(6)Emulsion Stability

7/27/2019 (6)Emulsion Stability

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Stability of Food Emulsions (1)

David Julian McClements

Biopolymers and Colloids Laboratory

Department of Food Science


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Definition and Importance ofEmulsion Stability

Definition: "Ability to resist

changes in properties over

time

Importance:Determines the

shelf-life and processing offood emulsions

May be desirable or undesirable

Arcocolors.com


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Emulsion Stability: Kineticversus Thermodynamic Stability

G

G*

Separated Phases

Emulsion

KineticallyStable

Kinetically

Unstable

Gi

Gf

ThermodynamicallyStable

Thermodynamically

Stable


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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., -3 oxidation, citral degradation


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Physical Stability Mechanisms

Flocculation

Stable

Emulsion

Coalescence

or

Ostwald

Ripening

Gravitational

Separation

PhaseSeparation


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Importance of Identification ofMajor Instability Mechanisms

Every food emulsion is unique!

There is no single strategy that can be used togenerally improve food emulsion stability

It is therefore crucial to identify the majorinstability mechanism for the specific foodemulsion of interest

Knowledge of emulsion science and technology

facilitates problem solving


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Emulsion Stability Testing:Diagnostic Approach

Phase separation

Oiling off

Rancidity

Creaming

Flocculation

CoalescenceOstwald Ripening

Process

Ingredient

Storage + +

Ca2+

Macroscopic Properties

Physicochemical Origin

Instability Mechanism

Solution

Determine instabilitymechanism(s)

Characterize product

defect

Identify physicochemicalorigin


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Gravitational SeparationPrinciples

UU== --22rr22((22--11)g/9)g/911

Stokes Law:Stokes Law:

Methods of Retarding Gravitational SeparationMethods of Retarding Gravitational Separation::

Reduce density difference (Reduce density difference ())

Reduce droplet size (r)Reduce droplet size (r)

Increase continuous phase viscosity (Increase continuous phase viscosity (11))

FG

FV


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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/r2 x

106(m-1s-1)

Sunflower oil-in-water emulsions containing weighting agents:

Ester gum, Damar Gum, SAIB or BVO

Density Matching


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Gravitational SeparationInfluence of Droplet Size

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3

r (m)

U(m

m/day)

Sunflower oil-in-water emulsions

Without thickening agent,

O/W emulsions are unstable

to creaming once r> 0.5 m.

U r2


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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 m, rfloc = 1.5 m

Thickening agents may

promote creaming instability

if they cause flocculation!Floc

Non-Floc


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Gravitational SeparationInfluence of Droplet Concentration

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4

U

/U0

Hexadecane oil-in-water emulsions (SDS)


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Strategies to Reduce Gravitational

Separation

Quality, sensoryAdd thickening

or gelling agent

Increase

Cost, qualityHomogenizeReducer

Stability, quality,

nutrition

Alter SFC

Regulations, costAdd weighting

agent

Reduce

ProblemsMethodPrinciple


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Food Emulsions Susceptible toGravitational Separation

High SusceptibilityHigh Susceptibility

BeveragesBeverages

Infant formulaeInfant 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


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Experimental Characterization of

Gravitational Separation

Indirect Methods (Prediction)

Stokes Law: U= -2gr2/9 Measure PSD, ,

Direct Methods (Measurement)

Visual observation

Physical sectioning Droplet profiling

0

5

10

15

20

25

30

35

0.1 1 10 100

Diameter (m)

VolumeFrequency(%)

Stable

Unstable


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Measuring Creaming StabilityVisual Observation

HM

HL

HUUpper

Creamed

Lower

Serum

Middle

Emulsion

Creaming Index: CI= 100 HL/ HE

HE

Long-term storage tests or accelerated (centrifugation) tests


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Two-layer

System

Three-layer

System

One-layer

System

05

10152025

3035404550

0 20 40 60 80

Time (h)

CI(%)

CIfinal

v = dCI/dt

CI


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Cream

Emulsion

Cream

Serum

Emulsion


Observation Problems:

Where is the boundary?

Which layer is which?

Subjective analysis

Container Requirements:

Flat bottomed

Graduated

Material (Glass/Plastic)


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(TurbiScan MA images from http://www.sci-tec-inc.com/)

Measuring Creaming StabilityOptical Imaging


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0

102030

4050

60708090

100

0 10 20 30 40

Height (mm)

BackSca

tter(%) 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 StabilityOptical Imaging


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Radial Position

Transmission

NIR LightSource

Sample

TimeColour Coded

TransmissionProfiles

CCD Sensor

ti

5 - 2300 g

Space and Time resolved Extinction Profiles

STEPTM - Technology

Time

Space

Measuring Creaming Stability:Accelerated Optical Imaging


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Measuring Creaming Stability

Ultrasonic Scanning / NMR Imaging

Quantitative

Concentrated Systems

0 hrs0 hrs

24 hrs24 hrs


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Droplet Flocculation

Aggregation of two or more droplets into a flocAggregation 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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ERR

OR

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stack

un

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:

(6)Emulsion Stability

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