A MANUAL FOR THE PROCESSING OF AGAR FROM GRACILARIA

The name agar originated from the Malay word “Agar-agar”, the local name in the Dutch East Indies for Eucheuma muricatum (spinosum) (Tseng, 1944) which was exported to China for more than a century. For the sake of simplicity agar-agar was shortened to just agar and is now accepted universally whether in the food and other industries or as culture media. The introduction of agar in bacteriology was achieved by a German housewife, Frau Hesse, who prepared the bouillon from agar-agar for her husband's bacterial cultures. The agar-agar came from Frau Hesse's mother who lived in America which was given to her by some friends who lived in Java. Dr. Walther Hesse was so excited about the efficiency of the new medium which his wife prepared, so he relayed his findings immediately to Dr. Robert Koch who was working at the time on the tubercle bacilli. In 1882, Koch reported on the tubercle bacilli and mentioned the new culture medium (Hitchens and Leikind, 1939). Hitchens and Leikind even suggested agar should be called Frau Hesse's medium in honor of the woman who discovered it. According to Tseng (1944) the agar-agar of Frau Hesse from Java could have been carrageenan from Eucheuma.

Tseng (1944) defines agar as the dried amorphous, gelatin-like, non-nitrogenous extract from Gelidium and other red algae, a linear galactan sulfate, insoluble in cold but soluble in hot water, a 1 to 2 percent solution of which upon cooking solidifies to a firm gel at 35° to 50° and melting at 90° to 100°.

The USP XVIII defines and describes agar as the dried hydrophilic colloidal substance extracted from Gelidium cartilagenium (Linne) Gaillon (Fam. Gelidiaceae), Gracilaria confervoides (Linne) Greville (Fam. Sphaerococcaceae) and related red algae. Unground agar usually occurs in bundles consisting of thin, membranous, agglutinated strips or in cut, flaked or granulated forms. It may be weak yellowish orange, yellowish gray to pale yellow, or colorless. It is tough when damp, brittle when dry. It is colorless or has a slight odor and has a mucilaginous taste. Powdered agar is a white to a yellowish-white or pale yellow, insoluble in cold water, but soluble in boiling water. When boiled with 65 times its weight of water for 10 minutes, with constant stirring, and adjusted to a concentration of 1.5 percent, by weight, with hot water, agar forms a clear liquid which congeals at 32° to 39° to form a firm resilient gel, which does not melt below 85°. Armisen and Galatas (1987) reported a wider range of 34° to 43° for the gelling temperarature. Actually according to our observation, the gelling temperature as well as the melting temperature of a 1,5 percent concentration of agar vary according to the seaweed source, the method of preparation and the purity of the sample. The gelling temperature of the agar sols ranges from 30° to 50° and the melting temperature from 82° to 92°.