Bottle closures combine two distinct functions. The cork is entirely practical, preventing leaks and keeping the spirit secure, while the stopper (on top of a cork) also provides various opportunities for aesthetics and branding.
"We work closely with new product development departments, brand managers and master distillers to advise on the elements that can be used as a stopper on top of the cork. This could be a wooden head, though we're also working with a range of bespoke materials to bring innovation into this area, and individualise the tops and the corks for each brand or company," says Hugo Mesquita, sales and marketing director, Top Series Division, Amorim, a Portuguese company specialising in cork closures. Utilising the broader potential of a stopper is a relatively recent development.
"Over the past ten years the technology has developed so much that it offers far more opportunities, and a lot more thought and detail now goes into the stopper. You can emboss something on the stopper, or do the opposite and deboss, you can have a matt finish, a silk finish and choose from up to seven colours, whatever is appropriate for the brand," says James Crilly, packaging supply manager, Inver House Distillers.
Ian Hamilton, purchasing and packaging development projects manager, Morrison Bowmore, adds, "stoppers provide an opportunity to add value and interest, and the vast majority feature a message. The stopper for Glen Garioch, for example, is a wooden top featuring the year the distillery opened, 1797. It's an important detail that makes up part of the packaging, and helps create an overall effect that makes our product stand out. We're likely to see more innovation in this area in the future."
Meanwhile, there are other practicalities to consider. When a bottle is sealed there is also a small 'headspace' between the cork and the surface of the liquid.
"The size of the headspace, which is an air chamber of around 15-20 ml, is very important. This allows for any possible expansion of the liquid in a warmer environment, which is important for exports to countries with a hotter climate," adds Hugo Mesquita.
But can air in the headspace of an unopened bottle have any influence on the malt whisky?
"I don't think this has any effect, but I wouldn't dismiss it either, as it's very difficult to devise a scientific experiment to analyse this. You can take an old, sealed bottle and compare it to today's equivalent bottling, but there can have been various subtle changes in the production process over the years, such as cask selection, which cloud the issue and you may not be comparing like for like. If there is any change in a sealed bottle it's very subtle, and happening over a very long time frame," says Brian Kinsman, master blender, William Grant & Sons.
Stuart Harvey, master blender, Inver House Distillers, adds: "any possible change would be so small that you wouldn't detect it. Alcoholic strength is the key factor in preventing air in the headspace from impacting the malt whisky, and as malts are bottled at 40 per cent ABV, or higher, the whisky is very stable. The higher the alcoholic strength the more stable the whisky is." Moreover, there could be other factors at play.
"Potentially some change is occurring in an unopened bottle, but rather than the influence of air in the headspace I think this would be more to do with the continued integration of malts in the bottle, as most malts are a recipe of various casks blended together," says Dr Bill Lumsden, Glenmorangie's Director of Distilling, Whisky Creation and Whisky Stocks.
Once a bottle has been opened, and malt whisky poured, the decreasing level of liquid does of course increase the headspace, accommodating a greater volume of air within the bottle. Additionally, each time malt whisky is poured, the existing air in the headspace is expelled, and replaced by fresh air, prior to the cork being replaced.
"I've often returned to a bottle that's been open for a while. Possible effects from changes of air, even over a longer period of time, would be so minimal that most of the time this wouldn't be noticed," says Dr Bill Lumsden.
The cork tree is indigenous to the Mediterranean, with the greatest concentration of cork tree forests in southern Portugal, where specialist cork supplier Amorim is located.
On average it takes 43 years after planting a cork tree before the bark can be used to produce cork closures. Moreover, cork can only be harvested every nine years, as it takes this long for another layer of bark to 'regenerate.' The life-cycle of a tree is 220-250 years, which means around 15-17 harvests can be used. Cork possesses a natural flexibility as it comprises millions of minute cells that each contain air, which is trapped within each cell. A cork is sized slightly larger than the bottle opening, and after the cork is compressed into a bottle neck, the air within each cell attempts to expand and regain its original dimensions. This results in a tight seal, while also allowing the cork to be readily extracted and replaced.