The right temperature for fermentation depends on various factors. The optimal temperature is around 25 °Celsius, but there are also cooled fermentations with 12 °Celsius and, in red wine production, warm fermentations even up to over 30 °Celsius. When the critical boiling temperature of 35 to 40 °Celsius is reached, fermentation faults occur. The yeasts are poisoned by the warm alcohol and the grape must gets stuck in fermentation as a result. Since the acetobacter (acetic acid bacteria) can tolerate a higher heat load than the yeasts, the fermentation of the sugar takes place with them. This leads to acetic souring and brown curd (brown discolouration). Furthermore, there is a loss of aroma and alcohol and the retention of any undesirable sugar residues (residual sugar) ( if the wine is to be vinified dry ). In extreme cases, the wine is spoilt. A boiled must must be inoculated with dry yeast for fermentation. See also under wine faults.
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