Forget closure OTR (oxygen transmission rate) for the moment. In closing up a wine bottle, TPO (total package oxygen) is where the TLAs (three letter acronyms) are at, and the bottling operation is the bigger oxygen issue by far.

TPO is the sum of dissolved oxygen (DO) in the wine plus oxygen in the air of the headspace (HO), and whilst many folk have got their heads around DO, the idea of TPO is still relatively new.

Warren Roget, technical manger at the AWRI (Australian Wine Research Institute) said “normal bottling processes entrain oxygen in the bottle. There is oxygen dissolved in the wine, and there is oxygen in the headspace. Our work shows that 60 to 70% of the total package oxygen is typically contained in the headspace.”  What is concerning, added Roget, is that “it’s typical industry practice to measure DO in wine, meaning the majority of the oxygen in the bottle is not being measured.”  In practice he added, this means QA specifications may state permissible DO levels at less than 1mg/l, but when “you look at TPO, they may be closer to 3mg /l because they’re not measuring oxygen in the headspace.”  Wine specifications should be moving to detail TPO instead of the less relevant DO.

Measuring TPO has only been possible for a few years with the development of non-destructive (i.e. not needing to open the bottle of wine) measurement kits, such as PreSens, OxySense and NomaSense. Standard laboratory equipment only measures DO.

Importantly, TPO is a snapshot measure immediately after the bottle has been packed. “Three months after bottling” said Roget “the TPO will virtually all be consumed by the wine. It is from this point forward that closure OTR becomes the important factor in regulating oxygen intake into the wine. However significant quality and shelf life impacts may already have been incurred.”

Management of headspace is therefore one of the most critical areas for control of oxygen ingress at bottling.  Sometimes remedies are simple, though incur costs: using inert gases, for example nitrogen, or carbon dioxide, to flush out oxygen in the filling tanks, in pipework, the empty bottle, the headspace, prior to the wine being transferred.

According to closure trials done on riesling at Geisenheim Research Centre, Professor Dr. Rainer Jung found that after nearly a year, cumulative OTR varied significantly across screwcap and synthetic, from “0.5 to 2.5 mg/l in total, which is not very much.  It is not enough to oxidise the wine.”

But a different picture emerged with headspace trials. Jung said “we measured 6mg/l oxygen in the headspace. It takes about 4mg/l SO2 to reduce 1mg/l oxygen, so if you have 6mg/l of oxygen, you need 24mg/l SO2.” The first AWRI closure trial identified oxidative characters developing in white wine at about 10mg/l free SO2, and whites are commonly bottled with 30 to 40mg/l free SO2, so, said Jung “if you don’t want to lose of 24 mg/l SO2, flush the headspace [with inert gas].”

Jung highlighted the snapshot significance of DO and TPO, saying wine can arrive at the bottling line with 1-2mg/l DO already in the wine, though this completely depends on what has happened to the wine before, and its style. For example a micro-oxygenised red versus a reductively made sauvignon blanc. And he said, of a reductively made wine, where oxygen has always been kept away “in the last step, you pump into the bottling tank, and get the same amount of oxygen uptake. This will directly react with wine components, so no DO is measured, but the aromatics and phenols can be oxidised.” So the wine is in specification, but its defining characters have been lost.

Even some reds won’t benefit from oxygen at bottling. Stéphane Vidal, global oenology director at Nomacorc, said “syrah is quite reductive. If you add oxygen at bottling, you are simply wasting sulphur dioxide” and shelf life. He added that improving bottling TPO by 2mg/l “could save one year of shelf life of the wine”, if the bottle is closed with their Nomacorc Classic+, for example, which has a 2mg/l OTR over the course of a year.

Headspace management is also a critical control point for traditional method sparkling wine. Michel Valade, responsible for viniculture at the CIVC in Champagne said “the quantity of oxygen that might enter the bottle at moment of disgorgement varies according to conditions of disgorgement.” CIVC studies showed that the amount of oxygen introduced at disgorgement varies from 1 to 10 mg/l, and averaging 2 to 4mg/l.  Valade said that during ageing [sur lattes] “only 1 mg/l per year of oxygen enters the wine, and is consumed by the wine, so 2 to 4mg/l is the equivalent of 2 to 4 years of oxygen transmission through the closure.”

Valade explained “at the moment of disgorgement some bottles let some mousse escape, in which case there will be no oxygen entering the bottle [as the effervescence expels headspace oxygen]. But if bottle is very quiet, or stays a bit longer on the line, up to 6 mg/l may enter the bottle.” This clearly creates big bottle variation which will directly affect the flavour profile. 

The ideal is to have zero oxygen entering all bottles at disgorgement.  And to this end, the CIVC are developing a technique already used by the brewing industry, and, according to Valade, by some of the big Champagne houses, though the CIVC are still completing trials. A tiny amount (20 μL, or 0.02 mL) of wine is injected into the bottle under pressure, after dosage, immediately before the bottle is closed. This provokes the wine to effervesce, which expels the headspace oxygen. 

“Very soon” Valade said “this technique will be widespread in Champagne.” It “is not very expensive, and can easily be installed on the disgorgement line.“ Added to which, he said, achieving a close to zero ingress of oxygen at bottling means producers can use less sulphur dioxide, to achieve a lower final sulphur dioxide measure.

The level of TPO that industry should be aiming for depends on individual wine style, though “generally the lower the better” said Roget. Vidal said their studies showed “on average, DO is 1-2ppm mg/l. And headspace is 1-4mg/l, giving a TPO of 2 to 6 mg/l.” So, he said “A TPO of 2 mg/l is therefore already a good situation – a bottling line that is working fine.“

For the majority of wine which is drunk within two years of bottling, this is all crucial, as high TPO at bottling quickly erodes shelf life. And for these wines, the wine should be ready for drinking at the point of bottling.  Jung said “During bottling and storage the lowest quantity of oxygen coming into the bottle would be the best way to keep the wine in a ‘ready to drink’ situation.”  Roget added “a high TPO can have a shelf life reduction equivalent to 10 years of oxygen transmission through a Saran-tin screwcap.” He said it’s a “completely different order of magnitude with TPO versus OTR.”

It’s clearly time to focus attention on the moment immediately preceding closure.

CASE STUDY – Reh Kendermann GmbH Weinkellerei

Reh Kendermann (RK) moved from measuring DO to measuring TPO in March 2010, with the purchase of NomaSense equipment.  Their winemaker Phillip Maurer explained that some years ago their contract customers wanted to know about oxygen management, so “we started to measure oxygen input at all the different situations in the cellar – racking, filtration, centrifugation, blending, the whole bottling – tank, filter, filler, bottling line.” 

Measurement, by sampling bulk wine was relatively easy, enabling RK to control the whole process. Flushing pipes, tanks, bottles etc., and blanketing wine with carbon dioxide minimised oxygen uptake. But this only measures DO.

With NomaSense, the TPO is measured and the “main goal is to have a TPO in bottle below 2mg /l.” Maurer added that such a TPO measure is now included as standard in the wine specification for Black Tower and the Kendermanns wines.

A flurry of activity, mostly generated by customer demand for innovation, has seen several new closures on the market, both for premium and high volume wines.  TCA (2,4,6-trichloroanisole, the chemical that causes musty off flavour in wine) has given way to OTR (oxygen transmission rate); new stoppers have been brought out for sparkling wines, and marketeers are getting back on the act as closure choice becomes a way to differentiate brands.

Three new sparkling wine stoppers have appeared.

Three new sparkling wine stoppers have appeared. Alcan’s Maestro stopper has all the pizzazz Champagne could want, and it is built around the utilitarian crown cap, which frequently does the stopper-work in maturation cellars before disgorgement.  The functional cap is dressed to the nines with thick foil and the new one-arm bandit opening mechanism. 

The business bit of of the Maestro

Crown caps with synthetic liners have been used in Champagne for more than 10 years. Maestro uses a single piece liner of “unique shape that gives it a better sealing, with a thicker layer of polyethylene” said Mathias Mélan Moutet, president of cap-maker Solocap-Mab, which, he said, gives it a CO2 loss well within the lowest third of the CIVC’s ‘acceptable’ range.

Bruno de Saizieu, the commercial and marketing director of Maestro-maker Alcan Packaging Capsules cheekily smiled “the effort to open it is easy enough for a woman to do”. The opening is made simpler without the foil to unwrap and the wire to unwind. Restaurants should love it.

Australian company Zork has brought out a plastic stopper for sparkling wine, based on the technology for its still wine stopper. It comes at a cost they say is comparable to cork-plus-wire, but, said marketing manager Jo Baker “the big cost saving for producers is to opt for no [foil].”

Zork sparkling

Evans and Tate have used this on their new sparkler Zamphire.  Rosemary Scott, their general manager for global sales and marketing, said “the re-sealable closure appeals to consumers as it allows them to have a glass or two, reseal the bottle and save the rest for another day. Equally, almost 50% of Australian sparkling wine consumers who we surveyed said they found it difficult to open bottles. The new closure allows more control when opening while still providing the traditional ‘pop’ and ceremony.”

The semi-sparkling (frizzante) market is big in Italy, and Guala Closures has brought out the ‘Moss’ (Italian for slightly fizzy) screwcap for semi-sparklers. Marketing manager Anne Seznac said the screwcap “was used for small formats, but not for big formats until requests came from Italy, Argentina and Brazil” for something easy to open and close for younger consumers.” A polyethylene liner was developed, but the shell of the screwcap and the bottle are the same, so, she said, only an adjustment to the block pressure of the capping heads needs to be made.

With Prosecco resurgent, as well as semi-sparkling usually attracting a lower duty rate in the UK than sparkling, this could be an interesting development, although Neil Bruce, wine director at Waverley TBS said he would want to know “firstly, if we have a need for a semi-sparkling. Is there enough innovation in the wine?” With 85% of Waverley’s business being in the generally slow-to-innovate, fragmented on trade, he added innovation in the on trade “tends to be supplier led.”

'Moss' screwcap for semi-sparklers

For still wines, a number of premium products have been launched and brand differentiation is emerging as a consideration for choice. After nearly five years without reported quality issues, glass stopper Vinolok’s general manager Siegfried Landskrone said the stopper’s “attributes combine the oenological standpoint of a proven system, and it meets the aesthetic view of a new trend for customers – it’s easy to open, there’s no extra tool, it’s a more modern and fashionable way, and it meets the emotional requirements for how a closure should look.”  

Landskrone said: “Vinolok is moving from a purely technical solution, where the main contact was the winemaker.  In the last 18 months this has changed to the sales and marketing guys. Where people are producing wines for export, more wineries are looking for marketing strategies to makes attractive packages.”  He added in terms of cost Vinolok is comparable to high quality natural cork, so it’s not an option for many.

Peter Gago, chief winemaker for Penfolds agrees that “aesthetically and psychologically people have a lot of trust in glass.” Indeed Penfolds are at the very early stages of trialling two prototypes of true glass-on-glass stoppers, though phase one is not yet complete, Gago’s confidence notwithstanding: getting a glass disc appropriately fixed onto the levelled top of a bottle. If this development is successful, said Gago, “it will be for wines meant to age long term, the upper end of our portfolio,” adding “the proof is we have bottles [Grange] under this seal for over a year now. It’s working.”

Another product aspiring to the premium niche is Econatur from cork producer Juvenal. Cork is undeniably the most environmentally-friendly wine stopper, and Juvenal have amalgamated cork harvested from their 600 hectare organically-certified forest with cork from FSC-certified forests to produce a range of corks they market as ‘chemical-free’. 

Econatur one-piece cork

Rui Pereira, sales director of Juvenal said “the idea had been just to do single-piece cork, but there was pressure from customers to include technical corks”, which has already given Juvenal orders for 1m stoppers.  

At the high volume end of things, cork giant Amorim have re-launched a single-piece cork stopper, targeted at the fighting end of the market where clients want whole cork not technical (cork particles/discs of whole cork), but have not been able to afford it. It is cited as being able to undercut the alternatives by up to 50%. 

Communications director Carlos de Jesus said Aquamark is a single piece “lower quality natural cork stopper wrapped in new technology, which is sensorially neutral and has a visually-appealing result while keeping costs at a price point that brings natural [single piece] cork stoppers to price sections from which they were previously excluded.”

The price range was given between €35 and €110/1000, and de Jesus added: “Since we launched five months ago, 100 new clients have started using it, including some brand new customers.”

Acquamark stopper

The cost competitiveness of such a product may be the only thing that gets the UK trade interested. Matt Dickinson, director at Thierry’s said: “I look at [innovation] from two perspectives: what will the consumer think, and what will our customer think? And what about the cost? Can we benefit because of the cost, or for profile?”

He explained: “For cost and profile, commercially it depends on type of product. If mass market, finding as perfect a closure as possible for as little as you can pay is the route to go down. Any saving we can make on any aspect of packaging is a good thing, especially if it improves the overall quality of the liquid.” But, he warned. “the consumer is key to all this. If they don’t accept a particular type of closure, you need to find ways of bringing them on board.”

A different strategy has been adopted by Nomacorc who are working with scientific institutions to understand oxygen management throughout winemaking to arrive at “the sensory profile of the wine that the winemaker wants” said Malcolm Thompson, vice president of marketing and innovation, adding “OTR is one aspect. We have learned that oxygen management upstream is critically important. Ultimately we’ll look at pre-bottle ageing, micro-oxygenation and try to bring the whole process under control.” Adding “we’re aligning the research to our closures, and we can imagine a range of closures with different OTRs.” In the meantime their customers get valuable insight.

Such total control for reliability and consistency should elicit the interest of the big brand owners. Though, having adopted screwcaps 18 months ago, Greg Wilkins, director of Brand Phoenix, owners of First Cape, said: “for brand owners, consistency is the most crucial factor. We evaluate new closures periodically [but] we’re fairly conservative. Once we’ve found one that works, then the person who is the most important is the consumer.”

Pernod-Ricard’s wines development director, Adrian Atkinson was equally circumspect, saying: “Whist we do keep up to date on new closures, we have put a lot of time and research investing in screwcap closures – it is a commercially viable investment for Jacob’s Creek both now and for the foreseeable future.”

Ultimately, no matter the innovation, it has to stand firm with consumers. David Gill MW, director of Bottle Green said “It’s all very well if we or the retail buyer thinks it’s a good idea, but will the consumer buy it?”

Oxygen ingress has become the new black for the closures industries.  TCA (the  mouldy, musty taint in wine) is last season’s outfit, and environmental considerations remain an underground movement, threatening to move into mainstream fashion at any moment. 

In its efforts to forge an environmental path, cork closure champion Amorim held a seminar on the sustainable future of natural cork, presented by Dr Miguel Cabral, their research and development director. According to communications director Carlos de Jesus, the seminar covered “anything that people want to talk about – carbon footprints, the latest research and development, TCA, new products, new oxygen permeability research.”

But cool chic is all about oxygen management, and OTR (oxygen transmission rate – how much oxygen passes through/past the closure into the wine) is the new three-letter acronym of closure cool.  It’s during the last year or two that oxygen ingress through/past the closure has quietly become the accepted norm.  Screwcaps are no longer considered hermetic by the mainstream, though it is difficult to pinpoint the origin of this, as the Stelvin brand of screwcaps have been available with two differently permeable liners for more than 30 years, and, according to Bruno de Saizieu, sales and marketing director of Stelvin makers Alcan Packaging Capsules, have been marketed as such.  

Why is OTR so ‘of the moment’? Managing oxygen is related to the mechanical and physical properties of the closure in contact with the bottle bore or top of the neck. Closures need to be consistent, and, by and large, it is the industrially made closures, where each and every one is the same as the others in a batch, that are likely to perform most consistently. With such consistency of closure, and therefore, it is thought, of OTR, trials and observations can be made from which wine behaviour in bottle can be predicted with confidence.  Dean Banister, the sales director for Oeneo, who make Diam, said: “at Oeneo we’ve been talking about OTR for a few years now and for us it is the next big issue for closures. For me the subject is being led by Oeneo and Nomacorc because we can both actually control OTR with our closures whereas it is not possible with natural punched cork.”

Indeed, earlier this year, synthetic stopper supremo Nomacorc, with G3 Enterprises and Lallemand, founded O2inWines, a group bringing together industry technical leaders to expand science-based understanding of oxygen’s relationship with wine. Inter-Rhône has just become the fourth industrial member of this Association, alongside academic members such as AWRI, INRA, UC Davis and Geisenheim (institutions in Australia, France, USA and Germany respectively).

Nomacorc’s vice president, marketing and innovation Malcolm Thompson, said: “there’s quite a buzz associated with the whole oxygen management in winemaking. Some as a direct result of the conference, a lot of it as a result of work Nomacorc has done specifically”, which includes four initiatives with leading institutes across the world, researching the effect of oxygen on different grape varieties and bottling conditions.  He added “the research is step by step globalising, with four programmes at AWRI (the Australian Wine Research Institute), INRA (the Institut National de la Recherche Agronomique) the Geisenheim Research Centre and UC Davis. And we’re nearing a position to announce a fifth programme in south America.” 

PLASTICS

As well as OTR research initiatives, a new model of Nomacorc closure – Classic+ – has just been released, after a three year development programme. It has an OTR about 30% lower than their Classic model and Thompson said: “the Classic+ driver is from performance to preservation. We’re capitalising on breakthrough technology when we developed Premium.”  Classic+ adds another 12 to 18 months shelf life compared to Classic.

Nomacorc may have five different models on the market, but competitor Neocork  have adopted a different paradigm, with just a single product. Mark Coleman, their director of global business development, said with: “a decade of commercial performance in the marketplace, we’ve been reluctant to start compromising a proven formula because when you start changing materials or densities on such a technically engineered product, there will be compromises.” He emphasised their single product met the age-ability needs of 85% of the wine market, that is, drunk within a couple of years of bottling.

But trials are under way, he said “to address market demands of both a lower cost cork and one more suitable for wines intended to age five-plus years with products whose densities, mechanical and sensory attributes mirror the proven performance of our current product.”  Expect some news in late 2009/early 2010.

SCREWCAP

A similar time frame is forecast for news on new screwcap liners. At least two types of liner are undergoing trials, one with an OTR between the two existing liners on the still wine market, Saranex (higher OTR), and SaranTin (lower OTR), and one with a higher OTR than Saranex. 

At one of the leading screwcap manufacturers, Guala Closures Group, different types of testing has involved different materials. Their marketing manager Anne Seznec said: “we’re working on a liner with a 100% hermetic closure because we need to achieve an hermetic seal first, before we decide the rate of OTR that we want to put in the liner.”

Alcan Packaging Capsules vie for global top dog slot with Guala Closures in the long skirt screwcap market. De Saizieu said “we are making some trials with customers on finished products. We’re testing regularly those liners to be sure that what we expect is right with the wine. But this takes time because of the ageing.”

CORKS

While both synthetic and aluminium closures are industrially made and therefore batches are expected to be consistent, natural cork, being natural, is argued to have some variation.  Single piece natural cork is still the most widely used cork closure with Amorim alone making about 1 billion of them, but, being single piece natural corks, they’re naturally subject to individual variation, though the amount is widely, and often inaccurately, debated. A two to three-fold variation is oft-discussed among academics, and there is also an argument that the high level of cork compression inside the bottle neck bore eliminates much of any natural differences between individual corks.

It may be difficult to offer a precisely measured OTR parameter for natural whole-piece corks, but it should be possible on high quality technical corks, such as Oeneo’s Diam, even though we don’t yet understand what a specific OTR means in terms of wine development in bottle.  

Nonetheless, OTR becomes a relevant topic of conversation to further that understanding.  Since the launch of Diam in 2004, Banister has been doing his bit to move OTR up the agenda. In fact, he said “we have not produced natural punched cork for over two years and we ended supply of this product in most markets at the end of 2007.” He added, “We will be a single technology company in the next few years, focusing only on the Diam technology” which means their ‘Reference’ product will be gradually phased out.

PROCEED WITH CAUTION

With Diam, Oeneo offer four levels of OTR, which, Banister said “because of the low understanding of the effect of OTR on wine we offer them on the basis of wine ageing potential [shelf life].” There’s also a differential cost “because of the density of each closure and the size of the cork grains.”

Carlos de Jesus says Amorim’s technical granular cork Neutrocork is their fastest growing stopper, though it’s long way behind the volumes of twintop.  On the various merits of single piece versus technical cork, he said: “Cork offers different products for different price points for different wines. You wouldn’t cross a desert with a 2CV [car], you might want something better adapted. We’re creating different products for the market, which is becoming more segmented. No one else can cover so many price points, we can offer something for £1 bottle or £1,000 bottle. 

And he doesn’t think the single piece natural cork stopper will fall out of favour any time soon. Using Bordeaux growths as his example, he said “If natural cork stoppers did not give you predictability and consistency, natural cork would have gone long ago. There has to be some consistency, there is a common sensory thread running through [different bottles of the same] wine.” He has a point. High level blind tastings would be tricky without some consistency across the board.

Though our understanding of oxygen management is at a foetal stage, OTR  and the closure is only one aspect, albeit a crucial one, of many, involved in managing oxygen. Before bottling, management through the vinification process and at bottling are crucial.  And oxygen ingress rates are affected by environment throughout the supply chain, from point of bottling to the consumer shopping basket and beyond.   

The reality is that we don’t yet know which level of OTR is good for a particular country or grape variety, or vintage, or vinification method, or bottle age. As Banister said: “”Do we really understand OTR, in short no not yet. Much work is being done on this subject and we know it matters but as yet it has not been quantified, so many variables, wine styles, winemaking style, expected consumption date etc. And remember that oxidation to one person can be seen as nice development by another.”

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.