To avoid the death of effective live bacteria due to excessive temperature during the drying process, the drying temperature of the material should not be higher than 60 ℃ (generally 50 ℃). For this reason, the temperature of the hot air in contact with the material should be different according to the moisture content of the material, generally no more than 130 ℃, so that the hot air temperature can be controlled in sections to improve the thermal efficiency as well as avoid the death of effective live bacteria.