Page 310 - Pharmacognosy 2 PG303

Pharmacognosy-2 (PG303) Level 2 Clinical Pharmacy-Pharm D

Agar is the dried mucilaginous substance prepared from Gelidium corneum
(Ruds.) Lam., G. cartilagineum Gaill. (Fam. Gelidiaceae) and other closely allied
red algae of Japan. Agar contains not more than 1 % of foreign organic matter.

Preparation
In Japan the sea weeds are collected in May and October. They are then spread
upon the beach to dry. The dried sea weeds are beaten and shaken to remove shells,
sand, etc., and are taken to factories where the weed is washed in water, bleached by
exposure to the sun and then boiled in open boilers for 5-6 hrs with about 50 times
its weight of very faintly acidulated water (dil. H 2SO 4 1/40,000). The liquor is
strained through cloth and transferred to wooden troughs about 100 x 40 x 8 cm,
where it is allowed to cool in the open air and the liquid congeals. The jelly is cut
into pieces about 40 x 8 x 5 cm using knives guided by a ruler. These rectangular
pieces of jelly are put into wooden cases of slightly larger internal dimensions, one
end of which is covered by wire netting; the jelly is then forced through the netting
by means of a wooden plunger pushed into the case. The narrow strips thus formed
are spread out on rush mats to dry and bleach in the frosty air and sunshine. The
alternate freezing and thawing of the product helps to remove the water from the
strips and for this reason the manufacture is conducted in the winter only.

Description
Japanese agar occurs in greyish-white, translucent strips about 60 x 0.5-1 x
0.1 cm, occasionally in flattened sticks, about 30 x 2.5 x 0.5-0.7 cm and having a
slightly yellowish tint. The surface is crinkled, and somewhat micaceous, and
various species of diatoms are found embedded in it, the most characteristic being
species of Arachnoidiscus, which is discoid and has a sculpturing on its valves in the
form of radiating lines and concentric circles. Agar may occur as greyish-white
granulated powder. Agar is tough and difficult to break. It has a slight odor of
marine algae and a faintly salty mucilaginous taste.

Solubility
Agar is insoluble in cold water, but is slowly soluble in boiling water, and
0.75-1.0 % boiled with water forms a colloidal fluid, which sets to a firm jelly on
cooling.

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