Yeast consists principally of a substance very similar in composition, and in many of its sensible properties, to gluten; and, when new or fresh, it is inflated and rendered frothy by a large quantity of carbonic acid. When mixed with wort, this substance acts upon the saccharine matter; the temperature rises, carbonic acid is disengaged, and the result is ale, which always contains a considerable proportion of alcohol, or spirit. The quantity of yeast employed in brewing ale being small, the saccharine matter is but imperfectly decomposed: hence a considerable portion of it remains in the liquor, and gives it that viscid quality and body for which it is remarkable. The fermenting property of yeast is weakened by boiling for ten minutes, and is entirely destroyed by continuing the boiling. Alcohol poured upon it likewise renders it inert; on which account its power lessens as the alcohol is formed during fermentation.
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