Winemaking: A Sparkling Transformation

Checking the lees after second fermentation in bottle, Pol Roger

There's something about bubbles that makes wine seem extra special; Caroline Gilby MW takes a look at how those bubbles get there.

Champagne has long been the choice for celebrations of all kinds, and Prosecco seems to have hit the spot for that 'welcome to the weekend' moment, never mind great sparklers from countries as diverse as England and Australia. All start their life as still wine but are transformed into something else by the presence of carbon dioxide.

In normal winemaking carbon dioxide (or CO2 for short) is a waste product, and a dangerous one at that, as it's heavier than air, invisible, suffocating, and has a habit of hanging around in the bottom of fermentation vats or low corners of the cellar. There is a real risk of asphyxiation and tragically every year there are reports of winery workers losing their lives around harvest time.

CO2 under pressure
However, in sparkling winemaking, the secret is to trap the CO2 and force it to dissolve in the wine by keeping it under pressure. What you end up with is a supersaturated liquid that is in equilibrium with the small amount of gas in the headspace of the bottle. When the bottle is opened, this CO2 rushes out of solution to try and form a new equilibrium with the wider atmosphere, and this is when all those delightful bubbles form.

A predilection for fizz scientifically proven
It seems there's a bit more to our enjoyment of fizz than just the theatre of the pop of the Champagne cork and the appearance of that lively foam in your glass. It was only as recently as 2009 that scientists discovered a protein receptor in mice (which have very similar taste buds to us) for the taste of carbonation.

They discovered a specific enzyme called carbonic anhydrase-4 that is attached to the sour-sensing taste buds. This enzyme binds CO2 to form carbonic acid, helping to give a mildly sharp taste perception. It also seems that CO2 triggers both touch and pain receptors so may give a little endorphin boost to add to our enjoyment of fizz.

What about the other physiological effects of fizz?
There is also some suggestion (though robust research into this is notoriously difficult due to the impossibility of double blind trials) that we get tiddly more quickly on sparkling drinks. Researchers speculate that this may be due to bubbles speeding the wine through the stomach into the small intestine where it is absorbed more quickly.

British contributions

Bottles maturing at Pol Roger; note the crown caps used to seal the bottles

In top-quality sparkling wines, the bubbles are trapped by carrying out a fermentation in a pressure vessel of some sort to trap the bubbles in the wine. There is also the option of carbonation from cylinders of CO2 just as in fizzy soft drinks, but this does not make for high-quality results in wine.

The importance of strong glass
Final pressure has to be at least three atmospheres and is more typically five to six atmospheres for sparkling wine. This highlights one of the major contributions of the British to inventing sparkling wine: strong glass.

In Britain, coal-fired furnaces for glassmaking were introduced sometime between 1611 and 1613 and these produced robust bottles much more able to resist gas pressure than the wood-fired furnaces in use by the French at the time.

... a good seal
The next British contribution was the use of cork tied down with string as a gas-tight seal (at the time the French practice was to use wooden bungs wrapped in cloth).

and ensuring the second fermentation
Finally, it seems that the British were first to work out how to ensure a reliable second fermentation by adding sugar to make wine bubbly (long before the role of yeast had been discovered).

Christopher Merret presented a paper to the Royal Society in 1662 where he noted, 'Our wine coopers of later times use vast quantities of sugar and molasses to all sorts of wines to make the drink brisk and sparkling.' This was at least six years before the famous Dom Perignon even made it to the Champagne region.

Fermented in this bottle
Most people agree that the very best sparkling wines are made by the traditional method, or 'méthode champenoise' for winemakers based in that region. It's sometimes labelled 'fermented in this bottle' and 'this' is the key word to note as it means the wine spent its life in the very bottle you are buying.

Making the base wine
Many readers will be well aware of the fancy terms used by the French to add an air of mystery to a practical process. In reality, it is simply one of making a still base wine (in tank or cask) and then putting this into a bottle, along with a dose of sugar (about 24 grammes per litre is typical) and some yeast, then sealing it, typically with a crown cap similar to those used on beer bottles.

The traditional approach to turning the bottles at Alfred Gratien

The second fermentation in bottle
A second fermentation then takes place with the yeast using up this extra sugar and producing more alcohol, and CO2, which is now trapped. Then the yeast runs out of sugar, so it dies and starts to break down in a process called 'autolysis'. This releases amino acids and flavour compounds into the wine, giving those biscuit and toasty notes that are so prized.

Also as the yeast cells' walls collapse, they release compounds called mannoproteins, which have a very important effect in stabilising the bubbles and helping to ensure that the wine stays fizzy in the glass for longer.

Lees contact adds complexity to the flavour
In this method of production, bottles are laid on their sides in a cool cellar to age on the dead yeast (or lees, called 'sur lie' in French), for a long time (a minimum of nine months for cava and 12 months for non-vintage Champagne), but the longer this process lasts, the more complex the flavours and the finer the bubbles. Vintage Champagne must age for a minimum of three years 'sur lie' but some wines may age for well over five years.

How to remove the dead yeast cells
One practical problem is the grey sludge of dead yeast that ends up left in the bottle, and this has to be removed as the wine is going on sale in that very bottle. Inevitably, there is a fancy French term for the next step, called 'remuage', but it's basically a process of twisting and gently shaking the bottle to tip all the grey sludge into the neck.

Some producers continue to do this by hand but other use machines called gyro-pallets. Once in the neck, the bottles are passed neck down into an ice bath to freeze this dead yeast into a plug of ice. The bottles are then opened (or 'disgorged') so this ice, complete with all the sludge, pops out. Finally the bottle can be topped up with wine and some sugar (the 'dosage'), and then closed with the evocative mushroom cork and wire cage.

The invention of a robust bottle and seal were crucial in the development of sparkling wine

Quality of the base wine is key
The secret of quality comes from the base wine - usually produced from fairly neutral grapes (chardonnay, pinot noir and meunier are rightly popular for sparkling wines) and from a cool climate; like Champagne, or Sussex, or Tasmania and giving wine that is high acid and low in alcohol as it is going to gain another 1-1.5% alc with the second fermentation. Gentle handling and pressing of the grapes, careful blending and that initial fermentation all make a difference too.

Then complexity comes from the long, slow ageing in the cellar. There's also another layer of complexity from ageing in the bottle after disgorging as the wine has gained just a touch of oxygen, plus there are reactions like the Maillard reaction which needs sugar and proteins to give hints of caramel.

The Maillard reaction (named after the French chemist who first described it in 1912) is a chemical reaction between amino acids and reducing sugars (like glucose) that gives browned foods their desirable flavour. Seared steaks, pastry crusts, breads, toast, malted barley and many other foods make use of the effect, including possibly bottle-aged Champagne.

Technical alternatives

The Charmat method
A more technical and cheaper approach is the Charmat method, also called Cuve Close or tank fermentation, a method patented by Eugene Charmat in the early 20th century, though possibly invented earlier by an Italian called Martinotti in the late 19th century.

Tanks rather than bottles
It substitutes the heavy labour load of bottle fermentation with a second fermentation in large enclosed pressure tanks. In some wines, it is simply a question of a second fermentation of still base wine with sugar and yeast and then possibly ageing 'sur lie' in the tank, with occasional stirring to release yeasty flavours more quickly, before filtering under pressure and bottling.

A single fermentation for Asti & Prosecco
In some areas, like Asti and some Prosecco, there's variation on this theme. Here the idea is to get the freshest, fruitiest flavours possible, so instead of making a base wine, the grape juice is chilled to around freezing and kept cold until the wine is required for sale. It then undergoes a single fermentation, but as this is happening in a pressure tank, the CO2 is trapped.

These wine styles are bottled quickly without time on lees as the focus is on fruity notes, not yeast derived flavours. Tank-fermented wines, without lees ageing, typically have lively foam in the glass, rather than long-lasting bubbles within the wine itself.

The transfer method
There are other variations on these themes. In the transfer method, the wine is fermented in bottles, but is then transferred to a pressurized tank to be filtered and put into fresh bottles ('fermented in the bottle' on the label is the clue here).This approach is also sometimes used for 'hard-to-handle' large-format Champagne bottles.

The rural or rustic method
There's another historic variation, used in a couple of places in France called 'méthode ancestrale' and in Portugal as 'método antigo', where the wine is bottled before the completion of the first fermentation, but with no additional sugar. The fermentation finishes off in the bottle and the CO2 is trapped, but the yeast removal stage is skipped (so be warned there may be a deposit in your bottle).

The Wine Society lists a Blanquette de Limoux 'méthode ancestrale' made from the local mauzac grape by the Antech family and ocassionally, Luis Pato's 'método antigo' made from the aromatic maria gomes grape.

So whatever your choice of sparkling wine, the bubbles and the complex process of getting them into the wine, is a fascinating one. Cheers.

Caroline Gilby MW

Caroline Gilby: Master of Wine

Caroline Gilby is a Master of Wine and a scientist by training. She is a wine writer with a passion for the wines of Central and Eastern Europe and contributes to several wine books, magazines and websites.

January 2015

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