Anthonares

While out to dinner last night (Valentine’s Day) a quick glance about revealed that more than half of the people in the restaurant were drinking wine. Some red, others white. But I’m sure that very few of them were thinking about what makes that wine so distinctive and delectable. Neither was I.

This morning, though, my scientific materialism kicked in again while looking at last week’s issue of Science. It featured a series of perspectives on the field of aromatic compound research, including a short review article on the molecular basis for wine grape quality. Since I’m always of the opinion that whenever I understand something I can appreciate it more thoroughly, as probably are many of you, here’s a Research Synopsis all about that Dionysian delight.

Citation (online at CiteULike.org):
Lund, S.T. and Bohlman, J. (2006). The Molecular Basis for Wine Grape Quality–A Volatile Subject. Science 311(5762), pp. 804-805.

Synopsis:
The flavor in wine comes from aromatic volatile organic compounds (VOCs) that originally resided in the skin (exocarp) and the fruit (mesocarp) of the berry. The figure on the left lists the primary flavor-inducing VOCs in each portion of the berry that have been isolated so far. The list on the left is by no means complete, indeed scientists are not yet capable of listing the entire suite of aromatics in a given berry because some of them are present in exceedingly small quantities. Humans are capable of tasting beta-damascenone, which imparts a honeylike, fruity taste to red wines, in concentrations as low as 1 part in a trillion. Another example is that of 2-methoxy-3-isobutylpyrazine, a VOC that lends a certain vegetative character highly prized in Sauvignon Blancs. Only recently have laboratory analytical equipment reached this sensitivity of detection.

Grape wines undergo what is called a biphasic maturation. First, the flesh and skin expand in volume very quickly. During that time, organic acids such as malate and tartrate are produced and stored in the mesocarp. These acids taste tart to the human tongue and are believed to have evolved in order to dissuade foraging creatures from eating the immature seed within the fruit. Once the expansion slows, the second phase of berry development begins. This is the ripening phase, in which (for reds at least) anthocyanins are produced, reddening the exocarp. (Remember those? They are what turn our leaves red in the fall). During this phase, the malates and tartrates are largely broken down and converted to other, less tart VOCs. The primary components of wine flavor are organic acids, proanthocyanidins (tannins), terpenoids, and various aromatic aldehydes, esters, and thiols (don’t know what those are? Neither do I, but that may mean something to the organic chemists among us).

If you’ve ever eaten a grape straight from the vine, you know that it bears little resemblance to the wine that it may eventually become. This is because, unlike citrus fruits or peppermint leaves, grapes do not store their VOCs in gaseous form. The VOCs are dissolved in water within the berry by being bonded to amino acids and sugars. During the ripening phase, enzymes within the fruit release these VOCs which gradually diffuse out of the berry. Thus, berry harvest, which usually occurs 12 to 14 weeks after the first phase begins, must be timed such that these enzymes are present but have not been active for too long. Also, the conversion of malates and tartrates must have been sufficient to balance the sweetness of the berry. Suffice to say that for a very long time the selection of harvesting time has been much more art and tradition than science.

The study of wine grape formation and maturation is known as vinology, and the cultivation of them as viticulture. Once the harvest is completed, those folks go home and hand the berries over to the vintner and his friend the enologist. Still with most of those VOCs trapped by conjugation to sugars and amino acids, the wine is very much incomplete. Physical crushing of the grapes exposes those VOCs to oxygen which aids in their volatilization, and the adding of grape and yeast enzymes along with others proprietary to the vintner, aid in this process as well. For those who have tasted the light, clean sweetness of a Beaujolais Nouveau, a mature wine still has a long way to go. That’s why wines are best when aged properly. The enzymes still present in the young wine will continue their action, albeit more slowly, for years and years. The wines become sweeter and their flavors more pronounced. Their prices increase as well because most of us don’t have the patience to wait for the wine.

Context:
I hope you enjoyed that short intro to vinology and enology as much as I did. Now, when I look at a bottle of wine I will see those VOCs still conjugated to the sugars and amino acids, waiting to be released. Also, knowing that there are dozens of known and perhaps hundreds of unknown compounds that contribute to flavor in astonishingly small concentrations is fascinating to me as well. To think that biological evolution has given us such sensitive detection instruments that we perhaps use for ends other than strictly survival makes me appreciate more deeply the wonder and mystery of life. This is scientific materialism at its best.