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The Collaborative International Dictionary
Bottom fermentation

Bottom fermentation \Bot"tom fer`men*ta"tion\ A slow alcoholic fermentation during which the yeast cells collect at the bottom of the fermenting liquid. It takes place at a temperature of 4[deg] - 10[deg] C. (39[deg] - 50[deg]F.). It is used in making lager beer and wines of low alcohol content but fine bouquet.

Wiktionary
bottom fermentation

n. (context brewing English) A slow alcoholic fermentation process carried out at low temperatures during which the yeast sinks to the bottom of the liquid. Used in brewing lager-type beers.

WordNet
bottom fermentation

n. a slow kind of alcoholic fermentation at a temperature low enough that the yeast cells can sink to the bottom of the fermenting liquid; used in the production of lager