Fermented citric acid can be refined and neutralized to a certain pH value, and then crystallized to obtain sodium citrate. If the refining conditions are not well controlled and the refining process requirements are not fully met, a small amount of protein will be dissolved in the sodium citrate solution. In the process of evaporation and crystallization, a small amount of protein will be precipitated together with sodium citrate and mixed into the sodium citrate product. After sodium citrate is dissolved, protein will not dissolve, and white floc will be produced.