Sauvignon BlancI think almost everyone is familiar with Sauvignon Blanc. It’s grassy and frequently laced with tropical fruit flavors when grown in warmer climes, which Bordeaux is not. While the terroir of Bordeaux varies from area to area, it is simply not as warm as New Zealand or Napa, that’s for sure. With these cooler temperatures Sauvignon Blanc tends to be crisper, offering more peach and citrus than tropical fruit, and very zesty and expressive in its youth. The herbal elements can also be quite prominent and pungent, with accompanying acidity levels that can seem elevated to palates more used to soft, warm climate wines.
SemillonUnlike Sauvignon Blanc, Semillon is a subtly-scented wine, lacking aromatic intensity in its youth, yet capable of developing alluring aromas of dried fruits, honeycomb, and pollen-rich flowers with bottle age. On the palate, Semillon is frequently noticeably low in acid, rich and almost oily in texture. What it does have, other than a severe susceptibility to the “noble rot” that allows for the remarkable dessert wines of Bordeaux, is a core of sweet, almost figgy fruit with nuanced accents of herbs and nuts. Sounds like an ideal blending wine, now doesn’t it?
The final grapes that are allowed in Bordeaux Blanc are Muscadelle and Ugni Blanc, though Ugni Blanc really doesn’t need to be covered here as it’s a grape that is generally grown for quantity over quality, producing a pleasant wine. In general it's a grape that has fallen out of favor with quality-minded producers of Bordeaux Blanc.
That leaves the unfortunately named Muscadelle. I say "unfortunately" because it shares a root with the Muscat family of grapes, and sometimes even a bit of aromatic similarity, though the grapes are in fact unrelated. Muscadelle can add zesty floral tones to the perfume of Bordeaux Blanc, and can contribute to the fruity character of the blend, but rarely makes up more than 10% of any wine.
So, it already sounds like blending Sauvignon Blanc and Semillon makes a lot of sense. You take two slightly imperfect wines and they marry perfectly, each making up for the shortcomings of the other while maintaining its own identity! A whole that truly is greater than the sum of its parts! So how do the Bordelaise -- the folks making these wines -- figure out how to blend these grapes?