Filtration in winemaking is used to accomplish two objectives, clarification and microbial stabilization. In clarification, large particles that affect the visual appearance of the wine are removed. In microbial stabilization, organisms that affect the stability of the wine are removed therefore reducing the likelihood of re-fermentation or spoilage.
The process of clarification is concerned with the removal of particles; those larger than 5-10 micrometers for coarse polishing, particles larger than 1-4 micrometers for clarifying or polishing. Microbial stabilization requires a filtration of at least 0.65 micrometers. However, filtration at this level may lighten a wines color and body. Microbial stabilization does not imply sterility. It simply means that a significant amount of yeast and bacteria have been removed.