Colloids

Mixtures• Not every mixture allows particles to dissolve• In colloidal dispersion, particles of one substance are

distributed (or dispersed) in another substance without dissolving

Colloidal Dispersions• Homogeneous mixtures that are not true solutions• Particles are relatively large• large molecules or clusters of molecules

Phases• 2 substances may be a combination of any of these phases:

solid, liquid, gas• Examples:• Italian salad dressing: liquid dispersed in liquid (vinegar and oil)• Beaten egg white: gas is dispersed in liquid• Gravy: solid in liquid• Baked custard: liquid in solid

Term Review• Dispersed phase: substance that is dispersed within another• Continuous phase: substance that extends throughout the

system and surrounds the dispersed phase (ex. Water)

How do colloids stay dispersed?1. The motion of the molecules in continuous phase

• Smaller the mass, the faster the speed• Smaller molecules in continuous phase move faster than colloids;

they collide• Collisions change direction of both molecules = keeps colloids

dispersed

How do colloids stay dispersed?2. Colloids are usually molecules of the same substance = have the same charge•Like charges repel – push away from each other•Water molecules align around colloids and form cushions

• Prevents the colloids from sticking together and settling

Colloids vs. Solutes

Colloids

• 1000x larger• Do not dissolve• No impact on

freezing/boiling points• Bend/reflect light onto

another path (Tyndall Effect)

Solutes

• 1 nanometer-1 micrometer• Dissolve in other

substances• Have impact on

boiling/freezing points• Too small to bend light

Tyndall Effect

Milk• Is a colloidal dispersion and a solution• Made up of water, lactose, mineral salts, protein, fats• Solutes: lactose and mineral salts• Colloids: proteins• Water, salt, lactose = solution that is continuous phase which

protein is dispersed

2 types of colloids• Foams• Emulsions

Foams• Dispersions of gas in liquid• Ex. Egg foam: egg white is the continuous phase and air beaten into the

white is the dispersed phase• Ex. Whipped cream: air is beaten into the cream

• Not all liquids can make a foam; the more viscous (resistant to flow) the better the foam• Viscosity of the fat is greater when cooled – chill

cream, bowl, beaters• Addition of solutes can alter foam stability (ex. Sugar)

Whipped Cream1 cup heavy cream2 teaspoon vanilla4 tablespoons powdered sugar

In a large bowl, whip cream until soft peaks are just about to form. Beat in vanilla and sugar until stiff peaks form. Make sure not to over-beat, cream will then become lumpy and butter-like.

**Works best if bowl and beaters are chilled first.

Review Clip• http://www.youtube.com/watch?v=kezsLzyR-Pc

• One type of colloidal dispersion• A mixture of 2 liquids that do not normally blend with each other• Because they can’t combine = immiscible liquids

• 2 types:• Oil in water

• Mayo, salad dressings, ice cream, cake batters

• Water in oil• Butter and margarine

Emulsions

• agitation (stirring/beating) stabilizes mixture• As mixture is agitated• oil breaks into tiny droplets • emulsifier is added and coats the droplets so they can not rejoin

How are immiscible liquids combined?

• Two ends:• Usually has an end that dissolves in water (polar) and one that

dissolves in fat (nonpolar)

What is an emulsifier?

• Egg yolk• Contains the phospholipid lecithin (good for oil in water)

• Honey• Mustard• Starch• Used for gravies and sauces• Must be heated for a long time

Emulsifiers in Foods

Good Eats Questions – Answer in your notebooks

1. Draw a picture of an emulsifier.

2. Why is lecithin important?

3. Describe the steps to make mayonnaise.

4. Why is mustard important?

5. Which foods use mayonnaise as a base?

• 1/4 teaspoon dry mustard powder• 1/8 teaspoon salt• 1 egg (pasteurized) • 1 tablespoon white vinegar• 1 cup vegetable oilIn a food processor, combine the mustard powder, salt, egg, and

vinegar. Mix until well blended. Gradually drizzle in the oil. Cover and store in the refrigerator for up to two weeks.

Mayonnaise