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Brewing is quite a robust process microbiologically due to a combi-
nation of factors such as low nutrient status, ethanol content and low pH
and there is a limited range of micro-organisms of concern to the brewery
microbiologist. Members of the Enterobacteriaceae such asObesumbac-
terium proteus,KlebsiellaandEnterobacterspecies are sensitive to the
low pH and ethanol of beer but can grow in wort producing off-odours
like dimethyl sulfide which can persist through to the final product. They
also contribute to the production of nitrosamines by reducing wort
nitrate to nitrite. Obesumbacterium proteus is particularly associated
with top-fermenting yeasts but can be controlled by acid washing.
Acetic acid bacteria of the generaAcetobacterandGluconobactercan
be found throughout the brewery. As obligate aerobes they are partic-
ularly associated with cask-conditioned beer where they cause spoilage as
a result of turbidity, ropiness and the oxidation of ethanol to ethanoic
(acetic) acid.Zymomonas mobilisis an anaerobic, Gram negative rod
which can ferment sugars to ethanol. It causes more problems in ale
brewing where it grows in the primed beer producing turbidity and off-
flavours.
Lactic acid bacteria of the generaLactobacillusandPediococcuscan
grow widely in the brewery environment and in beer where they produce
acid, diacetyl, which gives beer a sweet butterscotch flavour, and

Table 9.10 Possible taints in beer

Taint Associated Flavours Possible Cause

Acetaldehyde Apples, paint, grassy Bacterial spoilage: Acetic acid
bacteria/Zymomonas
Sulphur Bad eggs, drains Formed during fermentation,
wild yeasts/Zymomonas
Cloves Herbal phenolic Bacterial spoilage/wild yeasts
Musty/fungal Mouldy, stale, hessian Mould or bacteria: Usually in
water
Fruity Estery, pineapples, solvent,
bananas, pear drops

Wild yeast/Brettanomyes: Wort
bacteria/Enterobacter
agglomerans
Ethyl Acetate Solvent-like Wild yeast/Hansenula anomala
Diacetyl Toffee, butterscotch, honey Formed during fermentation/
lactic acid bacterial spoilage:
Lactobacillus/Pediococcus
DMS Sweetcorn, jammy Bacterial spoilage:Hafnia/
Obesumbacterium. Wort
bacteria
TCP Acetic Acid Medicinal, antiseptic Sour,
vinegar

Bacterial spoilage: Wort bacteria
Bacterial spoilage:
Acetobacter/Gluconobacter
Acidic Sourness and creaminess
Sourness and apples

Bacterial spoilage:Lactobacillus

Acetobacter

Courtesy L. Hargreaves

Chapter 9 355