In my last posting I provided some background on "wine diamonds" and suggested they are unattractive but totally harmless. Today, I will discuss the general idea of instabilities in wine and talk about what we do in the cellar to minimize one form of instability: tartrate stabilization - also called "cold stabilization".
You might be surprised that clear wine can result from what begins as cloudy, particle-laden, fresh-pressed grape juice. In this juice are a variety of sugars, acids and proteins that live together in a fermentation environment. Eventually with enough time the particles, that you can visibly see, begin to sink to the bottom of the fermentation tank yielding a clearer wine. Periodically throughout the fermentation and aging process the clearing wine is transferred off the settled particles (e.g., dead yeast cells, bits of grape skin, and other unsightly things) into a clean, new tank. The process continues somewhere between three and five times, with periodic filtering (to remove smaller and smaller particles) until the crystal clear wine is bottled.
Over time the crystal clear bottle of wine can undergo a change leading to crystal formation and haze in your wine. There are two main forms of instabilities we encounter in the cellar: protein and tartrate. We'll focus on how to manage tartrate stability or how to minimize/eliminate "wine diamonds".
Potassium Bitartrate (aka: cream-de-tartar) or tartartic acid are relatively stable in grape juice but less so in wine. Several factors, such as pH, temperature and alcohol content influence the crystallization (or precipitation) of bitartrate salts (i.e., wine diamonds). In other words, the clear wine you take home and place in your cool cellar can affect the stability of your wine and may cause crystal formation.
Preemptive Intervention in the Cellar!
Fortunately there are approaches winemakers can use to minimize the chances of crystal formation. (Some winemakers intentionally do not intervene because of winemaking philosophical reasons - yes, there are philosophies of winemaking).
Because crystal formation is temperature sensitive a winemaker could lower the temperature of the aging wine to help encourage tartaric acid crystallization. Perhaps you've been in a wineries cellar and noticed a frosty patch on some of their stainless steel tanks -- this preemptive measure, to cool the wine to near the freezing point (28-30F) for two to three weeks, is frequently employed. For high acid, Midwestern grapes this is nearly a must and yields a side benefit of reducing acidity.
Cooling a tank is somewhat effective for encouraging crystal formation; however, if you think of crystal formation as a process analogous to the formation of a hail stone in a summer storm it may help. Hail is formed as a little piece of ice crystal is lifted, repeatedly, inside the cloud. After several trips up and down inside the cloud the small ice crystal has grown in size and eventually falls to the ground. The second method to encourage tartaric acid crystal formation is similar.
Called the Contact Process, this cold stabilization process continues to chill the wine (as described above) but adds another step: seeding the wine with cream-de-tartar. Chilling the wine to near freezing temperatures (or lower) and adding cream-de-tartar crystals to the cold wine essentially works like a magnet for other tartaric acid crystals to adhere (i.e., just like a hail stone). The seeded wine is circulated, via closed pump-hose system for 15-30 minutes and is left to chill for a week, then transferred to a clean tank or filtered and bottled.
So, the next time you see wine diamonds in the bottom of your glass you can tell your friends they are harmless; and that crystal formation is pH, temperature and alcohol dependent; and that maybe, just maybe it was a philosophical decision not to cold stabilize the wine by the winemaker.
Brad Johnson is a contributing writer for Make Mine Wine Magazine, an artisan winemaker, researcher, teacher, and proud member of the Eastern Iowa Wine Club. He Tweets as "Iowine"