Agar or agar-agar is a gelatinous substance derived by boiling from a polysaccharide in red algae (usually found in Asian waters), where it accumulates in the cell walls of agarophyte and serve as the primary structural support for the algae’s cell walls. Agar is a mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin.
The gelling agent is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the genera Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema). For commercial purposes, it is derived primarily from Gelidium amansii. In chemical terms, agar is a polymer made up of subunits of the sugar galactose.
Agar 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.
Marketed as a dry powder, flakes or strips. Varies in colour from white to light amber, which may fade with prolonged storage. Normally shipped in press-packed bales, lined with tarred paper, covered with gunny, or in drums when in powder form.
Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work. Agar (agar-agar) can be used as a laxative, an appetite suppressant, vegetarian gelatin substitute, a thickener for soups, in fruit preserves, ice cream, and other desserts, as a clarifying agent in brewing, and for sizing paper and fabrics.
Agar is used throughout the world to provide a solid surface containing medium for the growth of bacteria and fungi. Microbial growth does not destroy the gel structure because most microorganisms are unable to digest agar. Agar is typically sold commercially as a powder that can be mixed with water and prepared similarly to gelatin before use as a growth medium. Other ingredients are added to the agar to meet the nutritional needs of the microbes. Many specific formulations are available, because some microbes prefer certain environmental conditions over others.
Used in the manufacture of medicinal preparations, preserves, paper and textiles, a dressing for silk and other fabrics and a culture medium for bacteria and fungi.
Agar is also used:
- As an impression material in dentistry.
- To make salt bridges for use in electrochemistry.
- In formicariums as a transparent substitute for sand and a source of nutrition.
- As a natural ingredient to form modelling clay for young children to play with.
Deteriorates rapidly when in contact with moisture, changing colour to yellow or pink. Dissolves in contact with hot water and in cooling sets to a jelly which renders the product useless for food purposes and as a culture media.