During the wine fermentation process, sugar from grape juice is converted into alcohol by natural occurring or added yeasts. If you've ever tried to make bread dough, you know that when you leave it to rise, the ball of dough heats up. This is true in the wine fermentation tank, which is made often stainless steel, sometimes made of oak, and increasingly, it's made from concrete.
While fermenting grape juice in concrete is a pretty ancient practice, it fell out of fashion in a big way. Now it's back. Winemakers like the miniscule little pockets of air across the surface of concrete, claiming that they have a beneficial effect on the resulting wine -- smoothing out rough tannins, letting the wine "breathe". While concrete isn't permeable, winemakers feel that it allows the fermenting juice to breathe much in the same way that oak does, but without borrowing any flavors. No oaky Chardonnay here! Concrete, like stainless, is neutral and does't lend any (wanted or unwanted) flavors to the wine.
In additions, concrete is a natural insulator against outside temperature shifts. Think of a cool concrete floor on your feet, especially if you have a basement. The egg shaped concrete vats provide the added benefit of temperature consistency on the inside, while square vats have "corners" where fermentation can speed up or stagnate. Many winemakers choose the egg shape because they look cool, of course, but also because they give a more consistent fermentation temperature throughout.
The most famous producer of the concrete vats is Marc Nomblot, who says vats are made “from washed Loire sand, gravel, non-chlorinated spring water” and cement. French winemakers and increasingly winemakers from all wine regions are choosing them. We know of more than one French winemaker who has shipped them to their vineyards in the Southern Hemisphere, to enjoy their benefits in the winemaking process.