Colloidal Dispersion SOLS, GELS, Emulsions, Foams PDF

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COLLOIDAL DISPERSION: SOLS, GELS, EMULSIONS, FOAMS Classification: Class

Phase

Size

Solution

Molecules

Monophase

1nm

Colloidal dispersion

Particles

Diphase

1nm-0.5μm

Coarse dispersion

Particles

Diphase

0.5μm

Diphase food dispersions: Dispersed phase

Continuous phase

Type

Example

Solid

Liquid

Sol

Milk, syrup

Liquid

Liquid

Emulsion

Mayonnaise

Gas

Liquid

Foam

Meringue

Gas

Solid

Solid foam

Bread

Solid

Gas

Aerosol

Smoke

Rheology: science of deformation and flow behaviour of matter Why study?: ● Study flow of materials in pipes ● Assess texture of foods, creams, capsules ● Make special products [e.g. sip feeds, infant foods, slow/rapid release gels for drugs] ★ Viscosity: resistance to flow or shear stress ★ Shear stress: resistance to an applied force - defined as force/area sheared ★ Shear rate: gradient of velocity in a flowing material Measuring viscosity: ● Capillary viscometer - dilute solutions ● Rotational viscometer - suspensions Rheological properties: ● Newtonian flow: shear stress ∝ shear rate [e.g. oil, syrups, dilute protein suspensions] ● Pseudoplastic flow: shear thinning - apparent viscosity decreases with shear [e.g. polysaccharide gums] ● Dilatant flow: apparent viscosity increases with shear ● Bingham plastic: a yield stress has to be overcome before material starts to flow

GELS

Gel = continuous phase of interconnected particles or molecules mixed with a continuous liquid phase ● Usually formulates on heating and subsequent cooling ● Gelatin: exceptional protein which melts on heating, and sets on cooling ○ Less than 4% gelatin is required for gel formation [e.g. jelly capsules for drugs] ● Reversible gels: cross linked by non-covalent bonds [hydrophobic, electrostatic, hydrogen] ● Non reversible gels: cross linked by covalent bonds [disulphide]

Polysaccharide gels: ● Polysaccharides from trees [gums], seeds, fruits [pectin] and seaweed [carrageenan alginates] are also used to make gelled food, pharmaceutical products and cosmetics ● Less than 1% polysaccharide is required for gelation ● Thermoreversible gels are usually produced [melt on heating and set on cooling] Properties of Gels: ● Can be rigid, elastic or brittle ○ Depends on gelling agent, salt, pH and temperature ● Stability: not stable as they lose water [syneresis] Formation of gels: ● Factors affecting formation: ○ Temperature ○ Chemical alteration of gelling agent [e.g. modified starch] ○ Reduction in number of charged groups by pH adjustment or addition of salt ○ Addition of water competitive compounds [e.g. sugar]

EMULSIONS

Emulsion = when an immiscible liquid is dispersed in small droplets [disperse phase] in another immiscible liquid [continuous phase] Types: ● Water in oil [W/O] emulsion [e.g. butter and margarine] ● Oil in water [O/W] emulsion [e.g. milk, salad dressing, ice cream, mayonnaise

Course emulsions: comminuted meat products [e.g. sausage] - mix made using a bowl chopper Emulsifying Agents: ● Proteins, phospholipids [egg yolk] ● Sterols, esters of fatty acids, glyceryl monostearate [GMS] ● Carboxymethyl cellulose Surfactant properties of emulsifiers: ● Hydrophilic head and hydrophobic tail forms micelles Properties of emulsifiers: ● Polar and non-polar groups adsorb at the interface and form a monolayer ● Emulsifiers lower the surface tension Requirements for good emulsion formation: ● Emulsifying capacity: the rate and contact angle at which the emulsifier penetrates into an O/W interface ○ Lowers the interfacial tension to lessen the work or energy required to produce a given particle size Surface/Interfacial tension: forces that reduce the surface or interfacial area - therefore increasing the surface area to create many small droplets ● Benefits of lowering: stable emulsion, less work required to create emulsion

Which emulsifier to choose? ● Hydrophilic - lipophilic balance [HLB number] = ratio of hydrophilic groups to hydrophobic

groups HLB number

Emulsion type

Examples

3-8

W/O emulsions

GMS

8-11

Both types

Gelatin

11-20

O/W emulsions

Gums

Emulsion Stability: ● Emulsion must be stable once made otherwise it will separate into different layers ○ Factors affecting drop collision: theory of colloidal stability and DVLO theory ● Stability depends on 2 types of interactions between colloid particles: van der waal attraction, and electrostatic repulsion between electrical double layers of identical charge ● Repulsion potential > attraction potential = stable suspension Assessing stability: 1. Measure change in dispersion - drop size analysis by microscopy / turbidimetric measurements / coulter counter / sedimentation rates 2. Charge on the droplet - electrophoresis 3. Application of stress centrifugation / heating FOAMS Foam: dispersion of gas bubbles in a liquid ● Typical food foams: beer [liquid], bread/meringue [solid] ● Foam formation and stability have similar principles to emulsion...