Agar

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Agar or agar-agar, is a jelly-like substance, obtained from red algae.

Composition

Agar consists of a mixture of two polysaccharides: agarose and agaropectin, with agarose making up about 70% of the mixture. Agarose is a linear polymer, made up of repeating units of agarobiose, a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Agaropectin is a heterogeneous mixture of smaller molecules that occur in lesser amounts, and is made up of alternating units of D-galactose and L-galactose heavily modified with acidic side-groups, such as sulfate and pyruvate.

Gelidium agar is used primarily for bacteriological plates. Gracilaria agar is used mainly in food applications.

Properties

Agar is a yellowish, beige or brown (depending on the source) gel, which dissolves slightly in water. It exhibits hysteresis, melting at 85 °C and solidifying from 32–40 °C. This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures. Since many scientific applications require incubation at temperatures close to human body temperature (37 °C), agar is more appropriate than other solidifying agents that melt at this temperature, such as gelatin.

Availability

Agar can be bought from many food stores, as well as online.

Projects/Uses

  • Food (deserts)
  • Vegetarian substitute for gelatin
  • Clarifying agent in brewing
  • Make agar plates
  • Grow microbial cultures
  • Measure microorganism motility and mobility
  • Make salt bridges and gel plugs

Handling

Safety

Agar is non-toxic and is used as food. You should not, however, consume lab-grade agar, though.

Storage

Agar should be kept in dry and clean places, to avoid rotting. A plastic bottle or bag is usually sufficient. Agar is best stored between +15°C to +25°C.

Disposal

No special disposal is required. Discard it as you wish.

See also

References

Relevant Sciencemadness threads