Winemaker Fiona Donald gently teases the bung from one of the hundreds of old barrels in Seppeltsfield's cathedral-like fortified wine cellar in the Barossa Valley. She inserts a long shining steel pipette to take a sample of the wine inside - and when she pulls it out it's smeared disconcertingly in a thick white creamy substance.
"Mmm," smiles the winemaker, unperturbed. "Cottage cheese. If the yeast looks like that, I know it's healthy."
The yeast Fiona Donald is talking about is called flor, a special kind of culture living on the surface of the wine in each of the barrels stacked high around us. This layer of creamy yeast protects the fortified white wine from oxidizing, keeping it fresh as it matures in the barrel; it also contributes a distinctive nutty, tangy flavour to the wine.
Flor-ageing is a technique of biological maturation that has been employed for centuries in the Spanish wine towns of Jerez and Sanlucar to produce bone dry fino and manzanilla sherry - and for 100 years or so here in Australia to produce what we used to call "sherry" but now call pale dryapera.
Seppeltsfield is arguably the best known producer, but you can also find traditional, flor-aged apera in Victoria's Rutherglen region at well-established wineries such as Chambers, Morris and Pfeiffer, and at a couple of fortified-wine-obsessed smaller newer producers such as 919 Wines in South Australia's Riverland.
It's not an easy style of wine to make. As we move to the next barrel, Donald tells me that managing the flor is like nurturing a living thing. "The culture is very sensitive to heat and humidity," she says. "It shrinks and grows depending on the season. We have to check each barrel regularly and make sure the yeast is healthy."
Fiona Donald at Seppeltsfield.
Flor-ageing might be tricky but it produces fortified wines with remarkable complexity: the wine inside the first barrel has been under flor for a short time and is light, floral, with a hint of green almond and lanolin; the second barrel has been under flor for longer and has a richer tang to it - really savoury, like toasted salted almonds.
Flor-ageing isn't just used to make fortified wines like sherry, though. In France's Jura region, it's also used on dry white wines made from grapes such as chardonnay and savagnin - most famously to produce the nutty, yeasty, searingly dry style called Vin Jaune, which spends over six years under flor (locally known asla voile, or "the veil") before being bottled.
Inspired by Jura, some Australian winemakers are experimenting with flor-ageing their dry whites - adventurous winemakers such as Iwo Jakimowicz and Sarah Morris from Si Vintners in Margaret River in Western Australia.
"We totally love drinking Jura-style wines," says Jakimowicz. "So when a flor naturally formed on our first semillon - a layer of that thin, cream cheese-looking stuff - we thought, awesome, let's run with it." The result was the 2011 Chincheclé Semillon, a fabulously nutty, vibrant take on a well-known regional style that helped establish Si Vintners as an exciting new name to watch.
In that same year, 2011, unbeknownst to each other, Crittenden Wines on the Mornington Peninsula in Victoria and Kangarilla Road winery in McLaren Vale in South Australia also started making wine modelled on Vin Jaune, using the savagnin grape variety they had both planted a few years before. Crittenden called their wine, aged for three years under flor before bottling, Cri de Coeur; Kangarilla's aged for just one year, was called The Veil. Both were delicious: fruitier, richer and less overtly sherry-like than the Jura wines they were inspired by, but with plenty of that nutty, briny tang flor-ageing brings.
"That first vintage impressed us so much that we now have three more vintages sitting in barrel under flor," says winemaker Rollo Crittenden. "It's not easy, though: keeping the flor culture alive is a challenge. We've had to cull one barrel because the flor just died and the wine oxidised. But we think we're getting better at it, understanding how the yeast responds to things like fluctuating temperature."
It has also been a steep learning curve for Kangarilla Road's Kevin O'Brien. He not only lost half of his first year's production because the flor didn't grow properly, but also had to deal with some negative reactions from his peers.
"It's an unusual style of wine," he says. "If you've never tried anything like it, it's quite confronting. When I started making it I did a tasting for a bunch of local winemakers and you should have heard the carry-on: 'What the bloody hell have you given us, O'Brien?' But for wine drinkers who are curious - especially younger customers - those with a willingness to try and understanding something new, it's a style that can really appeal."