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Wine Making Process |
| Harvesting
Fresh and fully ripened wine grapes are preferred as raw material for wine making. In
cool climates, as in northern Europe and the eastern United States, however, lack of
sufficient heat to produce ripening may necessitate harvesting the grapes before they
reach full maturity. The resulting sugar deficiency may be corrected by direct addition of
sugar or by the addition of a grape juice concentrate. Grapes that are allowed to reach
full maturity on the vine or that are partially dried by exposure to sun after harvesting
are high in sugar content as a result of natural moisture loss (partial raisining as in
the production of Málaga wines in Spain). A beneficent mold, Botrytis cinera, may
also be employed to hasten moisture loss (as in the production of Sauternes in France).
These grapes are used to produce sweet table wines. Special methods employed to produce
these wines include the addition of sulfur dioxide, the use of small fermenting vessels
during processing, or the use of cool temperatures--the objective being to stop the
fermentation before all the sugar is fermented.
Because of the effect upon grape composition, proper timing of the harvest is of great
importance. Premature harvesting results in thin, low-alcohol wines; very late harvesting
may yield high-alcohol, low-acid wines.
Harvesting may be completed in one picking or in several. The grape clusters are cut
from the vine and placed in buckets or boxes and then transferred to larger containers
(large tubs in Europe, metal gondola trucks in California and elsewhere) for transport to
the winery. Mechanical harvesting systems, based on shaking the berries from the clusters
or on breaking the stems, are widely used in California, Australia, France, and elsewhere.
At the winery the grapes may be dumped directly into the crusher or may be unloaded
into a sump and carried to the crusher by a continuous conveyor system.
Crushing
In modern mechanized wine production, the grapes are
normally crushed and stemmed at the same time by a crusher-stemmer, usually consisting of
a perforated cylinder containing paddles revolving at 600 to 1,200 revolutions per minute.
The grape berries are crushed and fall through the cylinder perforations; most of the
stems pass out of the end of the cylinder. A roller-crusher may also be used. Ancient
methods of crushing with the feet or treading with shoes are rare.
When red grapes are used to produce a white juice, as in the Champagne region of
France, crushing is accomplished by pressing.
Red grapes are sometimes introduced whole into tanks, which are then closed. The
resulting respiration in the fruit, consuming oxygen and producing carbon dioxide, kills
the skin cells, which lose their semipermeability, allowing easy colour extraction. There
is also some intracellular respiration of malic acid. This respiration process is slow and
in warm regions may result in wines of low colour and acidity and distinctive
odour.
Juice Separation
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. Occasionally, to increase flavour extraction, the white skins may be allowed to
remain in contact with the juice for 12 to 24 hours, but this procedure also increases
colour extraction, sometimes undesirably.
Two main procedures are employed to separate the juice from the solids. Much of the
juice may be drained off by placing the crushed grapes in a container having a false
bottom and often false sides. This juice is called the free run juice, and the mass of
crushed grapes is called the must, a term also used to refer to the unfermented grape
juice, with or without skins.
More commonly, the crushed grapes are placed in a press. The traditional basket press
is gradually being supplanted by a horizontal basket press, applying pressure from both
ends. Continuous screw-type presses are also employed, especially for drained pulp. The
Willmes press, widely employed for white musts, consists of a perforated cylinder
containing an inflatable tube. The crushed grapes are introduced into the cylinder, and
the tube is inflated, pressing the grapes against the rotating cylinder sides and forcing
the juice out through the perforations. Several pressings may be made without the
extensive hand labour required for basket presses.
Continuous presses are practical for production of red wines, in which skins, seeds,
and juice are all fermented together. Separation of the juice is simplified because
fermentation makes the skins less slippery, and the amount of free run juice obtained is,
therefore, much greater than for unfermented musts. Separation of the less slippery solids
from the juice by pressing is also simplified.
The drained pomace (crushed mass remaining after extraction of the juice from the
grapes), from white or red fermentations, may be used to provide distilling material for
production of wine spirits. Water is usually added, the fermentation is completed, and the
low-alcohol wine is drained off. The pomace may be further washed and pressed or may be
distilled directly in special stills.
Must Treatment
White musts are often turbid and cloudy, and settling is
desirable to allow separation of the suspended materials. Such measures as prior addition
of sulfur dioxide and lowering of the temperature during settling help prevent
fermentation and allow the suspended material to settle normally. In many areas wineries
centrifuge the white must to remove the solids. In this process a strong pulling force is
created by circular motion. Musts are sometimes pasteurized, inactivating undesirable
enzymes that cause browning. The addition of pectin-splitting enzymes to the musts to
facilitate pressing is uncommon. Bentonite, a type of clay, may be added to musts to
reduce total nitrogen content and facilitate clarification.
There is renewed interest in the prefermentation heat treatment of red musts to extract
colour and deactivate enzymes. This process, when performed rapidly at moderate
temperatures and without undue oxidation, may be particularly desirable in the production
of red sweet wines, which employs short periods of fermentation on the skins, and for use
with red grapes that have been attacked by the parasitic fungus Botrytis cinera, which
contains high amounts of the polyphenol oxidase type of enzymes that cause browning.
Fermentation
The process of alcoholic fermentation requires careful
control for the production of high quality wines. Requirements include suppression of the
growth of undesirable microorganisms, presence of adequate numbers of desirable yeasts,
proper nutrition for yeast growth, temperature control for prevention of excessive heat,
prevention of oxidation, and proper management of the cap of skins floating in red musts.
Grape skins are normally covered with bacteria, molds, and yeast. The wild yeasts such
as Pichia, Kloeckera, and Torulopsis are often more numerous than the wine
yeast Saccharomyces. Although species of Saccharomyces are generally
considered more desirable for efficient alcoholic fermentation, it is possible that other
yeast genera may contribute to flavour, especially in the early stages of fermentation. Saccharomyces
is preferred because of its efficiency in converting sugar to alcohol and because it is
less sensitive to the inhibiting effect of alcohol. Under favourable conditions, strains
of Saccharomyces cerevisiae have produced up to 18 percent (by volume) of alcohol,
although 15 to 16 percent is the usual limit.
Use of the yeast Schizosaccharomyces pombe has been proposed for the early
stages of alcoholic fermentation. Because it metabolizes malic acid, this yeast would be
useful in excessively acid musts, but commercial applications have not yielded
consistently favourable results. The addition of lactic-acid bacteria to musts, using
strains metabolizing malic acid, is now common.
The number of undesirable microorganisms is greatest in partially rotted or injured
grapes. Such damage may occur in harvesting or during transportation, particularly in warm
climates. Suppression of undesirable microorganism growth is required, and the most common
method used is the addition of sulfur dioxide to the freshly crushed grapes at the rate of
about 100 to 150 milligrams per litre. Sulfur dioxide is more toxic to undesirable
microorganisms than to desirable microorganisms. When it is used in musts, an inoculum of
the desired yeast strain, usually called a pure yeast culture, is added. Musts are rarely
pasteurized, although this process may be applied when they contain undesirable amounts of
oxidizing enzymes from moldy grapes.
Enologists, technicians in the science of wine making, do not agree on the most
desirable yeast species and strain, but strains of S. cerevisiae are generally
used. The chosen strain is allowed to multiply as much as possible in sterilized grape
juice and is then transferred to larger containers of sterilized grape juice, where it
continues to grow until the desired volume is reached. Suitable pressed yeasts of
desirable strains are added directly, avoiding the troublesome practice of building up and
maintaining a pure yeast culture. About 1 to 3 percent of a pure yeast culture, or
sufficient pressed yeast to provide a population of 1,000,000 cells per millilitre, is
used.
Temperature control during alcoholic fermentation is necessary to (1) facilitate yeast
growth, (2) extract flavours and colours from the skins, (3) permit accumulation of
desirable by-products, and (4) prevent undue rise in temperature, killing the yeast cells.
Optimum temperature for growth of common wine yeasts is about 25 C (77 F), and in many
viticultural areas of the cooler temperate zone, grapes are crushed at about this
temperature. Fermentation is seldom started at so high a temperature, however, because it
is then difficult to prevent the temperature from exceeding 30 C during fermentation.
Extraction of flavours and colours is not a problem in white musts; the crushed grape
mass is usually separated from the skins before fermentation. Fermentation of white musts
at relatively cool temperature (about 10 to 15 C [50 to 60 F]) apparently results in
greater formation and retention of desirable by-products. An undesirable feature of such
relatively low-temperature fermentations is the longer period required for completion (six
to 10 weeks compared to one to four weeks at higher temperatures) and the tendency for the
fermentation to stop while residual sugar remains. (This is not always considered
undesirable--i.e., in German wine production.) In practice white table wines are
usually fermented at about 20 C.
In red wine musts, the optimum colour extraction consistent with yeast growth occurs at
about 22 to 28 C (72 to 82 F). Alcoholic fermentation produces heat, however, and careful
temperature control is required to prevent the temperature from reaching a point (about 30
C) where yeast growth is seriously restricted. At still higher temperatures, growth will
stop completely. Modern temperature control is accomplished by use of heat exchangers.
Older methods include placing the fermenters in a cold room; using cold pipes in the
fermenter; pumping the must through double-walled pipes, with cold water in the
surrounding pipe; pumping the must through a sump containing cooling coils; and pumping
the coolant through jackets surrounding the tank.
Contact with air must be restricted to prevent oxidation during fermentation. In very
large containers, the volume of carbon dioxide given off is sufficient to prevent entry of
air. In small fermenters, fermentation traps are inserted, preventing entry of air but
permitting exit of carbon dioxide. These traps are particularly desirable during the final
stage of fermentation, when carbon dioxide evolution is slow. Following fermentation,
small amounts of sulfur dioxide are added to help prevent oxidation. Ascorbic acid (50 to
100 milligrams per litre) is sometimes employed to decrease the oxidation and thus the
amount of sulfur dioxide required as an antioxidant, but is not generally recommended.
The cap of skins and pulp floating on top of the juice in red-wine fermentation
inhibits flavour and colour extraction, may rise to an undesirably high temperature, and
may acetify if allowed to become dry. Such problems are avoided by submerging the floating
cap at least twice daily during fermentation. This operation, comparatively easy with
small fermenters, becomes difficult with large, tall fermenters of up to 100,000-gallon
(380,000-litre) capacity. In large units the fermenting must is drawn off near the bottom
and pumped back over the cap. The use of small fermentation vessels permits a greater
percentage of heat loss to the surrounding atmosphere, simplifying temperature control.
Postfermentation Treatment
With appropriate must composition, yeast strain,
temperature, and other factors, alcoholic fermentation ceases when the amount of
fermentable sugar available becomes very low (about 0.1 percent). Fermentation will not
reach this stage when (1) musts of very high sugar content are fermented, (2)
alcohol-intolerant strains of yeast are used, (3) fermentations are carried on at too low
or high temperatures, and (4) fermentation under pressure is practiced. Fermentation of
normal musts is usually completed in 10 to 30 days. In most cases, the major portion of
the yeast cells will soon be found in the sediment, or lees. Separation of the supernatant
wine from the lees is called racking. The containers are kept full from this time on by
"topping," a process performed frequently, as the temperature of the wine, and
hence its volume, decreases. During the early stages, topping is necessary every week or
two. Later, monthly or bimonthly fillings are adequate.
Normally the first racking should be performed within one to two weeks after completion
of fermentation, particularly in warm climatic regions or in warm cellars, as the yeasts
in the thick deposit of lees may autolyze (digest themselves), forming off-odours.
Early racking is not required for wines of high total acidity--i.e., those
produced in cool climatic regions or from high-acid varieties. Such wines may remain in
contact with at least a portion of the lees for as long as two to four months, permitting
some yeast autolysis in order to release amino acids and other possible growth factors
favouring growth of lactic-acid bacteria. These bacteria then induce the second, or
malolactic, fermentation.
Malolactic Fermentation
Enologists have known for some time that young wines
frequently have a secondary evolution of carbon dioxide, occurring sometime after the
completion of alcoholic fermentation. This results from malolactic fermentation, in which
malic acid is broken down into lactic acid and carbon dioxide. The fermentation is caused
by enzymes produced by certain lactic-acid bacteria.
Flavour by-products of unknown composition are also produced during this fermentation.
Malolactic fermentation is desirable when new wines are too high in malic acid, as in
Germany, or when particular nuances of taste and flavour are desired, as in the red wines
of Burgundy and Bordeaux in France. In other regions, some producers may encourage
malolactic fermentation, and others may discourage it, depending upon the particular
character desired in the wine. In all regions, this second fermentation is somewhat
capricious. One product, diacetyl (a flavour and aroma agent), is apparently beneficial at
low levels and undesirable at higher levels.
At low temperatures, malolactic fermentation proceeds slowly, if at all. German cellars
are often equipped with steam pipes, raising the temperature to encourage this
fermentation. The bacteria may fail to grow because of a deficiency or complete absence of
essential amino acids. Most lactic-acid bacteria growth can be inhibited by the presence
of 70 to 100 milligrams per litre of sulfur dioxide.
Excessive malolactic fermentation may produce wines too low in acidity (flat tasting)
or with undesirable odours (mousy, sauerkraut, or diacetyl). Such faults may be prevented
by earlier racking, filtration, and addition of sulfur dioxide.
Clarification
Some wines deposit their suspended material (yeast cells,
particles of skins, etc.) very quickly, and the supernatant wine remains nearly brilliant.
This is particularly true when 50-gallon wooden barrels, which have greater
surface-to-volume ratio than larger containers, are employed. The rough interior of wooden
cooperage facilitates deposition of suspended material. Other wines, particularly in warm
regions or when large tanks are used, may remain somewhat cloudy for long periods. Removal
of the suspended material during aging is called clarification. The major procedures
involved are fining, filtration, centrifugation, refrigeration, ion exchange, and heating.
Fining
Fining is an ancient practice in which a material that aids clarification is added to
the wine. The main processes involved are adsorption, chemical reaction and adsorption,
and possibly physical movement. Proteins and yeast cells are adsorbed on fining agents
such as bentonite (a type of clay formed mainly of montmorillonite) or gelatin. Chemical
reactions occurring with tannins and gelatin may be followed by adsorption of suspended
compounds. If an inert material, such as silica, is added to a cloudy wine, some
clarification will occur simply by the movement of the particles of inert silica through
the wine. This action probably occurs to a certain extent with the addition of any fining
agent.
Bentonite has largely replaced all other fining agents. Such fining agents as gelatin,
casein, isinglass, albumin, egg white, nylon, and PVPP (polyvinyl pyrrolidone) may be used
for special purposes, including removal of excess tannin or colour.
Excessive amounts of metals, particularly iron and copper, may be present in the wine,
usually from contact with iron or metal surfaces. These result in persistent cloudiness
and require removal by such special fining materials as potassium ferrocyanide (blue
fining), long recommended in Germany. Cufex, a proprietary product containing potassium
ferrocyanide, may be used in the United States under strict control. Phytates have been
used for removing iron. In modern winery operations excessive metal content is rare,
mainly owing to the use of stainless steel equipment.
Filtration
Filtration is another ancient practice, and early filters consisted of rough
cloth-covered screens through which the wine was poured. Modern filter pads are made of
cellulose fibres of various porosities or consist of membrane filters, also in a range of
porosities. The pore size of some filters is sufficiently small to remove yeast cells and
most bacterial cells, but filters operate not only because of pore size but also by a
certain amount of adsorption. Diatomaceous earth-filter aids, commonly added to the wine
during filtration, increase the functional life of a filter by retarding pore clogging.
Centrifugation
Centrifugation, or high-speed spinning, used to clarify musts, is also applied to wines
that are difficult to clarify by other means. This operation requires careful control to
avoid undue oxidation and loss of alcohol during the process.
Refrigeration
Refrigeration aids wine clarification in several ways. Temperature reduction often
prevents both yeast growth and the evolution of carbon dioxide, which tends to keep the
yeast cells suspended. Carbon dioxide is more soluble at lower temperatures. A major cause
of cloudiness is the slow precipitation of potassium acid tartrate (cream of tartar) as
the wine ages. Rapid precipitation is induced by lowering the temperature to -7 to -5 C
(19 to 23 F) for one or two weeks. If the resulting wine is filtered off the tartrate
deposit, tartrate precipitation will not usually cause clouding later.
Ion exchange
Another method of tartrate stabilization is to pass a portion of wine through a device
called an ion exchanger. If this ion exchanger is charged with sodium, it will replace the
potassium in potassium acid tartrate with sodium, making a more soluble tartrate. Usually,
if the potassium content of the blend of either treated or untreated wine is reduced to
about 500 milligrams per litre, no further precipitation will occur. Exceptions may occur,
however, and to be safe, tartrate and potassium contents and pH are included in the
calculation. The use of ion exchange is illegal in several countries.
Heating
Many wines contain small amounts of proteins that may cause clouding either by
precipitation or by reacting with copper or other metals to form aggregates that in turn
form clouds. The use of bentonite removes some protein, and protein adsorption is
increased if the wine is warm when fined. Pasteurization at 70 to 82 C (158 to 180 F) also
can be used to precipitate proteins, but in modern practice this process is seldom
employed to aid clarification. |
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