Emulsion Flashcards
Emulsion formed by
Emulsions do NOT form spontaneously
Energy input is required
High energy/short time (e.g. homogeniser)
Low energy/long time (e.g. paddle stirrer)
Droplet break-up due to induced turbulence
Homogeniser technology
high pressure homogeniser
Membrane emulsification
Rotor- stator (-rotor) devices
Ultrasound emulsification
Types of emulsions
= Simple emulsions (macroemulsions)
O/W or W/O
= Multiple emulsions
O/W/O or W/O/W
Stabilising emulsions
Can be stabilised by ?
Once an emulsion has been formed it must be stabilised
- initially at its time of formation
- in the longer term for product stability
stabilised by surfactant / polymer /solid particles
Emulsifiers frequently sold for which of their ability
their ability to lower surface tension
- however, it is usually their long term effects which are more important
Droplet formation equation
slide 7
Kolmogorov and Hinze:
Dm = (ε^-2/5) x (γ^3/5) x (ρ^-1/5) x(k)
Dm = smallest drop ε = energy density γ = interfacial tension ρ = density k = constant
a homogeniser can produce how much energy densities
billions of W per cubic metre
surface tension is measured in
mN
emulsifier’s primary role is NOT for ?
not on interfacial energy
Surfactants
Surface active agents occur widely in nature and have been used for over 1000 years as emulsifiers in cleaning and in foods.
Molecules with two distinct regions:
Hydrophilic head & hydrophobic tail
Three types of surfactants are usually used in food systems
a) proteins
b) polysaccharides
c) polyol derivatives of fatty acids
- Proteins found nearly all food systems
–> (c) usually in conjunction with (a) and/or (b)
they do not usually produce a system in their own right
Protein surfactant Ex & application
caseinate (O/W) Ice cream whey protein (O/W) cake batter Egg protein (O/W) mayonnaise
Polysaccharide surfactant Ex & application
Methyl cellulose (O/W) artificial cream
modified alginate (O/W) salad dressing
Small molecule surfactant Ex & application
monoglycerides (W/O) margarine
acid esters of MGs (W/O) & (O/W) bakery products
sorbitan fatty acid esters (W/O) confectionery
Kinetic view of emulsion formation
particles
a) large size small size
b) small size small size with surfactants
c) small size with surfactants small size with surfactants [Flocculation / de- flocculation]
d) small size with surfactants large size
see slide 11
Stabilisation of O/W systems using proteins may be considered as a how many stage process?
Any factor that can affect the process?
a three stage process
1) adsorption
2) denaturation
3) coagulation
Each stage is dependent on the type of protein involved [Globular protein (not denaturing) / Molten Globular/ Random Coil Protein]
Difference of high & low conc of protein emulsifiers
At low concentrations “true” surface behaviour may be seen. The surface may then be regarded a simple interface
At higher concentrations however the surface may be considered to be a “condensed liquid” even behaving as a viscoelastic or solid like surface
Very dependent on the protein conformation and packing
what may may compete for the surface present and hence inhibit protein stabilisation
small molecule surfactants
Instability types of emulsion
- Coalescence
- Creaming
- Breakaing
- Flocculation
[ Good Emulsion]
Describe Coalescence
can lead to?
instabilised emulsion with two or more separate particles merged into one larger particles
ultimately leading to the total phase separation of the two immiscible phases
Describe flocculation
instabilised emulsion: when particles stick together to form aggregation
This is reversible. Flocculated particles can be separated again by applying more heat to the system (increase the temperature, increase the kinetic energy and hence average speed of the particles) and they can move apart.
Describe creaming
instabilised emulsion: the migration of the dispersed phase of the emulsion, under the influence of buoyancy.
The particles float upwards or sink, depending on how large they are and how much less dense or more dense they may be than the continuous phase
Creaming = droplets moving upward; droplets density < continuous phase density; Oil or fat rises to the surface but remains in the form of globules Sedimentation = vice versa
Describe breaking
instabilised emulsion: Phase seperation
All colloid dispersions are governed by
their thermodynamics
can be unstable with respect to their bulk phases
–> All emulsions will collapse in time
Destabilisation is what kind of a process?
how to overcome?
Destabilisation is a kinetic process which requires the system to overcome an ”energy barrier” and “run down”
Creaming or sedimentation happens due to ?
and will followed by ?
Which instabilised emulsion can lead to creaming?
Creaming or sedimentation is due to the density mismatch between the drops and the continuous phase
Flocculation (aggregation) can be followed by creaming (O/W emulsions)
Describe Ostwald ripening
diffusion of dispersed molecules from small droplets to large ones across the continuous phase, because of the difference in Laplace pressure
P[L1] = 2 [sigma]/ r[1] >/< P[L2] = 2 [sigma]/ r[2]
slide 23
Four main processes of destabilisation of emulsions
1) separation
2) adsorption
3) aggregation
4) bridging flocculation
How to describe the Separation processes of emulsions
Need to determine the creaming rate
Rate may be CP viscosity dependent
How to overcome creaming emulsions
What factors can affect the creaming rate
Globules can be redistributed throughout continuous phase by shaking
Oils of low viscosity tend to cream more readily than those of higher viscosity
Flocculated droplets & Coalescenced droplets will cream rapidly; in comparison, good emulsion will cream slowly
Methods of assessing creaming
a) Meniscus (opaque, no conc. gradient)
b) Removing samples (differing heights)
c) Freezing (sectioning)
d) Ultrasonic testing
Adsorption phenomena can find in which food production ?
aeration of creams /
ice cream properties /
aeration of some cake batters
However not always beneficial (pipework, stirrers and pumps)
The driving force for adsorption is ?
Describe process
the achievement of the minimum surface energy
1) Drop approaches interface
2) Film thins between drop & surface
3) Film ruptures
4) Drop begins to spread
5) Spreading decreases
6) Drop approaches equilibrium
in the process of Film rupture, the drops in emulsion is move by ?
The initial contact with the surface is controlled by ?
When is the most critical stage for film rupture ?
What can promote film rupture?
moved by Brownian forces
controlled by diffusion or convection
The most critical stage is the film rupture which occurs when some critical film thickness is reached
Irregularities in the surface or the drop can promote film rupture (e.g. rough surface/ surface fat crystals)
What is Aggregation processes
This describes a number of particle/particle or drop/drop interactions, mainly:
- Flocculation:
bistate second order kinetics process i.e., it increases with increasing concentration - Coalescence:
unistate first order process which occurs independently of concentration
Define Bridging flocculation
In the presence of macromolecular surfactants during emulsion formation it is possible to observe “bridging” between the droplets by the surfactant
This may be either direct or indirect in nature (e.g. homogenised cream with protein bridging between globules)
Bridging flocculation can lead to ?
Overcome by ?
Bridging flocculation can lead to “network” formation
–> leads to high viscosity systems
This type of floc can disrupted by shear to give a low viscosity system. This is thixotropic behaviour
How to assess bridging or clustering
The viscosity of homogenised creams may be used to assess bridging or clustering
Bridging flocculation depends on
- Size of adsorbing species
- Conformation of the molecule
- Chemistry of the species
- For a given surfactant there will be an optimum concentration at which maximum bridging will occur
