What’s in the Bottle – Shards of Glass?

By Steven Hickman Ph.D., Staff Scientist, Harvard University and Fox Run Winery Intern emeritus

The story, as told by Peter Bell, goes as follows:  a woman calls the Fox Run tasting room one afternoon, in a state of some distress.  She had removed a bottle of Fox Run wine from her fridge, and as she was preparing to open and serve said bottle, noticed shards of glass suspended in the wine.  She was rather upset at the winery, accusing it of at best gross incompetence on the bottling line, and at worst an attempt on her life.  What had caused this strange circumstance?

The culprit was not a malicious staffer nor a malfunctioning bottler, but rather the combination of a common wine component, a clear bottle, and an abnormally cold fridge.  The small, clear crystals floating in the wine were not glass, but rather precipitated potassium bitartarate. 


Tartaric acid is one of the three principal acids found in wine – the others being malic and (sometimes) lactic.  The first three are found in the wine grapes, while lactic acid is produced during the transformation known as malolactic fermentation.

The author, before he was lured away by Harvard University

In both the grape juice and the post-fermentation wine, the amount of tartaric acid is often at or near the saturation point – that is, the concentration of dissolved tartaric is about as high as it can go.  This is a concern to winemakers, as the solubility of a solid, in a liquid, decreases as the temperature decreases.  Interestingly, this is the opposite of a gas – the solubility of a gas in a liquid increases as the temperature decreases.  This is why sparkling wine bottles can be opened and re-corked during disgorging, and only a small amount of carbon dioxide is lost. 

But with a solid, a decrease in temperature will cause a decrease in solubility, and if the amount of solid dissolved is already at the saturation point, then that cooling effect will cause some of the solid to precipitate.  In the case of the saturated tartaric acid, this causes two changes to the wine – one visible, and one sensory.  The visible change is the formation of crystals of the potassium salt of tartaric acid, often called tartrates.  Cream of tartar, a component of baking powder, is made up of this very salt.  


The second change is a decrease in the acidity of the wine, which in turn can affect the taste of the wine.  This may be a problem, because if the winemaker has carefully balanced the acidity of the wine before it is chilled, it will likely be out of balance after the wine is cooled and the acidity changes.
 

To prevent this, most winemakers perform a process called cold stabilization.  In this process, the wine is chilled to just above freezing – usually by circulating a refrigerant around the outside of the wine tanks (these cooling jackets are the dimpled jackets seen on most wine tanks, and when they are in action, a layer of ice can build up). 

The band in the middle of this tank, just below the fox run logo,

is the cooling jacket.

A cooling jacket in action, coated with a layer of ice.  The black foam rubber around the outside of the tank is a “blanket” to confine the cold air to the area around the tank.

In this process, the excess tartaric acid precipitates in the tanks at the winery, rather than in bottles in your fridge.  Most of the precipitated tartrates adhere to the sides or settle to the bottom of the tank, and any remaining solids are removed in the filtration process.

Looking up the inside of an empty tank after the cold stabilization process on a white wine.  The yellow specks on the side are precipitated tartrate crystals.

After the cold stabilization process, the wine is allowed to warm to cellar temperature.  Through sensory and chemical analysis, any necessary adjustments to the acidity, to account for the loss of some of the tartaric acid, are made.  This ensures that when the consumer chills the bottle at home, it will remain “stable” in taste and content.

Because detrimental changes can occur to the wine if it starts to freeze, cold stabilization is usually not done to below the freezing point of wine.  So, in all likelihood, the bottle at the start of this story had been in a sub-freezing portion of the woman’s fridge, and had reached a temperature below which the wine was cold-stabilized in the winery.  The problem was compounded by the clear glass bottle, which made it much easier to see the precipitated tartrates (this is one reason, albeit minor, that wines rarely come in clear glass bottles – the major reason being to protect the wine from degradation by light).


So the next time you encounter a few beautiful crystals floating around in your white wine, please don't panic. You can even amaze your friends by chewing on a few of them, to experience their mildly acidic crunch; or if you're really ambitious, you can use them to stabilize your next batch of meringue.