a term referring to the removal of a substance from the surface of a fermentation, usually one taking place in an open vessel. The precise meaning depends on the type of beer being brewed. For lager fermentations, skimming usually refers to the removal of proteinaceous cold break material that floats to the top of the wort soon after it is cooled into the fermenter. This material, which is brown and granular in appearance, is felt by some brewers to be detrimental to beer flavor. Others prefer to leave it intact, citing evidence that it contains nutrients that promote healthy fermentation. Once high krausen is underway, another residue may eventually form on top of the foam. Called brandhefe by German brewers, this may also be skimmed because it contains hop resins that may coarsen the beer’s bitterness.

In traditional British ale brewing, skimming refers to the recovery of brewing yeast toward the end of fermentation in a fermenting vessel where a “top cropping yeast” is used. Yeast is normally skimmed from relatively shallow square or round fermenters with wort depths of 2 to 4 m (6.5 to 13.0 ft); the vessel may be open or closed (with a top cover). Yeast is not skimmed from tall cylindroconical fermenters; yeasts in such tanks will usually sediment to the bottom of the vessels. When a beer is fermented using a top cropping yeast strain, the yeast will separate out from the body of the fermentation and rise to the beer surface. This will happen when most of the nutrients have been used up, so the yeast has no reason to stay in the wort; it will therefore flocculate (combine together in clumps) and form a thick, creamy yeast head on the beer. At a predetermined time, usually when a fermentation has reached a particular gravity, the yeast will be taken from the surface in a number of ways, depending on the brewery. The most common method is to suck the yeast off the vessel using a vacuum pipe attached to a tray that is floating on the surface of the beer; a special skimmer or paddle is used to pull the yeast into the tray and vacuum pipe. The yeast skimmed from a vessel may be collected and stored for use in following fermentations, with the excess often sold another brewery, a distillery, or as a by-product for animal feed.

See also brandhefe, fermentation, flocculation, and high kräusen.