It’s been pretty hectic recently, with two major Bordeaux tastings – of the 2007 and 2009 vintages – in the last few weeks. I’ll be writing them up imminently, with 2009 first in the queue, following the UGC tasting in mid-October. But first something tangential, an aspect of wine tasting I confess I haven’t considered in any great detail before, and that’s the effect of changes in atmospheric pressure on our assessment of wine.
Low atmospheric pressue is said, by some, to have a negative effect on the taste of many wines, and although the mechanism remains up for debate the most commonly proffered explanation is related to carbon dioxide moving out of solution with the arrival of low pressure (taking with it freshness and vigour) and moving into solution with the arrival of high pressure (reinvigorating the wine). Why would my mind turn to this, at a tasting of Bordeaux 2009 at two years of age? Simple; the tasting prompted me to think back to the 2009 primeur tastings in April 2010 when, in their Bordeaux 2009 reports, the low atmospheric pressure system which passed over Bordeaux during the tastings was reported as negatively influencing the showing of the wines by a number of critics. Perhaps the most notable comment on the effect of the low pressure system was from James Suckling, who has such tasting acuity he felt the wines change as the clouds rolled in, as documented in this blog post. And similar comments came from Robert Parker, in his Bordeaux 2009 report, but in this case describing the beneficial effect of the higher atmospheric pressure and clearer skies that blessed Bordeaux a week or two before the primeurs proper got underway, the wines tasting bright and fresh as a result. I would link to his comments, but they sit behind a paywall so there seems little point. They are within the introduction to his Bordeaux 2009 primeurs report, if you are interested.
Reviewing the atmospheric pressure over Bordeaux during the primeurs week in April 2010, however, suggests a story rather more complex than some have suggested. On the Sunday, as the main week of tastings kicked off, the pressure was a very reasonable 1018 mbar, before dropping to 1004 and 1003 mbar on Monday and Tuesday, rising again to 1010 and 1017 on Wednesday and Thursday respectively. The preceding week had seen a similar dip but also, in a similar fashion, some days where the pressure was respectable, whereas those tasting a week or two later enjoyed much higher pressures. Thus the atmospheric pressure was indeed on the low side, especially on the two days at the start of the week, although on many other days the recorded figures are, on retrospective examination, unremarkable. If you believe it is not absolute figures that are important but the changes in pressure, a detrimental fall at the start of the week was more than matched by a beneficial climb thereafter.
Either way, these figures look less than convincing to me. First of all, if the atmospheric pressure really has such an important effect, why did I not see (of course, I may have missed it – please let me know if any critics commented on this) reports that the wines tasted later in the week were showing much better than earlier in the week. Secondly, taking the measurements at face value, the pressure certainly varied, but these are very small changes. How likely is it that they would influence carbon dioxide solubility at all?
A little research led me to any number of documents and publications, but I will link to just one, the most useful and pertinent I feel, which is this paper, The Solubility of Carbon Dioxide in Water at Low Pressure (links to pdf, so you will need Adobe Acrobat if you click this link), published on the National Institute of Standards and Technology website. It’s from 1991, but as the laws of the universe haven’t changed too much in the last two decades, I think it is still relevant. “Low” in the context of this paper, by the way, means less than 1 Megapascal (which is 10 times atmospheric pressure) – you would be surprised how many papers focused on pressures much higher than this.
Looking at the paper, Figure 4 is the most important for my argument. To save you clicking through, I’ve reproduced it here, without the authors’ permission, I hope they will forgive me that.
Normal atmospheric pressure is 1013 mbar, so just above 0.1 MPa on the pressure axis going up the left-hand side (which is a logarithmic rather than linear scale, by the way). Trace the line across and you will see this corresponds, using the 20ºC plot, to a carbon dioxide solubility about 0.07 mol%. Now, consider these two situations:
(a) how wine tastes in an aircraft. It has long been held that wine does not taste as good at altitude, even in commercial airliners where the cabin is pressurised. Let’s suppose this is related to reduced carbon dioxide solubility because of the lower pressure within aircraft – although pressurised, they are not pumped up to 1013 mbar. Instead, a figure of 750-800 mbar would be more typical – that is why your ears still pop as you ascend and descend. Find 0.075 MPa (i.e. 750 mbar) on the Pressure axis on the left and, again using the 20ºC line, you will see this corresponds to a solubility of about 0.05 mol% – about a 30% reduction. I might be prepared to believe, therefore, that changes in taste of a wine under these conditions might be due to carbon dioxide solubility. It’s not proof, I’m merely suggesting plausibility.
(b) now consider this – Bordeaux primeurs, March-April 2010 vs Bordeaux UGC tasting, October 18th 2011. Let’s take the lowest pressure for the former, from the data presented above, which was 1003 mbar, or 0.1003 MPa. The latter figure, taken at midday on October 18th 2011 in London, was just a shade over 1010 mbar (0.101 MPa) – I’m using this figure as I haven’t heard any comments about how bad the wines tasted on the day, but you could take a higher figure if you wish – my argument would still hold. Look at the Pressure axis – both figures lie a mere hair’s-breadth above the 0.1 MPa marker. Tracing across to the 20ºC line illustrates that carbon dioxide solubility is essentially the same, about 0.07 mol%. In other words, no big change. A fraction different. I really do doubt that such minute alterations could have such a profound effect on the taste of a wine. I’m not saying I have disproved it, of course, but there is certainly a lack of plausibility here.
My conclusion is that if you believe the taste of a wine is negatively affected by low pressure weather systems, you might need to find a better explanation than carbon dioxide solubility. Big changes (such as on an aircraft) might have some effect, but I’m dubious about atmospheric fluctuations. The taste of a wine is an interaction between palate and wine, and I wonder whether weather doesn’t have more of an effect on the owner of the palate, than on the wine. This is particularly important when considering conditions during the London tasting, where tasters ‘endured’ pressures very similar to those during the primeurs (1010 mbar vs 1003 mbar), and yet I do not think we will be reading reports of how difficult the wines at the UGC event were to taste from all the acute and sensitive palates in attendance.
After all, it’s easy to comment on how atmospheric pressure affects your ability to taste, assess and comment when you need to generate a ‘story’ about the vintage, and outside there are gales blowing and rivers are bursting their banks (as was the case in Bordeaux in March-April 2010), but when the cues aren’t there (mild Autumn day in in London, October 2011), I don’t think it is such a prominent thought in many minds. Even though atmospheric pressure on the day of the tasting in London in 2011 was very similar to that experienced in Bordeaux in 2010.
Maybe some of these pressure-sensitive palates aren’t quite so sensitive after all?