Sentence:  It is important to allow beer to flatten, after it has ceased working. This is accomplished by leaving the casks open, when the small floating particles of yeast part with their fixed air, lose their buoyancy, and sink to the bottom. ... The beer having thus deposited its remaining yeast will not be liable to <WSD>fret</WSD>. 
Meaning of the word marked with <WSD> tags:  To have secondary fermentation (fermentation occurring after the conversion of sugar to alcohol in beers and wine) take place.