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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