Yeast flocculation: what brewers should know.

For many industrial applications in which the yeast Saccharomyces cerevisiae is used, e.g. beer, wine and alcohol production, appropriate flocculation behaviour is certainly one of the most important characteristics of a good production strain. Yeast flocculation is a very complex process that depends on the expression of specific flocculation genes such as FLO1, FLO5, FLO8 and FLO11. The transcriptional activity of the flocculation genes is influenced by the nutritional status of the yeast cells as well as other stress factors. Flocculation is also controlled by factors that affect cell wall composition or morphology. This implies that, during industrial fermentation processes, flocculation is affected by numerous parameters such as nutrient conditions, dissolved oxygen, pH, fermentation temperature, and yeast handling and storage conditions. Theoretically, rational use of these parameters offers the possibility of gaining control over the flocculation process. However, flocculation is a very strain-specific phenomenon, making it difficult to predict specific responses. In addition, certain genes involved in flocculation are extremely variable, causing frequent changes in the flocculation profile of some strains. Therefore, both a profound knowledge of flocculation theory as well as close monitoring and characterisation of the production strain are essential in order to gain maximal control over flocculation. In this review, the various parameters that influence flocculation in real-scale brewing are critically discussed. However, many of the conclusions will also be useful in various other industrial processes where control over yeast flocculation is desirable.