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Summary Food ingredient Functionality - emulsions and foams

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Summary lectures and reader Emulsions and foams of course Food Ingredient Functionality (FIF) at wur

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  • 17 november 2018
  • 24
  • 2018/2019
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Emulsions and foams
• Emulsions and foams
- Both dispersion continuous phase=liquid
- Emulsion: liquid-liquid dispersion (liquid droplets in liquid)
- Foam: air-liquid dispersion (air bubbles in liquid)
- Common characteristics emulsions and foams:
o Both systems have a large amount of interface between the phases
▪ For emulsion: interface
▪ For foams: surface
o Interfaces need to be stabilized by a surface active component
o Both are meta-stable  not in stable equilibrium and both will tend to phase
separate  can happen in days, weeks, months  stable over a certain period of
time (slowly evolving towards complete separation)
o Same type of physical phenomena occur to make the emulsions and foams unstable:
evolve by the same mechanisms (aggregation, coalescence, Ostwald ripening,
creaming)
- Differences emulsions and foams:
o Density difference between the two phases is much larger in foams
▪ Related to the difference in dispersed phase
▪ Emulsions: liquid, foams: air  air much lighter than liquid  large difference
in density between continuous and dispersed phase in foams
o Droplet size is much smaller in emulsions than air bubbles in foams (µm in emulsions
vs mm in foam)
- As a results of these two differences destabilization (creaming) is much faster in foam 
limits shelf-life for foams to large extent
o Shelf life foams: minutes, days, or weeks
o Shelf life emulsions: months or years
Emulsions
• Emulsions
- Two main types of emulsions:
1. Oil in water (O/W): oil droplets in a continuous water phase
o Milk
o Mayonnaise
o Sauces
o Salad dressing
o Ice-cream (also a foam  more complex)
o Large variety in foods
2. Water in oil (W/O): water droplets in an oil phase
o Butter (contains small amounts of water, >85% fat)
o Products are more fatty
o Margarine
o Spreads
o Less common in foods
- Hydrophobic oil and hydrophilic water are not miscible  one phase can
exist in the other one
o Requires the creation of an oil-water interface and these interfaces
are present in both types of emulsions
o Both emulsions are not in a thermodynamically stable state (meta-stable)  phase
separation (oil lower density  upper phase)
▪ Process of phase separation will occur for both oil-in-water emulsions as the
water-in-oil emulsion  but not at the same speed
• Preparation emulsions
- Oil and water have no affinity for each other  contact is not a favourable situation
- To create a surface between oil and water  costs energy:
o Energy can be given as Gibss free energy
o The creation of interface in a system costs energy:

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, o Creating interface (not favourable  not spontaneously  need do add ingredient):
▪ G (free energy) is positive: Process will not occur
spontaneously and not preferred
▪ System always try to keep the interfacial area to a minimum  achieved by
merging droplets  total volume will stay the same, but total surface area will
decrease
o Reducing interface:
▪ G (free energy) is negative: Process may occur
spontaneously
- When system contains an interface  will always try to reduce the amount of interfacial
area to lower the energy
- Why are emulsions meta-stable?
o Interfacial tension determines how much energy is needed to create an interface
o The energy required to create a surface:
▪ Decrease in surface tension
➢ Lower surface tension = related to how easy it is to
make droplets and create surface/interface
between 2 phases (low = easier)
➢  decrease in energy
➢  Easier to create surface
▪ Surface tension: the change of free energy resulting from a change in surface
area
▪ Interfacial tension between water and oil and surface tension
between water and air is quite large  difficult to make small
droplets/air bubbles (requires a lot of interfacial area)
▪ The interfacial/surface tension can be reduced by adsorbing different types of
molecules or particles at the interface  for the same energy input more
surface area can be created (as the surface tension is lower)
▪ The energy required to create a surface can be reduced by adsorbing
molecules or particles at the interface that reduce the surface tension
- Surface tension = results of the interactions of water molecules
o Interactions between water molecules quite strong due to hydrogen bonds
o In bulk phase: water molecules surrounded by other water molecules in oil directions
 due to strong interactions the surrounding water molecules will pull on the middle
water molecule  net force = 0
o When molecules are present at surface (air)/interface (oil)  only water molecules in
bulk water will be able to pull on the water molecule  greater attraction of water
molecules in the water than to the molecules in air/oil  inward force at the surface
 surface will acts as membrane
o As the interaction between water molecules is higher  surface tension between air
and water is relatively high (72.8 mN/m)




57

, - Role of surfactants
o Hydrophilic side (head group) and hydrophobic side (tail) 
able to absorb to interface between oil and water
o Have the capability to reduce the energy needed to form an
interface
o In water:
▪ Unfavourable mixing between water and hydrophobic tails 
micelles (hydrophobic interactions)
▪ Hydrophobic tails are clustered on the inside and hydrophilic
heads stick out
▪ Micel formation  Occurs naturally
o In emulsion:
▪ Tails are present in oil  prevents contact with water
▪ Favourable mixing of hydrophobic tails with hydrophobic oil
o Such surfactants will therefore be preferably present on the interface
between oil and water
▪ Surfactants adsorb to the interfaces between oil and water as much as
possible
o Comparing emulsions with and without surfactants:
▪ No surfactant: only energy costs for creating unfavourable contact between oil
and water  Gibbs energy positive (not preferred)  unstable situation
▪ Surfactant: energy costs for creating unfavourable contact between
hydrophobic oil and hydrophilic water  positive value for Gibbs energy +
energy gain by avoiding contact of hydrophobic tails and water  favourable
situation
▪ Presence of surfactants lowers the surface or interfacial tension  more
favourable  less energy needed to create the same interfacial area
o Energy cost for creating unfavourable contact between hydrophobic oil and
hydrophilic water  ΔG > 0  not stable
▪ Energy gain by avoiding contact of hydrophobic tails and water  ΔG < 0
o Surfactants will decrease the surface tension at low concentration  concentration
too high  molecules already covered  surfactants will stay in bulk phase (and
surface tension will not further decrease)




o How much interfacial tension will decrease depends on:
▪ The size of the molecules
▪ The hydrophobicity
▪ The distance between the molecules (as a result of the mutual interactions)
o Interfacial tension does not say anything about the stability of emulsions  stability
related to other properties of the interface and the interactions between oil droplets
- Emulsification process
o To make emulsion stable  have to create a lot of interactions  requires energy
input  commonly added by mixing process: stirring provides energy to create the
interfaces
▪ How much energy  depends on stirring speed (harder = more)
▪ This amount of energy determines how much interface can be created
depending on the interfacial/surface tension

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