What you see here are flakes of potassium bitartrate that precipitated out of my wines during a few weeks in December when the temperature in my basement got down to the mid-50’s. The yellower flakes are from the free run and press fractions of my Maryland Chardonnay. The grayer flakes are from a Chardonnay “second” that I’m attempting. Though harder to see, the light colored clumps of crystals are from the free run and press fractions of my Maryland Seyval. The crystals fell out of suspension as the room got colder and formed crusts on the bottoms of the carboys. I caught what I could in a colander during clean up and let them dry out for your viewing pleasure. (For all you REALLY boring people, I’ll post the webcam capture showing them drying out very soon. 🙂
The primary acid in all grapes is tartaric acid and it is generally found uniformly throughout the berry. Grapes can also be high in potassium with the skins being especially potassium-rich. As ripening progresses and during the course of making the wine, some of the tartaric acid and potassium react with each other creating potassium bitartrate — known more commonly as “cream of tartar”. Of course, the stuff in your baking cupboard is refined to a snowy white powder. The stuff here is full of grapey goodness and other contaminants. 🙂 At warmer temperatures the “KT”, as it’s sometimes known, remains in suspension. But when temperatures go down, the crystals form and precipitate out of the wine.
Had the temperature gone lower still it’s very likely that even more crystals (sometimes called “wine diamonds”, but not by me) would have precipitated. Most commercial white wines undergo a formal process of “cold stabilization” whereby the temperature of the wine is lowered to just above freezing for a week or two in order to force as much precipitation as possible to occur before the wine is bottled. (The picky consumer hates when all that crap falls out of the wine after it’s been sitting in the refrigerator for a few days. “What is that stuff? Sand? In my wine? Call the FDA, Helen!” 🙂 Red wines, on the other hand, are often not cold stabilized since they aren’t typically refrigerated before being consumed.
Because cold stabilization effectively removes some of the tartaric acid from the wine, it can be a simple way to modestly reduce the acidity of wine that tastes a bit too tart. But watch that pH! This is also one of the reasons why a white must that starts out at 8.5g per liter of acid may ultimately yield a wine with an acidity of 7g per liter. And in wines with low acidity and high potassium content it’s one reason to consider using citric acid or an acid blend to boost the wine’s acid levels — because potassium citrate is more soluable at these temperatures and so does not precipitate out.
Want to know more? There’s lots more to know! Try here for example.
P.S. I called them “cornflakes” but in the process of browsing for the “more info” link above, I came across a comment that the French refer to these flakes as “butterfly wings” or “angel’s wings”. I like “angel’s wings”… 🙂