Fermentation is defined as a form of anaerobic respiration, a process that breaks down carbon-containing compounds while producing energy. Alcoholic fermentation is a process in which yeast, or bacteria, convert sugars into alcohol and carbon dioxide. In this series of articles, I'll discuss principles of fermentation pertaining to wine, though all will be applicable to other products including beer, cider, and spirits.
A successful fermentation can be defined as one that proceeds at a desired temperature, achieves the desired sugar-to-alcohol conversion and concentration, and produces desired sensory qualities (appropriate balance, varietal character, and no undesirable aromas or flavors).
Problem ferments are those that have undesirable yeasts and/or bacteria present and active, too low or too high of temperatures, do not ferment to the desired sugar-to-alcohol concentration, and/or develop undesirable sensory qualities (spoilage, sulfides, and/or lack of fruitiness and complexity).
Several factors affect how well a ferment proceeds:
- Vineyard practices and pesticide residues
- Ratio of potassium ions to hydrogen ions
- Non-soluble solids and sedimentation
- pH of the juice/must
- Toxicity due to CO2, sugar, and/or alcohol
- Presence of wild yeast and/or bacteria
- Levels of oxygen and SO2
- Yeast preparation, strain, and population
- Fermentation rate and temperature
- Ratio of glucose : fructose
- Levels of nutrients and vitamins (amount and timing of additions if required)