Genetic Control of Flocculation

Abstract

Flocculation—or conversely, lack of flocculation—is a most important characteristic of yeast strains used in industrial fermentations. Unfortunately, a certain degree of confusion has arisen by use of the term “flocculation” in the scientific literature to describe different phenomena in yeast cell behavior. The term, as used most commonly within the brewing industry in particular, is defined here as “the (reversible) aggregation of dispersed yeast cells into floes, generally toward the end of fermentation, and subsequent separation of the floes from the suspending liquid.” The definition excludes other forms of yeast aggregation, particularly those of “clumpy growth” and “chain formation,” that is, the nonseparation of daughter and mother cells during growth, which are sometimes also referred to as flocculation. The term “nonflocculent” will, therefore, imply the lack of cell aggregation and consequently much slower separation of (dispersed) yeast cells from the liquid medium. Although yeast separation is very often, as expected, by sedimentation, it may also be by flotation because of cell aggregates entrapping bubbles of carbon dioxide, as in the case of “top-fermenting” brewing yeasts.

We wish to record our gratitude to the Science Research Council and to Allied Breweries (Production) Limited for their financial support, and to P. A. Martin of Allied Breweries, Burton-on-Trent, for his advice and assistance during these investigations.