Arnold’s Food ChemistryLesson 2: Emulsions, Gels, & Foams (Science in Everyday

Life)

Emulsions

• Emulsion – a colloid* in which liquids that do not normally mix are spread throughout each other.

• Emulsifying is done by slowly adding one ingredient to another while simultaneously mixing. This disperses and suspends tiny droplets of one liquid (known as the dispersed phase) through another (known as the continuous phase).

• *Colloid – a mixture of very tiny particles that are dispersed in another substance but do not settle out of that substance

Emulsions (cont.)

• To prevent the mixture from separating, emulsifier, which is attracted to both oil and water, is added, thus allowing the two to mix.

• The emulsifier functions by surrounding the oil droplets to form a protective coat which holds the oil droplets in suspension.

• One part of the emulsifier molecule (the polar end) is soluble in water and one part is soluble in the oil (the non-polar end).

Oil

Non-polarendPolar

end

Water

Emulsion Example: Mayonnaise

• Mayonnaise is an example of an oil-in-vinegar emulsion

• The basic ingredients in mayonnaise are: – Large quantities of oil

(dispersed phase)– Small quantities of an acid

(continuous phase), such as vinegar or lemon juice, and

– Egg yolk (the emulsifier)– Other ingredients may be

added for flavor

Emulsion Example: Mayonnaise (cont.)

• The egg yolk is added because it contains lecithin, a naturally occurring emulsifier.

• The lecithin functions by surrounding the oil droplets to form a protective coat which holds the oil droplets in suspension.

• One part of the lecithin molecule (the polar end) is soluble in water and one part is soluble in the oil (the non-polar end). Since lecithin is attracted to both the oil and the water it prevents them from separating.

Polar End

Non-Polar End

Foams

• A foam is a type of colloidal dispersion in which very tiny particles of gas are dispersed (scattered) in a liquid or solid substance and do not settle out of that substance.

• Examples of foods that are considered foams include ice cream, whipped cream, foamed milk, marshmallows, and beaten egg white.

Foams (cont.)

• A foam is made by agitating a liquid (by beating or mixing) which in turn traps air inside the liquid film.

• As air is trapped in the liquid the dispersion increases in volume.

• This increase in volume is known as overrun.

Egg White Foam

• Egg white foam is a type of foam used in meringues, soufflés, foamy omelets, angel food cake, and sponge cakes to make them light and porous (airy).

• An egg white foam is a colloid of bubbles of air surrounded by part of the egg white protein (albumen) that has been denatured during beating.

• *Denaturation is the change of a protein’s shape under stress

Gels

• Gels are defined as more-or-less rigid colloidal systems

• In the case of gels, solid particles are dispersed in a liquid. The solid particles form a network structure which traps the liquid and gives the gel its shape.

• In food preparation, gels are often formed by the proteins of eggs or flour in such products as puddings, batters, and doughs. Gelatin, a type of protein found in the bone and skin tissue of animals, also forms gels.

• Some types of carbohydrates such as alginate, starch, and pectin, also form gels.

Alginate Gels

• Alginate is a type of polysaccharide that occurs naturally in all brown algae as a skeletal component of their cell walls.

• Alginate is used in food because it is a powerful thickening, stabilizing, and gel-forming agent.

• Examples: ice cream, fruit-filled snacks, salad dressings, pudding, onion rings, and even the pimento strips that are stuffed into green olives.

Alginate Gels (cont.)

• Most alginate used in foods is in the form of sodium alginate

• In order to form a gel, sodium alginate needs to come into contact with divalent ions such as calcium (Ca2+).

Alginate Gels (cont.)

• The calcium ions are able to crosslink the alginate polymers because they can form two bonds, as opposed to monovalent ions such as sodium which can only form one bond.

• The longer the alginate is in contact with the calcium chloride solution, the more rigid the gel will become, as more crosslinks are formed.

• Also, depending on the concentration of calcium ions, the gels are either thermo-reversible (low concentrations) or not (high concentrations).