Floculacion

APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Feb. 1995, p. 718–728
0099-2240/95/$04.00 0
Copyright 1995, American Society for Microbiology

Vol. 61, No. 2

Mechanisms of Yeast Flocculation: Comparison of Topand Bottom-Fermenting Strains
´
PASCALE B. DENGIS, L. R. NELISSEN,

AND

PAUL G. ROUXHET*

Unite de Chimie des Interfaces, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium´
´
Received 27 June 1994/Accepted 11 October 1994

The flocculation of two brewing yeast strains, top-fermenting strain Saccharomyces cerevisiae MUCL 38485
and bottom-fermenting strain Saccharomyces carlsbergensis MUCL 28285, has been investigated by means of a
turbidimetric test. The two strains showed different electrical properties, a different hydrophobicity, and a
different surfacechemical composition. They flocculated according to completely different mechanisms; however, no correlation between the cell physicochemical properties and the onset of flocculation was found for
either strain. Flocculation of the bottom strain was governed by a lectin-mediated mechanism. It was inhibited
by mannose and some other sugars, required calcium specifically, occurred in a narrow pH rangedifferent from
the isoelectric point, and was not influenced by ethanol. The onset of flocculation at the end of the exponential
phase was controlled both by the appearance of ‘‘active’’ lectins at the cell surface and by the decrease in sugar
concentration in the solution. Flocculation of the top strain was not inhibited by mannose, did not require the
addition of calcium, and took place at thecell isoelectric point. Low concentrations of ethanol broadened the
pH range in which the cells flocculated, and flocculation was favored by an increase of ionic strength. Adsorbed
ethanol may induce flocculation by reducing the electrostatic repulsion between cells, by decreasing steric
stabilization, and/or by allowing the protrusion of polymer chains into the liquid phase. The onset offlocculation was controlled by both a change of the cell surface and an increase in ethanol concentration. The only
evidence for an adhesin-mediated mechanism was the specific requirement for a small amount of calcium.
between the isoelectric point of yeast cells and the N/P concentration ratio measured at the surface by X-ray photoelectron spectroscopy (XPS). Amory et al. (1) reported that the
influenceof the culture conditions on the tendency of the cells
to flocculate was related to the variation of the surface electrical properties. In an extended study, Amory and Rouxhet (2)
found that top-fermenting strains were systematically more
hydrophobic and less negatively charged at pH 4 than bottomfermenting strains. The onset of flocculation was found to
coincide with an increase in surfacehydrophobicity (27, 53,
62). Straver et al. (59) hypothesized that fimbriae were responsible for both specific interactions and hydrophobicity.
The aim of this work was to achieve a better understanding
of the relative importance of different types of interactions in
yeast cell flocculation. The flocculation behavior of a top-fermenting strain, which is not inhibited by sugars, and a bottomfermentingstrain, which is inhibited by sugars, is studied in
detail by means of a turbidimetric test. The influences of pH,
the concentration of sugars, cations, ethanol and other solvents, and the culture medium are investigated.

Flocculation of yeast cells is usually observed at the end of
beer fermentation and is of great importance in brewery (54).
Its mechanism is not yet completely understoodand is still the
subject of much controversy (for reviews, see references 10, 11,
46, 55, and 56). It is a very complex process which depends on
numerous factors: the yeast strain (genetics, physiological
state, and metabolism), the composition of the culture medium, and the culture conditions (temperature, agitation, and
aeration). The involvement of all of these parameters in the...