We're trying white wines, and one of the tasters gets a look on their face as if they just sucked down one of those Altoids lemon sours. The kind that are so bitter, they make you feel as though you will suck out your brains through the insides of your cheeks, and spit them out shorty after you jettison the candy from your mouth at upwards of 200 mph.
"This wine tastes bitter. Why is that?"
This is usually followed by a statement from me that starts with "Astringency..." and ends quickly with "...soooooo, let's move onto our next wine..." in order to prevent me from looking like I've got no idea what I'm talking about.
Explaining bitterness in red wines is fairly simple: red wines contain tannins, and almost every wine drinker knows that tannins (especially in young wines) impart a sometimes bitter, often gum-drying quality. Tannins come primarily from the skin contact that makes red wines, well, red. White wines typically don't undergo too much (if any) contact with grape skins when they're made. Sure, sometimes oak aging can impart tannins in white wines, but usually not enough to make them bitter.
So, what gives?
At a recent wine geek tasting party, my geeky friends and I discussed the very topic. And one of those geeks knows a guy, who knows a guy, who knows some people.
Important people. Dangerous people.
Well, not really dangerous, but people who have dangerous levels of wine guru knowledge. Like, for example, Jancis Robinson, one of the premier wine authorities in the known universe.
Now, before I get into Jancis' (and others') answer on the question of what makes some white wines bitter, we need to take a little side trip to the "She Blinded Me With Science" Department. Because when it comes to wines, you can't talk about the nitty-gritty of flavors and bitterness without talking phenols. And you can't talk phenols without talking science....
She Blinded Me With SCIENCE!!!
Just like you and me, wine is made mostly of water. It also has a good portion of alcohol floating around in there (just like me.. but maybe not just like you). The 5% or so left over are the chemicals (well over 900 in some cases) that make wine, well, wine. Because wine is made from grapes, it picks up chemicals from the skins, seeds, and stems of grapes called phenols. Phenols interact with other molecules in the wine, and those interactions help to define the taste, color, and body of a wine. When you taste bitterness and astringency in red wine, it's likely you're tasting phenols. Since the chemical reactions in wine can change over time (for example, when phenols interact with the small amount of oxygen present in the wine bottle), so can a wine's tastes. This, in part, explains why a wine is often said to "mellow out" and become less bitter and softer in mouthfeel over time - thanks, in part to phenols.
Still with me? OK, so how does this tie into bitterness in white wines? What makes some white wines bitter for Pete's sake??...
I've got three answers to that question, from three separate Masters of Wine, all of which are different but technically correct! Let's decode each one:
1) According to Julia Harding, MW: "Aromatic compounds called terpenes, particularly important in aromatic varieties such as Gewürztraminer, Muscat and Riesling (but also in the aromas of flowers), are said to contribute to bitterness. Their concentration is greater in very ripe grapes and the effect is likely to be more marked when grapes have been pressed less gently or after ill-judged skin contact."
Translation: If the wine gets a bit too much poorly-timed skin contact (from squishing the juice out too roughly, or from deliberately giving the juice contact with the grapes skins but at the wrong time), you might end up with some bitter white wine - especially if the grapes were very ripe at harvest. Terpenes can also be imparted from oak contact, so too much oak contact could also be the culprit.
2) Pancho Campo, MW's answer:
Translation: The grapes weren't ripe enough when the wine was made. This allowed the introduction of those IBMPs, without the right amount of flavor, etc. to counterbalance the 'greeness'.
3) And from the irrepressible Jancis Robinson herself:
Translation: Too much phenol action, baby. Either from mistake or from the winemaker gettin' a little too greedy, someone was trying to extract the maximum amount of flavor from their grapes - but they went overboard. Whoops - hello Mr. Bitter!
There you go - now, off to impress the guests at your next wine party with your wine chemistry smarties...
For more wine chemistry geek knowledge, I recommend the Oxford Companion to Wine.
(images: time-inc.net, wikimedia.org)