Foams and Emulsions

• Foams and emulsions will be covered

Definitions

• Foams are a dispersion of air (or gas) in a liquid or solid.
• Emulsions are a dispersion of a liquid in another liquid.

Foams

• Air bubbles obstruct water molecules.
• Foams prevents water molecules from flowing easily, thus holding its shape.
• Foams give the whole a thicker body.
• Volume of the liquid increases.
• Contact with air over a large surface area enhances the release of aromas
• A light, airy quality results.

Types of Foams

• Raw: meringues, whipped cream, mousse
• Cooked: zabaglione, mousse, soufflé, bread
• Pressure formed: espresso cream, beer head
• Artificial: any liquid can be made into a foam with a carbon dioxide or nitrous oxide canister

Foam Formation

• Air/gas must be introduced into a liquid or semi-solid.
• Whipping is a method of introducing air/gas.
• Other methods include chemical raising agents/leaveners and pressurised gases.
• Two phases make up foams: a dispersed gas phase and a fluid (water) or solid phase.
• Energy helps to form a foam, but stability depends on the fluid phase.

Foam Stability

• Air and water have different densities.
• Gravity pulls water down, bubble walls thin, and air rises, exacerbating the effect.
• The result is an unstable foam.
• Emulsifiers or thickeners can stabilize foams.
• Examples include lecithin, phospholipids, proteins, starch, pectin, gums, fat, and carbohydrates.

Foam Stability Mechanisms

• Foam collapse prevented by interfering with the free movement of water.
• Drainage and thinning of the bubble walls is slowed, preventing bubble popping.

Making meringue

• Beat egg whites until they reach soft or stiff peaks.
• Although meringues are relatively long-lasting, they need reinforcement to hold up for days.

Meringue Chemistry

• Meringue is made from egg whites.
• Egg whites comprise ~90% water and ~10% protein, plus mineral, fatty material, vitamin, and glucose traces.
• Proteins are responsible for holding the foam together.
• Whipping egg whites unravels the tertiary and quaternary structures of the proteins.
• The whisk drags the liquid, creating a pulling force on the proteins.
• Air creates an imbalance of forces, pulling proteins out of their folded shape.
• Unfolded proteins aggregate at the air-water interface, being amphiphilic.
• Unraveled protein chains bond, creating a network.
• Globulins and ovotransferrin are the main proteins involved.

Stability of Meringues

• A simple egg white meringue will eventually coarsen, settle, and separate into water and protein.
• Meringues can be permanently stabilised with heat, chocolate, or gelatin.

Stabilizing meringues

• Add sugar at the end of whipping.
• Whip in a copper or silver bowl: Cu and Ag ions prevent S-S bonds from forming, preventing proteins from bonding too tightly.
• Acids play the same role as Cu and Ag (forming S-H), such as cream of tartar added (potassium bitartrate KC4H5O6).

Heating Meringues

• Ovalbumin (immune to whipping) unfold and coagulate, doubling protein reinforcement.
• Water evaporates, leaving coagulated proteins as a foam (air in a solid rather than air in a liquid).

Chocolate mousse

• Chocolate consists of cocoa particles (starchy), sugar, cocoa butter (fat), and lecithin (emulsifier).
• The meringue is stabilized in many ways by the addition of chocolate to whipped egg whites.
• The starchy cocoa particles absorb water, preventing it from creeping down.
• Sugar dissolves in the water and forms a viscous syrup.
• Cocoa butter solidifies when cooled.
• Lecithin helps fat and water "mix" using an emulsion.
• Adding egg yolks increases the amount of fat and lecithin.
• Can remain firm for days, refrigerated and with no collapse.

Instability of Meringues factors

• Fats (egg yolk, detergent, oil) replace proteins at the air-water interface and interfere with protein-protein bonds.
• Dissolved Na+ and Cl- ions compete with protein bonding sites, weakening network.
• Extended whipping and over coagulation

Types of Emulsions

• Two types: Oil-in-water: milk, mayonnaise, cream, egg yolk Water-in-oil: butter, vinaigrette
• Emulsifiers are generally needed to prevent splitting.
• What is an emulsion?
• Made of two liquids that don't dissolve, such as oil and water
• oil and alcohol (ethanol) can be mixed and wont form emulsion
• Tiny separate oil droplets are in a continuous aqueous phase for oil-in-water emulsions.
• Tiny separate water droplets are in a continuous oil phase for water-in-oil emulsions.

Properties of Emulsions

• Droplets range from 1/10th to 1/1000th mm across.
• Droplets deflect light rays, making emulsions milky/opaque even when constituents are translucent.
• The more dispersed liquid, the more viscous the emulsion.

Forming Emulsions

• Lipophilic (fatty) and hydrophilic (watery) ingredients have a natural tendency to separate.
• Formation requires force to disperse liquid droplets.
• Shear force is used to achieve this.
• The higher is the shear, the product is more viscous.

Emulsier Role

• It is hard to keep emulsions stable.
• Emulsifiers are used to prevent is from happening
• Lecithin (egg yolks, soy)
• Proteins (egg yolks, milk etc.)
• Both are amphiphilic molecules
• Emulsifiers reduce surface tension, making dispersed liquids easier to not coalesce.
• Dissolved in both oil and water phases, creating a protective layer.

Homogenizing Milk

• Milk naturally separates into fat and skimmed milk (water)
• Homogenizing prevents to split
• Milk is mixed to create an even distribution of all its compounds
• It pass through very small holes at high pressures
• Creates a homogeneous mixture

Mayonnaise

• It made from oil, vinegar and egg yolks
• Vigorous beat to add the oil is required when prepare at home
• It can also be prepared with stick blender
• Produced commercially using a high-shear colloid mill
• Egg yolks are super important

Making Mayonnaise

• Viscosity increase to break oil into more droplets and stabilize in this way.
• Over thick and need to be thinned down with vinegar.
• Too many droplets mean droplets can be too close, making it splits

Mayonnaise preparation

• First beat egg yolk, salt, and vinegar. Then add oil drop by drop while beating well.
• More stable through above actions.
• Add vinegar ( or more vinegar) with it gets stiff

Mayonnaise Chemistry

• Yolks provide lecithin and proteins which acts as an emulsifier
• Beating that egg with salt causes yolk to fall apart making it clearer and viscous.
• The oil is added at the beginning
• Small additions will create large drops which is easy to avoid churning and rise the top

Mustard in Mayonnaise

• Mustard is added for flavour
• Gum naturally helps in stabilising the emulsion since it has proteins and carbohydrates

Making Butter

• Unusual to have a water-in-oil emulsion
• Happens by churning cream with fat content
• Fat is cream is globules and it disperses in the water
• Leaked fat is caused by churning

Cream to Butter

• The damaged globules will come in contact during churning, with the liquid fat flowing together to form a continuous mass
• The larger that continues mass the more churn
• After that the grains are filtered to remove the butter milk
• consolidate water into droplets
• Gives fat with small droplets of the fat in it a W/O Emulsion