Agar-Agar

AGAR-AGAR, a product obtained by drying and bleaching a number of algae from the group of red algae (Florideae family), which grow in East Asian and South Asian seas. The main component of agar-agar is gelose, a pectin-like substance close in chemical composition to carbohydrates. Agar-agar dissolves exclusively in boiling water, turning into a transparent, glue-like mass with a neutral reaction. In cold water, agar-agar does not dissolve, but only swells and softens. An important feature of agar-agar is its ability to form gels upon cooling after being melted by boiling. The solidification of agar-agar begins at 40–42°, and subsequent liquefaction occurs at temperatures above 90°. Prolonged boiling, as well as repeated boiling, can lower the density of agar-agar. Upon complete solidification, agar-agar releases a small amount of condensation water. Several varieties of agar-agar are distinguished: 1) Japanese—the best variety (the Chinese t'ien-t'ien, Japanese and Chinese gelatin, phacocol, etc.)—is extracted from the algae Gelidium corneum Lamour., Gelidium cartilagineum Gaillon, and others; it forms a gel with 200–300 parts water; 2) Ceylon agar-agar (Ceylon moss) is obtained from Gracilaria lichenoides and forms a gel with 50 parts water; 3) Makassar and Javanese agar-agar are obtained from Euchema spinosum and form a gel with only 17 parts water. The following types of agar-agar are available for sale: 1) four-sided bars (the best), 2) threads 3–20 cm long and 3–5 mm wide, 3) powder (the worst). Agar-agar appeared in Europe in 1840 and was used, mainly, as a substitute for gelatin in confectionery products. Agar-agar received its most widespread application after its introduction by Angelina Hesse into bacteriological technique for the preparation of solid nutrient media. The introduction of agar media was of great importance, since, due to the ability of agar-agar to fully maintain its density at 37°, it became possible to grow microbes on solid media at a temperature optimal for most of them. The basic medium prepared with the help of agar-agar is the so-called ordinary meat-peptone agar-agar. The procedure for its preparation is as follows: 1) add 1.5–2% agar-agar to the finished broth (see), previously crushed and slightly moistened with water or broth for several hours to soften; 2) boil the mixture on a fire, stirring, until the agar-agar is completely dissolved; 3) establish a slightly alkaline reaction (using litmus) with a saturated soda solution; 4) cool the agar-agar to 50°, add egg white for clarification, and mix everything; 5) boil in an autoclave at 115° for 15 minutes; 6) filter through cotton wool moistened with hot water, and check the reaction again with litmus; 7) pour into flasks and test tubes; 8) sterilize in an autoclave at 120° for 15 minutes. In addition to ordinary agar-agar, the same agar-agar with various additives is also used in laboratory practice. Thus, blood agar-agar is obtained by adding defibrinated blood to sterile agar-agar that has been melted and cooled to 50° (1 part blood to 4–5 parts agar-agar); serum agar-agar—by adding blood serum in the amount of 1/4–1/3 of the volume of agar-agar, also after melting and cooling it to 50°; ascitic agar-agar is prepared like serum agar; sugar agar-agar—by adding 0.5–2% of one or another carbohydrate before sterilization; glycerin agar-agar—by adding 2–5% glycerin before sterilization.