Preparing white wine for bottling - heat stability, cold stability, fining, and filtration
August at a winery means three things: 1) vacation time for the winemaker, 2) putting on the finishing touches in the vineyard, and 3) emptying the tanks in time for harvest (not necessarily in that order!). Last weekend at Antietam Creek Vineyards, we bottled four white wines - Sauvignon Blanc, Albariño, Chardonnay, and Vidal Blanc. But what is involved exactly in preparing a wine for bottling?
Once a wine has finished its élevage* continued aging serves no further purpose and the wine will start to deteriorate, even if maintained well in the barrel. At this point the best thing for the wine is to prepare it for bottling. The steps involved are mostly cosmetic in nature and ensure the wine will remain attractive in the bottle even when exposed to conditions that are less than ideal.
With white wines we're concerned with two things that can occur after bottling - 1) a wine's propensity to form crystals of potassium bitartrate (cream of tartar) and 2) to develop a haze, even if the wine is filtered. In the first instance, crystals of potassium bitartrate form because the wine is exposed to colder temperatures (like in your refrigerator) than it has experienced in the winery. All wines are solutions of dissolved potassium bitartrate, a function of the natural potassium and tartaric acid found in grapes. As grape juice becomes wine and increases in alcohol content its ability to keep this compound dissolved weakens. At this point the wine is considered a "supersaturated" solution of potassium bitartrate and crystals will begin to form. But this process is very slow, often taking many months. If the wine encounters a sudden change in its ability to keep the compound dissolved, like drop in temperature (or fortification) then this process can be rapid. These crystals are harmless but unsightly, and large amounts are considered a flaw in a professional winemaking operation.
After several months, a wine will naturally precipitate this compound and become "cold stable," meaning its ability to create more crystals drops to an acceptable level, even if chilled to temperatures lower than the wine experienced at the winery. Should a wine need to be bottled before that time, the winemaker would have to address this in the cellar. Traditionally this simply involves chilling the wine for a period of time before bottling. If you ever toured a winery cellar and noticed a stainless steel wine tank covered in ice, then you witnessed a wine going through cold stabilization. Once the wine is deemed cold stable it is filtered and removed to another tank.
The other cosmetic issue facing white wine is its ability to throw a haze, even after sterile filtration. There can be many reasons why a wine does this, but the most common is from a protein instability. All wines contain naturally occurring proteins. Some of these proteins are "heat unstable," meaning they are dissolved (and invisible) at cellar temperature but with heat become denatured. Proteins are long, tightly wound chains of amino acids. Heat can cause proteins to unravel. When this happens the wine can no longer dissolve them and they become visible. Sometimes the precipitate settles to the bottom of the wine bottle but often it remains suspended and the wine becomes hazy. The denatured protein can interact with a lot of other things in wine, such as tannin, polysaccharides, and color, exacerbating the issue. It doesn't take much heat to cause a haze. This most often occurs when wine sees the trunk of your car in the summer time.
Fortunately, excess protein in wine is easily addressed with an abundant and naturally occurring substance that is dug out of the ground - bentonite clay. Purified bentonite is swelled with water to make a slurry and is added to wine. It binds with protein via opposite charge attraction and the bentonite simply settles to the bottom of the tank. It doesn't dissolve in wine and the wine is filtered off the sediment. Protein-induced haze in wine is largely a thing of the past, due to the clay's abundance and low price. But while the clay itself is inexpensive, fining a wine with bentonite does entail some loss due to the fact that the sediment is rather loose and fluffy, but substantial enough to instantly plug a plate & frame filter. Most of us are familiar with bentonite in its less purified form - kitty litter.
Once a wine is heat and cold stable it is ready for bottling, provided that it is clarified enough to be attractive in the bottle and wine glass. To remove any doubt the wine is filtered at the winery and often sterile filtered on the bottling line. All wines benefit from filtration. The clarifying filtration performed in the winery makes the wine brilliantly clear and the aroma becomes more intense and clean. If the wine is sterile filtered on the bottling line the wine's shelf life after uncorking increases from overnight to several days. Filtration is relatively cheap and easy. There's no reason not to filter a wine. If the wine is sweet, sterile filtration is a must to prevent the wine from re-fermenting in the bottle. In a small winery filtration often happens with a plate & frame filter. This device is similar in design to a liquid heat exchanger in that it simply distributes the wine over a large surface area in a compact device. The actual filtration media is diatomaceous earth (the fossils of ancient diatoms) and enough cellulose to hold it together in the form of a pad. They resemble ceiling tiles.
Cheap and easy filtration isn't a given in the cellar. A wine must be relatively clear in the first place to be put through the filtration process, or the winemaker will be constantly starting and stopping the process to replace the filter pads. The method to ensure the wine is clarified enough for filtration is called fining. Fining is the addition of a substance to wine that binds haze elements and drags it to the bottom of the tank. Fining agents come from a variety of sources, many are proteinaceous in nature. By definition, they cannot dissolve in wine, so their addition to wine is really more of a processing agent. After the fining agents settle to the bottom of the tank, the wine is pumped or filtered to another tank.
Fining is common to both white and red wine and while they have traditionally been used for clarification, its modern use has taken on alternative purposes. White wine fining agents are often used to prevent oxidation in the bottle, or to remedy excessive oxidation that occurred in the winery before bottling. This includes removing the color effects of oxidation as well as the nutty, aldehydic aromas associated with oxidation. Casein, the main protein of cow's milk, and polyvinylpolypyrrolidone (long used in beer production) are two fining agents that are particularly good at these actions. Like bentonite, these fining agents are not additive to wine, they cannot dissolve and are simply processing agents. In addition to bentonite, the other fining agent used in white wine production is isinglass, a collagen protein derived from the swim bladders of fish. Isinglass is used at a rate of about 2.5-10% of other fining agents and is particularly good at rapidly clarifying white wine.
Preparing a white wine for bottling is a fast and furious process. Of all the time that a white wine spends in the winery, these processes of cold stabilization, fining, and filtration typically occur as close to bottling as possible - usually in a matter of weeks. Filtration is particularly susceptible to requiring another round in the filter, as the yeast and bacteria that was removed can reestablish itself in the wine if too much time elapses. For this reason you will often find me in the cellar the day before bottling babysitting a pump and filter (hopefully with a beer in hand!).
* A French term for aging, or more specifically, "education" of wine