The grapes are picked when fully ripened. This is when the
sugar level (baume) reaches the correct level, and the fruit
reaches its full flavour. Unlike table grapes, the fruit needs
to be lacking in moisture to intensify the flavours and sugars.
The fruit is harvested by hand picking bunches into buckets
and then transferred to larger containers and trucked to the
winery or by mechanical harvesters which shake the berries
from the clusters.
CRUSHING AND DESTEMMING:
In modern mechanized wineries the grapes are normally crushed
and destemmed by a crusher destemmer which consists of a perforated
cylinder containing paddles revolving at 600 to 1200 revolutions
per minute. The grape berries fall through the cylinder perforations
into a must pump while the stems pass out the other end of
the cylinder. In earlier times crushing was carried out by
using the feet. Sometimes fruit is not crushed or destemmed
at all. This occurs chiefly with Chardonnay and is referred
to as whole bunch pressing. Reds are introduced whole into
the tanks, which are then closed. The resulting respiration
in the fruit, consuming oxygen and producing carbon dioxide,
kills the skin cells, which loosens their superpermeability,
allowing easy colour extraction. There is also some intracellular
respiration of malic acid.
When the juice of white grapes is processed or a white wine
is desired, the juice is usually separated from the skins
and seeds immediately after crushing. The mass of crushed
grapes is referred to as a “must”. When the grapes
come out of the crusher they fall into a must pump which then
pumps the juice into the wine press. This is a cylinder with
an inflatable tube. The grapes are introduced into the cylinder
and the tube or membrane of the press is inflated, pressing
the grapes against the rotating cylinder sides and forcing
the juice out through perforations. The process is designed
to be gentle, so the press may take up to 3 hours to complete
White musts are often cloudy and settling is desirable to
allow separation of suspended materials. Addition of sulphur
and lowering of temperature during settling helps prevent
fermentation and allows solids to settle. Pectin splitting
enzymes are often added to help settling. Bentonite clay may
be added to reduce nitrogen content and facilitate clarification.
The process of alcoholic fermentation requires careful control
for the production of quality wines. Requirements include
suppression of the growth of undesirable micro-organisms,
presence of adequate numbers of desirable yeasts, temperature
control, prevention of oxidation and the proper management
of the cap of skins floating in the red musts. The desired
yeast strain is to be added to a quantity of grape juice and
allowed to multiply before adding to the tank of must. This
strain would have been chosen for its flavour producing properties.
Temperature control during the fermentation process is necessary
to facilitate yeast growth, extract flavours and colours from
the skins in red must, permit accumulation of desirable by-products
and to prevent undue temperature change which could kill off
the yeast. Contact with air must be restricted during fermentation.
The cap of skins floating on top of the juice in red wine
fermentation inhibits flavour and colour extraction and may
rise to undesirable high temperatures which may acidify if
allowed to become dry. This is avoided by submerging the cap
by mixing or pumping 2 or 3 times a day.
Alcoholic fermentation generally ceases when the amount of
fermentable sugar available becomes low. About 0.1%. Ferments
will not reach this stage if must of high sugar content are
fermented, alcohol intolerant yeast strains are used or if
fermentation temperature was too high or too low. Fermentation
can take from 10 to 30 days. In most cases the major portion
of the yeast cells will be in the sediment or lees. Separation
of the wine from this sediment is called ‘racking’.
The container vessels must be kept full from this time on.
Young wines frequently have a secondary evolution of carbon
dioxide, occurring sometime after the completion of alcoholic
fermentation. This results from the malolactic fermentation,
in which malic acid is broken down into lactic acid and carbon
dioxide. Malolactic fermentation is desirable when certain
flavour characteristics are required. This is usually in red
wines but is also used for barrel fermented Chardonnay. This
process can be left to occur naturally or the bacteria can
be introduced via inoculation to insure the process happens
in a shorter time frame.
Fining: Is an ancient practice in which material is added
to aid clarification. The main processes involved are absorption
and chemical reaction. Protein and yeast cells are absorbed
on fining agents such as bentonite clay or gelatin. Other
agents used are egg white, milk powder, casein and pvpp.
Refrigeration aids clarification in several ways. It prevents
yeast growth and the evolution of carbon dioxide, which tends
to keep the yeast cells suspended. Carbon dioxide is more
soluble at low temperatures. A major cause of cloudiness is
the slow precipitation of potassium acid tartrate (cream of
tarter) as the wine ages. Rapid precipitation is induced by
lowering the temperature for 1 or 2 weeks. If the resulting
wine is filtered off the tartrate deposit, tartrate precipitation
will not usually cause clouding later.