The Australian Wine Research Institute

The amount of contact allowed between white grape juice and skins and seeds before fermentation has a marked effect on the properties of finished wine. Skin contact increases wine flavour and viscosity or ‘body’ because many flavour and phenolic compounds are found in the grape skin. However, the process needs to be tightly controlled because excessive extraction can have a marked negative effect on wine quality.

The 2019 Chardonnay trial includes a wine made from crushed and destemmed fruit given 72 hours of pre-fermentation skin contact, which can be compared to wines made from crushed and destemmed fruit with no additional skin contact, and juice from whole-bunch pressing. Additionally, a batch of crushed and de-stemmed fruit was fermented on skins to make an ‘amber’ wine.

Background

With hand-harvested intact whole bunches the winemaker has a clear choice whether to crush the fruit and press as quickly as possible, or to crush and allow a period of skin contact before pressing. Modern winemaking equipment or traditional presses used in Champagne can result in the virtual avoidance of skin contact. At the other end of the spectrum, ‘amber’ wines are essentially white wines made in the same way as red wines, with post-fermentation skin contact periods of several months employed by some winemakers.

With machine-harvested fruit, however, musts receive a degree of skin contact during harvesting and transport to the winery, and when transported over long distances the amount of skin contact can be substantial. When machine harvesting was first introduced, this was regarded as largely negative for wine quality. Since then, the ability to machine harvest at cooler night temperatures, improved dosing of SO₂ with better dispersal through the must and other technological advances that allow more gentle grape handling have largely negated the earlier deleterious effects.

Practical considerations and helpful hints

In addition to increases in compounds associated with wine flavour and body, other changes in must and wine composition are consistently seen following skin contact and these should be considered before employing this technique. These include increases in must pH, potassium and total nitrogen (particularly ammonia), and decreases in titratable acidity and tartaric acid. In the resulting wines, both flavonoid and non-flavonoid phenolics and malic acid have been seen to increase proportionally to the length of 6, 12 and 24 hours of skin contact.

Skin contact may be conducted with and without the addition of pectinase and/or glucosidase enzymes, and it is recommended that it is performed under inert gas cover. Enclosed presses are ideal vessels but may not be available for the length of time required at the height of vintage. If presses are not available, an important consideration is how to move the must to the press after a period of skin contact, without excessive aeration or additional mechanical maceration. Overhead tanks from which the must can be dropped directly into a press are the next best alternative to enclosed presses.

Temperature has a major influence on the rate and nature of extraction, with higher temperatures resulting in a marked increase in phenolics and wines with increased colour which can mature more rapidly and have ‘coarser’, more astringent sensory characters. The best results are likely to be obtained between 15 and 20°C. However, while cooler temperatures are beneficial, they could result in a need to warm the juice prior to yeast inoculation.

Potential risks

The presence of Botrytis when conducting pre-fermentation skin contact can lead to rapid oxidation due to the laccase enzyme, and it should be remembered that even in dry conditions, Botrytis may be present on the inside of tightly filled bunches. Close inspection is therefore recommended. It is also important that grapes are fully ripe, because skin contact coupled with subsequent pressing leads to an increase in C6 compounds responsible for ‘herbaceous’ characters in white wines.

The over-extraction of phenolic compounds leading to overly astringent wines is also a risk if the contact period is too long, especially at an elevated temperature, or if the fruit is not otherwise treated gently during harvesting, transport, de-stemming, crushing and pressing. While every batch of grapes is different, a degree of standardisation of extraction can be achieved between batches and between vintages, by use of spectrophotometric measurements of phenolics.

High pH is also a potential risk if the initial juice pH and potassium are high, because precipitation of potassium bitartrate under these conditions will lead to pH increasing further. Measurement of pH and potassium before the fruit is harvested is therefore recommended.