The Collaborative International Dictionary of English v.0.48:

Top fermentation \Top fermentation\
   An alcoholic fermentation during which the yeast cells are
   carried to the top of the fermenting 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.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

top fermentation
    n 1: 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"