When I was a child, my mother would put out a can of cranberry sauce for Thanksgiving. The sauce would make a "schloop" sound as it left the can, releasing the vacuum. My mother always insisted that it be left in the cylindrical can shape until dinner began. I always thought this was a little embarrassing, but I later found out that food gels were very rare until the middle of the 20th century, as evidenced by the profusion of food gel recipes in cookbooks from the fifties and sixties. Until refrigeration became widespread, only wealthy people could afford the time, energy, and effort to create and consume jiggly food. Gelled cranberry sauce was considered to be a delicacy.
Cranberry sauce is made into a gel with the addition of a gelling agent. In commercially available cans of cranberry sauce this is gelatin, a substance rendered from animals. You may find that juices from your Thanksgiving turkey leftovers become gelled in the refrigerator. A substance called collagen is present in many parts of the bird, including bones, skin, ligaments, and tendons. When the bird is cooked, some of this collagen is rendered into gelatin. This mixes with water released from the tissues, and sets to a gel when the leftovers are cooled. Needless to say, foods with gelatin are not suitable for vegetarians.
Other gelling agents used with food include pectin, a soluble plant fiber used to make jams and jellies, and agar, a substance rendered from seaweed. Agar is also used as a growth medium in petri dishes.
Gels are a type of colloid - a mixture of materials, usually in different phases (gas, liquid, or solid). One material tends to dominate, and this is called the continuous medium. The material suspended within it is called the dispersed medium. There are eight types of colloids:
- Foam - gas is the dispersed medium and liquid is the continuous medium (gas/liquid). Soap lather is a foam
- Solid foam - gas/solid - the porous volcanic rock pumice is a solid foam
- Liquid Aerosol - liquid/gas - hairspray is a liquid aerosol
- Emulsion - liquid/liquid - milk is a classic example of an emulsion
- Gel - liquid/solid - cranberry sauce, cheese, and even opal gems are gels
- Solid Aerosol - solid/gas - cigarette smoke is a solid aerosol
- Sol - solid/liquid - ink is a sol
- Solid Sol - solid/solid - some types of pigmented glass are solid sols
