considerations that are often important to winemakers is a yeast's tendency to: Quickly begin fermentation, out-competing other "wild yeasts" for nutrients in the must Completely utilize all fermentable sugars with a predictable sugar-to-alcohol conversion rate Have an alcohol-tolerance up to 15% or even higher depending on the winemaking style Have a high sulfur dioxide tolerance but low production of sulfur compounds such as hydrogen sulfide or dimethyl sulfide Produce a minimum amount of residual pyruvate, acetic acid and acetaldehyde Produce minimum foaming during fermentation which may create difficulties for cap management during maceration or cause bungs to pop out during barrel fermentation. Have high levels of flocculation and lees compaction that makes racking, fining and filtering of the wine easier. Inoculated (or pure cultured) yeasts are strains of Saccharomyces cerevisiae that have been identified and plated from wineries across the world (including notable producers from well-known wine regions such as Bordeaux, Burgundy, Napa Valley and the Barossa Valley). These strains are tested in laboratories to determine a strain's vigor, sulfur dioxide and alcohol tolerance, production levels of acetic acid and sulfur compounds, ability to re-ferment (positive for sparkling wine but a negative attribute for sweet late-harvest wines), development of surface film on the wine (positive for some Sherry styles but a negative attribute for many other wines), enhancement of a wine's color or certain varietal characteristics by enzymes in the yeast cells and other metabolic products produced by the yeast, foaming and flocculation tendencies, yeasticidal properties (a trait known as "Killer yeast") and tolerance for nutritional deficiencies in a must that may lead to a stuck fermentation. Re-hydrating freeze dried yeast cultures Pure culture yeasts that are grown in a lab are often freeze dried and packaged for commercial use. Prior to their addition into must, these yeasts need to be re-hydrated in "starter cultures" that must be carefully monitored (particularly in regards to temperature) to ensure that the yeast cells are not killed off by cold shock. Ideally winemakers want to add enough inoculum to have a viable cell population density of 5 million cells per milliliter. The exact amount of freeze-dried culture varies by manufacturer and strain of yeast but it is often around 1 gram per gallon (or 25 grams per 100 liters). Wines that could have potentially problematic fermentation (such as high sugar level late harvest or botryized wines) may have more yeast added. Similarly, re-hydration