Random oxidation – the facts
This article was first published in The Drinks Business, August 2008.
Random, or sporadic post-bottling, oxidation is a misunderstood term. We speak of it confidently, yet not always with ful
l knowledge of its possible causes. Is random oxidation a convenient ‘cork-bashing’ tool now that those cork manufacturers who adopt best practice are finding ways to minimise TCA? Or are there are wider issues with implications for both bottle and closure and the process that joins them together? And what about the variability of natural cork?
It may seem obvious, but “random oxidation is bottle to bottle variation” said Dean Banister, sales director of Diam, “a lot of people misunderstand and confuse oxidation with random oxidation.” If a bottle is being tasted in isolation, without comparison to the rest of the batch, it’s impossible to say if it’s random oxidation, i.e. bottle variation, or a more widespread quality control issue.
A number of causes to consider – 1, 2 and 3: the closure, the bottle and bottling.
The closure
Everyone agrees that random oxidation can occur under any closure.
An inconsistent performance of the closure will increase the likelihood of random oxidation. Technical closures made on an industrial scale, such as screwcaps, synthetics, technical corks such as Diam and Twintop have much greater consistency than natural cork. Jim Peck, senior research scientist with G3 Enterprises, said: “dense technical corks such as the Diam and Neutrocork have very uniform exteriors and press against the bottle bore quite tightly, reducing the possibility of oxygen ingress no matter what the position of storage [see below].”
Natural cork is a potential culprit here; it’s a product of nature and its sealing capability is influenced by nine years of natural growing conditions.
The bottle
For driven closures, the neck bore of the bottle is key, for screwcaps, the top of the bottle. Geoff Taylor, managing director of UK technical lab Corkwise said: “one bottle mould could give a slightly defective bore [though still within specification]. Even with a perfect closure, this could cause random oxidation.” This is complicated by the fact that different neck bore sizes are used in different parts of the world.
Bottling
Joining closure and bottle together is a risky process for wine integrity. Bottling is, essentially, an exercise in oxygen-avoidance. The opportunities for oxygen pick up are manifold and if something goes wrong with the equipment even the best risk-management protocols can be breached.
Even with a perfect natural cork, said Taylor, “the cork may be OK, but if something as simple as the corker jaws are not set properly, the cork will be over-compressed. The cork is blamed for random oxidation, when the jaw is at fault. Synthetics are more of a problem with over-compression because they spring back less.” And if a crease develops along the length of the closure, then the opportunity exists for rampant and random oxidation.
Olav Aagaard, chief technology officer for Nomacorc said: “Bottling and closures are not yet integrated into winemaking. If something is wrong at bottling, people say the closure has to be guilty, but it is not always the case.” If 1 or 2 heads in a 24 head filler are badly tuned, this can lead to random oxidation down the road. And this is with any closure.
“Differences in tooling and maintenance can create differences” said Jacques Granger, consultant to the manufacturers of Stelvin, “the shape of the filling pipe in the bottle, jogging or shocking bottles as they move between filling and capping can allow in oxygen.” Banister added “if the labeller breaks, filled bottles wait [for the repair], then they’re sent to the capper.” If the vacuum doesn’t pull on one bottle, to evacuate the headspace, some bottles start out with a little more oxygen.
And the rest 4, 5, 6
Sulphur dioxide management
“No matter how good the closure is” said Banister, “if there’s not enough SO2 in use to begin with, then the wine will die.”
The first Australian Wine Research Institute (AWRI) closure trials identified oxidative characters developing in white wine at about 10mg/l free SO2. This is a potential risk for producers operating a minimum SO2 regime. “There’s no leeway” according to Taylor, so “if organoleptically there is no difference between 15mg and 10mg free SO2, why not bottle at 15? We need to take more notice of SO2 because it is related to oxidation.”
Control of sulphur levels is not a random issue. But if there is no leeway, then sporadic incidences of oxidation may occur as a result of other causes.
Ascorbic acid
Ascorbic acid is often used in conjunction with SO2 to keep wines fresher and more youthful than by SO2 alone, and there have been suggestions that ascorbic acid might be implicated in random oxidation events. Dr Geoffrey Scollary, now consulting to the wine industry, has done much research on ascorbic acid and said: “this is the critical thing. Ascorbic acid as ascorbic acid is fine and safe; as soon as it breaks downs, when more than 95% is consumed that’s when colour reactions [signifying oxidation] occur.”
But, he says: “ascorbic acid does not decay rapidly unless a large amount of oxygen is present.” Indeed, AWRI research showed wines were less oxidised if ascorbic acid was added at bottling than if it was not added. Industry consultant Richard Gibson, of Scorpex backs this up, saying “ascorbic acid is not the culprit – it cannot degrade SO2 unless it has been exposed to oxygen. And it is just not possible to get enough oxygen into the bottle at bottling to account for the extent of change that has been seen.”
Storage and transport
Incidences of driven closures moving within the neck of the bottle are well-recorded, especially as wine travels across the equator. Mai Nygaard, Nomacorc’s business development manager said she had observed where “there’s no temperature control in the container, there is more variation on the outside boxes than in the ones in the middle of the container” which suggests such an occurrence may lead to random incidences rather than the whole container being compromised.
In a shop environment, display bottles in the window or under bright lights will have a different evolution than those kept in their cartons in the storage area.
Back to natural cork
In addition to the risk factors that cross closure type, natural cork is created by a process of nature which give it unique attributes, both positive and negative. Does its natural variability affect random oxidation events?
Studies using Mocon Oxtran measurements ten years ago found a 1,000 fold variation in oxygen transmission rate (OTR) across a sample of natural corks. Yet other studies, using free and total SO2, and colour change measurements, show only a 2 to 3 fold variability in the OTR of natural cork.
So where is the science at now? Gibson, who was involved with the original Mocon research during his time at Southcorp said the “1,000 fold variation reflects the risk of random oxidation when upright storage with some batches of cork is used. Variation is less when laid down storage is used, but can still be considerable.” He emphasised the variation across cork batches, saying some batches are fine, but “I’ve continually said, some cork batches are worse than others. I’ve no idea why.”
One of the analytical issues with Mocon is that it only uses dry cork, that is, cork not in contact with wine. At G3 Enterprises in California, Peck has developed a ‘wet’ OTR cork method for the Mocon Oxtran, which is designed to more closely resemble horizontally stored wine bottles. He said “in an upright situation it is likely the lack of a good seal between the cork and glass that allows in oxygen through what I call micro-channels between cork and glass. As the bore of the glass bottle is decreased, a better seal is created by increased compression between the glass and the cork, sealing off these channels, lowering the OTR. In a laid down or inverted position, wine will help seal these and also soften the cork to conform to the glass more completely, resulting in low OTR.” The study is ongoing so he doesn’t reveal the actual OTR value.
A study by Elizabeth Waters, a biochemistry research manager at the AWRI, also 10 years ago, looked at different natural corks, bottling a wine with the corks, also bottling the same wine with screwcap and bottling the wine with cork plus screwcap, one on top of the other on the same bottle. The idea being any variability under cork-stopper should not be replicated with the cork+screwcap stopper.
The study found with some of the natural cork there was a big spread in the data for free SO2, total SO2 and colour, yet Waters said: “It is very hard to calculate an OTR variation rate from the spread of SO2 data, but with some assumptions, we can attribute a 2 to 3 fold variation in OTR to the largest spread of SO2 we saw in this study.”
In another study, using non-destructive spectrophotometry to monitor browning in white wine as a measure of oxidation, she said; “the spread of data was more like 2 fold. These values are a long way from 1,000 fold.”
Scollary’s observations of random browning also concur with the 2 to 3 fold variation.
Miguel Cabral, heading up research and development at Amorim, the world’s biggest cork producer, said: “After the publications of Paulo Lopes et al. [at the University of Bordeaux] in the Journal of Agricultural and Food Chemistry in 2005, 2006 and 2007, it became clear that the theories defending OTR values with a 1,000 fold variation in natural cork stoppers cannot survive a scientific analysis.
“This variability obtained by Hart et al. [the Southcorp work] was probably due to a methodology error, or something else. But one thing is clear, this information was not published in a scientific [peer reviewed] paper as Lopes’ study has been. Lopes’ data was clear: technical corks have less variability in permeability than natural corks but nothing even remotely comparable to a 1,000 fold variation.”
He added “we are looking into reasons for the possible variability in natural cork stoppers, but its too soon to present any definitive conclusions.”
What’s clear is the science does not agree and partisan proponents are able to pick the parameter that best promotes their cause (whichever cause it is). Speculation amongst scientists includes a rogue result in the 1,000-fold study, for example by a crease in one of the cork samples, or some unknown aspect of Mocon Oxtrans measurement that is not yet understood. Gibson emphasises he sees samples with oxidation rates that cannot be explained by oxygen ingress at bottling, and emphasises certain cork batches are worse than others for reasons we don’t yet know.
It’s clear the science is at odds on this issue, which is not necessarily unusual, and industry can draw no definitive conclusion until the science catches up to explain experience. It’s only in recent years that miniscule oxygen ingress has been widely accepted as a normal part of bottle age.
The reality is there’s not been much work done looking at the OTR variability of natural cork, apart from the studies mentioned. Another reality, perhaps related to the first, is the measurement of dissolved oxygen in wine is still a major technical and analytical challenge, whatever the methods used. But science is on the cusp of a new frontier of oxygen measurement and, therefore, management. This will undoubtedly help our understanding of non-random and random oxidation events. Peck even suggests emphasis might shift to the bottle, saying “I suspect that bottle bore may have been the culprit as much or more than the cork. We now do much of our OTR testing in precision bore glass sleeves to eliminate problems with bottle bore variation.”
We await results from Peck’s ‘wet’ Mocon measurements. And at the AWRI, Waters is beginning research using two new methods. Oxysense uses “oxydots, a fluorescent method, using scanning technology to measure oxygen through bottles” and the other new method has been developed at the AWRI, which uses a specific oxygen trap.
What is clear is that random oxidation can occur across closure types. Natural cork is more variable than industrially produced closures. More than this we cannot really say until science provides industry with consistent, and commercially-useful, information. The spotlight, or maybe the oxydot, is on.
