Cell separation: yeast flocculation

Abstract

Yeast flocculation is a process wherein cells are gathered together into multicellular masses, called flocs.

Due to their mass, yeast flocs sediment rapidly from the medium in which they are suspended. Flocculation is an easy, off-cost and eco-friendly process of cell separation that reduces the costs of equipment and energy demand associated with cell separation. Yeast flocculation can be seen as a self-immobilization process; in this way, flocculent yeasts can be used in high cell density reactors and increase the efficiency of the fermentation process. These advantages fit quite well with the idea of White Biotechnology, which advises the optimization of the processes and reduction of energy consumption in order to contribute to a more sustainable future. Yeast flocculation is a complex phenomenon, influenced by multiple factors. This review discusses and updates the current knowledge on the flocculation of Saccharomyces cerevisiae. The expression of several genes (such as FLO1, FLO5, FLO8, and Lg-FLO1), the cell surface characteristics, the chemical characteristics of the medium (such as pH and the presence of bivalent ions) and the fermentation conditions (oxygen, sugars availability, growth temperature, and ethanol concentration) that affect yeast flocculation are outlined. In addition, the present knowledge about the molecular mechanism of flocculation in S. cerevisiae, as well as the different phenotypes [Flo1, NewFlo, mannose insensitive (MI) phenotype and strains that only flocculate in the presence of ethanol] is presented. This review highlights the advantages of yeast flocculation when compared with yeast immobilization and other cell separation processes. In the last part of the work, the advantages and importance of flocculation in many industrial processes (such as in the production of wine, bio-ethanol, heterologous proteins, and particularly in the brewing industry) and environmental applications is reviewed and critically discussed.