VILA SECA, Portugal — In the cellar of a modern, concrete winery in this small town in the Douro Valley, Luis Seabra was drawing a sample of his 2018 Xisto Cru Branco from the large old barrel in which it was aging.

Even at such an early stage, this was a gorgeous white wine, made largely of rabigato , a grape grown almost nowhere else in the world, blended with a little of the equally obscure côdega, gouveio and viosinho.

The wine was saline and mineral, tightly coiled, with an opaque texture that was not quite ready to admit exploration. It was exactly the sort of fresh, vivacious white that has never been associated with the Douro Valley. Yet it was evidence of the unexpected evolution of this historic region, so long associated with the production of port.