Other Alleged Causes of Premature Oxidation


A wide variety of other potential causes of the premature oxidation problem have been floated as theories in the past few years. Some of the more serious ones (and some of the more ridiculous ones as well) are discussed below.

Shape of the Bottle Neck
Domaine Sauzet and Domaine Mallard, among others, believe that at least a partial explanation for some of the premature oxidation being experienced has to do with the shape of the neck of the bottle on modern mass-produced wine bottles. In order to facilitate easy cork insertion and removal, the neck of most burgundy bottles in use today is tapered, being narrowest at the top and, after a short distance, tapering slightly outward. This means that the cork seal is somewhat looser along the bottom two thirds of the length of the cork than it is at the top.

Initially, I gave this theory little or no credence because I didn't see any evidence, particularly on bottles of oxidized Sauzet whites from the 1995 to 1999 vintages (when Sauzet's incidence of premox was the highest) that the tapering appeared excessive or that there was any evidence of loose corks.

However, after experiencing a run of 50% or better premox on Fevre's 2004 Chablis Clos and Chablis Valmur bottled in 375ml bottles, I noticed that the corks withdrawn from those bottles had an excessive amount of tapering from top to bottom. Three days after removal from the bottle, the top half inch of the corks measured three-quarters of an inch (or 6/8") in diameter while the bottom of the corks was more than 7/8". In looking down the neck of the bottle, I saw that the glass visibly deflected outward about a half inch below the lip of the bottle. Bouchard and Fevre have noted some oxidation is being experienced with their 375ml bottles and they began using DIAM enclosures for 375ml bottles two years before adopting them for all bottles.

Both Sauzet and Mallard have begun using bottles in which the diameter is uniform along the entire length of the neck of the bottle into which the cork is inserrted. These bottles are a bit more expensive to produce than the standard mass-produced bottles, but they would seem to be well worth the investment given the cost of white burgundy these days.


Rot
Both noble rot and grey rot produce a glyco-protein enzyme (laccase) which accelerates the oxidation of wine. Unfortunately, laccase is difficult to counteract or control once it is present in the must. The best method of control is by vigorous sorting of the bunches/grapes to exclude rot affected bunches. To inhibit lacasse with SO2 the wine must be treated with at least 50mg/liter of SO2 (on a wine with a ph of 3.4.) Other control options are to fine with a tannin product such as Galalcool, casein and bentonite, or polyvinylpolypyrrolidone (PVPP), but the latter often strips body and mouthfeel and is considered toxic at high doses.

Many people in burgundy believe that the 1995 vintage has a very high incidence of oxidation because there was a lot of rot, and hidden rot in some cases, particularly in the late harvested grapes which were not properly triaged and/or treated by many domaines. While rot would seem to play a definite role in the overwhelming incidence of premox in the 1995 vintage, it cannot explain the oxidation which has occurred in wines from the 1996, 1999 and 2002 vintages, which all had remarkably clean, healthy grapes.


Growing grass between the rows of vines
One of the theories about premature oxidation that has been espoused by French wine critic Michel Bettane and others is that a change in cultivation practices, including method biodynamie and lutte raisonée, has led to a reduction in Glutathione, an important naturally-occurring sulfur-containing component in grape must which acts as an anti-oxidant. The theory is based on the work of Prof. Denis Dubourdieu and his colleagues at the University of Bordeaux. 1/

The work of Prof. Dubourdieu demonstrated that Glutathione plays an important role as an anti-oxidant in wine. According to Zoecklein's Enology Notes #101, April 20, 2005:

"Another very important sulfur-containing compound in grape juice is glutathione. This tripeptide is by far the most abundant source of reduced sulfur in the grape berry. It acts as an antioxidant and helps to stabilize 4-MMP, 4-MMPOH and 3-MH (Dubourdieu, 2004). Lees contain glutathione, one reason why lees storage keeps wines from oxidative degradation."

The work of Prof. Dubourdieu and his colleagues demonstrated that when grapes are subjected to extreme hydric stress they produce less Glutathione, and thus have less natural anti-oxidants in the grape skins and the lees. They also theorize that as many grape growers rebelled against the use of herbicides and adopted method biodynamie and lutte raisonée, allowing grass to grow between the rows of vines, the grass created additional competition for the scarce water and likely resulted in reduced levels of Glutathione, particularly in vintages subject to great hydric stress.

Others have criticized this theory, pointing out that this theory, if true, would result in across-the-board oxidation rather than the typical phenomenon of a given case having 2 to 4 oxidized bottles. Others have pointed out that some of the most well known proponents of method biodynamie have some of the lowest incidence of premature oxidation (e.g. Leroy/D’Auvenay), and that the high degree of variation in oxidation rates between producers seems inconsistent with the theory. An additional argument raised against the “grass” theory is that the premature oxidation is occurring even in vintages that did not suffer from significant hydric stress.

Personally, I don’t find the logic behind this theory very compelling versus the premature oxidation presently being observed in vintages from 1995-2004.

1/ D Dubourdieu and V Lavigne-Creuge, “The Role of Glutathione on the Aromatic Evolution of Dry White Wine,” http://www.infowine.com/default.asp?scheda=1148&provenienza=78


An unknown virus or biological cause unique to burgundy
This is one of the more bizarre explanations which has been floated on the Squires Board thread as a possible explanation more than once. However, the fact that the incidence of oxidation varies dramatically by produer and that a few producers have no higher incidence of oxidation than they did in the late 80's and early 90's rather simply refutes such a suggestion.

"Unfiltered Wines"
The argument has been made that the trend to make "more natural" unfiltered wines has somehow allowed elements that cause wines to prematurely oxidize to remain in the bottled wines where it was formerly filtered out prior to bottling when filtering wines was more common. Generally speaking, where there are problems with the health of the skins that might jeopardize the bottled wines, diligent producers go ahead and filter the wines, as happened widely in the 2001 vintage. Another problem with this theory is that there is no evidence that conventional filtering removes either laccase or the metal ions (copper and iron) present in wine which drive the oxidation of wine.

“High Acidity”
Pierre Rovani's initial post on premature oxidation of white burgundy on the Squires Board thread on premature oxidation listed this as a likely contributor to premature oxidation. No explanation was ever provided and, unless the argument might be that in high acid vintages vintners cut their SO2 usage, it frankly makes zero sense. Generally speaking, the higher the acidity level in the wine is (i.e., the lower the ph level is), the higher is the level of free molecular SO2 from a given fixed level of free SO2 addition. (See the discussion of sulfur levels above). Most burgundy producers have a standard or fixed free SO2 at bottling target that they apply in each vintage. So in "high acid" vintages, assuming the standard amount of free SO2 is added at bottling, the greater the level of molecular free SO2 protecting the wine from oxidation.


“High Yields”
This was another of the probable rationales for premature oxidation offered by Pierre Rovani in his initial post on the Squires Board premox thread. Here again, no explanation was offered as to the mechanism by which having high or maximum density yields might result in premature oxidation nor was there any attempt to correlate the incidence of oxidation predominately with high yield producers. There was also no explanation of how the onset of premature oxidation after the 1994 vintage might be explained by sudden changes in the averge yields.

[Text and Opinions by Editor Don Cornwell © Don Cornwell 2005-2014]