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Pressure-Sensitive?

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?

20 Responses to “Pressure-Sensitive?”

  1. Wow..actual reason, backed up with some basic data. It will never catch on in these kind of discussions. We simply have to bow to the greatness of the pallets that can also detect the position of the moon in the taste of wine.
    Interesting as always.

  2. Thanks Will for that comment. I’m glad someone has at least managed to read this post to the end – it ended up a tad more lengthy than I had anticipated!

    Another interesting point from the paper I link to is the significant effect of temperature on carbon dioxide solubility. Wines taste fresher when cooler, and carbon dioxide solubility is higher at cooler temperatures (at ‘normal’ atmospheric pressure, solubility at 6ºC was double that at temperatures over 20ºC). This is an even bigger effect than that seen with the low pressure experienced on commercial airliners, and perhaps carbon dioxide gives us the freshness found at cooler temperatures.

    It would also suggest the changes in solubility of carbon dioxide that result from changes in temperature of the wine would overwhelm any seen as a response to minor changes in atmospheric pressure.

  3. Chris,

    Your analysis of what occurs in an airline is why I do not open wines that I know have seen recent transport in this manner for several months after purchase to ensure that any effect of pressure on the wine dissipates and the wine re-equilibrates

    As far as the effect of daily pressure fluctuations on the solubility of CO2 in wine ( water) and the resultant effect on the taste of the wine—

    I hope atmospheric pressure in Chicago is at least 1010 on the particular day in Jauary when the UGC presents the 2009 wines or I must consider not attending. This is before considering amount of sunlight, temperature as well as, whether the air conditioning will be sufficient at the Drake Hotel to keep the wines from getting over heated. Alot to consider before going to taste or drink a bottle or wine. There are far fewer days to open a bottle of wine each year than I had originally thought.

    Gary

  4. Hi Gary – good to see you here. Sorry about your comment awaiting moderation.

    You’re right, there are few days left when we should actually drink wine. That’s before you even consider whether it is a root, leaf or flower day.

    If we’re not careful my yearly drinking will soon all have to be crammed into one Thursday afternoon. :)

  5. Hi Chris,

    I think its fair to say your post basically says ” thier arn’t half some pompous posturing critics out there”.
    I think wine; more so than most topics online, does seem to attract some seriously self inflated ego’s.
    I would say your own mood has much more of an effect on your perception of taste than a passing cloud.

    I think compulsory blind tasting should be instigated for all tasting events. I think that would seriously be an eye opener to read those reports. I think a “Cheval Blanc” label has a 1000 times the effect of a rain cloud

    Regards

    Richard

  6. Well Richard, I certainly think there are plenty of people ready to repeat what they hear without thinking about the plausibility of the argument. And I certainly agree that there would seem to be plenty of influences on how a wine tastes where the effect is much greater than that which results from changes in atmospheric pressure, not just mood but other physcial influences, especially temperature.

    One of the problems with your argument about blind tasting and how it relates to the primeurs is that the top wines won’t show their wines with their peers, and therefore tasting will never be blind. You have to visit all the firsts, including right bank firsts Ausone and Cheval Blanc, also Ducru, Montrose, Cos d’Estournel to taste their wines. All the UGC press tastings can be done blind or non-blind, whereas the trade tastings are non-blind.

  7. Hi Chris,

    I can see why the upper echalons of producers prefer to have thier tastings at home. They can run the tasting to thier satisfaction, plus the tasters know full well where they are. It makes you wonder though, I know these premier tastings are invite only, would a writer who gave a few less than flattering reviews to a Chateau still be on the invite list afterwards ? How seriously could or would a headline wine writer be taken if he was no longer on the A list ? Still I would love to see a yearly blind tasting with a broad selection of wines; say 3 years after release, and see what gets rated.
    I think thats why I really enjoy reading the decanter wine challange awards each year, a unbiased blind test.

    Regards

    Richard

  8. Good points Richard. I guess whether or not that happened would depend on the writer in question. You point out that the writer might not maintain credibility, but would a chateau maintain credibility if they blacklisted someone like Parker, Jancis or James Suckling?

    Less ‘famous’ names are perhaps more vulnerable. But I can’t think of this ever happening, and it would ultimately lead to negative publicity no matter how small the writer in question.

    Blind competitions such as the DWWA or IWC have their advantages, but the big problem – in the context of this debate – is that they don’t cover the wines we are discussing here. You won’t find many classed growth clarets in Decanter’s list of gold-medal-winning Bordeaux in their Wine Awards.

  9. All of this leads me back to something I learned long ago: Trust your own palate.

    One of the main reasons I’d rather buy a ” less exalted” wine that I or a trusted firend have actually tasted, rather than going off the opinions and ratings of the establishment.

    I may not get access to the same group of trophy wines, but so what with so much good stuff to choose from these days. In this respect Parker and the rest have done me a big favor as long as you are willing to go beyond the usual suspects.

  10. That’s very true Gary; there are lots of great wines out there, many from beyond Bordeaux’s borders.

    Having said that, none of them taste quite like the Leoville-Las-Cases – from a vintage when LLC was still affordable – I am drinking tonight. :-)

  11. Please don’t mention Mr Parker to me,

    ever since he “discovered ” my favorite wine and scored it 90+ the prices have shot up. Luckily I bought several cases from different years before this happened. Never mind about rising atmospheric pressure, lets talk about rising blood pressure :) . Still it must mean I have good taste ( regardless of what my wife says about my dress sense), think theres a job at eParker going :D ?.

    Richard

  12. Chris, gr8 article. A bit of science to explain rather than justify a point. I wonder how much of the ‘pressure differential’ arguement is used by some as a means to ‘cover’ their assessments of certain wines. Given the sky-rocketing prices of Bordeaux these days can this arguement be used by some to deflect potential criticisms of their assessment….?

  13. Richard; who knows, maybe there will be a vacant post at erobertparker.com sooner than we think?

    Matt, thanks for those comments. Obviously it’s impossible for me to answer your latter question, but I do think many people have a tendency to ‘externalise’ responsibility. Something goes wrong, and so someone/something must be to blame.

    In the case of wine tasting, a critic thinks all the wines feel a bit dull and flat. Looking around, the blustery weather outside suggests the atmospheric pressure might be a few millibars lower than usual. A-ha – the obvious explanation – case closed!

    Other explanations, relating to the taster (state of tiredness or alertness, hydration, hunger, number of wines tasted so far) are ‘strangely’ overlooked.

    Of course, if the weather outside is bright and sunny, there’s always the biodynamic calendar to blame. What did people do before this come along – blame the alignment of stars and planets, I suppose!

    I can hear it now: “there’s a waning Moon in Sagittarius, moving into Capricorn – no wonder this Léoville-Poyferré tastes rubbish”. :)

  14. Just a quick ‘heads-up’ for those posting comments that I have added a quick ‘captcha’ anti-spam box to the comment form. It’s short (four characters) and easy to read so I hope it won’t be too inconvenient.

    The reason for doing this is the level of spam experienced. The new blog has been running for just over ten days. The number of spam comments posted, and all manually binned by me?

    A rather remarkable 537 – just over 50 per day.

    Please let me know if the captcha box causes any grief – there is an ‘Email me’ link on the homepage.

  15. I’m a weather forecaster : seems I will have a new task in the future : predicting the best time to drink a wine ;)

    I don’t believe in the theory in normal circumstances, meaning the little change in air pressure at sea level, the effect of going from one place to another will usually be much greater than the change in pressure due to the weather, every 8 meters higher of lower means a difference of 1 hPa in pressure, so going up the hill to drink a bottle with your friend who lives overthere should mean that this wine will taste less good > anyone experienced such a thing? ;)

    this also means that is a shame/waste to drink Lafite in a Penthouse at the 100th floor or so of a skyscraper, because overthere you will have a fall of 40 hPa in pressure, and the “poor” people in Switzerland can’t enjoy there glass of wine because overthere the drop in airspressure is significant : eg in Sankt-Moritz (where I suppose they drink plenty of Lafite and Petrus) the pressure drops around 190 hPa in comparison to sea level

    looking at the graph a change in temperature is far more important : a big change in airpressure can be “corrected” by a very small change in temperature (only a few tenths of a degree)

    I think another theory is needed to explain this (if there is anything to explain ofc ;) )

  16. Chris – agree 100%, how often I have a tasted a wine one day and reached a conclusion, only to reach a different conclusion the next day – perhaps lack of sleep, a slight cold, hunger etc, etc – I guess another explanation is that tasting, whilst it can be a structured process, is an art and not a perfect science – we are merely human beings in the end – thanks again

  17. Hi Kris, good to see you here. So now we know your profession – and I thought you were in the wine trade!

    You point is just superb – a brilliant comment, thanks. I thought of pressurised aircraft, but did not think of the effect of simply ascending a hill. So an ascent of 8 metres = a fall of 1 hectoPascal (or 1 millibar)?! That’s hilarious! The difference between the UGC tasting 1010 mbar/hPA) and the 2009 primeurs (1003 mbar/hPa at its worst) is equivalent to a 56 metre climb!

    We must alert the wine tasting community!! All wine tastings to be held at ground floor level from now on! The Bordeaux UGC tasting in Covent Garden is held on the first floor – the Bordelais would sue if they realised the adverse conditions their wines are being subjected to! :)

    I knew there was a good reason not to buy a penthouse suite! :)

    You’re right about temperature – there’s another figure in the linked paper which makes this even more clear. Changes in temperature definitely have a big effect on solubility. Whether this truly affects taste is still an unknown though.

  18. Matt, thanks for that response. There are clearly plenty of variables with a potential role to play here.

    I’ll be viewing any comments on atmospheric pressure and the taste of wine with a very critical eye from now on!

  19. Win trade??? just drinking :)

    I think the weather impact on a wine tasting can be neglected, which is a shame for me ofc : would love to make weather forecasts for tastings > and I would have them pay me in wine ofc ;)

    the only impact ( a very important one) on wine from the weather is during the time that the grapes or still in the vineyards I think

    but to be sure I will drink a top level claret the next time I’m in Sankt-Moritz > just needing a sponsor to pay for the bottle :D any candidates? ;)

  20. Kris. I’m sure you will be able to cadge a bottle from your neighbours – they’re probably owners of a classed growth château. I know many of the Bordelais holiday down in Arcachon, but some of them must go skiing as well. :)