Yeast quality in industrial fermentations

Industrial pitching yeast for production of alcoholic beverages or fuel ethanol should ferment the sugars fast and completely. Fresh optimal propagated yeast can normally fulfill these industrial demands. However, cropped, treated, stored or dried yeast is in most cases not a good pitching yeast due to its bad physiological condition. The amount of storage polysaccharides - glycogen and trehalose - can be too low, the fluidity of the membrane for active transport of fermentable sugars can be insufficient, ATP regeneration can be too slow and the enzymes for glycolysis can have limited activity. Quality parameters for yeast are viability and vitality. Vitality is a measure for the metabolic capacity of the yeast. Yeast with high vitality can ferment well under stress conditions such as high temperature, high alcohol concentration, osmotic pressure and hydrostatic pressure. Pitching yeast with the same viability can have a completely different fermentation capacity. Most of the methods described in literature for determination of viability and vitality of yeast were tested by us. For the study of industrial yeasts, the following methods were used: plating (as a reference method for the viability), methylene blue staining (microscope and Cellometer), fluorescence flow cytometry with propidiumjodide-Syto 13 and determination of intracellular pH (ICP) and number of scars. The ICP is important for glycolysis and glucogenesis and thus for metabolic capacity of the yeast. The ICP is determined with the pH-sensitive SNARF fluorochrome. Average bud scar numbers are counted microscopically after Calcofluor white staining and allow cell age estimation of yeast. A further complication of industrial fermentations is that the exact composition of the fermentation liquid is not known. Even if the yeast is in good condition, fermentation can stuck due to shortening of zinc, essential amino acids and fatty acids. Also the sugar spectrum can be out of balance, yeast inhibitors present in the fermentation liquid or the fermentation liquid can have a PYF (Premature Yeast Flocculation) potential. We have shown for brewery fermentations that the differences between industrial used yeasts are huge, even under the same fermentation conditions, showing the importance of a good yeast management and the need for more investigation in this field. By using good prepared pitching yeast with the necessary metabolic capacity and by adapting the composition of the fermentation liquid, it is possible to guarantee successful fermentation. Case studies of industrial yeast fermentations will provide the essential information needed therefore. Especially for new industrial fermentations, as high gravity, short, and simultaneous saccharification fermentations, yeast quality and composition of the fermentation liquid are critical. This stresses the need for good control of the latter to solve the fermentation problems