home.cd3wd.ar.cn.de.en.es.fr.id.it.ph.po.ru.sw
GRAPE WINE
Grape wine is perhaps the
most common fruit juice
alcohol. Because of the
commercialisation of the
product for industry, the
process has received most
research attention.
The production of grape wine
involves the following basic
steps: crushing the grapes to
extract the juice; alcoholic
fermentation; maltolactic
fermentation if desired; bulk
storage and maturation of the
wine in a cellar; clarification
and packaging. Although the
process is fairly simple,
quality control demands that
the fermentation is carried out
under controlled conditions to
ensure a high quality product.
The distinctive flavour of grape wine originates from the grapes as raw material and
subsequent processing operations. The grapes contribute trace elements of many volatile
substances (mainly terpenes) which give the final product the distinctive fruity character. In
addition, they contribute non-volatile compounds (tartaric and malic acids) which impact on
flavour and tannins which give bitterness and astringency. The latter are more prominent in
red wines as the tannin components are located in the grape skins.
Although yeasts are the principal organisms involved, filamentous fungi, lactic acid bacteria,
acetic acid bacteria and other bacterial groups all play a role in the production of alcoholic
fruit products. Normal grapes harbour a diverse micro-flora, of which the principal yeasts
(
Saccharomyces
cerevisiae
) involved in desirable fermentation are in the minority. Lactic
acid bacteria and acetic acid bacteria are also present. The proportions of each and total
numbers present are dependent upon a number of external environmental factors including
the temperature, humidity, stage of maturity, damage at harvest and application of
fungicides . It is essential to ensure proliferation of the desired species at the expense of the
non-desired ones . This is achieved through ensuring fermentation conditions are such to
encourage
Saccharomyces
species.
The fermentation may be initiated using a starter culture of
Saccharomyces cerevisiae
in
which case the juice is inoculated with populations of yeast of 10
6
to 10
7
cfu/ml juice (cfu
refers to coliform forming units. It is a measure of the activity of micro-organisms). This
approach produces a wine of generally expected taste and quality. If the fermentation is
allowed to proceed naturally, utilising the yeasts present on the surface of the fruits, the end
result is less controllable, but produces wines with a range of flavour characteristics. It is
likely that natural fermentations are practised widely around the world, especially for home
production of wine. In natural fermentations, there is a progressive pattern of yeast growth.
Several species of yeast, including
Kloeckera, Hanseniaspora, Candida
and
Metschnikowia
,
are active for the first two to three days of fermentation. The build up of end products
(ethanol) is toxic to these yeasts and they die off, leaving
Saccharomyces
cerevisiae
to
continue the fermentation to the end.
S
.
cerevisiae
can tolerate much higher levels of ethanol
(up to 15% v/v or more) than the other species who only tolerate up to 5 or 8% alcohol.
Figure 1: Advice form ITDG Latin America helped
improve the grapes and the yields of this wine
producer in Peru. Ann Pilling/ITDG