Yeast flocculation: New story in fuel ethanol production

Abstract

Yeast flocculation has been used in the brewing industry to facilitate biomass recovery for a long time, and thus its mechanism of yeast flocculation has been intensively studied. However, the application of flocculating yeast in ethanol production garnered attention mainly in the 1980s and 1990s. In this article, updated research progress in the molecular mechanism of yeast flocculation and the impact of environmental conditions on yeast flocculation are reviewed. Construction of flocculating yeast strains by genetic approach and utilization of yeast flocculation for ethanol production from various feedstocks were presented. The concept of self-immobilized yeast cells through their flocculation is revisited through a case study of continuous ethanol fermentation with the flocculating yeast SPSC01, and their technical and economic advantages are highlighted by comparing with yeast cells immobilized with supporting materials and regular free yeast cells as well. Taking the flocculating yeast SPSC01 as an example, the ethanol tolerance of the flocculating yeast was also discussed.

Introduction

Yeast flocculation is a non-sexual and reversible cell aggregation in which cells adhere to each other to form flocs (Stratford, 1989). The brewing industry often takes the advantage of the flocculation of brewer’s yeast Saccharomyces cerevisiae as a simple and cost-effective way to separate yeast cells from fermentation products (Verstrepen et al., 2003). The proteins responsible for adhesion of cells to cells or cells to abiotic surfaces, which are called “adhesins” or “flocculins”, are cell wall proteins with different sugar binding properties. The mechanisms of adhesion were reviewed and categorized into two main groups by Verstrepen and Klis (2006): Lectin-like adhesion and sugar-insensitive adhesion. The lectin-like adhesion is caused by the binding of the lectin-like carbohydrate binding domain of the adhesins to sugar residues on the surface of other cells, whereas the sugar-insensitive adhesion depends on the binding of the adhesins to peptides or increased hydrophobic interactions between the cells and certain abiotic surfaces by the adhesion. Lectin-like yeast flocculation was further classified into two main categories: Flo1 and NewFlo (Stratford, 1989, Stratford and Assinder, 1991, Sieiro et al., 1995). The Flo1 type strains are constitutively flocculent, whose flocculation is only inhibited by mannose; whereas the NewFlo type strains start to flocculate in the stationary phase, and their flocculation is inhibited by both mannose and glucose. While most laboratory strains are of the Flo1 type, the majority of brewing yeasts belong to the NewFlo type (Sieiro et al., 1995), which need careful control of the conditions for the onset of their flocculation, because too early or too late flocculation leads to either sluggish fermentation or the need of centrifugation to recover the yeast cells (Heine et al., 2009); however, both the genetic and physiological control of the NewFlo type yeast flocculation have been proved to be highly complicated. The mechanisms of yeast flocculation and its application in the brewing industry were reviewed previously (Domingues et al., 2000, Verstrepen et al., 2003, Verstrepen and Klis, 2006). The recent years witness new progress in the genetic basis and regulation of yeast flocculation (Fichtner et al., 2007, Govender et al., 2008, Smukalla et al., 2008) as well as the genetic engineering applications of flocculating yeast for enzyme and ethanol production (Domingues et al., 2002, Guimarães et al., 2008, Wang et al., 2008).

Although the application of flocculating yeast in the brewing industry has a long history, its use in ethanol production is still very limited, particularly in the production of fuel ethanol at a commercial scale. In this article, updated research progress in the FLO genes responsible for yeast flocculation, their expression and regulation, recombinant manipulations and physiological factors affecting yeast flocculation is reviewed, and ethanol production with flocculating yeast from various feedstocks is evaluated. In the meantime, the self-immobilization of yeast cells through flocculation is revisited, and their advantages and disadvantages are illustrated by comparing with immobilized yeast cells with supporting materials and free yeast cells as well. Taking the self-flocculating yeast SPSC01 as an example, the ethanol tolerance of flocculating yeast was also discussed.