Citric acid is also one of the acids used in winemaking for the purposes of acidification. It is inexpensive but unsubtle and is used almost exclusively for inexpensive wines. It is always added after rather than before fermentation since it can be converted to acetic acid by the yeast. It is produced commercially by fermenting sucrose solutions; very small amounts are recovered from processing citrus fruits. It is also used for cleaning. In the eu it is not permitted for acidification but it can be used to prevent iron casse if blue fining is not possible. See also diacetyl.