Definitions: Colloid, phase, and colloidal systems • Food Colloids - classification • Emulsions and emulsifiers • Sols and gels • Foam

1. DAVID TRYSON TEMBO, PhD
MUBAS, PBS, MALAWI
1
PBS-FCH-311
FOOD COLLOIDS

2. Lecture overview
• Definitions: Colloid, phase, and colloidal
systems
• Food Colloids - classification
• Emulsions and emulsifiers
• Sols and gels
• Foams

3. Learning outcomes
• At the end of this session, you should be able to:
– Describe what gels, emulsions and foams are and
give food examples of each
– Link key drivers and quality parameters (texture) to
the structure of a number of food products
– Discuss the functionality of phospholipids and
proteins in the structure and texture of emulsions
and foams

4. What is a colloid?
"A colloid is a dispersion of large
molecules through a continuous phase"

5. What is a phase?
• A phase is a homogeneous amount of matter

6. Colloidal Systems
• Colloidal system: made up of large molecules of a
substance dispersed in a continuous phase
Think of it as "the container and the contained"
• The continuous phase (container) can be
• Liquid
• Solid
• The disperse phase (contained) may be:
• Solid
• Liquid
• Gaseous

7. Food colloids
Continuous phase: water
Dispersed phase: proteins, lipids, sugars
Continuous phase: oil
Dispersed phase: water
Most foods are multi-phase systems

8. Food colloids
• The main molecular components of food colloids are
proteins, lipids and polysaccharides.
• Colloids consist of particles substantially larger than atoms
but too small to be visible to the naked eye (10-7 to 10-3 m)

9. • Colloid: any substance consisting of
particles substantially larger than atoms or
ordinary molecules but too small to be
visible to the unaided eye (10-7 to 10-3 m)
Dispersed phase
Continuous
phase
Gas Liquid Solid
Gas N/A Liquid aerosol
Hair sprays
Solid aerosol
Liquid Foam
Cappuccino foam
Emulsions
Milk - Mayonnaise
Sol
Starch in water
Solid Solid foam
Meringue - Bread -
Ice cream
Gel / Solid Emulsions
Jellies / Butter
Solid sol

10. Emulsion
Foams
Gels
Emulsion
Emulsion

11. Solutions, dispersion and
suspension
Solution: the dissolved substance is < 1 nm (10-9 m) in
size (e.g. sucrose solution): not a colloid
SIZE!
Dispersion: the particles are between 1 nm and
1 m in size (e.g. proteins, starch)
Suspension, if the particles are > 1 m (10-6 m)
in size (e.g. flour in cold water)

12. Emulsions
• The surface tension causes a liquid to assume the
minimal surface area
• When we mix water and oil we increase the area of
contact
• Can be defined as
• O/W (salad dressing)
• W/O (Butter, Margarine)

13. Emulsions
– Oil-in-water (o/w)
• Oil droplets forming the disperse phase are dispersed
throughout the water
– Water-in-oil (w/o)
• Water droplets are dispersed through the oil
Oil droplets

14. Destabilized emulsions
Flocculation: small particles come
together form aggregates (reversible)
Coalescence: particles merge forming
larger spheres (irreversible)
Creaming: aggregates float / sink -
less dense than continuous phase
Breaking: separation of
phases

15. Emulsifiers
• To form a stable emulsion a third substance
called an emulsifying agent or emulsifier is
added
• Emulsifiers act by lowering the interfacial
tension
• The emulsifier becomes adsorbed at interface
and lowers the surface tension making the
system more stable

16. Emulsifiers
• Emulsifier molecules contain a hydrophilic and a
hydrophobic group
– The hydrophillic group is polar and is attracted to water
– The hydrophobic group is non-polar and is attached to
the oil
Hydrophillic
group
Hydrophobic
group

17. Nature of the emulsion
• The type of emulsion formed depends on a
number of factors:
– composition of oil and water phase
– proportions of oil and water
– chemical nature of emulsifier
• HLB (hydrophilic-lipophilic balance) is used to
choose a suitable emulsifier

18. Natural emulsifiers
• Phospholipids
– Lecithin (HLB values 4 to 9)
• Proteins
– Casein in milk
• Carbohydrates
– Acacia gum in fizzy drinks

19. Proteins as emulsifiers
• Diffusion to the interface
• Energy of the system is lowered by
– Having hydrophobic parts of the molecule
sticking into the organic (oil) phase
– The hydrophilic part remaining in the aqueous
phase
– Adsorption occurs, protein is denatured Water
Oil

20. Artificial emulsifier
• Glycerol monostearate (GMS)
– E471
Hydrophilic
Hydrophobic

21. Food emulsions
Example Type Main emulsifiers
Milk, Cream o/w Proteins (casein)
Butter w/o Proteins (casein)
Mayonnaise, Salad
cream
o/w Egg-yolk (lecithin), GMS, mustard
Margarine w/o Proteins (casein), lecithin, GMS
Ice cream o/w Proteins (casein), GMS plus stabilisers
(gums, alginates)

22. Strategies for low fat products
• Creamy mouthfeel linked to oil droplet size and
number
• Evidence that droplets are not sheared in the mouth
• Double or duplex emulsions to “empty” droplets of fat
Water
Oil

23. Sols and Gels
• A sol is a solid in a liquid (e.g. starch in warm water)
– The dispersed phase is formed by high MW molecules
(e.g. starch, protein)
– A sol can set to a gel if the concentration of DP is high
enough
• A gel is a liquid in a solid (e.g. jellies, jams, custard)
– The liquid phase (e.g. water and dissolved solids) is
dispersed in a solid network of polymer chains (high
MW molecules) such as gelatine, pectin, gums, starches,
which 'trap' the liquid
– They have stable structures and retain their shape

24. Gels - Gelatine
Gelatine forms thermo-reversible gels
Insoluble in cold water
Soluble in hot water

25. Stabilisers
• Prevent coalescence, creaming,
flocculation or breaking by increasing the
viscosity
– Pectins
– Starch
– Alginate

26. Gels – Starch - Alginate –
Carrageenan – Pectin
• Technological aims:
– Increase viscosity: creaminess / mouthfeel
– Improve freeze / thaw stability
– Increase water retention
• Thickeners
• Ferran Adrià
– Liquid olives
http://youtu.be/gKWgmx0kc1A

27. Foams
•Liquid Foam
• Gas dispersed in liquid
• e.g. whipped cream, beaten egg-white
•Solid Foam
• Gas dispersed in a solid
• e.g. ice cream, marshmallows
•Foams consist of an agglomeration of gas
bubbles separated by thin films

28. Foams
• Critical for product quality
– Ice cream / Cappuccino / Bread
• Aeration can be achieved by:
– whipping air in
– chemical reaction produces gas e.g. baking
powder produces CO2
– yeasts ferment food (sugar) to produce CO2

29. Foaming agents
• Foaming agents are required in order for
the foam to remain intact.
• They form a cohesive, deformable film
around the air bubbles.
• Similar mechanism as for emulsions
– Decrease surface tension

30. Foaming agents
• Proteins:
– Absorption at the interface, denaturation
• Cellulose derivatives
– CMC (carboxymethyl cellulose)
SDS

31. Foam stability and protein
iso-electric point
• Foam stability is enhanced when pH is
close to the i.p. of the proteins
– The proteins readily denature
• Lower the pH to reach the isoelectric point
– small amounts of acidic materials (e.g. lemon juice,
cream of tartar or vinegar) can be used to lower pH

32. Foamed foods
• Bread, ice-cream, whipped cream…
• Mint gazpacho with bacon and mushroom
foam

33. Structure in relation to sensory
properties
Food Structure
• Droplet size
• Droplet number
• Emulsifier type
• Viscosity
• Macromolecule type
• …..
Oral Processing
• Mechanical process
• Enzymes
• Chemical process
• Temperature
Sensory Properties
• Hardness
• Brittleness
• Viscosity
• Gumminess
• Chewiness
• Adhesiveness
• Grittiness
• Mouth coating

34. QUIZ
Take a paper (can be very small);
do NOT write your name on it

35. 1. In a solution, the substance that is being dissolved
is called?
a. solvent
b. emulsifier
c. solute
d. filtrate
2. An example of a colloid which is an
emulsion:
a. Whipped cream
b. Mayonnaise
c. Gelatin
d. Black coffee

36. 3. A glycerol backbone with 1 or 2 fatty acids plus another
group, possibly a phosphorous group produce?
a. sterol
b. trans fatty acid
c. phosphoric acid group
d. phospholipid
4. Which of the following is NOT an emulsifier:
a. triglyceride
b. bile acids
c. lecithin
d. monoglyceride

37. Answers
Swap papers with the person next
to you

38. 1. In a solution, the substance that is being dissolved
is called?
a. solvent
b. emulsifier
c. solute
d. filtrate
2. An example of a colloid which is an
emulsion:
a. Whipped cream
b. Mayonnaise
c. Gelatin
d. Wine

39. 3. A glycerol backbone with 1 or 2 fatty acids plus another
group, possibly a phosphorous group produce?
a. sterol
b. trans fatty acid
c. phosphoric acid group
d. phospholipid
4. Which of the following is NOT an emulsifier:
a. triglyceride
b. bile acids
c. lecithin
d. monoglyceride

40. Take Away Message
• Colloids are important structures which
are found in foods.
• Emulsions are mixtures of immiscible
liquids which constitute a major groups
of food products.
• Foams are mixtures of gases in a
continuous solid or liquid phase.
• Understanding colloid behaviour
enables technologist to alter the
properties of food products.

41. Further Reading
• Dickinson, Eric (1987) Food emulsions
and Foams, Royal Society of Chemistry.
London.