Sparkling science

Taste a glass of sparkling wine and uncork the science. Some of the 49 million bubbles found in each bottle will hit sensors on your tongue, creating a tingling sensation as enzymes convert the CO₂ to carboxylic acid.

The acid stimulates blood circulation in mucous membranes in your mouth, small intestine and stomach, so you’ll absorb the alcohol contained in the fluid quickly, giving you a noticeably faster high – even within five minutes of your first sip.

Even though the drink is cold, the alcohol might give you a sense of warmth in the mouth, and the mix of acids, sugars and other compounds, usually formed over years in the bottle, will stimulate different parts of your tongue.

Many things can affect the taste of your bubbly, from the depth and quality of the soil that the grapes were grown in, to the site micro-climate – even to the company you are keeping when you pop the cork.

Tasmanian winemaker Pat Colombo, of multiple award-winning winery Bream Creek, says grapes grown in thinner soils result in “broader and less intense” flavours.

“Thin soils probably result in a more sparse canopy (of foliage on the vine), which can lead to lower humidity within the bunch zone, and you probably get more light penetration, and those two things can result in excessive phenolic development,” he says.

Warmer climates mean faster fruit growth and more rapid sugar accumulation within the fruit, which can mean the flavours are less refined – more suitable perhaps to flavourful red wines than a refreshing bubbly. “In Tassie we have a slow growing period, so you’re accumulating a whole lot of refined flavour,” Colombo says. “The flavour is more delicate – it’s got more finesse.”

The main grapes chosen in Australia for sparkling wines are chardonnay, pinot noir and, to a lesser extent, pinot meunier. Chardonnay is a light-skinned grape, but the pinots have red skins, and so after harvest the grapes are only gently crushed to ensure minimal to no contact between the juice and skins, maintaining a white wine character. Leave the skins interacting with the juice, and you create delicious purple-red wines.

The grapes for sparkling wine are picked very early, while the sugar levels are still at or below 11 on the Baumé scale, so that when the juice is fermented it will keep the alcohol levels at about 11% and the flavour delicate. Grapes for still wines might be harvested at 12–14 Baumé. “Sparkling is always the first cab off the rank coming in,” Colombo says.

From the time that grapes are harvested, individual winemakers make hundreds of different choices that will affect the end result. Straight after pressing, the juice may be protected from oxidation, or encouraged to oxidise for broader flavours and colour reduction. “There are so many options – so many paths that you can go down,” says Dr Fiona Kerslake, head of agronomy at Bitwise Agronomy. She previously conducted 15 years of wine research at the University of Tasmania, including a major project on sparkling wine, in order to scientifically understand what was happening at each step. “We decided to pull it apart so we could understand it and give industry the tools to make decisions,” she says.

Kerslake says the gentle crushing of the grapes helps prevent phenolics that are found in the grape skins from entering the wine and giving it tannin properties that you would expect to find in a heavy cabernet or shiraz. “We need phenolics in sparkling wine, but we need the right ones and not too many,” she says. “In the fruit skin, phenolics can act as a sunscreen, so you can have a lot of them in warmer climates. In the cooler climates you can get a better balance.”

But very little work had previously been done to identify which phenolics are present or desirable in sparkling wines. “We just had some gut feels,” Kerslake says. With no rapid assay then available to even determine what phenolics were present, Kerslake worked with Bob Dambergs at Charles Sturt University to create a spectral phenolic fingerprint. “It’s like a scan of all the wavelengths in the UV spectrum and gives an indication of what phenolics we’re looking for,” she says.

Once a base grape juice has been established, yeast is introduced (Colombo uses champagne yeast from France) for the first fermentation, a stage common to all winemaking and beer brewing. As the yeast “eats”, it converts almost all the glucose sugar (C₆H₁₂O₆) to alcohol (CH₃CH₂OH) and carbon dioxide (CO₂), fermenting the juice to wine. The resulting base wine is acidic, with delicate flavours.

Some winemakers will then choose to inoculate the wine with an Oenococcus bacteria to bring on a malolactic fermentation. This converts the harsher malic acids that are present in the wine to creamier lactic acids. “It’s a way of softening that acidity,” says Kerslake. “It gives a softer, rounder mouth feel, like milk.”

Following this are multiple stages that may include ageing, adding sulphur, removing some protein, and chilling the wine to -2°C in order to remove wine crystals. But the main stage that separates sparkling from other wines is the secondary fermentation. A mix of some of the base wine, cane sugar and more yeast is brewed up and added to the rest of the wine at a final concentration of approximately 1.5-2 million yeast cells per millilitre. The wine is kept under pressure for the ensuing fermentation.

In traditional sparkling winemaking (a method used by Colombo and most Tasmanian winemakers) the yeast-sugar mixture is put into the bottle that you end up buying. The bottle is made with a thicker glass than many wine bottles to withstand the pressure of six atmospheres (more than twice that of the average car tyre) that builds up as the yeast again turn the new sugars into alcohol, creating lots of CO₂.

“It’ll have 21–23g/L of sugar thrown at it,” Pat says. “Generally, about 4g/L of sugar will give you an atmosphere in your bottle.” This stage may also be done in a pressurised tank, with the wine kept at 13–16°C.

As the yeast finishes its transformative work, it undergoes autolysis (destruction of dead yeast cells by their own enzymes), forming so-called “lees”. “All those little yeast cells are under a whole lot of stress now, so they rupture and ooze out all their characters,” Colombo says. “That’s the yeast cells spilling their guts.”

In traditional champagne making, the wine must stay in contact with the lees for at least 18 months. But when Kerslake studied this revered, ancient process at the University of Tasmania, she discovered a surprising fact.

“The line of thought had been that as the yeast breaks down it puts compounds into the wine that are hard to describe – not floral, not cherry, a bit nebulous. But we found that a lot of the character that winemakers described came from the ageing of the wine, not necessarily the ageing of the wine on the yeast lees.”

If the wine has had its secondary fermentation in the bottle, it’ll be gradually upended over time so that the lees end up in the neck of the bottle. The end of the neck is then frozen for about eight minutes at -25°C, and the top popped, ejecting the frozen plug with all the lees. Within 30 seconds, winemakers have then topped up the bottle with a final blend of sugars and other base wines to create the final flavour. It is then resealed, maintaining about 4–5 atmospheres of pressure within the bottle.

You’d think by this stage, we’d all know exactly how the wine is going to taste when we open it. But according to wine educator and sommelier at Wine Experience Lab Dr Marcell Kustos, even the serving temperature can affect the tasting experience.

“CO₂ is more soluble at lower temperatures, so serving sparkling wine at low temperatures will give us more sensory pleasure because the bubbles are finer,” he says. The ideal temperature is usually around 7°C. “If it is 10°C it will have a coarser feel, because the bubbles are larger.”

On top of this, there are individual interactions with saliva that will vary from person to person. Kerslake says there are also psychological interactions with the experience, as the nerve stimulation sparks neural pathways that bring back memories or create sensations.

“There’s an emotional factor,” she says. “You can drink the same wine with someone you love and then with someone you hate and it will taste completely different.”