FST 1800 Fundamentals of Food Science

Week 2 – Food Colloids: Emulsions

Learning objectives
• To study emulsion and type of emulsions • To understand interactions between dispersed droplets
• To classify emulsifiers and mechanism of action for emulsifiers

Lecture outline
• • • • • • Terminology Interactions between dispersed droplets Types of emulsion Emulsifiers Mechanism action HLB values

Terminology
• Colloidal dispersion – dispersion of two or more immiscible materials, containing a dispersed phase in the size range of 1 nm – 1 µm
• Simplest type of colloidal system – a single dispersed phase of particles in a continuous phase (dispersion medium)

Dispersed phase

Continuous phase Interface

Terminology cont.
• A system containing particles > 1 µm that prone to settle under gravity – suspension
• A dispersion or suspension of liquid droplets in a liquid continuous phase – emulsion

• Foam???

Food Colloids
• In food systems, “colloids” includes dispersions containing larger particles (> 1 µm)
• Food dispersions encompass suspensions such as sugar fondant (solid sugar particles in a saturated sugar solution), emulsions such as milk, cream and spread, and foams such as in beer, ice cream & bread

Interactions Between Dispersed Droplets • Two main interacting forces in affecting colloid systems – (1) van der Waals attractive forces and (2) electrostatic • Attractive forces – destabilize colloids
• Repulsive forces – impart stability

Van der Waals Interactive Forces
• Originate interactions from dipole-dipole

• Force between two droplets arising from van der Waals interaction is always attractive for like droplets • The attractive force increases more rapidly as the droplets approach close to each other

Van der Waals Interactive Forces

-

+

-

+

F = -β/r6 β = constant; r = distance in between two droplets

Electrical Double Layer
---- ++++++ --- ++++++ --- ++++++ --- ++++++ --

• Oppositely charged ions – attracted towards surface
• Same charge ions repel away

Electrical Double Layer
• Double layer is important for the stability of colloid • Double layer is sensitive to electrolytes & temperature • The stability of colloid may be manipulated by adding electrolytes or changing temperature

Electrostatic Forces & DLVO Theory
• DLVO theory – named after four scientists: Derjaguin, Landau, Verwey & Overbeek • When two charged surfaces approach so that double-layers overlap, a repulsive force is induced
• DLVO theory suggests that electrical doublelayer repulsion will stabilize emulsion, when the electrolyte concentration phase is less than a certain value

Electrostatic Forces & DLVO Theory cont.

Increasing interdroplet distance

Electrostatic Forces & DLVO Theory cont.
• DLVO theory – the stability of emulsified droplets with two independent potentials that come into action when two droplets approach each other • For small distance, the sum of the two energies is always negative (i.e. net attractive) – aggregation
• When the sum of two energies is +ve – repulsive force

Surface & Interfacial Tension
• Surface tension (N/m) – force acting over the surface of the liquid per unit length on the surface perpendicular to the force

Surface & Interfacial Tension
• Surface tension (γs) – the amount of energy (ΔE) required to increase the surface area between a liquid & gas (e.g. air & water) by an amount ΔA ΔE = γsΔA • Interfacial tension (γi) – the amount of energy required to increase the interfacial area between two immiscible liquids (e.g. oil & water)

Importance of Large Interfacial Area
For a fixed composition: • Decrease size, increase number of particles • Increase area of interfacial contact

Emulsion
• Is thermodynamically unstable • Must be stabilized using emulsifier • Type of emulsion – (1) w/o; (2) o/w
Oil Water

Water

Oil

Identifying Emulsion
• Dilution test – An emulsion can be diluted using same