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.