Wine under a veil

Fill a barrel of white wine from Savagnin grapes, deliberately leave a gap of air at the top, and wait. Within weeks, a wrinkled, cream-coloured film spreads across the surface. This is the voile — the veil — a community of yeast, principally Saccharomyces cerevisiae var. beticus and related strains, that have metabolised the last of the dissolved oxygen and now form a continuous living lid between the wine and the air above it.

Elsewhere in winemaking, a film like this would be a disaster. Winemakers elsewhere top up their barrels obsessively, excluding oxygen to prevent oxidation and spoilage. Here in the Jura, in the appellation of Château-Chalon and the broader Côtes du Jura, the gap is the point. The barrel is left part-filled, typically to about five-sixths of its capacity, in a cellar where temperature shifts with the seasons — cool stone walls, no climate control, the same conditions that have governed this production for centuries.

Over the next six years and three months — the legally required minimum — the veil does its work. It protects the wine from outright oxidation while simultaneously allowing a slow, controlled exchange: the yeast consumes glycerol and ethanol, releasing acetaldehyde and generating the compounds that build the wine's characteristic walnut, curry and dried-fruit profile. The wine concentrates as it slowly evaporates, losing roughly a third of its volume by the time it is bottled. That loss is called la part des anges — the angels' share — although in the Jura they make no romance of it; it is simply the cost of the process.

The resulting wine is bottled in a clavelin, a squat 62-centilitre vessel whose volume corresponds, roughly, to what remains of a litre after the ageing losses. This bottle shape is unique to vin jaune and functions as a unit of measure as much as a container.

The grey marl slopes below the plateau edge are where Savagnin grapes find the drainage and mineral uptake that make them suited to this process. The slope is the vineyard: without the right geometry of aspect and soil, you never get the grape, and without the grape, no veil is worth cultivating. Geology comes first, and the yeast film follows.