I recently had a few wineries who brought me examples of bottles of wine that resembled snow globes. Though I realize that Minnesotans are missing snow this winter, I doubt that seeing it in a bottle of wine will make up for it! Haze formation or deposits in wine are caused by the wine not being properly stabilized, and are quite varied in origin. I had to brush-up on my detective skills on how to identify each different type of deposit, and thought I’d share them with you. I imagine that this problem might be more widespread than the examples I received this week.
I can’t stress the importance of stability testing before bottling your wines. Though “stability” is a fairly obscure term, in general, a wine that is stable would be expected not to undergo any undesirable physical or sensory changes from the time of bottling until it is consumed. Although it is impossible for a winemaker to imagine what the life of each particular bottle of wine will have or the conditions it will be exposed to, a series of stability tests have been devised that will predict physical changes to the wine throughout typical wine storage.
Stability tests are carried out pre-bottling in order to predict protein stability, tartrate stability, oxidative stability, color stability, and metal stability. It is important to learn more about the causes of each of these instability problems in order to prevent them from occurring in your wine. However, small wineries or home winemakers sometimes overlook the importance of doing these stability tests – not realizing their wine is unstable until a deposit forms in the bottle. Yes, I know they are time-consuming and tedious, but once a deposit forms in your wine there is really little you can do but decant each bottle, filter out the deposit, and re-bottle the wine (after having to do it on a few hundred bottles of wine, you will perhaps realize that stability testing isn’t a waste of time after all). However, understanding the source of the deposit may help you understand the underlying cause so that you can prevent the problem in the future.
Perhaps the easiest type of deposit to identify are crystalline deposits. These are either Potassium tartrate crystals or Calcium tartrate crystals. If the latter, the crystal may appear in a more rhomboid shape. However, the only real way to determine what mineral is involved is by a flame test. Calcium burns red, while potassium will burn violet.
The amorphous (or non-crystalline) deposits that form in wine are a bit more difficult to differentiate. There are four different origins of these amorphous deposits: proteins, tannins/pigments, mineral (copper or iron), or microbiological.
Protein precipitation is perhaps the most common cause of a hazy deposit. It’s typically light (white to tan) in color and consists of very fine particles that remain in suspension for a long period of time. To be sure, if you are able to collect the deposit by either decanting the wine or by centrifugation, then try to dissolve it in a 0.1 M sodium hydroxide (NaOH) solution. If it dissolves, it is likely protein in origin.
If the deposit is due to tannin and/or pigment precipitation, the color of the deposit will often be darker in color – more brownish than white. If you can collect the deposit, it should be soluble in a 50% ethanol solution. The source of this deposit is often due to oxidation of some of the pigments in the wine. However, it is also possible that if you overfined your wine with a protein fining agent, some of the excess protein can react with the tannins in natural cork closures when you bottle it. They will combine to form a precipitate in the bottle.
A haze that is microbial in origin will typically be accompanied by a wine that has excess carbon dioxide. It is fairly unmistakeable when a wine shows a bit of unintended fizz. Microorganisms (yeast and bacteria) are never the cause of protein instability in a wine. Haze due to microorganisms is usually due to improper filtration or sanitation of your bottling line. You can confirm this type of deposit by looking at it under a microscope.
Excess iron or copper can also form a deposit in your bottle. During bottling, exposure to oxygen causes iron to change from its soluble state to an insoluble state. In red wines, it interacts with the tannins to cause a precipitation that is blue in color. In white wines it interacts with phosphates. You need a significant quantity of excess iron for this reaction to occur – more than 7 mg/L for white wines, and more than 10 mg/L for red wines. Typically these levels are only obtained if a vineyard or winery uses old equipment constructed from iron. Another possible cause is if vineyard soils are treated with iron, and dust from the earth is somehow deposited on the grapes prior to harvest. Copper deposits are also rare, and form in conditions of reduction (opposite of oxidation) and are accelerated when the bottle is exposed to light. Again, this type of deposit is rare in modern winemaking. Older cellars might still have equipment made of brass, which can cause excess copper to form in the wine. Residual copper from vineyard sprays may also make their way into the winery if the spray was done too close to harvest. It’s also possible that when treating your wine with copper sulfate you added excess copper. If your wine contains more than 0.5 mg/L of residual copper, it should be treated with a protective colloid such as gum arabic. Note that the legal limit of residual copper in your wine is 1 mg/L. Thus, the importance of running bench trials prior to using copper sulfate to treat reduction odors! A deposit that is mineral in origin will dissolve in a solution of 25% hydrochloric acid (HCl).
Hopefully this short tutorial on identifying deposits in your wine will help with preventing them in the future!
Iland, P., N. Bruer, E. Wilkes. 2004. Chemical Analysis of Grapes and Wine: Techniques and Concepts. Patrick Iland Wine Promotions.