emulsion droplets: characteristics and importance · size-class: number ( n), surface ( s) or...

Emulsion Droplets: Characteristics and

Importance

1

*Food Structure and Functionality LaboratoriesDepartment of Food Science & BiotechnologyUniversity of HohenheimGarbenstrasse 21, 70599 Stuttgart, Germany

Jochen Weiss

Emulsion Workshop

November 13-14th, 2008, Amherst, MA

DropletCharacteristics

QualityAttributes

Concentration

Particle size distribution

Electrical charge

Interfacial properties

Physical state

Shelf-life

Texture

Appearance

Flavor

Nutrition

Organization

Interactions

FoodStructure

Need to know how to control, measure and report droplet characteristics!

Influence of Droplet Characteristics on Emulsion

Properties

2

f0% 100%

fC

Dilute Concentrated

FLUID-LIKE SOLID-LIKE

Skim

Milk

Hea

vy C

rea

m

Ma

yo

nn

ais

e

Milk

Infa

nt F

orm

ula

Be

vera

ge

Lig

ht C

ream

Dre

ssin

g (R

eg

ula

r)

Dre

ssin

g (L

igh

t)

20% 40% 60% 80%

Droplet Concentration: Influence on Emulsion Properties

3

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40

Rel

ati

ve

Cre

am

ing

Vel

oci

ty

0

10

20

30

40

0 20 40 60

Droplet Concentration (%)

80

85

90

95

100

0 5 10 15 20 25

Texture Appearance Stability

Droplet Concentration: Influence on Emulsion Properties

Rela

tive V

isco

sit

y

Droplet Concentration [%] Droplet Concentration [%]Droplet Concentration [%]

Lig

htn

ess

Rela

tive C

ream

inV

el.

4

� Disperse phase volume fraction:

� Disperse phase mass fraction:

Emulsion

Droplets

V

V=φ

Emulsion

Droplets

mM

M=φ

Volume% = 100 ×××× φφφφ

Mass% = 100 ×××× φφφφm

Droplet Concentration: Specification

5

Of major importance to:

� Fat crystallization in O/W emulsions (impurities/droplet)

� Distribution of reactants in emulsions (pro-oxidants/droplet)

∼∼∼∼ 1012 – 1020 m-3

0

5

10

15

20

0 5 10

Tm

(ºC

) Het. Nucl

Hom. Nucl

Droplet Concentration:

Number of Droplets per Unit Volume

N = 3 φ / 4 π r3

Tm

[ºC

]

Radius [µµµµm]

6

Of major importance to:

� Amount of emulsifier needed to cover surface

� Rate of surface mediated reactions e.g., lipid oxidation, lipid digestion

S = 3 φ / r

∼∼∼∼ 102 – 107 m2 m-3

00.1

0.20.3

0.40.5

0 20 40 60 80 10

0

Small S

Large S

Droplet Concentration: Droplet Surface Area per Unit Volume

Time [min]

FA

Rele

ase

7

Shelf-life: Creaming, Droplet Aggregation

Quality: Appearance, Flavor, Texture

Droplet Size:

Influence on Emulsion Properties

0

20

40

60

80

100

0 1 2 3

r (µµµµm)

Radius [µµµµm]

U [

mm

/da

y]

Small Particles Large Particles

Particle Size Distribution Specifications

0

2

4

6

8

10

0.01 0.1 1 10 100

Particle Diameter (µµµµm)

Mean particle diameter < a critical diameterMedian particle diameter < a critical diameterPercentage of droplets < a critical diameter

Problem

Droplets

A manufacturer should establish a particle size distributionspecification based on known product performance (e.g., stability,

optics, texture, flavor, mouthfeel):

Particle Diameter [µm]

Volu

me F

ractio

n [

%]

Representing Droplet Size Data

Size Range

(µµµµm)

di

(µµµµm)

Ni

0 – 1 0.5 6

1 – 2 1.5 12

2 – 3 2.5 20

3 – 4 3.5 25

4 – 5 4.5 19

5 – 6 5.5 6

6 – 7 6.5 3

7 – 8 7.5 2

8 – 9 8.5 1

9 – 10 9.5 0

0

5

10

15

20

25

30

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5


Nu

mb

er %

Tabulate

Graph

Particle Size Distribution

Average

Measure

dN = 3.4 µµµµm

Prepare

2 µµµµm

Nu

mb

er

%

Particle Diameter [µµµµm]

10

Droplets all same size

Report d or r

Droplets different sizes

Report PSD

Report mean d or rReport width

Representing Droplet Size Data:What to Present?

Monodisperse Polydisperse

11

Particle Size DistributionsGraphical Representation

0

5

10

15

20

25

30

0 4 8 12


F (

µµ µµm

)

0

20

40

60

80

100

0 4 8 12Particle Diameter (µµµµm)

0

5

10

15

20

25

30

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5

Vo

lum

e%

V%i = Fi ×××× ∆∆∆∆di

FrequencyHistogram Cumulative

Ci = % < di

Histogram:Amount in specific size categories is

reported

Frequency:Amount is equal to the area under the

curve

Cumulative:Amount below a specific size is

reported


Nu

mb

er

%

Particle Diameter [µµµµm] Particle Diameter [µµµµm]

F [

µµ µµm

-1]

C %

12

Representing Droplet Size Data:Specifying Size and Frequency

1. Size Parameter. A measurement of the

size of the particles in the size-class: radius (r) or diameter (d)

2. Frequency Parameter. A measurement of the amount of particles present in the

size-class: number (N), surface (S) or

volume (V)

Size Range

(µµµµm)

di

(µµµµm)

Ni

0 – 1 0.5 61 – 2 1.5 122 – 3 2.5 20

3 – 4 3.5 254 – 5 4.5 19

5 – 6 5.5 66 – 7 6.5 37 – 8 7.5 2

8 – 9 8.5 19 – 10 9.5 0

dAmount can be absolute, fraction or

percentage.

13

Particle Size Distributions: Use Appropriate Frequency Parameter!

0

2

4

6

8

10

12

0.01 0.1 1 10 100


Nu

mb

er %

0 mM NaCl

150 mM NaCl

0

2

4

6

8

10

12

0.01 0.1 1 10 100


Volu

me%

0 mM NaCl

150 mM NaCl

Vi = (ππππ/6) Ni di3

0 mM 150 mM


Nu

mb

er

%

Vo

lum

e %


14

Representing Droplet Size Data: Using Mean Particle Sizes

0

20

40

60

80

100

120

140

160

0.01 0.1 1 10 100


Vo

lum

e %

0 h

7 h

24 h

48 h

0

1

2

3

4

5

0 10 20 30 40 50

Time (h)

d3

2 (

µµ µµm

)

Average

Average values simplify presentation, but information is lost!Which mean is the best to use?


Vo

lum

e %

Time [h]

d32

[ µµ µµm

]

15

Representing Droplet Size Data: Defining Mean and Width

0

2

4

6

8

10

12

14

0 5 10


Nu

mb

er %

d N = 5 µµµµm

σσσσN = 0.3 µµµµm

d N = 5 µµµµm

σσσσN = 1 µµµµm

i

i

i xx ∑= φ

( )2

xxi

i

i −= ∑φσ

Mean = Measure of Central TendencyWidth = Measure of Spread of Data Around Mean (x/s)

Mean:

Width:

Here: xi = size parameterfi = frequency weighting parameter

σσσσ

x


Nu

mb

er

%

16

Representing Droplet Size Data: Defining Mean and Width

Name of

Mean

Frequency

Weighting Parameter

Size

Parameter

Definition of

Mean

dN φN,i = Ni/ΣNi di ΣφΝ,i × di

dS φS,i = Si/ΣSi di ΣφS,i × di

dV φV,i = Vi/ΣVi di ΣφV,i × di

Name of Width

Definition of SD

σN √[ΣφN,i ×(di-dN)2]

σS √[ΣφS,i ×(di-dS)2]

σV √[ΣφV,i ×(di-dV)2]

Mean Width

17

Defining Mean Particle Sizes:Equivalent Ways of Expressing Means

Name of

MeanDefinition

dN Σ(Ni/ΣNi × di)

dS Σ(Si/ΣSi × di)

dV Σ(Vi/ΣVi × di)

Name of Mean

Definition

d10 ΣNidi/ΣNi

d32 ΣNidi3/ΣNidi

2

d43 ΣNidi4/ΣNidi

3

Si ∝∝∝∝ Nidi2

Vi ∝∝∝∝ Nidi3

18

Representing Droplet Size Data: Use Appropriate Mean Size!

0

1

2

3

4

5

6( µµ µµ

m)

3 4 5 6 7

d32

Stable ?

Effect of pH on Emulsion Stability:Protein stabilized emulsion containing cationic polysaccharide

Mea

n P

art

icle

Dia

met

er

[ µµ µµm

]

pH

19

Representing Droplet Size Data: Use Appropriate Mean Size!

0

1

2

3

4

5

6( µµ µµ

m)

3 4 5 6 7

d43

d32

Effect of pH on Emulsion Stability:Protein stabilized emulsion containing cationic polysaccharide

Mea

n P

art

icle

Dia

met

er

[ µµ µµm

]

pH

Using appropriate

means can helpIdentify issues with

particular particles size classes !

20

Representing Droplet Size Data: Choose Appropriate Mean Size!

0

1

2

3

4

5

0.01 0.1 1 10 100


Vo

l%

d10 = 0.09 mmd32 = 0.21 mmd43 = 1.20 mm

Note:d10 and d32 are sensitive

to the majority of particles

d43 is sensitive to the presence of a few large

aggregates

d10d32d43


Vo

lum

e %

21

d10 = 5 µµµµmσσσσ10 = 2 µµµµm

Representing Droplet Size Data: Always Examine Full PSD!

0

2

4

6

8

10

12

0 5 10


Nu

mb

er%

Monomodal

Bimodal

These distributions have the same mean

diameter and the same

distribution width!!!


Nu

mb

er

%

22

Representing Droplet Size Data:Some Factors to Consider

• Size Parameter

– Diameter or Radius?

• Frequency Parameter

– Number or Volume?

• Always Examine PSD

• Mean Size & Width

– d10, d32, d43?

Using and Reporting Droplet Size Data:

0

1

2

3

4

5

6

( µµ µµm

)

3 4 5 6 7

d43

d32

Mea

n P

art

icle

Dia

met

er [

µµ µµm

]

pH

0

2

4

6

8

10

12

0.01 0.1 1 10 100


Volu

me%

0 mM NaCl

150 mM NaCl

Vo

lum

e %


23

Representing Droplet Size Data:Choice of Appropriate Mean Size

• Instrument: The sensitivity of an instrument to the

particles in a particular size category depends on its underlying physical principles

• Feature of PSD of Interest: Majority of particles or presence of large aggregates?

• Utilization of Information: Sometimes a particular mean size is required in a calculation, e.g., d32 is used

to calculate the surface area of an emulsion.

Factors Affecting Choice of Appropriate Mean Size:

24

Droplets interactions have a large impact on

emulsion microstructure and overall properties

Non-flocculated Flocculated

Low ViscosityStable to Creaming

High ViscosityUnstable to Creaming

Droplet Interactions and Organization

25

ATTRACTIVE

van der Waals

Hydrophobic

Depletion

REPULSIVE

Electrostatic

Steric

h

Aggregation occurs when attractive interactions outweigh repulsive interactions

w(h)

Aggregation Depends on Balance of Colloidal Interactions

26

-150

-100

-50

0

50

100

150

0 5 10 15 20 25

Attractive

Repulsive

Overall

h

Predicting Colloidal Interactions

• Sign

• Magnitude

• Range

w(h

)/k

T

h

27

Knowledge of colloidal interactions used to:

Stable Unstable

Colloidal Interactions

• Predict stability to droplet aggregation

• Identify origin of emulsion instability

• Develop strategies to prevent instability

• Predict emulsion rheology

28

� Influences droplet stability to aggregation, and therefore physicochemical properties

� Influences droplet interactions with other charged

species - biopolymers, mineral ions, vitamins,

antioxidants, etc

++

+

+

+ +−−−−

−−−−

−−−−

++

+

SO42-Ca2+

Fe2+

100

1000

10000

3 4 5 6 7

pH

Dia

me

tete

r

Droplet Charge:Importance

29

� Adsorption of charged species to droplet

surfaces

�Emulsifiers, biopolymers, small ions (e.g. OH-)

-CO2-

-NH3+

-SO4-

-PO4-

+

+

+++

+

−−−−−−−−

−−−− −−−−−−−−

+

+-

-

Protein

Hydrocolloid

Surfactant

-

OH-

pKa

Droplet Charge:Origin

30

� Charge measured by electro-kinetic techniques

� Effective electrical potential on droplet

� Depends on charge density and ionic strength

+

-

- -

-

-

-

-

-

-

--

-+

+

+

+

Measure Direction and Velocity gives Sign and Magnitude

+−−−−

ElectrodeElectrode

Droplet Charge:

Measurement of ζ-Potential

31

-100

-75

-50

-25

0

25

50

75

3 4 5 6 7 8

Knowledge of droplet charge can be used to:

� Predict emulsion stability to droplet aggregation

� Characterize interactions between droplets and other charged species

-60

-40

-20

0

20

40

60

0 0.01 0.02 0.03 0.04

Droplet Charge: Utilization in Emulsion Scienceζζ ζζ-

po

ten

tial

pH

ζζ ζζ-p

ote

nti

al

Chitosan (wt%)

32

� Influence emulsion stability to flocculation, coalescence, partial coalescence, Ostwald ripening &

creaming

� Influence partitioning of food ingredients

� antioxidants, flavors, colors, minerals etc

� Influence release characteristics

Characteristics:Composition

ThicknessRheology

Charge

Interfacial Characteristics

33

� A better understanding of the role of droplet characteristics on emulsion properties would enable

manufacturers to improve product shelf-life, appearance, flavor, texture and develop reduced fat products

Droplet Characteristics: Practical Significance

34

emulsion droplets: characteristics and importance · size-class: number ( n), surface ( s) or...

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