Dave Miller's Homebrewing Guide by Dave Miller

Author:Dave Miller
Language: eng
Format: epub
Publisher: Storey Publishing
Published: 1995-03-24T16:00:00+00:00

Flocculation and Secondary Fermentation

Flocculation is the joining together of yeast cells into large clumps. Yeasts vary greatly in their tendency to flocculate. Many yeast strains do not flocculate at all. Others flocculate strongly, very early in the fermentation. When this happens, the evolving carbon dioxide gas carries the flocs to the surface of the beer, where they form the pancake so typical of top-fermenting yeasts. Bottom-fermenters flocculate later, when most of the sugar in the yeast has been consumed and much less gas is evolving. As a result, the flocs fall to the bottom of the fermenter.

The slow end stage of fermentation (when all the simple sugars and most of the maltose has been used up and the yeast is left to chew on ends of maltotriose and other complex, hard to ferment sugars) is called “secondary fermentation.” In the case of ales it is usually rather brief, because most ale yeasts cannot handle the complex triple sugars very well. Lager yeasts, however, can work their way slowly through the less edible foodstuffs, and so lagers have a rather lengthy secondary fermentation. This is especially true if it is conducted at very low temperatures, as is typically the case.

The long secondary fermentation of lager beers is important because lager yeasts produce large amounts of by-products during the active part of fermentation. On the third day of fermentation, lager beer will reek of diacetyl, acetaldehyde, and a host of other compounds. During the cold secondary fermentation, the yeast reabsorbs and further metabolizes these substances, and the end result is a clean, crisp flavor in the finished beer.

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