White grapes: more colorful than you think

Our society has a bizarre habit of mislabeling things by color. The familiar case in point: white people are never white but always various tints of pink, peach, and yellow; black people are invariably not actually black but some shade of brown or tan. Less familiar case in point: white wines are really always somewhere in the yellows, and the grapes themselves range from green through yellow to pink. (Red wines are, at least, red, even if the grapes which give them birth are more aptly blue, black, and purple).

We do talk about the color of white wines, from the pale straw of a light-bodied sauvignon blanc to the amber of an elderly riesling. Anything not firmly lodged on the green-yellow to brown-yellow spectrum, though — including forays into pink — is considered a fault. Now, that judgment (like some other “wine fault” decrees) seems a bit arbitrary to me: would I really mind sipping a pinkish chenin blanc? (No, I would not). But “pinking,” as it’s called, is a problem, if for no other reason than consumers might have a hard time coming to grips with it. Gewürztraminer grapes are unquestionably (and beautifully) pink, but gewurztraminer isn’t one of the grapes prone to pinking and, in any case, we’re not talking about color derived from skin contact — those wines are “orange,” not pink. Real pinking qua pinking can show up before bottling or suddenly after pouring and seems to be the result of exposing a reductively-made wine to oxygen.

A group of chemists from Portugal have done a convincing job of demonstrating that — at least in the Siria grapes they tested — pinking is caused by…anthocyanins. Yes, the very same pigmented molecules that make red wines red. But, what are anthocyanins doing in white wine?

In some sense, they were there all along. All grapes have the genetic machinery to be red. White varieties are mutations, the result of genetic changes in the genes responsible for the production of red anthocyanin pigments in grape skins. The same mutations seem to exist in all white grape varieties, which suggests that they probably all descended from a common ancestor.

Anthocyanins would seem the obvious culprit for pinking, even if it does seem odd to think of them being found in whites — that genetics of grape color research isn’t especially new. One of the classic wine science textbooks, Ribéreau-Gayon and company’s Handbook of Enology, says that pinking is caused by unknown compounds that can’t be anthocyanins because they don’t respond to sulfur dioxide and pH in the expected ways. That book was published in 2006, though, and folks like James Kennedy at Fresno State University and Jim Harbertson at Washington State University (along with a good many other researchers) have, since then, made a fair bit of headway into figuring out how anthocyanins react with each other and other wine components. (It remains a terribly complex and incompletely understood topic.) This team of Portuguese researchers could still observe that the pinking-related anthocyanins they observed didn’t act exactly like “normal” anthocyanins because they polymerize over time in the wine, which makes them more resistant to the color-bleaching effect of sulfur dioxide. Suffice it to say that they go through some complicated chemical acrobatics to show that the molecules they isolate from their pink Sirias are indeed anthocyanins.

The researchers responsible for this study speculate that Siria, the rather obscure Portuguese white grape variety with a persistent pinking problem that they chose to examine, may have regained the ability to manufacture some anthocyanins. Not enough to make the grapes overtly pink in the vineyard, but enough to belie their presence after at least some kinds of winemaking operations. (Anthocyanins are unstable molecules susceptible to changing in the presence of oxygen and other molecules.) Though they haven’t substantiated that speculation with molecular analyses, it’s not out of the question that additional mutations in those anthocyanin-producing genes might restore some of their functionality or cause them to be transcribed under specific circumstances.

If you’re not a winemaker with pink problems, why is this research interesting? It’s a good reminder that white grapes aren’t necessarily simpler than red ones, as it’s so easy to imagine, and that we’re still learning a lot about the very complicated pigments that make wine color happen. But it also makes me stop and think about how flexible plants really are. We can select for and preserve features we want through careful clonal selection of the most highly desirable plants, but vines are still going to change and mutate and do new (or redo old) things on the sidelines.

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