In protein biochemistry study, it is important to concentrate the initial protein extract into small volume.Methods for concentrate protein extract into small volume:- Freeze drying: this method removes water by sublimation.- Dialysis: this method utilizes the semi-permeable membrane to exchange small molecules between two solutions with different concentration. It is commonly used to desalt the protein solution and for buffer exchange.- Ultrafiltration: It can be used for efficient buffer exchange and protein desalting.- Protein precipitation by PEG (polyethylene glycol)- Protein precipitation by ethanol and acetone.