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Exciting Clay

©Richard Gawel ....Email this article to a friend....

Wine abuse is rife in the community. How many times have you bought a bottle of wine for that late summer afternoon café jaunt with a few friends, and then remembered that you left it on the back seat of the car in the searing sun. Being at work on the 20th floor you think that it's just too much of a hassle, so you leave that "hope that none of my real wine friends show up, Chardy" to natures' mercy.

However winemakers fully appreciate the slackness that pervades the wine drinking community and counteract it by undertaking a series of processes collectively known as stabilisation. So what specifically would happen to your white wine if it wasn't stabilised and you left it in the car to fry? In all likelihood your wine would end up containing a mass of fine white flotsam with the occasional chunkier bit floating around for good measure. The wine would taste fine, only that it would have the texture of the leftover water after poaching a few eggs. These fine fluffy particles are actually wine proteins that precipitate out of the wine after it has been inadvertently heated. Grapes contain natural proteins, and these are transferred to the wine when it is made. At room temperature these proteins are soluble, so we can't see them nor taste them. However, when heated, they denature (i.e. break up), and the resultant bits are far less soluble. Consequently, they precipitate out of the wine forming a fluffy haze. Not surprisingly if you cool the wine down, the haze does not disappear as the original proteins cannot magically reassemble themselves.

A way of avoiding the formation of heat hazes is to take the proteins out of the wine before it is bottled. This is achieved during winemaking by adding a unique clay (yes, folks, clay) called bentonite to the wine during its making. Bentonite is a special type of very fine clay of the aluminum-silicate type. While deposits of bentonite are found in various parts of the world, there are a few deposits including the one from which the clay is named, Fort Benton in Montana, that are most suitable for wine stabilisation. Bentonite is a natural substance which has been very intricately assembled by the forces of nature. Formed from fine volcanic dust that just happened to come in contact with the right type of water at the right time, it consists of miniscule plates consisting of silicon, magnesium and aluminium. There are two things about these plates that make bentonite just right for the job. Firstly, when it is hydrated the trillions of plates in their totality provide a huge surface area, and secondly, the plates are negatively charged. Wine proteins have a slight positive charge, and as opposites attract, they bind electrostatically to the negatively charged bentonite plates. The huge surface area makes bentonite the fining agent of choice, as it provides it with a very effective 'search and destroy' action. If you were a wine protein and someone added bentonite to the tank you were in, then there wouldn't be too many places you could hide. Each gram of bentonite has an effective surface area of 800 square metres. So to put this in perspective, if a big company winemaker added bentonite at a typical rate (of 1/2 gram per litre), to a typical wine tank (of 100,000 litres), then the surface area of the added bentonite will be a mere 4,000 ha. So it is easy to get excited over clay!

Bentonite fining does have some disadvantages. Firstly, some aroma and flavour molecules are not immune from the attractive forces of the bentonite plates. Therefore, its use to fine out unwanted proteins can also produce an inadvertent loss of varietal character in the wine. A French study released this month, quantified the loss, and showed that up to 1/5 of the varietal defining terpene compounds in Gerwurztraminer were stripped from a wine that was fined with bentonite. While this doesn't mean a loss of 1/5 of the aroma and flavour of the wine, a perceptible loss of aroma and flavour would be expected. The loss of varietal character following bentonite fining is generally pretty obvious. As a result, winemakers always conduct small scale laboratory trials in which they determine the minimum amount of bentonite required to achieve protein stabilisation (the loss of sufficient proteins so that a heated wine will not go hazy).

A second disadvantage is that following mixing bentonite with the wine, the bentonite settles to the bottom of the tank taking the unwanted proteins with them. Known as bentonite lees, this sludge is actually a bentonite/wine mixture. I'm sorry to tell you this, but in medium to large scale wineries, this wine is not wasted but recovered from the bentonite and added back into sub-premium products.

But look on the bright side. This wine has had the most intimate contact with the worlds most exciting clay. However, they wouldn't have to get so intimate if you didn't leave your wine in the bloody car!