Rocks in My Wine?

During a Grape of the Night gathering, where Chateauneuf du Pape Blanc was the star, it was discovered that there was a key taste in most all of these wines. All of the bottles brought were from Southern France giving them commonality in the terroir leading one to believe that they would share some characteristics. The key feature that was found at this gathering was a taste of minerality which varied in degree and flavor between the different wines poured.  

I made a comment, which may or may not have been correct, stating that the mineral taste is the result of the terroir.  The term minerality is very often used, or misused, as a wine descriptor providing a link between a wines terroir, or region, that a wine comes from.  "The Oxford Companion To Wine", a book that is used by many of us, does not even have the term "minerality" in it.  Yet we hear many knowledgeable people comment on a wines minerality. For some wines, it is an important part of the wine's profile.

We know that there are minerals such as sulfur, magnesium, potassium, calcium, manganese, iron, copper and zinc in wines but the levels are too low to be detectable by humans. So where do people come up with taste descriptors such as chalk, slate, wet rock, flint and limestone with certain wines?  

Enter chemistry!  One of the best explanations, and there are many, comes from the research by Denis Dubourdieu at the University of Bordeaux on mercaptans.  Mercaptan (thiols) is a sulfur containing organic compound.  The key word here is sulfur.  Mercaptans are formed by yeast during and after alcohol fermentation if there is a deficiency of nutrients, such as nitrogen. Due to a lack of nutrients, especially nitrogen during fermentation, the yeast uses the sulfur in amino acids (Cysteine) that are present forming what are called mercaptans.  This is what creates the mineral profile that is detected in wines. Since wines, such as Chateauneuf du Pape Blancs, are grown in soil that consists of pebbles and sand with smooth rocks on the surface, called galets, there may be a nitrogen deficiency.  Thus, giving the answer to the question on why we get a mineral flavor profile from these wines.  

In looking at my statement on the terroir being the cause of minerality, I would not say that I was incorrect.  The fact that the soil lacks nutrients can be viewed as part of the terroir making my statement correct.  It would be incorrect if the assumption is that the minerals from the pebbles and rocks are being absorbed and concentrated in the grapes.  Even today there are differences of opinion on what causes minerality.  As an engineer with a chemical background this seems to be the most valid.  

Cheers,

Rusty Sly