Emulsion Flashcards

1
Q

Emulsion formed by

A

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

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2
Q

Homogeniser technology

A

high pressure homogeniser
Membrane emulsification
Rotor- stator (-rotor) devices
Ultrasound emulsification

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3
Q

Types of emulsions

A

= Simple emulsions (macroemulsions)
O/W or W/O

= Multiple emulsions
O/W/O or W/O/W

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4
Q

Stabilising emulsions

Can be stabilised by ?

A

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

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5
Q

Emulsifiers frequently sold for which of their ability

A

their ability to lower surface tension

- however, it is usually their long term effects which are more important

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6
Q

Droplet formation equation

A

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
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7
Q

a homogeniser can produce how much energy densities

A

billions of W per cubic metre

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8
Q

surface tension is measured in

A

mN

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9
Q

emulsifier’s primary role is NOT for ?

A

not on interfacial energy

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10
Q

Surfactants

A

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

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11
Q

Three types of surfactants are usually used in food systems

A

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
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12
Q

Protein surfactant Ex & application

A
caseinate  (O/W)  Ice cream
whey protein  (O/W)    cake batter
Egg protein  (O/W)    mayonnaise
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13
Q

Polysaccharide surfactant Ex & application

A

Methyl cellulose (O/W) artificial cream

modified alginate (O/W) salad dressing

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14
Q

Small molecule surfactant Ex & application

A

monoglycerides (W/O) margarine
acid esters of MGs (W/O) & (O/W) bakery products
sorbitan fatty acid esters (W/O) confectionery

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15
Q

Kinetic view of emulsion formation

A

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

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16
Q

Stabilisation of O/W systems using proteins may be considered as a how many stage process?

Any factor that can affect the process?

A

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]

17
Q

Difference of high & low conc of protein emulsifiers

A

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

18
Q

what may may compete for the surface present and hence inhibit protein stabilisation

A

small molecule surfactants

19
Q

Instability types of emulsion

A
  • Coalescence
  • Creaming
  • Breakaing
  • Flocculation

[ Good Emulsion]

20
Q

Describe Coalescence

can lead to?

A

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

21
Q

Describe flocculation

A

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.

22
Q

Describe creaming

A

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
23
Q

Describe breaking

A

instabilised emulsion: Phase seperation

24
Q

All colloid dispersions are governed by

A

their thermodynamics
can be unstable with respect to their bulk phases
–> All emulsions will collapse in time

25
Q

Destabilisation is what kind of a process?

how to overcome?

A

Destabilisation is a kinetic process which requires the system to overcome an ”energy barrier” and “run down”

26
Q

Creaming or sedimentation happens due to ?
and will followed by ?
Which instabilised emulsion can lead to creaming?

A

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)

27
Q

Describe Ostwald ripening

A

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

28
Q

Four main processes of destabilisation of emulsions

A

1) separation
2) adsorption
3) aggregation
4) bridging flocculation

29
Q

How to describe the Separation processes of emulsions

A

Need to determine the creaming rate

Rate may be CP viscosity dependent

30
Q

How to overcome creaming emulsions

What factors can affect the creaming rate

A

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

31
Q

Methods of assessing creaming

A

a) Meniscus (opaque, no conc. gradient)
b) Removing samples (differing heights)
c) Freezing (sectioning)
d) Ultrasonic testing

32
Q

Adsorption phenomena can find in which food production ?

A

aeration of creams /
ice cream properties /
aeration of some cake batters

However not always beneficial (pipework, stirrers and pumps)

33
Q

The driving force for adsorption is ?

Describe process

A

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

34
Q

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?

A

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)

35
Q

What is Aggregation processes

A

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
36
Q

Define Bridging flocculation

A

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)

37
Q

Bridging flocculation can lead to ?

Overcome by ?

A

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

38
Q

How to assess bridging or clustering

A

The viscosity of homogenised creams may be used to assess bridging or clustering

39
Q

Bridging flocculation depends on

A
  • 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