Today our class split into three groups. One brewed, one learned about draught systems, and my group filtered. Filtering was really quick, so we only had a half day of class today as a result. The filter we used was a plate and frame filter (basically, the frame holds plates, and sheets of cellulose are placed in between the sheets to form a filter bed). We used diatomaceous earth as a prefilter to help remove some of the yeast before the beer reached the cellulose sheets. This helps prevent the filter from clogging and creating a pressure spike. The beer we filtered was a hefeweizen (the yeast will be added back into the beer for bottle fermentation) and it was sterile filtered.
Sterile filtration means that the beer was filtered through a very fine filter (.45 micron), and all of the yeast, protein, and potential bacteria was taken out. This type of filtration is the most commonly complained about type by beer geeks because of the perceived loss of flavor. My theory is that the flavor doesn't change, but the mouthfeel does as a result of stripping out proteins and smaller yeast cells which contribute to the viscosity and body of beer. It was definitely a good day for a half day since it is 75 degrees and sunny here in Munich!