Most people assume that harvest is the most stressful time of year for a winemaker, except for this winemaker. I really enjoy harvest, maybe because I drink a lot of beer, the most stressful part of making wine, getting the wine ready for bottling. Since we are bottling our first wines of the year, I have been in the cellar a lot and todays blog is dealing with the stability of wine, namely protein and cold stability.
The issue is, if the wine is not stable, chemical reactions might occur that will alter the wine, haze might develop or even tartrate crystals if the wine is subjected to very low temperatures. Ever forgotten your white wine in the freezer, only to discover crystals hanging onto the cork for dear life, well one of my responsibilities is to ensure that this does not happen.
To sound very smart then; clarification and stabilization of wine involves removing insoluble and suspended materials that cause a wine to become cloudy, gassy or form unwanted sediment in the bottle. Most of this is done after primary fermentation has occurred. This is not always the case though, timing of these processes are very much dependent on the grape variety and the quality of the fruit and juice as it enters the cellar.
Proteins are precipitated by heat so one of the easiest methods to determine stability is through a simple heat test whereby a sample of wine is heated and held for a period of time, cooled and then observed to see if there is clouding or precipitation [protein instability]. There are other tests, including the TCA and Bentotest but since I have never used them I will not discuss them [ I already tend to write novels for blog entries ]
So how do we go about making a wine protein stable? Well, we do that by adding a reactive or absorptive substance to remove the concentration of undesirable constituents, namely BENTONITE, also known as “Wyoming clay” due to the fact that it is principally mined in Wyoming. Bentonite is a volcanic material and when added to water forms a colloidal suspension with a large surface area. Through adsorption between positively charged proteins and negatively charged plate surfaces, proteins are removed.
Racking Pinot Gris off bentonite to tank on right
Bentonite and dead yeast
Since protein removal is proportional to the amount of bentonite added, we need to make sure that we do not add to much to the wine, as this may reduce the wines aroma and flavor compounds, I probably do not need to point out why this might be a bad thing. We normally add, depending on testing, 24g/HL. We make a slurry mixing bentonite with warm water and add it to the wine. Due to the density of the bentonite, relative to that of the wine, it settles to the bottom of the tank and the wine is subsequently racked off the bentonite into another tank.
Tartaric acid is the main acid present in wine, and in solution are available in three forms  undissociated tartaric acid  bitartrate ions and  tartrate ions. We as wine makers have to be aware of tartrate precipitation in the bottle, although harmless, the wine admittedly looks quite undesirable to the consumer. To prevent this from happening, we expose the wine to cold temperatures [28 degrees for a period of 2 weeks]. Precipitation occurs in 2 stages, concentration of potassium bitartrate nuclei increase due to chilling, which is followed by a crystallization stage, where crystal growth and development occur.
Wine with tartrates clearly visible on tank wall
By ensuring the wine is now cold stable, you should be able to pop it into the fridge and it should come out as clean as clear as it went in. All this in the name of appearances. And just to put your mind at ease, here is a view of the now clean wine as seen through the site glass on the rack valve. i promise you, it tastes quite good.
Clean wine being racked off, YUMMY
Almost ready for bottling, all that is left to do now is filter the wines, but I think I will wait to bore you with those details in another post.