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February’s 29 days of Chardonnay

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The Problem with Premox (excerpt)

Authored by Arne Ronold

On behalf of Filip Verheyden of TONG

BEST WINE MAGAZINE IN THE WORLD 2011

GOURMAND WORLD COOKBOOK AWARDS

Over the past 10 years or so, wine lovers have had to add a new word to their vocabulary: “premox”, or “premature oxidation”. The premox problem has affected white wines in Burgundy to such an extent that many consumers now look to other regions for high-quality age-worthy white wines. Cork quality and vinification techniques have been blamed, but more thorough investigation has shown only one reasonable explanation: a lack of sulphur dioxide (SO2).

It all started with the 1996 vintage. In retrospect, some wines from the 1995 vintage were already affected, but it was in 1996 that the extent of the problem became apparent. That year was characterized by a long and slow ripening period with sunny weather and a cool north- to north-easterly wind, and cold nights, producing perfectly ripe grapes with very high acidity. Acidity levels of around 8 grams per litre and pH values of 3 were common, and because of this many producers had trouble with malolactic fermentation, meaning that many wines were bottled later than usual. But the quality of most of the wines was considered outstanding, and because of the acidity levels, they were thought suitable for very long ageing.

Tasting the wines just after bottling, and also a year later or even two to three years later, confirmed this assessment. But then, after three to four years in bottle, the situation changed. Many of the wines showed huge bottle variation and there were an alarmingly high number of prematurely oxidised bottles, which appeared to be distributed randomly among wines and producers.  With time, the number of these oxidised bottles increased. A similar pattern was detected in other vintages originally considered ageworthy, such as 1999, 2002 and 2004.

The problem can be described as follows: white Burgundies from what are considered good to great vintages appear as they should in cask, promising and apparently age-worthy. They show similar properties just after bottling as well as a couple of years later. But then, after several years in bottle, a high proportion of the wines appear to oxidise prematurely, a problem that seems to accelerate with time.

What is causing this and why did the problem appear only from the mid-1990s onwards? Older vintages, such as 1993, do not appear to be affected. Premox has been the object of much attention over the past 10 years, in wine publications as well as on internet discussion forums, such as oxidised-burgs.wikispaces and dat.erobertparker.com. In Burgundy, the Bureau  Interprofessionel des Vins de Bourgogne (BIVB) has made significant investments into research in this area.

There are a number of theories; many point to the corks as the most likely cause for premox, as this would explain the random nature of the problem. Many claim that cork quality has decreased significantly since the mid-1990s, mainly due to increasing demands from the wine industry. Others suggest that new bleaching material used for the corks – from chlorine-based products to hydrogen peroxide, a strong oxidant – may leave traces of peroxide residues that could cause premature oxidation. Yet others suggest that it is the new cork-coating material paraffin coating rather than silicon-based material – which facilitates the intrusion of oxygen into the bottles and thus triggers the premox process.

However, according to the Enology Department at Forschungsanstalt Geisenheim, Germany, corks are not to be blamed. “At Geisenheim, we have run trials and comparative studies over the last 20 years that eliminate the cork as the problematic factor in this problem.” If premox was caused by poor-quality corks or any particular treatment then oxidation would appear almost immediately after bottling and not several years later. Many observers claim that the problem is due to vinification issues such as the improper use of SO2 during vinification or the extensive use of bâtonnage. In 1996 in particular many of the wines were aged extensively on the lees due to delayed malolactic fermentation, and many producers did extensive bâtonnage due to the clean fruit and high acidity of the wines. But again, if this were the cause, you would expect the oxidation to occur much more rapidly.

Others believe that the new wine presses contribute to the oxidation process. The older basket presses would allow some exposure of the must to oxygen. When pressing the grapes, phenolic compounds would oxidise and settle and the remaining must – and the resulting wine – would be less prone to oxidation. Today most producers use pneumatic presses, which operate in a more reductive atmosphere and yield a very clean juice and a wine that could be more prone to oxidation. But I don’t believe the type of press makes any significant difference, as long as the wines are protected by high-enough levels of free SO2. The wines are likely to oxidise if there is not enough SO2.

We are left with only one logical cause for the premox in white Burgundies: too-low levels of SO2. Or more precisely: too-low levels of free SO2 at the time of bottling. SO2 has several useful and protective qualities in winemaking, among them its antioxidative properties. It exists in different forms in wine and it is common to speak about “bound” SO2 and “free” SO2, which together constitute the “total” level of SO2. It is the free SO2 that has the antioxidative properties. SO2 can be used at several stages during vinification, but It is particularly important to ensure that free SO2 at the time of bottling is at a satisfactory level if the wine is destined for a long maturation period in bottle. Just after bottling, the level of free SO2 will decline, initially quite significantly, later more gradually. If there is not enough free SO2 at the start, then the level will become insignificant at some stage during bottle maturation and the wine will grow prone to oxidation. Wines contain enough free SO2 to protect them for some years, but not enough for the entire desired maturation period. That is most probably the cause for premox in white Burgundy wines.

But what about the seemingly random appearance of oxidised bottles? How can this be explained? The answer is that the level of free SO2 will vary from bottle to bottle, even if all the bottles are filled in one batch. The reason for this are small variations in fill levels, as well as the small but uneven amounts of oxygen released into the wine through the corks after bottling. So the SO2 levels will vary, and it is those bottles with the lowest levels of free SO2 at the outset that will oxidise first. This model also explains why magnums seem to hold better than standard 750mL/75-cl bottles. If we accept that premox is caused by too low levels of free SO2 at the time of bottling, the next question is:

Why?

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Arne Ronold is founding editor of the Norwegian wine magazine Vinforum (since 1986) and was the first Master of Wine (1993) from the Nordic countries. He is responsible for running the Wine & Spirit Education Trust courses in Norway and lectures regularly on wine in many other European countries. Arne is also the author of numerous books about wine and is considered an expert on wines from Austria, Burgundy, Germany and Italy.

TONG is a magazine about wine.
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