Most wine chemistry inside the bottle is determined by Oxidation-reduction reactions. History of glass bottles is itself not an old one for it was introduced in 17th century and only by 1900s the mass production emerged.
All wines will receive maturation inside the bottles. For majority, the ageing may be intended for several months while for premium wines, several years of maturation inside the bottle may be necessary before the release and the debate is whether the maturation depends on the type of closure used. (Philips, 2000)
With screw cap invented in 1856, it was considered as a wine closure only in 1959, when French winery Le Bouchage Mecanique used generic stelcap for its wine bottles. However, when Chateau Haut Brion screw capped 1969 vintage oxidized after the breakdown of plastic film in the cap, French abandoned the usage. However Switzerland achieved success in usage of screw caps when it tried it by 1970. Australian wineries tried commercial usage of screw caps by 1976; however the usage ceased in response to public response. However Swiss market continued to flourish with screw caps in 1995 with New Zealand and Australia joining the cause in later 2000s. (First Screw Caps appear in Bordeaux, 2004)
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Compared to cheaper and efficient screw caps, Cork is manufactured from the bark of Quercus suber and is the traditional closure for wines for years. It is permeable to allow minuscule amounts of oxygen into wines while Screw cap with facing of a layer of tin that is inert to any gas transfer. However the main disadvantage with usage of Cork is the cork taints that it imparts on wine. The new bottle closure, screw caps started to gain prominence mainly due to cork taints. However the increased incidence of reductive characters in screw capped wine is shifting the focus back to regular natural corks.
In this writing, problems faced by corks and screw caps were reviewed & the inherent advantage of one closure over another is briefed. I personally prefer corks and the review may be biased towards corks.
Role of Oxygen inside the bottle
Pastuer declared in 1863 that "it is the oxygen that maketh the wine". 70 years later, in 1930 J. Ribéreau-Gayon et al stated "Reactions that takes place in bottled wine do not require oxygen".
50 years later in 1984, Emile Peynaud, in his book "Knowing and Making Wine" suggested "It is a process of reduction by which wine develops in bottle".
In a recent study by Allen et al on "The role of oxygen in the aging of bottled wine", Red wine will continue to mature and develop both with and without additional oxygen, increased availability of oxygen greatly increases the rate at which a red wine will mature. In an anaerobic environment such as a bottle of red wine sealed with a screw cap or crown seal, some wines may develop reductive characteristics. (Allen Hart, 2005)
Singleton suggested that the consumption of oxygen would continue unabated in wine exposed to air with spoilage oxidation by around 10 saturations (60 mL O2/L) for white wines and 10 to 30 saturations (60 - 180 mL O2/L) for red wines. Whilst such high levels of wine oxygenation (and subsequent oxidation) are rarely encountered in practice, it is widely accepted that limited oxygen exposure, particularly early in the life of a red wine is desirable for phenolic development (Singelton, 1987).
In addition to reactions that affect colour and tannin structure, the presence or absence of oxygen also influences wine aroma development. Ribereau-Gayon et al suggest that the bouquet of a majority of great wines develops as a result of reduction, whereas in contrast flatness (particularly attributable to aldehydes) is an oxidative phenomenon (Allen Hart, 2005).
A continued supply of oxygen may not be required by a wine throughout its bottle development; however a perfect oxygen balance is needed at the time of bottling to ensure that wine remains stable throughout its lifetime. Too little oxygen can add reduced characters to the wine like rubbery, struck flint, and even rotten egg. Which are described in later part under Sulphide reductivity. Also too much can make the wine taste vinegar, flat, loss of fresh fruity aromas etc. Also dissolved oxygen is closely related with colour change in white wine. Due to "Piston" effect of insertion, there is good chance for significant increase of Dissolved oxygen in wine.
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The requirement to better understand the practical role of oxygen in determining wine style during bottle aging has been brought to the fore with the increasing use of alternative wine bottle closures by the AWRI. Previous research on bottled wine has generally been limited to white wine, ranging from 25 years ago to the more recent closure trial conducted by AWRI. These two studies focused on white wine, and in particular aromatic wine styles.
The AWRI study used screw cap, synthetic and cork closures, and also found that screw cap preserved the wine better than both synthetic and cork based closures. However, they found the wines stored under screw cap showed a reduced character, probably due to the limited amount of oxygen in the wine's environment (Madigan, 2004).
Problems associated with Cork
Corks have been used as a closure for wine bottles for over centuries. The new bottle closure, screw caps started to gain prominence mainly due to cork taints. The cork taints were due to 2,4,6-trichloroanisole, a compound produced when moulds infects the corks and reacts with chlorine which was used to bleach the corks.
(Mark A.Sefton, 2005)
TCA was the first compound to be identified as a cause of cork taint (Buser, 1982) and is now the principal contributor to cork taint. In addition five distinctly different taints were reported in the closure assessments of more than 145,000 corks over nine years (Simpson, 2005). TCA is a chemically stable substance that will not significantly degrade over time. TCA changes in wine with temperature and time as that can lead to desorption and absorption of wine by the Cork with the former released in to wine.
2,4,6 trichloroanisole (TCA) is created when chlorine comes in contact with moulds that form in the bark of oak tree (Quercus suber). TCA often exists outside of the cork due to chlorine washing. However, due to usage of organic chlorine pesticides, TCA can exist within the cork. Effect of TCA is characterized by mouldy, wet cardboard, wet dog etc. TCA will suppress the fruit and shortens the length of finish in wine. Below are the threshold levels of TCA in wines.
2,4-DCA, 2,6-DCA, TeCA, PCA are the other compounds responsible for cork taints on some occasions and among these, TCA has the lowest aroma threshold followed by TeCA .
(Mark A.Sefton, 2005)Apart from TCA , 2 Methoxy 3,5 dimethyl pyrazine referred to as Fungal must) is important form of cork taint and it is second only to TCA. It has a low aroma threshold of 2ng/L. Its origin in corks is still unknown. (Mark A.Sefton, 2005)
Portugal is the leading producer of corks with a share of about 54 % of the total corks produced and many companies in Portugal are now employing steam cleaning techniques by which removes 75-80 % of rapidly releasable TCA from granules.
The biggest cork producer Amorium of Portugal who produces 25 % of total corks has discontinued chlorine bleaching replacing it with peroxide bleaching. Dramatic decline in cork taint has been absorbed with number of problem reported were near 4 % which dropped to less than 1 % in 2010 reports APCOR, Portugal Cork consortium. (Amorium, 2007)
The term random Oxidation was invented in Australia in 1990s in response to perceived increase in sporadic oxidation in some bottled white wines. This problem remains as the most significant after cork taints due to usage of corks. It occurs as a result of ingress of oxygen through or around the closure that may lead to premature browning, madeirised characters, loss of primary fruits, general flattening of flavours, shortening of length of finish, Wine may taste vinegar
The important reasons for random oxidation were due to imperfections during corking process that might result in folds and ridges on sides of corks ultimately leading to leakage and oxidation where as Screw caps seal around bottle making every cap to provide consistent seal
In a study by Allen et al on the oxygen permeation through the closures for 3 yrs post bottling, Corks used in the study allowed 0.001 to 1.0 mL of oxygen per day and Screw cap permitted less than 0.001 per day. They found that consistency in screw cap is mainly due to mode of sealing. (Allen Hart, 2005)
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Sporadic post bottling oxidation is caused by one or more malfunctions in the bottling operations rather than by physical and chemical properties of Cork like excessive internal bottle pressure, defects in the compression jaws and inadequate cork closure diameter. (Casey, September 2009)
Other reasons attributed are temperature variation of wine which may cause expansion which will push against the cork. Also natural corks may have insect holes or lenticels leading to permeation.
Corks may impart woody, sappy, dusty to extreme vanilla, coffee or fungal musty aromas to wine. The other disadvantage of cork includes stripping or scalping effect in wine resulting in loss of fruit flavours.
Natural cork largely absorbs TDN (2,5,8-trimehtyldihydronapthalene) which gives kerosene-like flavour to wine. In addition, natural cork can dull grassy, herbaceous characters in wines.
In comparison, Screw caps are inert to maintain the true characters mainly in Aromatic white wines without addition or removal of any characters of wine. (Dimitra capone, 2003)
Problems associated with Screw caps
Sulphide aromas are the most commonly associated problem with screw caps. Volatile sulphur compounds are extremely odorous and they are the integral part of varietal characters especially as thiols in SB, Chenin blanc etc,
In the form of Hydrogen sulphide, they can manifest themselves as aromas of struck flint, burnt match, rubber, garlic, onion, cooked cabbage, cauliflower or rotten eggs. These aromas and flavours were categorised under reductive since hydrogen sulphide tends to form under low oxygen levels (Goode, 2007).
A report by Maggie Rosen suggests that problems affecting wines sealed with screw caps have probably been underestimated, according to IWC 2006. Faults with screw caps are as common as cork taints, as 2.2 percent of wine tasted at IWC showed high sulphide faults related to screw caps -creating vegetal and eggy flavours as compared with 2.8 % related to cork taints due to usage of corks (Maggie Rosen)
Skouroumounis et al found that white wines under screw cap had a tendency to develop a rubbery or reduced aroma .The wines sealed under cork displayed relatively high fruit intensity and negligible reduced characters (G.K. Skouroumounis, 2005).
Low levels of sulphide remaining in the wine may get oxidisied with the remaining dissolved oxygen in the wine or by oxygen ingress through the closure. The imperfectness of cork may facilitate such oxygen ingress and this kind of imperfect oxygen barrier is providing protection for wine against formation of reduced characters in wine.
Closure trials by AWRI revealed low-level reduced characters developing in some screw capped bottles eighteen months post bottling. They noticed rubbery or struck flint characters in wine. The important reasons for reduced characters can be attributed to absence of cobber fining, strongly reductive conditions, inherent sealing properties of screw cap closure, the addition of ascorbic acid, conventional SO2 additions, low pH, and low head space can lead to the above characters.
AWRI comparisons between wine sealed under screw cap and under air tight glass ampoules revealed higher levels of reductive characters in the later one which suggests that small level of ingress can suppress these characters favouring the usage of permeable screw caps the replicate corks. (Casey, A Commentry on the AWRI closure report, 2005)
In a Publication , Paul White (Paul White, 'The tail wags the dog', Harpers, December 2006) says that the solution that settled on to overcome the major obstacle facing screw caps - post bottling sulphide reduction is to dose the wines with as much as heavy metal in the form of copper sulphate, as they can bear. He further claims that copper fining has always been considered a radical treatment as a last resort to salvage undrinkable wine, for it targets the by-products of yeast fermentation mainly thiols. In targeting Sulphides, winemakers can strip away good sulphides that can give its distinctive terrior characters
"The issue came up in late 2007 when New Zealand's Te Kairanga Wines had 4,000 screw cap bottles of pinot noir rejected by Germany for over-the-top copper amounts "reports imbibe. (Imbibe, 2008)
Consumer preferences and the bottle closures.
In a study on effect of wine bottle closure type on Consumer price expectation and purchase intent, it was found that liking rating for the wine played a more important role in impacting the purchase intent of a consumer. One unit increase in liking rating increased 10 % probability of buying the wine, and the study showed that type of closure had a very limited impact on the purchase intent. They concluded "Natural cork had the most influential and positive impact on purchase intent, but only marginally more than a screwcap, while synthetic cork had a substantial negative influence on purchase intent" (Durham, 2007)
In another study conducted by Tragon Corporation and AWRI, on the consumer perceptions in US and Australia, both of the consumers rated natural corks higher over synthetic or screw cap for special occasions. However, under normal circumstances, the purchase decisions for US consumers were ranked in the order of Closure, price, varietal and region, where as for Australian consumers, the purchase decisions were ranked in the order of price and then "Closure", followed by region and varietal. (Bleibaum, 2005)
Recent advances in closures technology
They are an alternative to natural corks. Manufactured in a co-extrusion process, the inner core will be filled with one extrusion and the outer core by another. LDPE (Low density polyethylene) and LDPE based thermoplastic elastomeric coating were used in making synthetic corks. They allow oxygen to permeate and are more associated with sporadic oxidation, and often used for bottles that are to be opened with in 12 to 18 months after bottling.
Zork is relatively a new entrant in the wine bottle closures. It was created by an Australian company called Zork pty ltd. Zork performs functions of both screw cap and cork as it has three main components, A cap to protect the wine, a foil to work as oxygen barrier and a plunger application. With the plunger application creates a pop after opening and can be resealed. However, it is a recent closure technology and much of the research needs to be done to evaluate its effectiveness. Few researches suggest oxidative development within the wine bottle and wine does not exhibit any reductive aromas and flavours (Gardner).
Another company called Alcoa recently released a closure called Vino-seal, which sits on a synthetic O-ring and held in place by removable aluminium cap (Alcoa, 2007)
It was evident that Screw cap has more advantages than Cork. However Consumers expect the screw caped wine priced relatively lower than the corked bottle. New Zealand, Australia and Switzerland are moving faster towards screw caps while other countries are still more biased towards to Corks.
However each wine variety will interact in a specific way with the different closures used and industry needs to adopt closure based on conditions, wine variety and to the market where it is consumed. I conclude that the "Corked Glass bottled" wine may prevail as long as tradition and story remains attached with wine.