A reading of Paul White’s “Scientifically Speaking” article in Harpers noted that “a whole range of good thiols derived from grapes (not yeast) that heavily determine a wine’s flavour and aromatic profile are stripped out by copper fining,” thereby prompting me to google “grape thiols minerality,” which disclosed the following: http://www.gvstudios.com/octnov08/feat.asp" onclick="window.open(this.href);return false;
… winemaker and wine educator John Buechsenstein …
Buechsenstein lays out his most compelling theory, based on mercaptans (also called thiols) last. Simply put, mercaptans are molecules containing hydrogen and sulfur - hydrogen sulfide (H2S), for instance, is not uncommon in wine. When it’s present in large enough amounts, it smells like rotten eggs. But in very small amounts? Buechsenstein observes that, “Wines with minerality often come from Old World, cool climate sites that have been farmed for centuries, even thousands of years. Their soils are thin and rocky - worn out from so many harvests.”
He explains that sulfides (mercaptans) are created in wines via musts that are poor in nitrogen. Yeast needs a certain amount of nitrogen to grow. If a must is nitrogen poor, then the yeast will start cannibalizing amino acids. “At the core of amino acids is a nitrogen group,” he continues. “So the yeast goes after the nitrogen in the aminos. They use the nitrogen to survive.”
Two amino acids have sulfur in their molecules - methionine and cysteine. When the yeast deconstructs methionine and cysteine to get their nitrogen, this sulfur is released into the must, which then goes through a reduction process to form mercaptans. That’s why winemakers treat a stinky must with yeast-assimilable nitrogen. The yeasts are given enough nitrogen to stop attacking the amino acids and liberating the sulfur."
Buechsenstein further notes that in the old days in Europe, grapes and olives were planted where nothing else would grow, such as on rocky hillsides. “These sites are naturally low in nitrogen, and that may be the cause of nitrogen scavenging by yeasts and the production of low amounts of mercaptans. So, minerality could be trace amounts of mercaptans in the wine,” he sums up.
I’ve sniffed enough hydrogen sulfide in wine to know that it is a really unpleasant offense to the nose. That in very tiny amounts it might actually be the cause of minerality is an interesting thought.
Buechsenstein’s explanation may not comport completely with White’s, but it nevertheless provides much food (amino acids) for thought.
Interesting topic. I agree that Thiols are important to the character of many/most European wines…or at least many/most French wines. I don’t associate Thiols with minerality tho, but that could be debated (endlessly! .
My various thoughts/comments:
Thiols are a very large group…mercaptans is one example of a thiol, but not all thiols are mercaptans. And none of the thiols that we find appealing in wines are mercaptans.
The description that Buechsenstein gave was a good one for the formation of H2S (hydrogen sulfide) during fermentation. But it’s not clear that there is any relationship between generating H2S in the fermentor, and developing thiols in the barrel (or in the bottle). The high level description of the formation of thiols (my understanding of it anyways) is: Wine is stored in barrels in part because the barrels provides a small amount of oxygen to the wine. This oxygen is required by various chemical reactions that occur when the wine is young. Some varietals need more oxygen than others (cab for instance)…hence cab is ‘racked’ one or more times before being bottled. If a wine in barrel becomes ‘reduced’ (reduction, aka reduced, is the opposite of oxidation… i.e. reduced is just the absence of oxygen) then sometimes the ‘oxygen requiring’ chemical reactions occurs…but sulfur takes the place of the oxygen in the reaction, since there is no available oxygen, and the result is a thiol (I’m a little vague whether the thiol is a direct result here, or if this is a multi-step process…but at a high enough level, it’s all the same).
Chemist-winemaker Dr Alan Limmer 'has frequently seen wines described with reference to their particular acidity and/or mineral tones when the presence of sulphides is apparent. … The sulphides impact from an organoleptic perspective, towards the end of the palate imparting a “mineral” or bitter/hard/astringent aspect.
“Minerality” is a frequently used descriptor in tasting notes, but it is the source of a lot of confusion. Where do "mineral flavours come from? One explanation could be the presence of reduced sulphur compounds, described commonly by tasters as “reduction” flavours. Wine contains a wide range of these (see chapter 18 for a full discussion of the subject) which, at certain levels, in specific contexts, may be mistaken for terroir character. … On other occasions it’s likely that sulphur compounds aren’t the explanation for “mineral” flavours, and wines with high acidity levels are described as “mineral” by tasters seeking an apt descriptor.
Very interesting.
Might be worthwhile doing a little research using CA wines as there AVAs such as Sta Rita Hills and Paso where soils vary quite a lot. I’d imagine that there are places (perhaps Carg) with nutrionally deficient soils.
A red wine closure/storage study, which included screw caps with different air headspace volumes, has shown that there is no significant sensory difference, and only marginal differences in phenolic composition, between the natural cork-sealed wine and the wine bottled under screw cap with an intermediate headspace volume. The effect of a small headspace volume under screw cap was to enhance slightly > a struck flint/rubbery ‘reductive’ aroma> , while storage under a very large headspace volume conferred a slightly elevated oxidative character to the wine, which was evident at 12 months’ post-bottling.
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