Sentence:  When the pitching heat is high, and the yeast is of a good quality and in sufficient abundance, the fermentation proceeds so rapidly and with such energy that it becomes ungovernable; some means must therefore be employed to check the heat. For this purpose coils of pipe, through which water circulates, are fitted up in the tun. Unless this is done the whole of the glutinous constituents of the gyle is not removed in the yeast, and the liquor does not cleanse satisfactorily, in consequence of an after fermentation which sets in, which is technically known as the "<WSD>fret</WSD>." 
Meaning of the word marked with <WSD> tags:  Agitation of the surface of a fluid by fermentation or some other cause; a rippling on the surface of water.