Source : Webster's Revised Unabridged Dictionary (1913)

Top fermentation \Top fermentation\
   An alcoholic fermentation during which the yeast cells are
   carried to the top of the fermening liquid. It proceeds with
   some violence and requires a temperature of 14-30[deg] C.
   (58-86[deg] F.). It is used in the production of ale, porter,
   etc., and of wines high in alcohol, and in distilling.

Source : WordNet®

top fermentation
     n : a violent kind of alcoholic fermentation at a temperature
         high enough to carry the yeast cells to the top of the
         fermenting liquid; used in the production of ale; "top
         fermentation uses a yeast that ferments at higher
         temperatures than that used for bottom fermentation"