Soft and elastic, the size in the same pot is roughly uniform, without umbrella opening mushroom, deformed mushroom, desmoped mushroom, spotted mushroom, pest infested mushroom, oxidized mushroom, hollow mushroom and thin skin mushroom not more than 10% whole mushroom, button mushroom: the diameter of the fungus is not more than 40 mm, the shape of the cap is complete, the section of the stipe is flat, the length of the whole mushroom stipe is not more than 8 mm, the length of the stipe is basically the same, and the stipe of button mushroom is flush with the cap.

Pour fresh mushrooms of different grades into 0.03% sodium metabisulfite solution, gently turn up and down, and wash away sediment, impurities, waxy substances and lipids on the surface of mushrooms. Rinse for 2 minutes and then put it into running water for washing. First boil the prepared 0.1% citric acid solution in the pre boiling machine, and then put the rinsed mushrooms. The ratio of water to mushrooms is about 3:2. Continue to boil until it is thoroughly boiled, 8-10 minutes in total, and then quickly cool down.