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Derived from red algae or seaweed, agar agar (often referred to as simply agar) forms rigid, brittle gels which do not melt in the mouth. It has long been a traditional ingredient in Asian cooking, primarily for sweets making, and is also used as a growth medium for laboratory cultures. To use, agar is stirred into cool liquids and then heated to a boil. The gel will set once the mixture cools to 95°F (35ºC), and must be heated to 185°F (85ºC) before it melts again. Start with 0.5 percent of agar agar by weight for a gel of average firmness; acidic liquids will decrease its gelling power. To make pudding-like sauces, set the liquid with agar agar and blend the resulting gel until smooth and translucent.
Large Asian grocers generally carry two types of agar agar; sticks, and powders. Although the sticks can be used for most molecular gastronomy applications, powders tend to be easier to work with as they are more readily dispersed in liquids. The sticks often are brightly colored, whereas the powders are sold in relatively small, hermetically sealed pouches. Alternatively, Japanese grocers sell them as "kanten" (寒天), as agar agar is typically used in Japanese sweets.
Stock filtration: Dissolve about 0.10 % agar (that's one thousandth) in any flavored liquid, and bring to a high simmer. Simmer two minutes. Freeze overnight. Line a strainer with cheesecloth and put it over a bowl in the refrigerator. Put the frozen stock in the strainer. Leave for a day or two. The liquid part of the stock will go through the cheesecloth, leaving behind an ugly lump which can be discarded. The liquid should be totally transparent.