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54 p r a c t i c a l w i n e r y & v i n e ya r d J U ly 2 0 1 4 problems in wine production due to their isolation from spoiled wines. Although it was known that some non-Saccharomyces yeasts could form beneficial metabolites for wine quality, this was outweighed by the high levels of volatile acidity and other negative compounds produced. At the same time, non-Saccharomyces yeasts were considered to be sensitive to SO 2 , known to be poor fermenters of grape must and intolerant to ethanol. Therefore it was accepted that those non- Saccharomyces yeasts, not initially inhib- ited by the SO 2 , died during fermentation due to the combined toxicity of the SO 2 and alcohol. However, research has highlighted the high numbers (10 6 to 10 8 cells/mL) and sustained presence of non-Saccharomyces yeasts in modern wine fermentations leading to revisiting the role of these yeasts in winemaking. Newer research showed that concen- trations between 50 and 100 ppm SO 2 , while effective in white wine fermenta- tions, do not prevent growth of non-Sac- charomyces in red wine fermentations. Generally, SO 2 concentrations between 0 and 50 ppm have been successfully used for wild fermentation. During crushing, the non-Saccharomy- ces yeasts on the grapes, on cellar equip- ment and in the cellar environment (air- and insect-borne) are carried over to the must. However, cellar surfaces play a smaller role than grapes as a source of non-Saccharomyces yeasts, as S. cerevisiae is the predominant yeast inhabiting such surfaces. Dominant yeasts in must after crushing should therefore be the same as those that are found on grapes. Despite all the variables in grape har- vest and wine production, the yeast spe- cies generally found on grapes and in wines are similar throughout the world. However, the proportion or yeast popu- lation profile in various regions show distinct differences. The contribution by non-Saccharomyces yeasts to wine flavor will depend on the concentration of metabolites formed. This in turn is affected by how active the non-Saccharomyces yeasts are. The specific environmental conditions in the must include high osmotic pres- sure, equimolar mixture of glucose and fructose, presence of SO 2 , non-optimal growth temperature, increasing alcohol concentrations, anaerobic conditions and decreasing nutrients. All these factors play a role in determining what species can survive and grow. The clarification of white must (centrif- W I N E M A K I N G Left to right: Metschnikowia and Pichia.