During alcoholic fermentation, Saccharomycestransforms grape sugars to alcohol and CO2. The winemaker’s challenge is to optimize those conditions that favor growth of fermentative yeasts resulting in complete alcoholic fermentation. Usually, completion of alcoholic and malolactic fermentations should result in a wine that is nutritionally insufficient to support further microbial activity. Although musts contain low populations of Saccharomyces, all will carry out fermentation under suitable conditions. However, initiation of these fermentations may require more time than most winemakers are willing to accept, and the outcome is not always predictable. For this reason, yeast starter cultures are used. Overall, the goal of using a starter culture is to start fermentation as quickly as possible while reducing the potential for spoilage by formation of numerical dominancy over native species. Yeast strains isolated from wineries or research institutes that possess good qualities for winemaking have been commercialized. Hundreds of strains have now been isolated, and are marketed to winemakers by several international companies. Unfortunately, the origins of some strains are no longer known. Although winemakers often have strain preferences for particular applications, the issue continues to be one of debate. Each strain possesses different characteristics including varying fermentation rate and production of H2S. Before the development of commercial active dry yeast, winemakers were forced to propagate to use starter inoculum from stock cultures maintained in the winery. This process involves transferring the pure culture to sterile juice. When the appropriate volume of starter was ready and was not microbiologically contaminated, it was then used to inoculate grape musts. Given the difficulty of preparation from stock cultures, enologists appreciated development of commercial fresh wine yeasts starters as early as 1950s and dry wine yeast in 1963s. Nowadays, 85 % of wine makers are using commercial active dry wine starters globally. Viabilities of active dry yeasts in most preparations contain 10-40 billion CFU/g. For vacuum packed products, the monthly loss of fermentation activity is around 1% when stored at 5ºC and 2% when stored at 20ºC. If stored at a high temperature (37ºC), they can lose 80% of activity after 16 months.
The method of starter preparation and propagation is critical to successful fermentation. The practice of spreading pellets into or over the surface of the must should be avoided. In general, most recommend use of water at 37ºC and rehydrate for 15 to 20 min before inoculation. Rehydrated yeasts should not be transferred directly to chilled musts. As a general advice, yeasts should be adapted to the must temperature before inoculation. In addition, caution should be exercised if agitation is used because this practice may result in decreased vitality of starter cultures.