polymeric material known as rope. Yeasts that are not used for the
fermentation can cause hazes and off-odours and are known as wild
yeasts. Normally these are described as beingSaccharomycesand non-
Saccharomyces. Saccharomyces wild yeasts can be detected using a
medium containing copper sulfate to inhibit the brewing yeast. Non-
Saccharomyces yeasts such as Pichia, Hansenula, Brettanomycesand
others can be detected with a medium containing lysine as the sole
nitrogen source whichSaccharomycescannot utilize.
Some of the possible taints in beer and their causes are presented in
Table 9.10.
9.11 Vinegar
Vinegar is the product of a two-stage fermentation. In the first stage,
yeasts convert sugars into ethanol anaerobically, while in the second
ethanol is oxidized to acetic (ethanoic) acid aerobically by bacteria of the
generaAcetobacterandGluconobacter. This second process is a common
mechanism of spoilage in alcoholic beverages and the discovery of
vinegar was doubtless due to the observation that this product of
spoilage could be put to some good use as a flavouring and preservative.
The name vinegar is in fact derived from the Frenchvin aigrefor ‘sour
wine’ and even today the most popular types of vinegar in a region
usually reflect the local alcoholic beverage; for example, malt vinegar in
the UK, wine vinegar in France, and rice vinegar in Japan.
In vinegar brewing, the alcoholic substrate, known as vinegar stock, is
produced using the same or very similar processes to those used in
alcoholic beverage production. Where differences occur they stem largely
from the vinegar brewer’s relative disinterest in the flavour of the
intermediate and his concern to maximize conversion of sugar into
ethanol. In the production of malt vinegar for example, hops are not
used and the wort is not boiled so the activity of starch-degrading
enzymes continues into the fermentation. Here we will concentrate on
describing the second stage in the process, acetification.
Acetification, the oxidation of ethanol to acetic acid is performed by
members of the generaAcetobacterandGluconobacter. These are Gram-
negative, catalase-positive, oxidase-negative, strictly aerobic bacteria.
Acetobacterspp. are the better acid producers and are more common
in commercial vinegar production, but their ability to oxidize acetic acid
to carbon dioxide and water, a property which distinguishes them from
Gluconobacter, can cause problems in some circumstances when the
vinegar brewer will see his key component disappearing into the air as
CO 2. Fortunately over-oxidation, as it is known, is repressed by ethanol
and can be controlled by careful monitoring to ensure that ethanol is not
completely exhausted during acetification. Most acetifications are run on
356 Fermented and Microbial Foods