emulsion droplets: characteristics and importance · size-class: number ( n), surface ( s) or...
TRANSCRIPT
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
Particle Diameter (µµµµm)
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
Particle Diameter (µµµµm)
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
Particle Diameter [µµµµm]
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
Particle Diameter (µµµµm)
Nu
mb
er %
0 mM NaCl
150 mM NaCl
0
2
4
6
8
10
12
0.01 0.1 1 10 100
Particle Diameter (µµµµm)
Volu
me%
0 mM NaCl
150 mM NaCl
Vi = (ππππ/6) Ni di3
0 mM 150 mM
Particle Diameter [µµµµm]
Nu
mb
er
%
Vo
lum
e %
Particle Diameter [µµµµm]
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
Particle Diameter (µµµµm)
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?
Particle Diameter [µµµµm]
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
Particle Diameter (µµµµm)
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
Particle Diameter [µµµµm]
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
Particle Diameter (µµµµm)
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
Particle Diameter [µµµµm]
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
Particle Diameter (µµµµm)
Nu
mb
er%
Monomodal
Bimodal
These distributions have the same mean
diameter and the same
distribution width!!!
Particle Diameter [µµµµm]
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
Particle Diameter (µµµµm)
Volu
me%
0 mM NaCl
150 mM NaCl
Vo
lum
e %
Particle Diameter [µµµµm]
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