Generally, four to six days after high kraeusen begins (six to ten days after pitching), the formation of the foam cover ceases. As the availability of fermentable extract drops during the post-kraeusen stage, the yeast adapt to changing conditions by accelerating their secretion of extracellular enzymes capable of splitting off glucose from dextrins in solution. Reasonable levels of diacetyl and the related diketone 2,3-pentane dione are also absorbed and metabolized by the yeast. It is important that the fermentation temperature not be prematurely lowered and that the beer not be racked off its yeast sediment until the diacetyl has been reabsorbed. With a clean fermentation, it is usual for the beer to be held in the primary fermenter for two or three days after the krausen head has fallen, with the temperature being lowered from 45 degrees F (7 degrees C) to 38 to 40 degrees F (3 to 4 degrees C). The extract drop over the final twenty-four-hour period of primary fermentation should be about .5 ºPlato (SG 1.002), and the density should be about one-third what the wort density (OG) was.