his intriguing question came up last summer, months before the 2007 harvest of grapes at Matanzas Creek Winery (part of Jess Jackson’s Artisans and Estates portfolio), and led to the fascinating conclusion: There is dirt in all wine, and the amount depends on various factors.

But more importantly, the two wine makers who discovered this little-discussed fact also say that not dealing with it could result in the crafting of wines that may well have a higher pH than is preferable. This discovery (and it may not be the first time it’s been discovered) was made by Matanzas Creek wine maker Francois Cordesse and his assistant, Alex Reble, two talented, French-trained viticulture and enology experts.

They observed during the course of the growing season that, in almost all vineyards, there will inevitably be various human impacts to the soil - from tractors and other mechanical equipment making trips hither and yon, to vineyard workers who kick up dust as they walk through the vine rows tending to various tasks. Dirt flies everywhere.

The dirt and dust sprayed into the air settles on grapes and stays there. “This doesn’t happen as much in France,” said Cordesse, “because there is usually an August rain that washes most of the dirt off the fruit.”

Coming Clean with “Dirty” Wine

Dirt in wine is commonplace (it occurs universally around the world), and has never, to my knowledge, caused harm to humans. Indeed, when I mentioned this to a number of wine makers, most didn’t know what I was talking about. Dirt is harmless because alcohol acts as an antibacterial agent.

It’s unlikely that the “dirt” in wine adds any terroir characteristic to the wine, but that’s another idea that needs to be tested.

Cordesse and Reble were concerned about how dirt could affect the wine in other ways. For instance, the acid-pH balance could be affected. The pH of soil is close to that of water, above 6.0 and below 8.0. This “about-7.0” pH means that it is neither acidic nor alkaline. However, since all wines have a pH range of about 2.8 to 4.2, any dirt left in wine is going to have the effect of raising the wine’s pH. The rise in pH may be marginal, but it could be significant, notes Cordesse. The difference between two red wines, one with a pH of 3.6 and another at 3.7, is significant. For one thing, the lower-PH wine will be more stable and requires less sulfur dioxide for protection. It also will age less rapidly.

Growers can reduce the amount of dirt and dust they have on their wine grapes by a short burst of water from overhead sprinklers just before harvest. I know of no growers who do this. Moreover, such a tactic only applies to those who have overhead sprinklers, which have become far less useful in recent years than the more cost-efficient drip irrigation systems.

To test their theory that lots of dirt gets into wine, Cordesse and Reble made a tank of a Merlot rosé in 2007 in which whole (uncrushed) grapes were placed into a tall, slender tank and the free-run liquid swirled around the berries. After an overnight of this activity, the tank was allowed to settle, and grape juice was drained from a lower racking valve.

That there was dirt in the bottom of the tank wasn’t as much of a surprise to them as was the amount they saw. “It looked like mud,” said Reble. The wine makers could well have filtered the first-drain juice, but it looked so awful they discarded it. They stopped draining when the juice ran relatively clear.

Then the wine makers fermented the remaining juice in the tank. The result was a Merlot rosé with a pH lower than what had been tested in vineyard. (The wine is not commercially available. Cordesse said he was considering selling a few cases of it at the winery’s tasting room.)

Dirt in wine may sound sinister, but since few if any wineries deal with this issue, I don’t envision it will prompt any health officials to intervene. Nor do I expect environmentalists to become concerned, since all this is doing is providing a bit of the land for wine consumers!

What could play a role here is the impact of dirt on pH levels.

California wine makers seem less than concerned about minor changes in the pH levels in their wines, even though UC Davis, Fresno State and most other enology schools preach the necessity of keeping pH levels low enough so sulfur dioxide use may be held to a minimum. Moreover, lower pH levels protect wine from early spoilage.

Since a change in pH of just 0.1 is significant, especially when the pH of the grapes coming in from the vineyard have an average pH that is fractionally above 3.6, it stands to reason that wine makers should be aware of any technique that, with minimal intervention, can keep pH levels low. It is clear, of course, that even the most sophisticated pre-harvest field sampling can only provide a rough approximation of the sugar content and pH. A few thousand berries looked at through a refractometer are clearly no help in determining pH; refractometers measure sugar content. Even a more thorough analysis can only provide the wine maker with a rough guide as to what sort of pH he or she is dealing with.

Wine makers know that it’s best to deal with the pH of the wine before the onset of the primary fermentation; dealing with pH after the completion of the primary fermentation is not only tricky, but may well be impractical.

Cordesse and Reble believe there is no point in running the risk of higher pH when it should be easy to set up a system for “washing” the fruit before it gets to the crusher. So, in 2007, Reble set up a Rube Goldberg sort of contraption at Matranzas Creek that sprays water on grape clusters as they are delivered to a conveyor belt leading to the sorting table. He said the system worked well, and he’s hoping to refine the system in time for next year’s harvest.

True, some wineries don’t have sorting tables, but any water spraying device that can clean off the grapes before they hit the crusher or press will wash off most dirt and dust and can help to lower the pH in the resulting wine without any change in viticultural practices.