Wine Production
Table of contents
Red wine production
White wine production
White wine grapes are harvested when the sugar, acid and flavour levels are determined to be suitable for the wine style being produced.
The fruit is usually picked at cool, ambient temperatures, around 10 to 20 degrees, often during night by machine or is otherwise chilled upon arrival at the winery, especially in warmer climates.
Sulphur dioxide, \(\ce{SO2}\), may be added to the picking bins at the time of mechanical harvesting to provide some protection against oxidation and browning and to minimize microbial growth (vineyards are full of different microbes, including yeast which conduct fermentation).
At the winery, grapes are inspected and sampled, to determine the basic chemical parameters such as total soluble solids, an indication of the sugar content, pH, and acidity. So appropriate additions can be made prior to fermentation. These variables will differ based on the great variety, growing region, and level of ripeness.
Grapes are selected by the winemaker according to quality and style considerations before being de-stemmed and crushed prior to pressing to obtain the juice for fermentation. This produces free run juice (no pressure is applied) and pressing which tends to be a bid harsher due to phenolic compounds, and they are lower in quality.
Pressings are usually fermented separately, but can be blended back in some proportion later in the process. Adjustments of tartaric acid or sugar concentration can be performed at this stage, depending on local regulations. And \(\ce{SO2}\) may be added prior to pressing. \(\ce{SO2}\), usually added in its bisulfite form, is used to prevent oxidation and microbial growth.
In some cases, such as in the production of champagne, chardonnay, or Riesling, bunches are pressed without crushing or de-stemming to minimise the extraction of phenolic compounds, providing higher quality juice.
After pressing, white juice is typically cold settled to remove suspended matter and decanted (racking) to another vessel to provide clarification of the juice prior to inoculation with yeast.
Further clarification by filtration or centrifugation of the settled juice may also happen to further eliminate suspended solids or to speed up the settling process. The level of clarification determined by measuring the turbidity is depended upon the wine style being produced and can be adjusted by altering the settling time or adding back some sediment. This sediment is known as Lees.
Some suspended solids are necessary to provide nutrients to yeast for optimum fermentation conditions and wine sensory properties. But, high levels of turbidity can lead to off flavours.
Pectin splitting enzymes are often used before or after pressing to assist in juice extraction and clarification process. The juice is inoculated with selected yeast. Many different strains are available. That is usually pre-cultured by the winery or added as active dry yeast just like bread making. Although wild and inoculated, fermentations with native yeast from the vineyard or a winery environment are also performed for certain wine stalls like sauvignon blanc or chardonnay.
Fermentation occurs in stainless steel tanks or oak barrels, depending on the wine style being produced. Yeast nutrients, a usable form of \(/ce(N)\), may be added if the grapes are deficient. And a steady fermentation rate is preferred. This can be maintained by manipulating the temperature of the ferment. Around or below 20 degrees is usually ideal.
Aeration during the first few days of fermentation can ensure a healthy yeast population and favourable fermentation conditions. That is efficient conversion of sugar and non-stressed yeast.
Temperature and specific gravity are monitored daily along with other parameters to track the progress of fermentation. Once a ferment has reached the desired residual sugar level, usually 2 grams per litre for dry wine, the wine is cooled and allowed to settle then racked off gross Lees, the left over grape solids and yeast remnants under an atmosphere of inert gas such as \(\ce{N} \text{ or } \ce{CO2}\) to prevent oxygen from spoiling the wine. For reasons stated before, \(\ce{SO2}\) is added product to racking as well.
The exact operation at this point depends upon the type of wine. The wine may be matured in oak barrels or undergo Lees aging for several months. This is fine Lees, which is mostly expired yeast cells with some form of occasional agitation of the leaves. This can be used to enhance the flavour and mouth feel of the wine.
Spontaneous or inoculated malolactic fermentation (MLF) conducted by lactic acid bacteria, like those in milk, can also occur. And this deacidifies the wine by metabolising malic acid into lactic acid. MLF can also play a role in the mouth and formation of aerobic compounds, buttery characteristics, among other effects.
Wine is blended and undergoes what is known as fining where phenolic compounds or other compounds are moderated to give the desired sensory profile. Wine is then stabilised against precipitation of natural grape proteins, which cause cloudiness and of tartrates, which can lead to crystal formation in the bottle.
Wines are analysed for pH, free \(\ce{SO2}\), and heat and cold that is protein and tartrate stability. Any adjustments are made. And the wine is clarified and stored in readiness for sterile filtration prior to bottling.
Sweet white wines such as Sauternnes, and Tokaji, which are usually served with desert, arise due to grape desiccation from infection by noble rot, a fungus called Botrytis, or from deliberately over-ripened grapes. These wines rely on high sugar grape levels. These juices are not fermented to dryness. So residual sugar, around 10 to 30 grams per litre, but up to 200 grams per litre, remains in the wine, affording various levels of sweetness. Alternatively, concentrated grape juice can be used to sweeten the wine that is fermented to dryness either before or after stabilisation and just prior to bottling. This gives greater control over the final sweetness of the wine., but generally produced inferior wines compared to fermenting a high sugar juice to obtain the desired sweetness.
Drying and freezing grapes is also used to yield wines with high residual sugar. Wines containing residual sugar, a food source for microbes, require special attention to ensure they do not re-ferment at some later stage.