Yeast and Winemaking
(book excerpts)Wine yeasts are a group of fungi that are predominantly uni-cellular (one cell) organisms which are widely distributed in nature. They are fermentative organisms that can metabolize a variety of sugars. Yeast has an equally important role to play in the formation of secondary metabolites of importance to wine, as well as in the conversion of grape aroma precursors to varietal aromas in wine. Many varieties of yeasts occur in nature. Some of the commonly found yeasts on grapes and in wines belong to the following genera: Saccharomyces, Kloeckera, Hanseniaspora, Candida, Hansenula, Pichia, and Brettanomyces. It is commonly believed that in a natural or spontaneous fermentation non-Saccharomyces yeasts such as Kloeckera, Hansenula, Candida and many others grow and participate in earlier stages of fermentation. As the fermentation proceeds the ethanol concentration increases. The high ethanol limits the growth and activity of native non-Saccharomyces yeasts thus creating a condition favorable to the growth and domination of Saccharomyces cerevisiae yeast, which then conducts the fermentation. Advocates of natural fermentation believe that a mixed culture fermentation produces more complex flavors in wine. Other winemakers find natural fermentations unpredictable and prone to the development of off odors. For these and other reasons mentioned later, vintners prefer to use pure Saccharomyces yeast strains in the form of a starter culture in order to conduct the alcoholic fermentation. Inoculation of a must by pure yeast strains assumes that the fermentation will be carried out by the inoculated yeast.
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Topics Within This Chapter:
- Yeast Role in Wine Production
- Life Cycle of Yeasts
- Wine Yeast
- Wine Yeast Strain Selection
- Acetaldehyde Production
- Ester Production
- Ethanol Tolerance
- Fermentation Temperature
- Fruit Condition
- Hydrogen Sulfide Production
- Malolactic Bacteria Compatibility
- Nutrient Requirements
- Sulfur Dioxide Production
- Type of Wine
- Volatile Acid Production
- Volatile Thiol Production
- Spontaneous Wine Fermentation
- Benefits and Risks of Native Yeasts
- Inoculated Wine Fermentation
- Sequential Inoculation
- Commercial Non-Saccharomyces Yeast Strains
- Wine Yeast Culturing
- Dry Wine Yeast Rehydration
- Rehydration Procedure
- Starter Tanks
- Inoculation Rates
- Wine Yeast Viability and Vitality
- Microscope and Hemocytometer
- Automated Yeast Cell Counting
- Wine Yeast Nutritional Requirements
- Nitrogen
- YAN Requirements
- Ammonium and Amino Acid Uptake
- Vitamins
- Minerals
