I came across this interesting article which is quite technical but may explain and clarify screw tops and its effect on bottle ageing. Now I do warn you to stay with it as it is complicated in its narrative.
I acknowledge the academic author Volker Schneider. I have edited the story the best I can without losing its context.
Screw caps differ in the oxygen barrier effect of their sealing inserts. Too much oxygen promotes oxidative ageing, too little oxygen promotes reductive ageing. Reductive ageing under hermetically sealing bottle closures is due to the formation of increased levels of volatile sulphur compounds, in particular thiols and H2S, whose odour causes reductive taints. Their prevention by the addition of copper salts before bottling is commonly opposed due to a variety of concerns. As an alternative, a functionalised liner for screw caps has been developed. It traps the thiols responsible for the occurrence of post-bottling reduction flavour and at the same time protects the fruity varietal aromas from oxidative ageing through its perfect oxygen barrier effect.
Fruity white wines develop different sensory expressions of ageing. The best known of these is typical ageing, which is intensified by oxygen uptake via the bottle closure. Conversely, it is largely prevented by the widespread use of screw caps, in some cases with hermetically sealing inserts. However, such closure systems promote the so-called reductive ageing through the formation of reductive taints in the bottle. The development of a functionalised liner for screw caps opens up a way out of this dilemma.
From the moment a wine is bottled with different closures, the emergence of different wines from the same initial wine begins. In white wines, essentially four different sensory expressions of maturation and ageing develop:
- Typical or oxidative ageing,
- Atypical ageing (ATA),
- Petrol flavour,
- Reductive ageing.
Without any doubt, every white wine is subject to ageing. The only question is which of the above-mentioned types of ageing it is and how quickly it develops. In most wines, the bottle closure, and particularly its oxygen permeability, plays a prominent role.
Different chemical reactions and odour-active compounds are responsible for the formation of the various forms of ageing. Two of them are predetermined by viticultural factors. Petrol flavour, for example, occurs almost exclusively in Riesling wines obtained from physiologically ripe grapes grown under hot-climate conditions, whilst the development of atypical ageing is observed exclusively in wines obtained from stressed fruit. The occurrence of these two very specific types of ageing is not related to the availability of oxygen. Consequently, it is not influenced by the oxygen permeability (OTR) of the bottle closure.
Regardless of this, the development of petrol flavour is significantly increased with screw caps because, in contrast to internally sealing closures such as corks, these have little material that could absorb the compound responsible for petrol flavour (TDN).
Typical or oxidative ageing
The situation is completely different with oxidative ageing. It has always been known and is, globally speaking, the most common sensory expression of white wine ageing. It is mainly due to the oxygen ingress through the bottle closure. In this process, odour-active compounds are formed under the influence of oxygen, of which methional, benzaldehyde, 2-phenylacetaldehyde, 3-methylbutanal and furfural are the most important ones and are considered indicator substances. They are higher aldehydes formed by oxidation of their corresponding alcohols. Their aroma notes of nuts, dry herbs, honey, cooked vegetables and boiled potatoes increasingly mask the fruity varietal aroma and cause a distinct madeirized aroma in extreme cases.
In contrast to the well-known acetaldehyde, which in its free form elicits its typical smell reminiscent of bruised apples and sherry and which can be bound by sulphur dioxide, these higher aldehydes barely react with SO2. Therefore, their formation cannot be effectively prevented by bottling with increased levels of free SO2. The reactions that lead to their formation are largely irreversible. They are controlled by oxygen supply and significantly accelerated by warm bottle storage.
Screw caps counteract oxidative ageing because they protect the bottled wine relatively well or even hermetically from absorbing atmospheric oxygen. This is one of the reasons for their almost universal acceptance in some countries. On the other hand, the assumption that the well-sealing screw caps protect the wine against any kind of adverse ageing is wrong.
Screw caps as an answer to oxidative ageing
In oxygen-sensitive white wines, differences in oxygen uptake of more than 5 mg/L O2can be discriminated by sensory means. This led to the initial assumption that the ideal closure for such wines would seal hermetically and prevent any oxygen ingress in order to preserve the fruity primary aromas for as long as possible. Since screw caps fulfil this requirement better than most other closures, there was initially nothing against their widespread introduction. This was especially the case when, a few months after bottling, the oxygen dissolved in the wine and the oxygen trapped in the bottle headspace has been completely bound and consumed by the wine, the closure takes control of oxidative ageing. With increasing bottle storage, the influence of the bottle closure comes more and more to the fore.
Significance of the sealing insert in the screw cap
Contrary to popular belief, however, screw caps are not a uniform type of closure, but are distinguished from one another by different sealing systems with different oxygen barrier effects.
Each screw cap consists of an outer aluminium cylinder and a single or multi-layer sealing insert. The outer cylinder fixes the insert in the correct position and presses it onto the bottle rim with the required pressure. The sealing insert provides the seal between the product and the closure, seals the bottle and prevents the diffusion of gases and liquids. It determines the tightness and functional quality of the screw cap. Thanks to their specific characteristics, the inconspicuous sealing inserts are the central element of screw caps and their functional end. In other words: Screw caps are as good as their sealing inserts. The latter are produced by specialised companies. The multitude of screw cap manufacturers is supplied by only a few manufacturers of sealing inserts.
Originally, the sealing insert consisted only of simple elastomers such as PVC or PE, which were injected into the aluminium cylinder. In the wine business, such inserts are mainly found in the short-skirted MCA or roll-on pilfer proof screw caps, which are preferably used in the segment of simple and inexpensive wines.
The gold standard for screw caps is now considered to be the long-skirted (60 x 30 mm) variants such as “Stellvin” or “Longcap”, which require a BVS bottle neck finish. Instead of injected elastomers, multi-layer sealing liners are predominantly used in these closures. Two main variants of such sealing liners are known (Figure 1):
|Figure 1: Structure of common liners used for screw caps.|
At the turn of the millennium, when the hermetically sealing screw cap with a tin-saran liner was introduced, Australia’s and New Zealand’s wine industries took a pioneering role. One of their reasons was the pursuit of better preservation of the fruity varietal aroma of white wines under absolute oxygen exclusion. After a short delay, the wine industry in several European countries also adopted this logic.
The initial euphoria ‘Down Under’ soon gave way to more sober reflection when a greater tendency of the wines to developing aromas described as reductive or sulphurous was demonstrated under conditions of absolute air exclusion such as under the tin-saran lined screw caps.
The oxygen supply through the selected bottle closure determines whether the ageing of the wine is driven more in the oxidative or more in the reductive direction.