Winemaking

The production of wine is a long process and goes through many stages, from harvesting, fermentation, pressing, filtration and bottling. Although most wine is made from grapes, other fruits or non-toxic plants can also be used to make wine. For example, Mead is a wine that is made with honey and water, honey being its main ingredient.

The science of winemaking is known as oenology and is generally divided into two categories; sparkling wine production, being with carbonation and still wine production, being without carbonation.

The grapes

Grape quality is of high importance, more so than any other part of the winemaking process, as this determines the quality of the wine. The quality of the grapes can be affected by variety, as well as weather conditions during the growing season. The time of harvest, soil minerals and acidity, and the pruning method combined, are often referred to as the grape's terroir.

Grapes are usually harvested from the vineyard in the autumn, from early September to early November, in the northern hemisphere. In the southern hemisphere, harvesting takes place from mid February to early March.

Red wine is made from the pulp of red or black grapes that go through fermentation together with the grape skins. White wine is made by fermenting juice which is pressed from white grapes. White wine can also be made from the pulp extracted from red grapes but without the grape skin. Rose wines are made from red grapes, where the juice is allowed to stay in contact with the dark skins, long enough to pick up a pinkish colour but little enough not to pick up the tannins contained in the skins.

Harvesting and destemming

Harvesting is the first step in wine production, where the grapes can be picked mechanically or by hand. When to harvest the grapes is decided upon by the wine maker and informed by the level of sugar, acid and pH of the grapes. Other things taken into account for harvesting include ripeness, berry flavour and the development of tannins, e.g. seed colour and taste.

Mechanical harvesting involves large tractors straddling grapevine trellises by using plastic or rubber rods. These rods strike through the grapevine, dislodging the grapes from the rachis. The advantage is that the mechanical harvesters cover a large area of vineyard ground, in rather a short period of time.

Mechanical harvesting is rarely used in America, due to the way the vines are picked and the increased oxidation of the grape juice. It is more common in countries such as Australia and New Zealand, purely due to general labour shortages.

Manual harvesting is the hand picking of grapes from the grapevines. Its advantage is that the workers have the knowledge to pick only the ripe grapes and to leave the not so ripe grapes and grapes containing bunch rot behind. This can be quite an effective way to pick grapes as it prevents any poorer quality fruit from contaminating the wine.

Destemming is the separating of stems from grapes. Depending on the winemaking process this method is normally carried out before crushing. Its purpose is to lower the development of tannins and vegetal flavours in the resulting wine.

Destemming differs for red and white winemaking. Generally for whites, the stems are not removed and are kept intact throughout the crushing process. For red wine making, the stems are removed before fermentation as they are high in tannins and can also give off a vegetal aroma. Now and again the stems can be left on if they contain less tannins than required.

Crushing and primary fermentation

Crushing uses the procedure of softly squeezing the berries, breaking the skins to start, to release the contents of the berries. With traditional and small wineries, the harvested grapes are crushed by walking on them barefoot. However, in larger wineries, a mechanical crusher is used. For whites, the fruit is crushed and the stems are placed in the press along with the berries. With the stems in the mix, they aid the pressing, allowing juice to flow past flattened skin. This then builds up around the edges of the press. For reds, the grapes are past between two rollers which squeeze the grapes, separating the skin and pulp. With some delicate red wines such as Pinot Noir or Syrah, all or part of the grapes maybe left uncrushed, encouraging the release of fruity aromas through partial carbonic maceration.

Red wines are produced by destemming and crushing the grapes into a tank, leaving the skins in the juice throughout the fermentation process. It is possible to produce white wines from red grapes by the fastidious pressing of uncrushed fruit. For example, sparkling wine is derived from Pinot Noir, a red vinifera grape.

There is normally yeast present on the grapes, which the fermentation process can be done with, however, this can give unpredictable results due to the exact type of yeast that is present, and therefore cultured yeast is often added. One main problem with the use of wild ferments is that sometimes sugar remains unfermented. This results in making the wine sweet, instead of the dry. Often, wild ferments lead to the production, as a by product of unpleasant acetic acid which is vinegar.

Throughout the primary fermentation process, yeast cells feed on the sugars in the wine and multiply, making carbon dioxide, gas and alcohol. The temperature is crucial during this process, as it affects the speed of the fermentation as well as the taste of the end product. The temperature for red wines should be between 22C and 25C and for white wines, between 15C and 18C. For every gram of sugar that is converted, half a gram of alcohol is produced. For example, to get a 12% alcohol concentration, the wine needs to contain 24% of sugar.

Pressing

Pressing is where pressure is applied to the grapes in order to separate wine from grapes and grape skins. It can be optional, for if the grapes are crushed there is a large amount of juice instantly released which can be used for winemaking. This juice is called “free-run-juice”, and is a higher quality than the pressed juice. However, the majority of wineries use presses in order to increase their production, as pressed juice can represent between 15-30% of the total juice volume from the grape.

Presses work by placing the whole grape or the grape skins between a rigid and a moveable surface, slowly decreasing the volume between the two surfaces. Modern presses these days, tend to follow a pressing program which dictates the duration and pressure at each press cycle. Sometimes winemakers choose pressures where they can separate the stream of pressed juice, known as “press cuts”. When the pressure increases on the grape skins so too does the amount of tannins extracted into the juice, making the wine tannic or harsh.

Before modern presses came about most presses were basket presses, made from wood, operating manually. Basket presses are made from a cylinder of wooden slats on top of a fixed plate, with a moveable plate that can be forced downwards. The grapes would be loaded into the wooden cylinder with the top plate put in place. It would then be lowered until the juice began to flow from the wooden slats.

Red wines are pressed after the primary fermentation, separating the skins and other solids from the liquid. With white wines, the liquid is separated from the pulp before fermentation. With rose, the skins are kept in contact for a shorter period in order to give colour to the wine.

Pigage

Pigage is a French winemaking term which involves the stomping of grapes in open fermentation tanks. This technique is used to make certain types of wine, and the process involves grapes being put through a crusher and then poured into open fermentation tanks. When fermentation begins, carbon dioxide gases are released, pushing the grape skins to the surfaces. This sheet of skins is known as the cap. As the skins are the source of the tannins, the cap needs to be mixed through the liquid each day, which is traditionally done by stomping through the vat.

Cold and heat stabilisation

This process is used to reduce tartrate crystals in wine. They look like grains of clear sand and can also be known as “wine crystals” or “wine dimonds”. They often look like sediment in the wine, however, they are not. Throughout the cold stabilisation process, the temperature of the wine is dropped near to freezing for 1 to 2 weeks. This causes the crystals to separate from the wine and stick to the sides of the container holding the grapes. As the wine is drained from the containers, the tartrates are left behind. Throughout the heat stabilization, unbalanced proteins are removed by adsorption onto bentonite, preventing them from precipitating in the bottled wine.

Secondary fermentation and bulk aging

The second fermentation and aging process is very slowly, taking 3 to 6 months. The wine is kept under an airlock in order to protect the wine from oxidation. During this process, proteins from the grape are broken down and the remaining yeast and other fine particles are allowed to settle. The result is that the original cloudy wine becomes clear.

The secondary fermentation generally takes place in large stainless steel containers or oak barrels, depending on the aim of the winemakers. Unoaked wine is usually fermented in a barrel made out of stainless steel, or some other type of material which has no influence with the final taste of the wine. Depending on the required taste, the wine could be fermented just in stainless steel or in stainless steel and then briefly put into oak. Cheaper wine is often made from adding oak as chips and using a non-wooden barrel.

Malolactic fermentation

Malolactic fermentation is a process carried out by bacteria where tart-tasting malic acid, naturally present in grape pulp, is converted to softer-tasting lactic acid. The resulting wine is softer in taste and has greater complexity. This process is used in most red wines and is optional for white wines.

Laboratory tests

Tests are done throughout the winemaking process, at regular intervals to check the status of the wine. This is done so that if more sulphur (see preservatives below), needs to be added or other minor adjustments need doing it is possible to do them at the correct stages and before the wine is bottled. Common tests include, pH, titratable, acidity, Brix, residual sugar, sulphur, volatile acidity and alcohol.

Blending and fining

This involves the mixing of wine before it is bottled, in order to achieve the required taste. The winemaker can correct any wine that simply lacks the correct acids or tannins by mixing wines from different grapes and batches that were produced under different conditions. These adjustments can also be as complex as blending different varieties or vintages to accomplish a consistent taste.

Fining agents are used to remove tannins, reduce astringency (wines which are harsh and coarse to taste) and remove microscopic particles that can cloud the wines. It is the winemakers’ decision on which fining agents to use and these often vary from product to product and even batch to batch, depending on the grapes of that particular year.

Preservatives

Sulphur Dioxide is the most common preservative used in winemaking, as well as Potassium Sorbate. Sulphur Dioxide has two primary actions; it is an anti microbial agent and an anti oxidant. When making white wine, small amounts of sulphur dioxide can be added before fermentation, usually 30 mg per litre. Add too much and you could stop the malolactic fermentation from taking place as well as bacterial spoilage. For red wines, higher levels of around 100 mg per litre can be added, helping even out the colour. However, it is recommended to add small amounts, around 20 mg, to avoid bleaching red pigments. Without Sulphur Dioxide, wines can suffer bacterial spoilage no matter how hygienic the winemaking process.

Potassium Sorbate is used for the control of fungal growth, such as yeast. However, one potential hazard is the metabolism of sorbate to geraniol a potent and very unpleasant by-product. To avoid this, the wine must be bottled in sterile bottles with the use of filtration or it must contain enough Sulphur Dioxide to inhibit the growth of bacteria.

Filtration

This part of the process involves cleaning up the wine using clarification and microbial stabilisation before bottling. Clarification is where large particles that affect the appearance of the wine are removed. Microbial stabilisation is where organisms that affect the stability of the wine are removed. This reduces the likelihood of re-fermentation or spoilage.

Bottling

At this stage, a final dosage of sulphate is added to help preserve the wine and prevent unwanted fermentation in the bottle. The wine bottles are sealed with a cork or a plastic stopper, and a capsule is added to the top of the bottle which is heated for a tight seal or screw cap.