Grape processing: destemming, crushing, and pressing
In the second part of our articles on pre-fermentation steps, Westgarth Wines wine specialist Maurizio Broggi delves into the specifics of destemming, crushing, and pressing in white wines.
When grapes are ready to be processed, the winemaker must decide on the destemming and crushing process.
Destemming involves removing the stems (or stalks) from grape bunches, essentially turning whole grape clusters into single berries. Crushing is the operation of breaking the skins to release the juice which produces the must, the mixture of grape juice, skins, seeds, and pulp. Although destemming and crushing can be carried out separately, they are commonly done together using crusher-destemmer equipment which combines both operations.
The decision whether to destem and crush the fruit has different implications for white and red wines. Both processes affect the amount of skin maceration the wines will be subjected to and thus ultimately impact the style of the wine. Indeed, one of the essential differences between white and red winemaking is the role and processing of the grapes’ skins.
In white winemaking, grapes are crushed and/or pressed to release juice before alcoholic fermentation, which means fermentation is carried out without the skins. Depending on grape variety and stylistic goals, skin contact for white wines may be entirely avoided or carried out for a short time. If skin contact is required, white grapes are destemmed and crushed before pressing to allow a certain amount of maceration between juice and skins. If skin contact is not desired, whole grape bunches are directly loaded into the press, a process known as whole bunch pressing. Whole bunch pressing is essential for sparkling wines and many white wines because it minimizes skin contact resulting in a delicate, brighter, high-quality juice that has lower phenolic content.
In red winemaking, grape juice is fermented on the skins to extract color (anthocyanins), flavor, and tannin. Traditionally, black grapes are destemmed and crushed to release juice to promote extraction from the skins. Today, however, winemakers can adopt various approaches to process black grapes depending on the stylistic goal and grape variety. Sometimes grapes are destemmed but not crushed, thus whole berries are fermented (whole berry fermentation). Black grapes may also undergo whole bunch fermentation, without destemming or crushing, such as in the case of wines made by carbonic maceration.
Whether or not to destem also depends on tannin management, ripeness of the stems, and aromatic profile of the wine. Since stems are rich in tannins, they can be included to increase the tannin levels in some red wines. Stems may also provide aromatic compounds such as herbal, wood, and spice notes which, depending on the varietal, complement the grape flavors. If stems are included, they must be ripe to avoid green, herbaceous aromas and astringency in the wine. Pinot Noir, Grenache, and Syrah are some of the most well-known cultivars that are often fermented with a proportion of whole bunches.
As previously mentioned, the winemaker may decide to crush the grapes to release the juice from the berries or leave the berries fully intact. The crushing operation marks the beginning of the maceration between skins and juice, thus it will impact the juice’s composition, and ultimately the wine’s style.
For white wines, crushing allows the immediate release of the free-run juice – extracted by crushing grapes without pressing them. The free-run juice is considered to be of superior quality as it has the lowest content of phenolics compared to juice obtained by pressing. Depending on the wine style and grape variety, winemakers may want to avoid crushing white grapes to reduce extraction of phenolics from the skins, opting instead for the more gentle whole bunch pressing.
In red winemaking, crushing allows for better maceration between juice and skins, resulting in optimal extraction of color, tannin, and flavor from the skins. Black grapes are usually crushed, unless carbonic maceration, whole bunch or whole berry fermentation are required.
Pre-fermentation skin contact and maceration
Pre-fermentation skin contact in white winemaking is the process whereby grape skins remain in contact with juice after the berries are crushed. Skin contact in white winemaking is performed at cool temperatures (typically less than 10-15°C/50-59°F) and lasts from a few hours up to 24 hours or more. Skin contact allows the extraction of skin components in the juice before skins are removed at pressing. It results in an enhanced aromatic varietal character, additional texture, a fuller body, and increased phenolic bitterness. Skin contact is often considered ideal for aromatic grape varieties that have valuable varietal compounds in their skins such as Sauvignon Blanc, Riesling, and Muscat among others.
In red winemaking, pre-fermentation maceration on the skins, known as cold soak, allows the extraction of skin components prior to fermentation, before the yeasts begin producing alcohol and heat during the fermentation. If cold soak is required, black grapes are crushed and kept at cool temperatures (5-15°C/41-59°F) for a few days. Many winemakers believe that cold soak enhances fruit character and color in the final wine. Some grape varieties such as Pinot Noir and Syrah are considered especially suitable for this technique.
Pressing in white wines
In red winemaking, pressing comes after fermentation is complete and refers to the pressing of new wine. By contrast, in white winemaking pressing is performed before fermentation with the purpose of releasing all the remaining juice held in the crushed grapes after the free-run juice has been drained off.
The quality of the white juice released by pressing depends on the pressure applied by the press. The higher the pressure, the higher the proportion of phenolics (tannins) and color pigments extracted from the skins. The juice obtained by applying the minimum of pressure will be of the highest quality. Later fractions, obtained by applying increasing pressure, will contain more bitter and astringent phenolics, more solids, and will be of lesser quality.
Various types of presses are available, depending on the wine’s style, quality, and volume of wine produced. In Champagne, the basket press (or vertical screw press) is traditionally used. It is a shallow press with a large diameter that works by applying pressure from the top. The pressure is achieved by screwing down a lid on the grapes. The juice drains through the vertical slats, collecting at the bottom before draining into a vat. The basket press is highly valued by producers thanks to its slow and gentle pressing that minimizes extraction of phenolics and color pigments, thus releasing a clear, delicate juice.
Many wineries around the world use pneumatic presses. They are typically composed of a horizontal, cylindrical body with an internal bladder or membrane that once inflated, squeezes the grapes. This press needs only moderate pressure to release juice from the grapes and produces a delicate juice with low levels of phenolics. The pneumatic press can be enclosed in a tank to avoid exposure to oxygen and can be pre-flushed with inert gases such as nitrogen to maintain anaerobic conditions. The advantage of pneumatic presses is that the amount of pressure applied is controlled, which results in a more efficient pressing process that is achieved with a lower, more uniform pressure.
The winemaker must make several important decisions about transportation, delivery, and processing of the grapes before fermentation even begins. These preliminary steps are nonetheless vital to supply healthy, undamaged grapes that, based on the stylistic goal, will be turned into the must for fermentation through a combination of destemming, crushing, and pressing.
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