Factors Affecting Food

Emulsion Properties

D. Julian McClements

Department of Food Science

University of Massachusetts , Amherst

Emulsion Properties Depend on

Droplet Characteristics

• Fat droplets affect:

– Appearance

– Flavor

– Texture

– Shelf-life

Need to Understand Role of Fat Droplets !

Emulsion Appearance

• Product Desirability

• Perceived Product Quality

• Consumer Sensory Expectations

Optical Properties Determine:

Influence of Fat Droplets on

Emulsion Appearance

Incident Wave, I(λ)

Reflected Wave, R(λ)• Absorption

• Scattering

Transmitted

Wave, T(λ)

Hexadecane oil-in-water emulsions (d = 0.3 µm, red dye = 0.1%)

Emulsion Appearance:Effect of Droplet Concentration

Emulsion Appearance:Effect of Droplet Concentration

40

60

80

100

0 5 10 15 20 25

Droplet Concentration

(wt%)

L

0

10

20

30

40

50

0 5 10 15 20 25

Droplet Concentration

(wt%)c

Hexadecane oil-in-water emulsions (d = 0.3 µm, red dye = 0.1%)

Lightness Color Intensity

Fading

Emulsion Appearance:Influence of Titanium Dioxide

40

50

60

70

80

90

0 5 10 15 20 25

Droplet Concentration (wt%)

L

0%0.04%0.10%0.30%

Hexadecane oil-in-water emulsions

(d = 0.3 µm, red dye = 0.1%)TiO2

• Emulsion opacity can be increased by incorporating particles

that effectively scatter light, but are legally acceptable, relatively

inexpensive and compatible with the product

Emulsion Appearance:Effect of Droplet Size

25

30

35

40

45

50

0 10 20 30

Droplet Diameter (µµµµm)

C

Hexadecane oil-in-water emulsions (φ = 10%, red dye = 0.1%)

50

60

70

80

0 10 20 30

Droplet Diameter (µµµµm)

L

Lightness Color Intensity

Emulsion Appearance:Effect of Refractive Index

nd/n

aq= 1

Hexadecane oil-in-water emulsions (φ = 4 wt%, d = 1 µm, 0.002% Red Dye)

Important for high sugar or polyol foods, or for analysis

Main Factors Affecting Emulsion

Appearance

• Droplet Size & Concentration

• Relative Refractive Index

• Dye Type & Concentration

Theoretical models available to

predict emulsion color

Emulsion Flavor

• Product Desirability

• Perceived Product Quality

Flavor Determines:

• Aroma

• Taste

• Mouthfeel

Flavor is:

Reduced Fat Products

Factors Affecting Flavor

• Composition

– Type and concentration of

flavor molecules present

– Food matrix

• Flavor Distribution

– Partitioning of flavors between

different environments

• Molecular Transport

– Molecular transport of flavors

between different environments

Flavor Distribution

Dispersed Phase

Continuous Phase

Interfacial Phase

Flavor Modifiers

Head-space Phase

Partition Coefficient

KHE = cH / cE

Flavor DistributionInfluence of Fat Content

0

0.5

1

1.5

2

2.5

3

0 20 40 60 80 100

Fat Content (%)

Head

Sp

ace C

on

c (

10

-4)

Non-polar

Polar

• Initial flavor intensity of emulsions with fixed flavor

content is strongly dependent on fat content

Flavor type:

Molecular Transport Kinetics

�Equilibrium: Partition coefficients

�Kinetic: Molecular transport

Dilution in

Mouth

Taste &

Mouthfeel

Aroma

Volatiles

Non-

Volatiles

Molecular Transport Kinetics

Influence of Fat Content

0

20

40

60

80

0 20 40 60

Time (s)

Fla

vor I

nte

snit

y

0%

1%

5%

15%

30%

• Headspace flavor intensity versus time profile is strongly

dependent on fat content

Malone et al (2000)

Control of Molecular Transport:Encapsulation Technology

• Release rate can be controlled by creation of novel microstructures

Faster Slower

Gelled

Particles

Multiple

Interfacial

Layers

Complexity of Flavor Perception

Factors Influencing Flavor

• Dilution with Saliva

• Change in pH, I, T

• Mixing & Fragmentation

• Swallowing/Breathing

• Transport to Receptors

• Interaction with biological surfaces

Appreciable progress has been made – but more applied and

fundamental research is still required.

Factors Influencing Emulsion

Flavor

• Flavor concentration profile

• Food matrix composition

• Food microstructure

On-line flavor analysis

Rapid progress has recently

been made in understanding

food matrix effects on flavor

Emulsion Rheology

Emulsion Rheology Influences:

Processing operations

– Flow through pipe, stirring, homogenization

Quality attributes

– Texture: “spreability”, “pourability”

– Shelf life: creaming, sedimentation

Sensory attributes

– Mouthfeel: “creaminess”, “smoothness”

Understanding Emulsion Rheology

Mathematical Modeling

Droplet Droplet

characteristicscharacteristics

Continuous phaseContinuous phase

characteristicscharacteristics EmulsionEmulsion

RheologyRheologyMathematicalMathematical

ModelModel

Concentration, size,

interactions

Viscosity,

gel strength

Viscosity,

gel strength

Emulsion RheologyInfluence of Droplets on Viscosity

• Emulsion Droplets Increase Fluid Viscosity

No Particle

Isolated Rigid

Spherical Particle

Greater

Energy

Dissipation(Einstein Equation)

Fluid

Friction

Losses

Greater

Energy

Dissipation

Interacting Rigid

Spherical Particles(DK Equation)

0

10

20

30

40

50

0 10 20 30 40 50 60

φ φ φ φ (wt%)

η/η

η/η

η/η

η/η

00 00 (

Pa

s)

Einstein & DK

Models

DK

Model

Einstein

Model

Emulsion RheologyModeling Influence of Fat Content

Doughery-Krieger

Equation:

ηη = = ηη00(1 + 2.5(1 + 2.5φφ))

Einstein Equation:

ηη = = ηη00(1 (1 -- φφ//φφcc))--[η]φ[η]φcc

• Continuous phase rheology (η0)

• Particle concentration (φ)

• Close packing concentration (φc)

φφφφc

Emulsion RheologyInfluence of Droplets on Viscosity

φφφφ0% 100%

φC

Dilute Concentrated

FLUID-LIKE SOLID-LIKE

Sk

im M

ilk

Hea

vy

Crea

m

May

on

naise

Milk

Infa

nt F

orm

ula

Bev

erage

Lig

ht C

ream

Dressin

g (R

egu

lar)

Dressin

g (L

igh

t)

20% 40% 60% 80%

0

0.005

0.01

0.015

0.02

0.025

0.1 1 10 100

Normalized Shear Stress

η

η

η

η (

Pa

s) 21%

33%

40%

Factors Affecting RheologyFat Content & Shear Stress

Shear

Random

(Low shear)

Ordered

(High shear)

Less frictional losses

0

10

20

30

40

50

0 10 20 30 40 50 60

φ φ φ φ (wt%)

η/η

η/η

η/η

η/η

11 11 (

Pa

s)

High Shear

Limit

Low Shear

Limit

Factors Affecting RheologyFat Content and Shear Stress

ηη = = ηη00(1 (1 -- φφ//φφcc))--[η]φ[η]φcc

It is important to specify & control measurement stress

Note: Small change in φ

gives large change in η

Factors Affecting RheologyDroplet Concentration

0.001

0.01

0.1

1

10

100

1000

0 20 40 60 80

Droplet Concentration (wt%)

Sh

ear

Mo

du

lus (

kP

a)

• Emulsions have solid-like behavior at high droplet concentrations

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80

Droplet Concentration (wt%)P

ha

se

An

gle

(º)

Solid-likeLiquid-like

Factors Affecting RheologyDroplet Size

0

0.5

1

1.5

2

2.5

3

97 91 89 84 80 78 76

Droplet Radius (nm)

Sh

ear

Mo

du

lus (

kP

a)

0

10

20

30

40

50

60

70

80

90

Ph

ase A

ng

le (

º)

Emulsions can exhibit solid-like behavior below critical droplet size, if there are

strong repulsive interactions

LiquidSolid

Factors Affecting RheologyFlocculation

0

5

10

15

20

0 0.2 0.4 0.6

Volume Fraction

Rel

ati

ve

Vis

cosi

ty

Non-Floc

Floc

Hexadecane Oil-in-Water Emulsions, d = 0.53 µm (SDS)

Factors Affecting RheologyModeling Flocculated Emulsion Rheology

3

3

4flocfloc rV π=

droplets

floc

VV

VR =

Non-Flocculated

ηη = = ηη00(1 + 2.5(1 + 2.5φφRRVV))

Modified Einstein

Equation:

3

3

4dropletsdroplets rNV π=

Flocculated

Flocs trap continuous

phase, which

increases their

effective volume

Factors Affecting RheologyFlocculation

0.001

0.01

0.1

1

10

0 20 40 60 80

φφφφ

G*

(P

a)

NF

F

Hexadecane Oil-in-Water Emulsions, d = 0.53 µm (SDS)

Factors Affecting RheologyFlocculation: Shear Thinning Behavior

2

0 //

/1)(

+

+=

∞

∞

i

i

ττηη

ττητη

+=

kT

U

r

kTi

min

31τ

1

10

100

0.01 0.1 1 10 100

Shear Stress (Pa)

ηη ηη/

Pa

s

Flow field

Deformation

Partial

disruption

Complete

disruption

Major Factors Affecting

Emulsion Rheology

ηη = = ηη00(1 + 2.5(1 + 2.5φφeffeff + 6.2+ 6.2φφeff eff +...)+...)

• Continuous phase rheology (h0)

• Particle concentration (φ)

• Particle interactions (w(h))

• Particle size (r)

• Applied shear stress (G)

φφeffeff = = ff((φφ, , ww((hh), ), rr, , GG))

φφeffeff

Influence of Fat Droplets on

Emulsion Mouthfeel

How do fat droplets

interact with tongue

surface ?

Bulk Rheology

or

Specific Interactions?

Thin Film Rheology - Tribiology

Emulsion Properties Depend on

Droplet Characteristics

• Fat droplets affect:

– Appearance

– Flavor

– Texture

– Shelf-life

Need to Understand Role of Fat Droplets !