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Stability of Food Emulsions (1)
David Julian McClements
Biopolymers and Colloids Laboratory
Department of Food Science
Definition and Importance of
Emulsion Stability
Definition: "Ability to resist
changes in properties over
time”
Importance: Determines the
shelf-life and processing of
food emulsions
May be desirable or undesirable
Arcocolors.com
Emulsion Stability: Kinetic
versus Thermodynamic Stability
∆G
∆G*
Separated Phases
Emulsion
Kinetically
Stable
Kinetically
Unstable
Gi
Gf
Thermodynamically
Stable
Thermodynamically
Stable
Physical stability:
Ability to resist changes in spatial distribution
of ingredients over time
- e.g., creaming, flocculation, coalescence..
Chemical stability:Ability to resist changes in chemical structure
of ingredients over time
- e.g., ω-3 oxidation, citral degradation
Physical Stability Mechanisms
Flocculation
Stable
Emulsion
Coalescence
or
Ostwald
Ripening
Gravitational
Separation
Phase
Separation
Importance of Identification of
Major Instability Mechanisms
• Every food emulsion is unique!
• There is no single strategy that can be used to generally improve food emulsion stability
• It is therefore crucial to identify the major instability mechanism for the specific food emulsion of interest
• Knowledge of emulsion science and technology facilitates problem solving
Emulsion Stability Testing:Diagnostic Approach
Phase separation
Oiling off
Rancidity
Creaming
Flocculation
Coalescence
Ostwald Ripening
Process
Ingredient
Storage + +
Ca2+
Macroscopic Properties
Physicochemical Origin
Instability Mechanism
Solution
Determine instability
mechanism(s)
Characterize product
defect
Identify physicochemical
origin
Gravitational SeparationPrinciples
UU = = --22rr22((ρρ22--ρρ11)g/9)g/9ηη11
Stokes Law:Stokes Law:
Methods of Retarding Gravitational SeparationMethods of Retarding Gravitational Separation::
•• Reduce density difference (Reduce density difference (∆ρ∆ρ))
•• Reduce droplet size (r)Reduce droplet size (r)
•• Increase continuous phase viscosity (Increase continuous phase viscosity (ηη11))
FG
FV
Gravitational SeparationInfluence of Density Difference
-0.5
-0.3
-0.1
0.1
0.3
0.5
-100 0 100 200
Density Difference (kg m-3
)
U/r
2 x
10
6 (
m-1
s-1)
Sunflower oil-in-water emulsions containing weighting agents:
Ester gum, Damar Gum, SAIB or BVO
Density Matching
Gravitational SeparationInfluence of Droplet Size
0
20
40
60
80
100
0 0.5 1 1.5 2 2.5 3
r (µµµµm)
U (
mm
/da
y)
Sunflower oil-in-water emulsions
Without thickening agent,
O/W emulsions are unstable
to creaming once r > 0.5 µm.
U ∝ r2
Gravitational SeparationInfluence of Continuous Phase Viscosity
0
5
10
15
20
25
30
0 0.005 0.01 0.015 0.02 0.025 0.03
Biopolymer Concentration (wt%)
U (
mm
/day
)
Predictions: RV = 1000; CFC = 0.004 wt%;
r = 0.5 µm, rfloc = 1.5 µm
Thickening agents may
promote creaming instability
if they cause flocculation!Floc
Non-Floc
Gravitational SeparationInfluence of Droplet Concentration
0
0.2
0.4
0.6
0.8
1
0 0.1 0.2 0.3 0.4
φφφφ
U/U
0
Hexadecane oil-in-water emulsions (SDS)
Strategies to Reduce Gravitational
Separation
• Quality, sensoryAdd thickening
or gelling agent
Increase ηηηη
• Cost, qualityHomogenizeReduce r
• Stability, quality,
nutrition
Alter SFC
• Regulations, costAdd weighting
agent
Reduce ∆ρ∆ρ∆ρ∆ρ
ProblemsMethodPrinciple
Food Emulsions Susceptible to
Gravitational Separation
High SusceptibilityHigh Susceptibility
•• BeveragesBeverages
•• Infant formulae Infant formulae
•• Salad DressingsSalad Dressings
•• Soups & SaucesSoups & Sauces
Low SusceptibilityLow Susceptibility
•• Margarine & ButterMargarine & Butter
•• MayonnaiseMayonnaise
• Low droplet concentration
• Low continuous phase viscosity
• High droplet concentration
• Gelled continuous phase
Experimental Characterization of
Gravitational Separation
Indirect Methods (Prediction)
• Stokes Law: U = -2gr2∆ρ/9η
• Measure PSD, η, ∆ρ
Direct Methods (Measurement)
• Visual observation
• Physical sectioning
• Droplet profiling
φ φ
0
5
10
15
20
25
30
35
0.1 1 10 100
Diameter (µµµµm)
Vo
lum
e F
req
uen
cy (
%)
Stable
Unstable
Measuring Creaming StabilityVisual Observation
HM
HL
HU
Upper
“Creamed”
Lower
“Serum”
Middle
“Emulsion”
Creaming Index: CI = 100 ×××× HL / HE
HE
Long-term storage tests or accelerated (centrifugation) tests
Measuring Creaming StabilityVisual Observation
Two-layer
System
Three-layer
System
One-layer
System
05
1015
2025
30
3540
4550
0 20 40 60 80
Time (h)C
I (%
)
CIfinal
v = dCI /dt
CI
Cream
Emulsion
Cream
Serum
Emulsion
Measuring Creaming Stability
Visual Observation
Observation Problems:
• Where is the boundary?
• Which layer is which?
• Subjective analysis
Container Requirements:
• Flat bottomed
• Graduated
• Material (Glass/Plastic)
0
1020
30
4050
6070
8090
100
0 10 20 30 40
Height (mm)
Ba
ck S
catt
er (
%) 0.9 hr.
5.7 hr.
8.7 hr.
13.8 hr.
24 hr.
46.1 hr.
70.3 hr.
123.7 hr.
Cream
Layer
Serum
Layer
Emulsion
Layer
Measuring Creaming Stability
Optical Imaging
Radial Position
Tra
nsmission
NIR LightSource
Sample
TimeColour Coded
TransmissionProfiles
CCD Sensor
∆t i
5 - 2300 g
Space and Time resolved Extinction Profiles
STEPTM - Technology
Time
Space
Measuring Creaming Stability:
Accelerated Optical Imaging
Measuring Creaming Stability
Ultrasonic Scanning / NMR Imaging
• Quantitative
• Concentrated Systems
0 hrs0 hrs
24 hrs24 hrs
Droplet Flocculation
““Aggregation of two or more droplets into a floc Aggregation of two or more droplets into a floc
where the droplets retain their individual identitieswhere the droplets retain their individual identities””
Stable Flocculated
• Fraction
• Size
• Strength
• Shape