CLOSURES: Firmly on the screwcap

The last decade has seen tremendous progress in research into and understanding of wine closures. Jamie Goode highlights industry advances to date.

The noughties have been the decade of the screwcap. Ten years ago, if you’d gone into a supermarket and scanned the wine aisles, they would have looked very different.

Screwcaps would have been non-existent, and the only alternative closures in use would have been the first-generation injection-moulded synthetic corks, which were first introduced in 1996 by Safeway. The vast majority of bottles then were cork sealed, and cork wasn’t doing a particularly good job.

How things have changed. The global picture now is one where cork is still the dominant closure, but alternative closures have made significant inroads.

Estimates are that of a global market for bottled wine of some 18 billion per annum, screwcaps are now sealing just over two billion of these bottles, while synthetic corks seal four billion.

But if we look at a more local level, there are countries where cork is in danger of extinction: screwcaps now seal around nine out of 10 bottles of New Zealand wine, and in Australia the situation is fast heading that way. In UK supermarkets, alternative closures are gaining a dominant position.

But perhaps even more significant than the change in closure usage has been the change in our understanding of what wine bottle closures actually do.

Ten years ago, the general consensus was that the role of the closure was simply to seal the bottle, and the better it did this the better the closure.

Received wisdom was that a total (hermetic) seal, allowing no oxygen transmission (OTR), would be the ideal closure. While a few still believe this, the results of a significant scientific study conducted by the Australian Wine Research Institute (AWRI) have blown this idea out of the water, and changed the way we think about closures forever.

Groundbreaking study

This study, the AWRI closure trial, began in 1999, just when screwcaps were being reintroduced to Australia after their previous (unsuccessful) adoption back in the early 1970s.

The trial took a single wine – a Clare Valley Semillon made by Kerri Thompson at Leasingham – and bottled it under 14 different closures, including synthetic corks, natural corks, technical corks and a screwcap with a metal layer in the liner.

This metal layer in the liner is important, because it is the liner that creates the seal and determines the property of the closure. In this sense, the screwcap itself is not the closure: just like a crown cap, a screwcap is merely a means of holding a liner in apposition to the rim of the bottle neck.

There are two liners currently used with screwcaps: one with a metal layer (now tin, but aluminum has also been used in the past), allowing very little OTR, and one made of Saranex, which allows more OTR. This is a significant difference that is often (mistakenly) overlooked.

The researchers at the AWRI followed these 14 versions of the same wine over a number of years, using both sensory and chemical analysis.

The findings were clear: from the moment of bottling, the wines set out on rather different developmental trajectories, such that by the time of the first major publication (results from 21 months post bottling), they were different wines.

The synthetic-cork-sealed wines developed quickly and showed evidence of oxidation, while at the other extreme the screwcap-sealed wine was freshest and was developing most slowly.

“An important outcome of the AWRI’s research on wine closures is the recognition that when a wine is bottled under different closures, different wines begin to be created from that point onwards,” stated the study authors.

“Other workers have apparently expanded this concept to other bottling variables such as the filling height, the concentration of free sulphur dioxide at bottling, and the mixture of gases in the headspace of bottles post filling.

"The ability to link such variables to wine development post bottling creates the possibility of reliably predicting, and therefore optimising, wine development in bottle.”

But there was a surprising observation. While the screwcap-sealed wines were the freshest and fruitiest of all, they also showed a trace of what was described as burnt rubber/struck match character in the sensory analysis.

After some head scratching, the study’s authors identified this as being due to some complex post-bottling wine chemistry involving sulfur-containing compounds, known more widely in the wine trade as reduction.

At five years, the distinctions were even more marked. The synthetic-sealed wines had gone by this stage, but the remaining closures showed distinct differences.

As an aside, I recently tasted one of the screwcap-sealed bottles from this trial, and it was still fresh, fruity and attractive, some 10 years and eight months after bottling, with the reduction noted in the trial being merely a tiny, and not unattractive, distraction from the fruitiness.

The clear conclusion from this study is that perhaps the most important property of its closure is its oxygen transmission properties. (I’m taking for granted here that the closure itself is not tainting the wine.)

At one extreme, the first-generation synthetic corks were allowing too much oxygen transmission, and the wines were developing fast and then oxidising, and at the other the screwcaps were allowing very little oxygen transmission, with the result that the wines developed more slowly, and in this case at least, this slow development came at the cost of some reduction.

This groundbreaking study prompted further investigation of wine development post-bottling, and the role of the closure in this process.

The conclusions of other similar studies have all been in agreement, reinforcing the fact that the rate of OTR is critical in determining the development of wines post-bottling.

Australian wine scientist Richard Gibson, a closures expert, summed up this concept in a nice graph, plotting closure OTR against wine sensory characteristics.

How much OTR by the closure do we want? Too much results in oxidation; too little and the risk of reduction is high.

Somewhere in between is just ideal, but how much is just enough? This has been a question that the trade has been grappling with for some years now.

Designer closures

The observation that closure OTR is a key property that influences how wine develops post-bottling has led to the concept of “designer closures”: the idea that if we were able to gather enough information about how different wine styles respond to oxygen post bottling, and couple this with our knowledge of closure oxygen transmission, we could then offer winemakers the information they need to match closure type to wine style. In this scenario, the winemaking continues post bottling, and closure choice becomes an active winemaking decision.

With this in mind, one of the world’s leading closure companies, synthetic cork manufacturer Nomacorc, has recently initiated a multicentre research project that aims to provide just this sort of information.

Its vested interest here is quite clear: it is currently offering products with a range of different OTRs, and so would see itself as well placed if winemakers really were to start choosing closures offering a specific level of OTR to match their wine styles.

To achieve its goal it has partnered with respected wine science institutions across four continents, each of which will be looking at particular grape varieties.

In addition, Nomacorc is one of the members of a new group called Oxygen in Wines, or O2W. This is an international non-profit association made up of suppliers and service providers to the wine industry, with the stated objective of “the promotion of scientifically-based solutions for oxygen management challenges in the wine industry”.

Participating members include G3, Lallemand, Perrier Bottling Machines, Inter Rhône, Appe and Nomacorc. As yet, O2W isn’t funding research directly, but may do so in the future. So far it has held two conferences on the topic, in 2008 and 2009.

“Oxygen management is an essential aspect of winemaking,” says Malcolm Thompson, CEO of Nomacorc. “Our ultimate objective is trying to gain an understanding of how oxygen, in combination with the closure, influences winemaking.”

The firm’s research agenda has two different levels. The first is to gain a greater understanding of ways to control oxygen during winemaking in terms of quality control.

Thompson describes this as the “low hanging fruit”: it will be relatively easy to measure oxygen during the winemaking process given the technology that now exists, and this will be a promising avenue for improving the consistency of wine development.

The second level is what Thompson dubs “winemaker intention”: to put winemakers in a position where they can integrate closure design into winemaking. What sensory attributes are winemakers looking to achieve? Nomacorc would like to be in a position to advise them on the right closure to use.

“This is not a fantasy,” says Thompson. “We are starting to see a strong correlation between the analytical work we are doing and sensory analysis.”

The technology

One of the keys to Nomacorc’s research agenda is the development of a relatively new way of measuring oxygen non-invasively, by means of oxy-luminescence. It has partnered with German company ProSens, which is one of the manufacturers of this technology, and is helping to market it.

How does it work? An optical fibre emits a blue light pulse, which is directed at reusable dot sensors. These sensors emit red light in the presence of oxygen, and the amount of light produced correlates with the oxygen level.

The sensors, which cost e20-25 (£17.50-£22) each, can be positioned inside wine bottles which can then be filled on a normal bottling line.

It is therefore possible to measure both headspace oxygen and oxygen in the wine in a non-invasive way, making repeated measurements on the same bottle over time. Two sensors ara available: one for 0-4% oxygen to a sensitivity level of 1 ppb, and the other 0.5-50% at 15 ppb.

The scope of Nomacorc’s research is quite broad. As well as comparing the development of different wine styles under different closure ox-trans levels, it is also aiming to look at the influence of winemaking practices.

As an example, with Cabernet Sauvignon it is aiming to look at 16 different wines. Winemaking styles are chosen that will produce a wine with high polyphenols and one with low polyphenols.

Then both these wines are made with and without microoxygenation. Finally, each of the four resulting wines is bottled under four different oxygen transmission scenarios (low, moderate, medium and high oxygen transmission).

This is repeated for each variety under study, with some variations in the protocol where this is appropriate.

These sorts of studies will hopefully provide winemakers with the sort of information they need to make the right closure choice for their wines, and reinforce the fact that there is no one perfect closure for all wines.

But if “winemaker intention” like this is to become reality, then the trade will need to have tighter control over issues such as shelf life and the distribution chain if closures allowing relatively high oxygen transmission are used.

If a wine is bottled with a closure that aims to show the wine at its best at the point of consumption, then the winemaker will need to know in advance how long it will be until the wine is on a shop shelf, and presumably the window of peak drinkability will be finite.

So the picture that is emerging is a complex one. It’s amazing to think how much more we now know about closures and their role in post-bottling wine chemistry than we did a decade ago, and for that we have the screwcap to thank.

Had it not been for the screwcap revival, which kicked off in earnest in 2000, much of this research would probably never have been done. What will the next 10 years bring?

Jamie Goode, February 2010