Culture medium is an artificial nutrient for the growth and maintenance of microorganisms, plants and animals, and generally contains nitrogen-containing substances, carbohydrates, inorganic salts, water, vitamins and so on. In culture medium, agar is used as coagulant to transform liquid culture medium into solid culture medium or semi-solid culture medium. Its main function is to solidify liquid culture medium.

Compared with the liquid medium, the solid medium containing agar is simple to operate, the ventilation problem is easy to solve, and it is easy to observe and study. The stability of agar is good.

Definition and characteristics of agar;

Agar, the scientific name of agar, also known as agar-agar, Haidong, freeze-dried powder, agar, gelatin, Phalaenopsis, foreign powder, cold weather, shredded potato, is a kind of plant glue, which is commonly made from marine eucheuma, agar and Gracilaria. It is colorless and does not exist.

Characteristics: The most useful characteristic of agar is the huge temperature difference between its freezing point and melting point. It melts when heated to 95℃ in water, and solidifies when the temperature of the melted solution is reduced to 40℃. It is the best coagulant for preparing solid culture medium. The solid culture medium prepared with agar can be used for high-temperature culture, which does not melt, and the culture will not be scalded to death after inoculation before solidification.

Therefore, agar is the most widely used coagulant in the preparation of various biological media. The concentration of agar is usually 1 ~ 1.5% of the liquid medium.