The most useful characteristic of agar is the significant temperature difference between its freezing point and melting point. It needs to be heated to 95 ℃ in water before it begins to melt, and the temperature of the melted solution needs to be lowered to 40 ℃ before it begins to solidify. Therefore, it is the best coagulant for preparing solid culture media. Solid culture medium prepared with agar can be used for high-temperature cultivation without melting, and the culture should not be scalded to death when inoculated before solidification. Therefore, agar is the most widely used coagulant in the preparation of various biological culture media. The concentration of agar is usually 1-1.5% of liquid culture medium.