It should not come as a surprise that the type of soil into which a grapevine is planted can influence the taste and overall quality of wine made from grapes the vine produces. Soils that provide just enough organic and inorganic nutrients in the vineyard are every grape grower’s dream. Soils that are too rich in nutrients, or get heavily fertilized, tend to spur grapevines into growing lots of stems and leaves to the detriment of developing fruit—the grapes needed for wine. An old adage suggests that poorer soils, in which grapevines struggle a bit, are typically better for wine grape production. Another long-held axiom in the grape growing business is that grapevines prefer “dry feet.” Grapevines benefit from soils that allow significant drainage during heavy rain episodes, yet retain a limited amount of moisture even in more arid periods. Vine roots sitting in soggy, heavy soil (think wet clay) are not very effective at delivering nutrients up the trunk to the cordon, shoots, leaves, and grape clusters. Soils with a mix of sand, clay, loam, and gravel are often considered optimum for grape production.
After stating the obvious above, there is another type of “dirt” that plays an important role in making quality wine, an activated clay material known as bentonite. Bentonite has myriad uses, such as: a moisture adsorbent in cat litter, a thickening agent in drilling muds, a binder material in metal casting, a water-retention barrier in sealant layers for ponds and landfills, and with its powerful absorbing properties, a purification and decolorizing agent for numerous liquids, like vegetable oils, dirty water, and many beverages. Bentonite’s primary use in wine making is for clarification—removing particulate materials and certain protein molecules that can cause haziness or cloudiness in the resulting wine.
When wine grapes are crushed to free the juice, or pressed to separate the juice from the grape seeds and skins, lots of solid materials remain suspended in the liquid, making it very cloudy (sort of like lemonade). Given time, most of these suspended particles will precipitate to the bottom of the tank or aging container. However, by adding a measured amount of bentonite in aqueous slurry, these suspended solids will attach to the activated sites in bentonite clay and readily precipitate, leaving an essentially clear juice or wine. So, bentonite provides wine makers a tool for efficient and rapid clarification to deliver a wine that is clear and bright.
Bentonite will also attach to and absorb relatively large amounts of protein molecules that are present in aqueous solutions, like wine. If these protein molecules are left in the wine, exposure to warm temperatures can denature the proteins and cause them to create annoying flocculent clouds or hazes in both red and white wines. Consequently, bentonite is useful in the process of heat stabilizing wines so they will remain clear and bright. Winemakers typically test the heat stability of wines by treating samples with varying amounts of bentonite, and then heating the samples to a certain temperature for a prescribed period of time to check for haze formation. The sample that remains clear after treatment with the least amount of bentonite will reveal the preferred treat rate in grams per liter. The winemaker then determines the total amount (liters) of wine to be treated, measures the appropriate amount (grams) of typically light tan bentonite powder, stirs the bentonite into warm water to create a slurry, and pours the slurry into a gently stirred tank of wine. After the addition, stirring is stopped and the bentonite, with any attached particulates, is allowed to settle to the bottom. The now much clearer juice or wine is then racked (decanted) away from the solids to another clean tank or vessel.
You might be curious as to the source of commercial bentonite. Most high-grade natural bentonite mined in the U.S. comes from the northwestern states of South Dakota and Wyoming. Georgia, Mississippi, and Alabama also produce good quality bentonite. Other countries, such as Turkey, Greece, Australia, India, Russia, and the Ukraine produce significant amounts of commercial bentonite.
In closing, it should be noted that haze or a film inside a wine bottle that occurs from heat instability—the denaturing of protein molecules—is different from the more commonly found sediment formed in wines via the precipitation of potassium bitartrate. Tartaric acid is a natural product found in most fruits, especially grapes. During the winemaking process, the acid can be converted to its very insoluble mono-potassium salt. When potassium bitartrate precipitates from a wine, the resulting sediment is often called wine sand or wine diamonds. Such sediment is most often the result of incomplete cold stabilization of a wine, an issue that will be addressed in a separate Texas Wine Lover contribution.
A contribution from Carl Hudson, Ph.D. and former assistant winemaker, currently the Wine Educator for 4.0 Cellars in Fredericksburg, Texas.