Readers of International Wine Cellar and Wine Advocate will likely recall reading in the mid-90's glowing comments from burgundy winemakers about their wonderful new computer-controlled bladder presses that allowed them to have an infinite range of control over the pressing process. Many touted that these new presses allowed them to produce lighter, cleaner juice and to eliminate the coarser lees, with the intended result of making wines which were more immediately approachable and attractive to consumers on release.

Steven Tanzer and Pierre Rovani, among others, have suggested that gentler pressing practices enabled by these computer-controlled bladder presses has likely contributed to premature oxidation. Although this factor was not well explained by either author initially, once you understand the science behind the comments, it is far more plausibe cause of premature oxidation than it first it might appear. At the urging of a respected wine chemist and a seller of computer-controlled wine presses, I have re-examined this issue and have been very surprised by what I have read

Wines contain three different types of phenols. The first type is known as flavonoid phenols, which are polyphenolic compounds located in the grape skins, seeds and stems. This type of phenolic compound is generally associated with pressing or extractive techniques. Thus,wines which are subject to more vigorous pressing exhibit considerably higher levels of flavonoid phenols and wines which are made with very gentle pressing and no extractive techniques exhibit very little in the way of flavonoid phenols. When flavonoid phenols are oxidized they result in color changes in the wine (browning), which change the wine over time from bright yellow to increasingly deeper shades of gold and finally brown color. The oxidation of this type of phenol produces the color changes in wine and the browning phenomenon which occurs with premature oxidation.

Ther second type of phenols in white wines are called non-flavonoid phenols, which make up the largest amount of the phenols present in white wine and are found primarily in the pulpy part of the grape. While non-flavonoid phenols also oxidize over time in wine, they do not produce any detectable changes in color, aromas or flavors.

The third type of phenols in white wines come from wood tannins. These tannins are absorbed or leeched out of the wood barrels during the barrel fermentation and aging process.

While flavonoid phenol levels in the skins can vary a bit by vintage, generally speaking, the more firmly the grapes are pressed (and the longer period of time that the grapes are pressed), the greater the level of flaovonoid phenols that will be extracted. More firm pressing will also mean that greater amounts of lees are extracted from the grapes. Flavonoid phenols can also produce astringent, drying or somewhat bitter elements on the palate, so obviously it can be counterproductive to press the grapes too hard, particularly if the pressing breaks the seeds which tend to have the most bitter components.

Phenols act as the initial or primary oxidation barrier in wine. The phenols, together with the SO2 which is added to the wine, prevent the formation of acetaldehyde (what we call oxidation) over time. Generally speaking, the phenols protect the wine initially from oxidation by aborbing the oxygen present, which slowly oxidizes the phenols. As the phenols are oxidized they produce small amounts of hyrdogen peroxide as a byproduct. The SO2 which is added to the wine prevents the hydrogen peroxide from oxidizing the ethanol in the wine.

If the level of flavonoid phenols extracted from white wine grapes during the pressing process is lowered by using gentle pressing and extraction techniques, then the primary oxidation buffer in the wine inevitably will be lessened to some degree. This at least suggests that where a given winery changed its practices from stronger or more extractive pressing (e.g. vertical presses) to more gentle computer controlled bladder pressing, the level of flavonoid phenol extraction will be lessened, with an associated loss of a portion of the primary oxidation buffer. Stated alternatively, if the level of extracted flaovonoid phenols is lowered, and no other changes are made (such as an increase in SO2 to offset the loss of the phenols), the ethanol in the wine will inevitably oxidize at a faster rate than its historical average. However, since flavonoid phenols are the smaller of the two groups naturally occurring in white wine must and the effect of the loss of these phenols has thusfar not been quantified in any study, this remains a controversial subject.

This is a current a hot topic of discussion among burgundy producers since a 2009 press release by the burgundy wine trade association BIVB stated that its preliminary studies show that gentler pressing techniques have played a definite role in causing premature oxidation. Some producers like Coche-Dury and Roulot have always pressed their grapes firmly to extract a proper level of phenols and those producers have had few premox problems (aside from oxidized 1996's from Roulot which they attribute to unique cork problems). Coche, for example, uses an old Vaslin vertical press rather than the computer controlled pneumatic presses. Others, including Le Moine and Faiveley, have taken recent steps to make sure that the grapes are pressed sufficiently firmly so that “the juice doesn't come out too clean.” Recent anecdotal evidence reported back to me suggests that several producers have taken to measuring phenol levels in their juice from the press and are pressing their grapes a bit more firmly in order to get more phenols in the juice.

It also appears that many producers have taken the approach that adding more SO2 as an oxidation preventative is a “substitute” for the loss of phenols resulting from more gentle pressing of the wines. Whether additional SO2 can be effectively substituted for lower levels of phenols resulting from more gentle extraction in the pressing process remains an open issue.

This issue has received scant attention from those writing on the subject of oxidation, yet the underlying science here seems pretty basic and yet compelling (yes, it is very difficult to wade through for non-scientists.) MORE TO COME HERE. STAY TUNED.

  • gentler_presses.txt
  • Last modified: 2018/03/14 12:41
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