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