Agar producing seaweeds are available from a wide variety of sources (table 1). Agar weeds are typically wild harvested although commercial farms have been used in Chile and Namibia.
|Table 1: Primary agar producing species|
|Gelidiella Acerosa||Japan, India|
|Gelidium Cartilagineum||USA, Mexico, South Africa|
|Gelidium Caulacanthum||New Zealand|
|Gelidium Corneum||South Africa, Iberia, Morocco|
|Gelidium Pristoides||South Africa|
|Gelidium Sesquipedale||Portugal, Morroco|
|Gracilaria Conferviodes||South Africa|
|Pterocladia Capillacea||Egypt, Japan, New Zealand|
|Pterocladia Lucida||New Zealand|
Agar can be extracted in a variety of ways but the basic methodology revolves around dissolving the agar from the seaweed with hot water, separating the agar from the cell wall residues by filtration and then isolating the agar from the dilute solution. Various methods have been developed to isolate the agar from solution, the traditional method relies on cutting the gel into strips and allowing it to freeze overnight and thaw out the next day in the sun. Due to the high level of syneresis produced in an agar gel the strips loose water on each freezing and cooling cycle until a dry strip is formed. this strip is known in Japanese as Kanten which literally translates as "Frozen sky".
Industrially Agar can be made in several ways. One method involves freezing agar solutions in ice tanks in a simply scaled up version of the traditional method. A newer method, which only works for agar types that have significant syneresis such as gracillaria, involves forming a blocks of gel wrapped in clothes and literally pressing the water out of the blocks. The pressing is usually done with large static concrete weights. The pressed agar is then usually pressed again in hydraulic presses to reduce the water content even further prior to drying. A variation of the gel pressing method involves pumping broken agar gel into large filter presses and using the pressure from the feed pumps to force water out of the matrix. This technology was pioneered by Hispanagar in the 1960's and is now the dominant method of pressing agar. Another methodology involves roller drying the extracted agar. This method has the advantage in that it can utilise a variety of agar species including gelidium which cannot be pressed easily.
Agar gel strength can be improved by removing some of the ester sulphates from the agar chain by alkali treatment. Alkali treatment also increases syneresis and makes pressing the agar easier.
Traditional agar is sold in strip or block form. Commercial agar is normally milled into a coarse powder. The agar gel press method is the basis for gel repss methods used in the newer carrageenan processing techniques. As in carrageenan processing not all agar weed types can be pressed and some of the weeds favoured for their low syneresis such as gelidium types are very difficult to press. Very low syneresis agar is favoured in microbial plates.