Daily temperature variations and wine storage

It is often stated that daily variations in cellar temperatures are more damaging to wines than consistently higher temperatures. As far as I know empirical evidence for this is lacking, but that is not to say it is necessarily wrong.

One possible mechanism could be that the resulting pressure variations pump out oxygen-depleted air, and suck in air that is richer in oxygen – or something analogous if the bottle is stored in the correct horizontal position and the cork soaked in wine. Unfortunately, I find it difficult to get a handle on how likely an explanation that might be.

However, the reason most often proffered is that the pressure variation caused by the temperature changes can loosen the cork, making it more likely to leak.  It is a lot easier to do a quick calculation to establish the plausibility of this explanation. It turns out that a 10ºC variation in temperature will result in a force of around 100g acting on the cork – see the bottom of this post for the workings. That is, for example, the force equivalent to the weight of a smallish apple.

100g apple

A better way of understanding it might be to push on some kitchen scales with a finger until they read 100g. You can then compare that with the force you need to get a cork moving when extracting it with a cork screw. Judge for yourself, but I find it totally implausible that the 100g force would shift most corks – however slightly, and however many times that force is applied. Perhaps very old corks that a cork screw would push into the bottle might be affected, but I am still not totally convinced of that either.

air pressure variation

Ah, but you say there are also pressure variations outside the bottle due to the weather, and when added to the pressure variations due to temperature inside the bottle they could be a lot more significant. Well, above are shown the atmospheric pressure variations measured at the National Physical Laboratory for 2015. It turns out that the pressure variations due to weather are about the same order of magnitude as those due to 10ºC temperature variation – again, see my workings below – and they are lot less frequent than every day, so they will not make that much difference.

So, in conclusion, I don’t know of any evidence for daily temperature variations being  worse for wines than consistently high temperatures. That does not rule out the possibility, but it also seems unlikely that those temperature fluctuations will loosen corks.

If you’re up for it, here’s…

The science bit

Gay-Lussac’s law says that for gases at a constant volume, pressure is proportional to temperature. Here the temperature must be measured on a scale where zero means absolute zero, so we will use Kelvin for temperatures. Room temperature is around 300 K, and we are looking at a temperature variation of 10 K, which is the same as 10ºC. Thus the relative change in temperature is 10/300 – around 0.03 or 3%.

Atmospheric pressure is approximately 100,000 N per square meter, and according to Gay-Lussac the relative change in pressure is the same as the relative change in temperature. So the absolute change in pressure is 100,000 x 0.03 = 3,000 N per square meter. (At this point you might also like to note that the atmospheric pressure variations shown above are also of the order of 3%.)

That pressure change acts on the area of the cork, which is about 1 cm, or 0.01 m, in radius. The area of the cork is 3.14 x 0.01 x 0.01 = 0.000314 square meters.  3000 N per square meter acting on an area of 0.000314 square meters gives a force of 3000 x 0.000314 = 0.94 N, which is approximately 100 g force.

Edit 24/07/19: It later occurred to me that although liquids generally expand much less than gases, the volume of wine is a lot greater than the ullage in the bottle, and that the wine expanding may significantly compress the air, and thus increase the pressure by more than calculated above. So I redid the calculation to take account of the above effect, and also with some other improved assumptions. Overall, including the effect of using a specified temperature change of 15-25ºC, a high-fill ullage volume, a pressure at 15ºC of 1 bar, and an expansion coefficient I found for white wine, the change in force on the cork would be 160 g. This reduces to 115g for a low-shoulder ullage. So the force is larger than originally estimated, but not one so much greater that it invalidates my general conclusions. I’m not going to publish my workings here, but let me know if you would like to see them.

Wine fridge buying advice

I am assuming here that you have considered the alternatives, and have come to the conclusion that you want a fridge for the long-term maturation of wine.  I am not here writing about fridges for holding wine at serving temperature.  My advice is based on discussion I have seen on various online forums, and on personal experience with a couple of Liebherr WKr4676 wine fridges.

Size

It is vital to get the size right.  Of course the fridge will need to fit into the space available, but it is also important to have some idea of how many bottles you need to house.

You can do a quick back-of-the-fag-packet calculation based on the rate at which you think you will be drinking wines from your fridge, and how many years on average you think each bottle will need to mature.  If you drink one a week, and the average time each bottle stays in the fridge is 10 years you will need storage for around 500 bottles, which is 10 years divided by 1 week.

One way of getting by with a smaller fridge is to store wines that do not need to be aged elsewhere, and you can do the same with other wines as a temporary measure to cope with variations in drinking and buying rates.  Alternatively, if you get through a lot of mature wine you would probably do better to store most of it off-site in a specialist storage facility, and just use your fridge as a buffer for the contents of the cases you have taken out of storage.

You should bear in mind that the fridge bottle capacity is usually quoted in terms of Bordeaux bottles, and if you mix bottle shapes in your fridge you will not achieve the advertised capacity.  I can get about 175 bottles in a fridge that is rated for 196 Bordeaux bottles.

Temperature

Don’t bother paying for more than one temperature zone.  Wines of different types do not need different temperatures for storages, and providing your fridge can hold your wine at around 12-14ºC you will be fine.  In fact, if I see a wine fridge being offered with more than one temperature zone it makes me suspicious – either the fridge is really designed for holding wine at serving temperatures, or the manufacturer is adding unnecessary features.

And while on the subject of temperature, be sure to check the range of ambient temperatures your fridge needs, particularly the lower end if you are planning on keeping it in an unheated outhouse. When I bought my fridges, some of the cheaper models were not very good at dealing with low temperatures.

Internal fittings

Top-end brands and models offer the possibility of drawers in your fridge.  They add to the cost and are not necessarily advisable.  Drawers make it a lot easier to access your wine – do not underestimate the effort it takes to dig out a bottle from the bottom of a pile on a wine fridge shelf, even assuming you can remember which shelf it occupies.  On the other hand, drawers will severely reduce the number of bottles your fridge can contain.  By far the most efficient method from a storage point of view is a stable pile of bottles that is as big as possible.

Also, from a stacking efficiency point of view, you should favour thin wire shelves over more stylish wooden ones.  The difference in thickness might only be a few millimetres, but on at least one shelf it will probably be the difference between an extra layer of bottles or not.

External appearance

External appearance is largely a matter of personal taste, so I will restrict myself here to saying that I personally would not go for a glass-fronted wine fridge, for a number of very different reasons.  They tend to be more expensive.  They insulate less well.  Light is damaging to wine – OK, glass doors use darkened glass which are claimed to prevent light damage, but why not simply plunge your bottles into complete darkness?  To use my fridge at full capacity I have to cram bottles in however I can, and the result is not very visually appealing.  And my final reason is that they announce to potential thieves that you might have wine worth nicking.

But if you are going to keep the fridge inside your house, and really prefer the aesthetics of a glass door, I’d say go for it.

Brand

When I bought my first wine fridge the choice of brands was quite narrow.  Eurocave dominated the top end of the market, while Transtherm had similar products but a little cheaper.  Then at the lower end of the market, but still offering a credible product for long-term storage, there was Liebherr.  Since then, many other names have appeared offering cheaper products, and I find it very difficult to determine if they are designed for, or even suitable for, long-term storage.  Often there are no claims or guarantees made on what I would consider to be key questions:  Is the fridge vibration free so sediment will settle properly in the wine; and will the fridge maintain a high degree of humidity so the corks will not dry out, while having good ventilation to prevent mould formation.  I think I would rather go for a model that is specifically claimed to be suitable for long-term storage, and one that offers technical information to support the claim.

I have never regretted buying my Liebherr fridges and have found their UK agents to be helpful on the couple of times I have had technical queries about the fridges.  I have blogged negative comments on the price of their filters, but I suspect Liebherr filters are no more expensive than any other brand.  The favourite fridge amongst UK wine geeks appears to be the Liebherr WK5700, largely I think on the basis of the price per bottle.  It is a bit of a beast, with all dimensions that little bit larger than a domestic fridge, and few compromises for style.  The main reason I chose the WKr4676 over the  WK5700 was that at the time the WKr4676’s ambient temperature operating range went a bit lower, but the external size and appearance was also a factor.

Temperature variation in wine cellars

The above diagram is taken from p303 of David MacKay’s book Sustainable Energy – without the hot air.  It shows how temperature in the ground varies with depth and time of year if the surface temperature varies as a sine function with a minimum of 3°C and maximum of 20ºC.  You can see the surface variation by reading off the temperatures from the horizontal slice through the diagram at depth 0.  Further into the ground, at depth 2 for example, you can see that the temperature is colder, varies in the range 10-13ºC, and the minimum temperature lags behind the surface temperature by about 5 months.  More rapid temperature variations, within a day and from day to day, are ignored, but these would be very small at almost any depth.  The correspondence between depths 0, 1, 2, 3 and depths in meters depends on the ground material; examples from MacKay are given in the table below.

Granite Damp sandy soils
or concrete
Dry or peaty soils
Depth 0 0.0m 0.0m 0.0m
Depth 1 3.0m 2.6m 1.3m
Depth 2 6.0m 5.2m 2.6m
Depth 3 9.0m 7.8m 3.9m

Of course, this would be a good predictor of temperature variations in your wine only if you had a few bottle buried outside, and perhaps in some commercial cellars.  Domestic cellars are usually directly below houses, and thus have surface temperatures that are both higher and more constant than those assumed in the diagram, and there are not metres of earth or rock between the cellar and the house.  But even then, the diagram does perhaps give some idea of the cooling effect from cellar walls.  If you are keener than me you could try doing calculations more relevant to wine cellars – MacKay gives you some equations you could use at a starting point.  Or you could just put a thermometer in your cellar.

Another wine application of these calculations would be to predict the temperature of vine roots at various depths.  I have no idea how important root temperature is to the vine, but if it is you now know where to get the equations.

Finally, a big thank you to David MacKay for allowing his material to be used under a Creative Commons licence.  See the website for details, but amongst other things it allows you to download the book free of charge.  I admit I have by no means read it all, but it looks well written and very interesting.

Liebherr wine fridge filters

As any owner of a Liebherr wine fridge should know, you are recommended to replace the air filter cartridge every year.  But they are are not cheap: a couple of months ago I bought 4 for £87.64 including delivery from Coolectic Ltd, and you can currently buy one from Wineware for £26.50 plus £5.45 delivery.  When you are paying the price of a decent bottle of wine per fridge on filter cartridges every year, it makes you wonder exactly what the purpose of them is, and if you can do without them.  Strangely I cannot find anywhere in the handbook, or on the web, that directly explains what their purpose is.  There are just vague comments about the filter ensuring that the air remains at optimum quality.

Let me help Liebherr out.  I understand that it is important to keep air circulating through the fridge to prevent the build up of mould.  And given that the air has to be sucked into the fridge somewhere I would imagine that if the air were not filtered you would get a black mark on the bottles adjacent to the air intake.  I can also see that if the filters are not replaced regularly they will eventually clog up and reduce the air circulation.  But the filter cartridges also contain activated carbon.  The only purpose for activated carbon in an air filter as far as I can make out is to remove odours from the outside air, so I really struggle to understand why that is so important for a wine fridge.  If you would be happy storing your bottles outside the fridge from an odour point of view, why would you want to remove odours as the air is sucked in?

So my view so far is that filters may be useful, but I would question the need for activated carbon.  My cynical mind suspects that the activated carbon is mainly there to help justify the high price of the filter cartridges.  The other reason for paying the high price is that the value of the contents of a wine fridge will typically run to over a thousand pounds, and you would not want to ruin all that for the price of a filter, now would you?

Motivated by an enquiring mind and not a little stinginess, I decided to investigate what I was getting for the twenty-odd quid I was spending on a filter cartridge by sawing open an old one.  Here is the result:

At each end is a circle of filter paper that looks and feels for all the world like the filter paper you use in cooker hoods.  And the gap in between is filled with 3mm diameter pellets of activated carbon.  According to the wikipedia article the technical term is apparently extruded activated carbon.  So how much would it cost me retail to get the bits I really need to replace – the paper and the activated carbon?

The filter paper is the easy bit.  I could buy 2 sheets of cooker hood filter paper for £2.81 including shipping from Amazon.  That would be enough for over 150 cartridges.  Activated carbon is more tricky, not least because it is sold by weight, and the carbon in the filter is so light I do not have the equipment to weigh it.  However, according to this table the bulk density of activated carbon is 0.26g/cc and I calculate that I would need 31.4cc per cartridge, which would be 8.478g.  So in a kilo there should be easily enough for 100 cartridges, and I can buy a kilo of 3mm pellets for £9.85 plus £5.25 shipping, £15.10.  I think you can probably see the direction this is heading.  I could buy the contents of a Liebherr filter cartridge for around 20p retail, even including the activated carbon that I am not convinced is necessary.

So most of the £20-£25 obviously covers the cost of the plastic and manufacturing.  Oh, and profit of course.

All I need to do now is work out how best to reassemble an old cartridge with new contents.  I think the easiest thing is not to saw at the flange as shown above, but rather to remove and discard the other end.  Then I could insert a disk of cooker hood filter paper, add the activated carbon, and seal the end with more filter paper, holding it in place with tape around the circumference of the cylindrical bit of the cartridge.  Or I could simply forget about the activated carbon, and sellotape filter paper over the hole in the fridge, or hold it in place with a bit of an old cartridge.

The cult of the natural

For as long as I can remember in the relatively short life of my wine geekdom, the standard temperature advice for long term storage of wine has read something like: ideally around 12 or 14°C, avoid extremes of heat and cold and diurnal temperature variation, but seasonal temperature variation will cause no damage.

So as I was reading Jasper Morris’s “Inside Burgundy” over Christmas, I was rather surprised to see the idea that “Arguably [natural conditions that vary somewhat from Summer to Winter] is to be preferred: it reflects the rhythm of the earth.”  After my standard sceptical harrumph to such notions, I thought little of it.  Until I stumbled across an article by Steven Spurrier for The Wine Society in the Society News January 2011 where I read something similar: “Today (October 24th) my cellar is 10°C and will descend slowly to 6°C by January, rising to 16°C by August. I feel that such variation suits the wines, as a constant temperature cannot be natural.”  What’s going on here?  Since when did seasonal temperature variations not only be harmless, but become desirable?  Did Jasper plant the idea into Steven’s mind, or does the meme originate elsewhere?  Some biodynamical proclamation perhaps?  I bet there is no basis in empirical evidence.

In the absence of that hard evidence, what might science teach us about temperature and the ageing of wine?  Well, many different complex chemical reactions contribute to the maturing of wine.  Each one will progress at a rate that depends on temperature, and as it takes place the products of the reaction become available for other reactions.  Roughly speaking, if you increase the temperature by 10°C the rate of any chemical reaction will increase by a factor of two, so your wine will mature about twice as fast.  But it is not quite as simple as that.  Because the rate of each reaction will respond to the increase in temperature slightly differently, the paths taken by the reactions will never be the same for wine matured at different temperatures, and they will always result in a different wine.  And if you vary the temperature with time, you will get yet another wine.  How big these effects are I wouldn’t like to say.  You will certainly notice a wine maturing faster at a higher temperature, but the difference between a wine cellared at 5°C for 10 years and one matured at 15°C for 5 years is going to be more subtle.

And what is so natural about laying down bottles of wine in a cellar under a domestic house anyway?  Wouldn’t leaving them lying around outside be more natural, and make the wine even more subject to the rhythms of the Earth?  Does that mean the wine would be even better?  In fact maybe we should abandon the nasty industrial glass bottles and revert to goat skins.

OK, maybe I am reacting to a couple of offhand comments with undue sarcasm.  And I certainly don’t really want to single out Messrs Morris and Spurrier for criticism.  It is a trend in wine writing, and also in society in general, to glorify the natural – man-made artifacts and environments are rejected, and natural is treated as being almost synonymous with good.

Meanwhile I am wondering if I need to adjust the temperature of my Liebherrs for the seasons.  I don’t think I’ll bother, but I fear that future models might come with annual temperature programmes as standard.  Until then, 12°C it is.