lec 2 food dispersions Flashcards
what are Food Dispersions
Dispersions are physically heterogeneous multi-phase
systems.
Common structure in both tissue-based and formulated
foods
Are dispersions stable or unstable?
Dispersions are physically unstable – kinetic phenomenon
what food has different properties when mixed with air compared to just being frozen?
The properties of ice cream are very different compared to the ice cream mix, air and the molten ice cream.
give Examples of colloidal dispersions
beer foam, bread, ketchup, mayo, chocolate, butter
what is a gas in a liquid called
foam, eg beer foam
what is a liquid in a liquid called?
Emulsion
(e.g. Mayonnaise)
what is a solid in a liquid called?
Suspension
(e.g. Toamato Ketchup)
what is a gas in a solid called?
Solid foam
(e.g.Bread)
what is a liquid in a solid called?
Solid emulsion
(eg Butter)
what is a solid in a soliod called?
Solid suspension
(Chocolate bars)
What characterizes a dispersion?
-Properties of continuous phase
- Properties of dispersed phase
-Properties of interface
- Volume fractions of dispersed phase
name Properties of the continuous phase
-Rheology
-Ionic strength
-Thermal properties etc
name the Properties of dispersed phase
- Solubility in continuous phase
- Size of droplets/bubbles/particles
- Thermal properties etc
what are the Properties of interface?
- Surface tension
- Surface rheology
- Stabilizing molecules
what are some Structural elements in dispersions?
-gas bubble
-emulsion droplets
-polymer molecule
-solid particles (amorphous)
-oil droplet with fat crystals
-crystal
-floc or aggregate of particles
-fibers
which foods are have very complex structures?
tissue-based foods, eg an apple
what else is important in dispersions?
Size and polydispersity is important
in dispersions
what are two methos used for Determination of size distributions?
Microscopy
Light Scattering/Diffraction
describe the Microscopy method
-Divide at least 200 particles into size classes
-Slow
-Easy to distinguish aggregates
- provides Shape information
-The Bergenståhl rule-of-thumb rapid method
descibe the Light Scattering/Diffraction method
-Fast
-“Black box” measurement in many cases
- Does not distiguish aggregates
-No shape info
-Unreliable data when high polydispersity
-Refractive index
do Colloidal dispersions have a large surface area?
yes!
Example: Homogenized milk (oil-in-water emulsion, containing 3% fat in
droplets with d approx. 1μm) has a surface area of 180 m 2/L milk
what are some Instability problems in colloidal dispersions
-Sedimentation/Creaming
-Flocculation (aggregation)
Flocculation is influenced by surface properties!
Stability is a kinetic phenomenon!
what does the size of the particles influence?
-Influences sensory properties – ice crystals, sandyness etc.
-Influences colloidal stability
what does Stokes law describe?
sedimentation and creaming
The volume fraction of disperse phase has a huge
influence on sedimentation/creaming!
what characterizes Surface active molecules?
*has Hydrophobic and hydrophilic parts
* Hydrophobic parts are excluded by water
* Can be small or large molecules
* In principle all proteins are surface active.
* Often referred to as emulsifiers
give Examples of low molar mass emulsifiers
-Phsopholipids
-Mono- and Diglycerides
-Monoglyceride esters of citric or tartaric acid
(DATEM and CITREM)
-Sodium stearoyl lactylat
what can emulsifiers do in aqeous environments
self assemble!
name some food emulsifiers
1.Macromolecules
-Proteins (Contain both hydrophilic and hydrophobic amino acids)
2.Polysaccharides
Naturally surface active (unusual!): Gum Arabic and
other exudate gums.
3.Hydrophobically modified polysaccharides.
what do Surface active molecules do?
they adsorb at interfaces, ex:
Oil-Water
Air-Water
Solid-Water (depending on interface properties)
An adsorbed layer of molecules is created.
what are emulsifiers used for?
Mixing of two liquids with low solubility, Addition of emulsifier is for stabilization.
which phase is the emulsifier more soluble in?
The phase in which an emulsifier is more soluble
constitutes the continuous phase
Consider a low-fat spread, an emulsion with
volume fraction oil = volume fraction water. How
can you find out which is the continuous phase?
If the continuous phase is oil, then adding more oil (the non-polar solvent) will further dilute the sample without causing phase separation. However, if the continuous phase is water, then adding oil will cause phase separation, with the oil forming distinct droplets in the water.
Conversely, if you add water to the sample and observe that it readily mixes without phase separation, then the continuous phase is water. However, if adding water causes phase separation, with water droplets forming in the oil, then the continuous phase is oil.
what are the five basic mechanisms by which emulsions destabilize?
-gravitational separation/shaking
-flocculation
-coalescence
-oswald ripening/disproportionation
-phase inversion
what 3 different types of foams are there?
1.gas dispersion
2.transient foams
3.stable foams
what does The viscosity of the continuous phase play an important role for?
stability
what 3 different mechanisms affect the stability of foam?
- drainage (gravity driven)/evaporation of continuous phase
2.coalescence (film rupture)
3.disproportionation (oswald ripening)
