The high-temperature fermentation process refers to fermentation at a temperature between 15 and 20 °C (59 and 68 °F), at which time the yeast will float.
Of course, there are also fermented at 24 °C (75 °F). During fermentation, a layer of yeast foam is formed on the surface, called “barm”.
High-temperature fermentation is also called top-level fermentation. Usually high-temperature fermented beer is naturally turbid, and some beer even has obvious flocculent precipitation (common in Belgium ALE beer).
The low-temperature fermentation process refers to the beer fermentation process at 10 °C (50 °F).
Unlike high-temperature fermentation, which is available in three weeks, low-temperature fermented beer will last longer in the cellar. Interestingly, lager itself means wine cellar.
Because of the temperature, most of the yeast sinks at the bottom of the fermentation vessel during the fermentation process, so low-temperature fermentation is also called bottom fermentation.
Technically high-temperature fermentation can also allow yeast to sink to the bottom of the fermentation vessel, but brewing is traditionally divided into this way.
Low-temperature fermented beer is more clear than high-temperature beer. As for the color of the liquor, it is related to malt, roasting process and other raw materials.