crystallisation kinetics
Supercooling
Induction time
Supersaturation
Growth rate
Processing
Convective heat transfer h
Shear rate g
Temperature of the coolant
Qualitative analysis
60
60
(60/40)
21°C
T (°C)
50
45
55
50
100
200
300
40
(60/40)
26.5°C
50
T (°C)
55
35
30
45
40
35
30
25
25
20
0
10
20
30
40
20
0
10
Time (min)
55
50
45
55
50
40
35
45
30
50
100
200
300
40
35
30
30
25
25
20
20
Time (min)
(100/0)
26.5°C
60
50
100
200
300
T (°C)
(100/0)
21°C
60
T (°C)
50
100
200
300
20
0
10
20
Time (min)
30
40
0
10
20
Time (min)
30
4
Anova on the induction
time
Enter method
Stepwise method
Conclusion:
The induction time is affected by
agitation but mainly by an increase in
heat transfer rather then an effect of
mass transfer
Anova on the growth
rate
Enter method
Stepwise method
Conclusion
The growth rate is influenced by shear
rate rather than by the convective heat
transfer coefficient, which suggest the
growth rate is more affected by the
mass transfer than by the overall
release of heat towards the coolant
600
600
500
500
400
21°C
300
26,5°C
200
crystal aggregate size (µm)
crystal aggregate size (µm)
Microstructure
100
400
21°C
300
26,5°C
200
100
0
0
50
100
200
300
50
100
200
300
agitation rate (rpm)
600
600
500
500
400
21°C
300
26,5°C
200
crystal aggregate size (µm)
crystal aggregate size (µm)
agitation rate (rpm)
400
21°C
300
26,5°C
200
100
100
0
0
50
100
agitation rate (rpm)
200
300
50
100
agitation speed (rpm)
200
300
Anova on primary
crystal aggregates
Size decreases with temperature of the coolant and more agitation
No effect on the lipid composition
Effect of agitation = effect on primary or secondary nucleation???
Effect on shear on
crystals
High shear
+/- homogeneous
size distribution
Low shear
More heterogenous
size distribution
Post crystallisation
Depends on:
The difference between crystallisation
temperature and the storage temperature
 Van der Waals – Solid bridges
 The cooling rate
 The specific surface area

Anova Rheology
Power-law models
Relation between SFC and G’ can be
described by power-law models
G'  A  SFC
where A is the interaction parameter and µ is
the scaling exponent
µ
Fractal nature of fat crystal networks

Applicable on this system?
G'  g
1
 SFC d  D
Regression analysis
The effect of agitation is larger when the
degree of post-crystallisation is small
Longer storage leads to space filling of initial
pores
The interaction term A
8
6
4
Log A
2
0
0
50
100
150
200
-2
-4
Tw=21°C
Tw=26,5°C
-6
-8
agitation rate (rpm)
250
300
350
The scaling exponent µ
9
8
Tw=21°C
Tw=26,5°C
scaling exponent (µ)
7
6
5
4
3
2
1
0
0
50
100
150
200
agitation rate (rpm)
250
300
350
Relation between process
parameters, crystallisation
kinetics and rheological
properties
T=low + shear=low
T=high + shear=low
T=low + shear=high
T=high + shear=high
Lipid composition
Temperature of the coolant
Shear rate
Crystallisation kinetics
Agitation
Heat transfer
Storage temperature
Primary crystal aggregates
Final microstructure
Rheological properties
Acknowledgements
IWT (Institute for the Promotion of
Innovation by Science and Technology
in Flanders)
Aveve Dairy products, Belgium
Special thanks to Wouter Pillaert,
Brecht Vanlerberghe, Leo Faes and
Frank Duplacie