They’re called quintas, Joana tells us, because the rich families who owned the land along this stretch of the Douro river used to let others work it in return for a fifth of the profits. And in this part of northern Portugal, ‘work’ means only one thing: wine. We’re here in the Douro Valley to learn more about it all, including this trip to Quinta do Bomfim, the winery where Dow’s port is made.