Stability of Food Emulsions (2)
David Julian McClements
Biopolymers and Colloids Laboratory
Department of Food Science
Droplet Coalescence
“Oiling
Off”
“Coalescence”
““Aggregation due to fusing together of two or Aggregation due to fusing together of two or
more individual droplets to form a bigger more individual droplets to form a bigger
dropletdroplet””
Droplet Coalescence
Oil
Oil
Water
Droplet coalescence depends on precise molecular details
of droplet interfaces - difficult to predict!
A few
nms
Factors Influencing Droplet
Coalescence
•• Relative magnitude of forces between dropletsRelative magnitude of forces between droplets
•• Resistance of interface to disruptionResistance of interface to disruption
•• Duration of contact between dropletsDuration of contact between droplets
•• Shearing and tearing of interfacesShearing and tearing of interfaces
Evolution of Coalescence
Homogeneous
Coalescence
Heterogeneous
CoalescenceCoalescence rate
increases with droplet
size
Coalescence rate
independent of droplet
size
Without chitosan
(1-month old emulsion)0.01 wt. % chitosan
(the same magnification)
Factors Influencing Droplet
Coalescence: Ingredient Interactions
Factors Influencing Droplet
Coalescence: Freezing & Thawing
0 wt% sucrose 20 wt% sucrose
Hydrogenated palm oil-in-water emulsions stabilized by
WPI (-40 ºC/40ºC) – sucrose modifies ice crystal formation
Factors Influencing Droplet
Coalescence: Dehydration
0 wt% sucrose 20 wt% sucrose
Oil-in-water emulsions stabilized subjected to freeze-drying
0
20
40
60
80
100
1º 2º 3º
Emulsion
% C
oale
scen
ce
-10º
-40º
• 1º Highly Unstable
• 2º Stable to Coalescence Only
• 3º Stable to Coalescence, Flocculation &
Creaming
3 Cycles: -10ºC/30ºC
Droplet Coalescence: Influence of
Interfacial Membranes
Features of Coalescence
• Bimodal PSD Evolution
• “Oiling off”
0
5
10
15
20
25
30
35
0.1 1 10 100
Diameter (µµµµm)
φφ φφ (%
) 0 hours
24 hours
Strategies to Reduce Coalescence
Use polymeric emulsifier(4). Increase resistance of
membrane to rupture
Add thickening or gelling
agent
(3). Decrease droplet contact
• Alter pH or I (E/S)
• Increase thickness (S)
(2). Increase Repulsion
• Avoid depletion
• Avoid bridging
• Avoid hydrophobicity
(1). Reduce Attraction
MethodPrinciple
Measurement of CoalescenceTechniques & Protocols
Instrumental Techniques
• Microscopy
• Particle Sizing
• Creaming stability/Oiling Off
Experimental Protocols
• Storage Tests
• Accelerated Storage Tests
• Environmental Stress Tests
Characterization of Coalescence: Microscopy Methods
Particle Size Distribution, Flocculation vs. Coalescence
(Image Analysis Software)
Characterization of Coalescence: Particle Size Analysis
0
2
4
6
8
10
12
14
0.01 1 100
Particle Diameter (µµµµm)
Volu
me%
37
0
-2037 ºC : All liquid
-0 ºC: Fat crystallizes
-20 ºC: Water crystallizes
Potential Problems: Sampling, Distinguishing from flocculation
Characterization of Coalescence: Distinguishing from Flocculation
0
0.5
1
1.5
2
2.5
3
3.5
4
0.01 0.1 1 10 100
Particle Diameter (µµµµm)V
olu
me%
0
2
4
6
8
10
12
0.01 0.1 1 10 100
Particle Diameter (µµµµm)
Volu
me%
0
0.5
1
1.5
2
2.5
3
3.5
4
0.01 0.1 1 10 100
Particle Diameter (µµµµm)
Volu
me%
Add
Deflocculant
(e.g., surfactant)
Initially Flocculated Initially Coalesced
Emulsion to
Be Tested
Measurement of “Oiling Off”
Stable
EmulsionOiling OffCoalescence
Extensive droplet coalescence can lead to the formation of
a thin layer of oil on top of a product (sometimes with little
change in PSD of bulk emulsion)
Petroleum
ether
Emulsion
shake
“Oiling Off”:Solvent Extraction Method
• Measure amount of oil that can be extracted by an
organic solvent
“Oiling Off”:
Dye Dilution Method
Cuvette
Mix Centrifuge Absorbance
• Measure dilution of dye solution by free oil
Add Dye
Emulsion
Characterization of Coalescence: Coalescence/Oiling-off by DSC
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-10 0 10 20 30 40
Temperature (ºC)
Hea
t F
low
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
-10 0 10 20 30 40
Temperature (ºC)
Hea
t F
low
Fat
Crystallization
Water & Fat
Crystallization
Fat crystallization behavior of oil-in-water emulsions
Characterization of Coalescence: Coalescence/Oiling-off by DSC
0
20
40
60
80
100
-40 -20 0 20 40
Temperature (ºC)
Oil
ing
Off
(%
)
Tween 20
Casein
Fat
CrystalnWater
Crystaln
Characterization of Coalescence: Centrifugation Methods
Oil Cream Serum
HReleased HC
( )A
ReleasedTotal VVgP
CR
OSM
−∆=
ρ
Centrifuge
• Time
• Speed
Partial Coalescence
FusionAggregation
““Clumping of partially crystalline droplets due Clumping of partially crystalline droplets due
to penetration of fat crystal from one droplet into to penetration of fat crystal from one droplet into
another dropletanother droplet””
40ºC (Liquid droplets) 0ºC (Partially Crystalline Droplets)
SEM Images of Partial
Coalescence
O/W Emulsions viewed by SEM (John Coupland, Penn State)
0
20
40
60
80
100
0 20 40 60 80 100
SFC (%)
EC (
%)
Influence of Droplet SFC on
Partial Coalescence
Influence of Interfacial Membrane
on Partial Coalescence
Thick Membrane
-Resistant to PC
- e.g., casein
Thin Membrane
-Prone to PC
- e.g., Tween 20
0
20
40
60
80
100
-10 0 10 20
Temperature (ºC)
Oil
ing
Off
(%
)Tween 20
Casein
Case Study:
Ice Cream Manufacture
Add Surfactant
& Age
Fat globules
covered with thick milk
protein membrane
Fat globules
covered with thin
surfactant membrane
Partial Coalescence in Ice Cream
Ice cream viewed by cryo-SEM (Douglas Goff. Guelph)
Air bubble
Partially coalesced
droplets around
air bubble
Partially coalesced
droplets in continuous
phase
Methods of Controlling Partial
Coalescence
FusionAggregation
•• Control droplet crystallization (SFC)Control droplet crystallization (SFC)
•• Control thickness & Control thickness & viscoelasticityviscoelasticity of membranes of membranes
•• Control dropletControl droplet--droplet interactionsdroplet interactions
•• Control droplet collision frequency or contact timeControl droplet collision frequency or contact time
Measurement of Partial Coalescence:
Techniques & Protocols
Instrumental Techniques
• Microscopy
• Particle Sizing
• Creaming stability/Oiling Off
• Solid fat content versus temperature
Experimental Protocols
• Storage Tests
• Accelerated Storage Tests
• Environmental Stress Tests
Ostwald Ripening
““Growth of large droplets at the expense of Growth of large droplets at the expense of
small droplets due to molecular diffusion of oil small droplets due to molecular diffusion of oil
molecules through the aqueous phase driven by molecules through the aqueous phase driven by
differences in Laplace pressuredifferences in Laplace pressure””
Ostwald Ripening
““Growth of large droplets at the expense of Growth of large droplets at the expense of
small droplets due to molecular diffusion of oil small droplets due to molecular diffusion of oil
molecules through the aqueous phase driven by molecules through the aqueous phase driven by
differences in Laplace pressuredifferences in Laplace pressure””
Time
Features of Ostwald Ripening
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 200 400 600
Time (hours)
d3
( µµ µµm
3)
• Monomodal PSD Evolution
• d3 proportional to time
0
5
10
15
20
25
30
35
0.1 1 10 100
Diameter (µµµµm)
φφ φφ (%
)
0 hours
24 hours
48 hours
Influence of Oil Type on
Ostwald Ripening
0
1
2
3
4
0 25 50 75 100
Time (hours)
d3
( µµ µµm
3) Decane
Hexadecane
Food Emulsions Susceptible to
Ostwald Ripening
High SusceptibilityHigh Susceptibility
•• Emulsions containing oils with high water Emulsions containing oils with high water
solubility, solubility, e.g., e.g., flavor oils, essential oils, SCFAflavor oils, essential oils, SCFA
•• Emulsions containing alcohol in the aqueous Emulsions containing alcohol in the aqueous
phase, phase, e.g., e.g., cream liqueurscream liqueurs
Low SusceptibilityLow Susceptibility
•• Emulsions containing oils with low water Emulsions containing oils with low water
solubility, solubility, e.g., e.g., TAGSTAGS
Ostwald Ripening
Methods of Retarding Ostwald Ripening:Methods of Retarding Ostwald Ripening:•• Reduce oil solubility in waterReduce oil solubility in water
•• Reduce interfacial tension Reduce interfacial tension
•• Incorporate low solubility oil into dropletsIncorporate low solubility oil into droplets
•• Use membrane resistant to deformation Use membrane resistant to deformation
δ<r>3/ δt = 8 γ Vm S D / 9 R T
Measurement of Partial Coalescence:
Techniques & Protocols
Instrumental Techniques
• Microscopy
• Particle Sizing
Experimental Protocols
• Storage Tests
• Accelerated Storage Tests
• Environmental Stress Tests
Conclusions
• Many different physicochemical processes contribute to the instability of food emulsions
• For a particular food product it is necessary to identify the dominant instability mechanism
• Emulsion science can then be used to improve food emulsion stability