Pressing-Clarification of White Grape Must
Pressing the Grape Must
The purpose of pressing is to recover the juice associated with the pulp and skin section of the grapes that are not readily released by natural draining. The juice that comes out of the press by using no pressure or just minimal pressure is called free-run, whereas the pressed juice is called press-run. Free-run fractions are clearer, possess lower levels of suspended solids, phenolic contents, and flavorants principally derived from the skins. Subsequent press-run fractions contain increasing amounts of suspended solids, anthocyanins, tannins and skin flavorants. Press-run fractions also are more likely to possess less total acidity (higher potassium contents), and have higher concentrations of polysaccharides, gums, and soluble proteins.
Whole-cluster pressing (foregoing the step of crushing and destemming the grapes) is most often done to make highend white wines (Figure 4.1). This method is often used for sparkling wines and Pinot Noir, because it creates a more delicate, less astringent wine. Producers indicate that this technique minimizes maceration and provides more efficient extraction.
Sulfur Dioxide Additions
Currently, most winemakers add sulfur dioxide in the juice phase during settling (i.e., addition of a pectinase enzyme) to inhibit the oxidative browning reaction facilitated by the enzyme polyphenol oxidase (PPO) (Section 3.9, Chapter 18). It registers a 90 percent decrease in activity when 50 mg/L of sulfur dioxide is added to the juice (Schneider, 2019). As a result, the juice remains green.
Oxidation of Juice
The solubility of flavonoid phenols and the extent to which they remain dissolved in the juice largely depend on the redox potential of the juice (Section 17.2). When freshly pressed grapes are entirely left to themselves without the addition of sulfur dioxide, they undergo browning. The cause of browning is an enzymatic oxidation of phenols by PPO.
Trends in using passive oxidation have become widespread for improving white wine quality and color stability. Passive oxidation occurs during pressing and handling (e.g., juice left in the press tray for two hours post-pressing without sulfur dioxide protection). As a result, there is no interference with the natural redox-balance of must. Under these conditions PPO activity is not inhibited by sulfur dioxide, and as a result one obtains a satisfactory precipitation of flavonoids.
Hyper-oxidation is a winemaking practice that involves forced oxidation of white juice prior to fermentation in order to reduce the phenolic content of the juice which can be bitter and lead to astringent wines (Sections 3.7, 17.3).
When practicing passive and hyper-oxidation microbial risks are a serious concern impeding grape and must processing without sulfur dioxide additions. The risks can be minimized by harvesting when the temperatures are low and efficient must cooling.
Sensory Effects of Must Oxidation
Musts that undergo oxidation result in wines that are more resistant to oxidative aging. Bitterness and astringency decrease markedly with oxygen addition in wine, and that this difference becomes greater during ageing of the wine (du Toit et al., 2006).
Depending on the intentions of the winemaker, and characteristics of the grapes, a portion of the second and possibly third pressing may be incorporated with the free-run. Based on all parameters, better quality wine is made from free-run, whereas the use of press-run must be considered according to its quality.
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