Stability of Food Emulsions (1) David Julian McClements Biopolymers and Colloids Laboratory Department of Food Science
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)
(TurbiScan MA images from http://www.sci-tec-inc.com/)
Measuring Creaming StabilityOptical Imaging
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
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