Fermented foods
Practical Action
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Figure 2: Equipment for production of wine a) demijohn, b) airlock. Photos: Pete Fellows.
Wines should be stored in airtight containers until they are bottled to prevent contamination by
acetic acid bacteria that would convert the alcohol to vinegar. Grape wine may be bottled
directly, or it can be aged to reduce its acidity and develop a characteristic bouquet. The main
acid in most grape wines is tartaric acid but in some red wines malic acid is present in a high
concentration. A secondary ‘malo-lactic’ fermentation by lactic acid bacteria converts malic
acid to lactic acid, which reduces the acidity and improves the flavour and aroma. Wines made
from other raw materials are usually bottled directly after they have cleared.
Filter aids are used to ensure that most wines and beers are crystal clear. Typically,
diatomaceous earth and perlite are used for beers. For wines, the filter aids are bentonite,
which is added first to the wine, and then perlite and isinglass are stirred gently into the wine
without disturbing the sediment. The wine should then be left for 30-60 minutes to clear. If a
haze remains, this is due to pectin and it may be necessary to use a pectic enzyme to remove
it (often named ‘wine enzyme’ or ‘wine clearing enzyme’).
Fermented dairy products
There are a large number of fermented milk products produced throughout the world using
lactic acid bacteria (Table 1). The bacteria use milk sugar (lactose) to produce lactic acid and
aroma chemicals that create the distinctive flavours found in products such as yoghurt and
cheeses. The lactic acid in turn changes milk proteins to produce the characteristic textures of
these products. The type of starter culture, incubation conditions and subsequent processing
conditions each controls the texture and flavour of the fermented milk, and hence produces
the many different fermented dairy products. Preservation is due to the increased acidity (in
yoghurt and cultured milks), the reduced moisture content (in some cheeses) and by chilling.
Further details of yoghurt and cheese production are given in technical briefs described in the
introduction.
Fruit and vegetable pickles
A large number of pickled vegetables, including olives and cucumbers (gherkins) are produced
by submerging them in brine, which inhibits the growth of spoilage bacteria and allows the
growth of salt-tolerant lactic acid bacteria. In each type of vegetable pickle, preservation is
achieved by the combination of lactic acid and salt, and products that are packed into jars
may also be pasteurised. Either naturally occurring lactic acid bacteria are allowed to grow in
the pickling brine or it is inoculated with a starter culture of different lactic acid bacteria (e.g.
Lactobacillus plantarum with Pediococcus cerevisiae). The amount of added salt controls the
type and rate of the fermentation: for example a brine containing 2-5% salt encourages the
initial lactic acid bacteria that produce the lactic acid, and this may be increased to 10% salt
to select salt-tolerant Lactobacilli. In some processes, sugar is added to increase the rate of
fermentation or to make the product sweeter. Vegetables are fermented for ≈ 5 weeks in
ceramic or earthenware jars at household- and micro-scales of production, or in fibreglass or
tiled concrete tanks at a larger scale.
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