A few weeks ago, we received the following email from a customer (edited to protect identity):
A couple of days ago we sampled a split of [Esprit de] Beaucastel Blanc at ... in ..., CA. We enjoyed the wine until the moment we poured the remains of the bottle, and a glob fell into my glass, with a noticeable plunk. The substance can best be described as several, 1 inch-long fuzzy-looking strands. I have seen crystalline material and sediment in wine, but nothing like what plopped into my glass. Needless to say, the restaurant did not hesitate to strike the cost of the wine from our tab.
I am just curious to learn what that substance could be.
Not the most pleasant of emails to receive. And I'm sure that to the customer who wrote, the surprise at the end of the bottle colored his experience of the wine he'd been enjoying. I was surprised, though, that the restaurant didn't have an explanation of what he'd encountered, because it's one of the most common things that can happen to a wine. He'd encountered tartrate crystals that had precipitated out of a wine and settled into the bottle.
Tartaric acid is one of the three main acids found in wine grapes, and along with malic acid and citric acid provides the tartness in both grapes and wine. Wine grapes are notably acidic; at harvest, their pH is typically between 3.25 and 3.5 at Tablas Creek. While much or all of the malic acid is transformed to the softer-tasting lactic acid in the fermentation process, tartaric acid is relatively stable and is therefore responsible for the maintenance of a finished wine's pH and its resistance to various kinds of spoilage. Still, much of the tartaric acid settles out of a wine during fermentation and aging as a part of the lees.
Tartaric acid's solubility is temperature-dependent. So, when wine is chilled down, some of the tartaric acid drops out of solution as fine white powder or crystals and does not under normal conditions dissolve again. For red wines, which are rarely subjected to cold temperatures and which may anyway be expected to throw a sediment over time as suspended color particles fall out of the wine, this is rarely a problem. But for white wines, which are often refrigerated for days or weeks, and in which consumers aren't expecting to see any sediment, the tartrate crystals can be alarming. We hear occasionally from customers who've seen these crystals wondering if they are shards of glass. These crystals -- sometimes called, a bit romantically, "wine diamonds" -- are not glass, and are harmless. In fact, they are largely potassium bitartrate, whose common name is cream of tartar and which can likely be found in your spice cupboard. Nearly all commercial cream of tartar is harvested from wineries.
Most mass-produced wines are routinely stabilized in various ways, typically through sterile filtration (to eliminate any chance of refermentation in bottle), heat stabilization (for proteins that can cause a haze in a wine if it's exposed to high temperatures) and cold stabilization (for tartrate crystals). The cold stabilization process typically involves chilling a tank of wine down around freezing for several days shortly before bottling. This extended chill causes tartaric acid to drop out of solution in tank, and the resulting tartrate levels are then low enough that no more is likely to precipitate out in a customer's refrigerator or a restaurant's wine room. Of course, like most interventions, cold stabilization has other less desirable consequences, and a lower concentration of tartaric acid in the resulting wine changes the wine's flavors somewhat and can impact its long-term ageability.
How a wine is stored impacts whether a customer even notices tartrates in bottle. If a cork-finished wine is stored upside-down, any tartrate crystals typically adhere to the cork and are removed with the cork when the wine is opened. However, if a wine is stored upright, any tartrates that form are likely to be visible, and the last glass (like the one at right) will show the evidence. And tartrates won't adhere to screwcaps, so no matter how a screwcap-finished wine is stored any tartrates will be visible in the bottle.
At Tablas Creek, we prefer not to cold-stabilize the whites that we hope will have the longest aging curve. These wines -- principally the Esprit de Beaucastel Blanc, Roussanne and Antithesis Chardonnay -- are also wines that we typically finish in cork. It should not be surprising to see these wines throw tartrate crystals over time. Of course, if they're stored upside-down, you may never notice unless you look at the underside of the cork carefully. Our whites that we intend for earlier consumption, and our Rose -- all of which which are typically finished in screwcap -- do receive a light cold stabilization, like the one pictured to the right. We would prefer not to have the tartrate crystals floating around in these bottlings, though it could still happen if a bottle is put in a cold refrigerator for a long period.
So, back to our initial customer. What do I think happened? I think that the restaurant likely stored the half-bottle upright in their white wine refrigerator for several weeks or longer, plenty of time for the tartrate crystals to settle out. And neither the customer nor the waiter had seen enough minimally-processed wines to know that what they had encountered was normal.