2.2.3. Fermentation
Conversion of sugars in to ethanol usually takes place in a fed-batch fermentation process with cells recycle; in this process configuration, yeast (Saccharomy cescerevisiae) recovered from a previous fermentation batch is fed to the fermentor prior to the juice; after addition of the juice, the mixture remains in the reactor fo rafew hours, and the sugars are fermented in to ethanol and by products (other alcohols, organic acids, etc.), simultaneously producing carbon dioxide. The process takes place at temperaturesaround30–34 1C and produces wine with relatively low ethanol content (up to 10 1GL) due to yeast inhibition related to substrate, product and temperature. Fermentation vats are usually closed and carbon dioxide recovered is washed in absorption columns in order to recover carried over ethanol, which is sent to distillation. The fermented liquor (wine) produced in the fermentation is centrifuged to remove yeast cells, which undergo a chemical treatment using water and sulfuric acid to reduce bacterial contamination. Yeast cells are used on the next batch, while centrifuged wine is sent to distillation. An alternative to this process is the continuous fermentation, which can be found in some mills and is based on the use of 3–5 reactorsinseries, using the same system for cells recycle described above.