Storing wine: the effects of temperature and humidity  

(This article is modified from a piece I published in Harpers' 2005 Warehousing and cellaring supplement)

In researching this piece, I came across a recurring theme: the science of wine ageing is poorly understood. Surprising as it seems, no one has really researched the effects of various environmental variables – such as temperature, humidity, light and vibration – on the ageing trajectory of wine. But if we look a little closer, potential reasons for this void become apparent. First, fine wine takes a long time to develop, in the order of years and even decades. That’s a long and rather boring timescale for an experiment – you’d start it at the beginning of your career and write it up in time for your retirement party. Second, fine wine is expensive, and by the time you have enough replicates to achieve statistical significance, and you have investigated a range of different parameters, you’ve got through a lot of costly wine. Third, there are many different ‘types’ of fine wine, and you’d really have to do your experiments on representative samples of each. That’s more expense. Which leads to the big question, who’s going to pay for all this research?

As a result, we can guess at which are the crucial environmental variables for successful ageing of wines, but we can’t know for sure in the absence of solid data. What we do know, however, is that wine ages the way we like in good old-fashioned underground cellars. These offer a constant year-round temperature of around 11 ºC together with dark, rather damp, vibration-free conditions. So if we are thinking about storing wine for any length of time, these are the conditions we should be aiming at. It’s the gold standard of wine storage, around which the current ‘system’ of fine wine has developed.

However, what professionals involved in storing wine commercially would really like to know is how playing with these variables affects the wine in their care. Clearly, to maintain a warehouse at 11 ºC is going to be more expensive than, say 16 ºC, and both options are more expensive than a storage space that has a low ambient temperature and which shows only a small fluctuation with seasonal variation. Does variation in temperature matter, and how will wine be affected by being kept at higher temperatures than a classic cellar? And is humidity important? There are clearly practical implications associated with keeping a warehouse as humid as a cellar. In the absence of experimental data, the best attempt to answer these questions we can make is to cobble together a guess based on practical experience, scientific principles and our relatively limited knowledge of the chemistry of wine ageing.

Of course, it’s only a tiny fraction of wine that is kept for more than a couple of years. Most wine is only warehoused for a short while – a brief-as-possible stop-over in the supply chain. The big question here is how fussy do we need to be about the storage conditions for short turn-over wines? If you ask people in the trade about some of the warehousing conditions big brands are subjected to, horror stories are common. One commentator remarked that ‘all sorts of shitty sheds are being used for warehousing: because of the need to save money in the supply chain, warehousing isn’t on the agenda’. While we can guess that large temperature fluctuations and extreme heat and cold are going to damage wine, it would be helpful to have guidelines about the sort of tolerances should be permitted for safe short-term storage of fast-turnover wines to ensure they are reaching the consumer in good condition. I’m beginning to repeat myself here, but in the absence of data, we’re just making educated guesses.

Temperature
Everyone seems to agree that temperature is the crucial variable. It’s likely that relatively few wines are spoiled by low humidity or light exposure, but many are ruined through storage at high and/or variable temperatures. Again, though, we don’t know how hot things can get before wine is damaged. What is known is that the rate of chemical reactions increases with temperature, and not just in a linear fashion. That is, doubling the temperature doesn’t just double the rate of the reaction; in fact, as a rule of thumb the speed of chemical reactions doubles for every 10 ºC rise in temperature.

However, this doesn’t just mean that wine ages faster along the same trajectory when the temperature is higher. If it was as simple as this, we’d be able to use temperature to hurry our fine wines along so that they are ready for drinking sooner. The problem with elevated temperatures is that reactions take place that wouldn’t take place otherwise, and these are almost always undesirable.

The other impact of temperatures is of variation, either over the short term (e.g. diurnal variation) or longer term (e.g. seasonal). Temperature variation is likely to be more critical for older wines where the seal of the cork is less tight (corks lose some elasticity with age). The problem with temperature swings is that the volume of the wine expands as temperature rises, decreasing the ullage volume, and then contracts as temperature falls, increasing the ullage volume. If the closure is providing a completely airtight seal then this is of relatively little consequence. But closures don’t, generally. Thus the change in ullage volume increases the oxygen transmission by mass transfer of gas.

Oxygen transmission is important in wine ageing, which is partly oxidative (I should add here that this is a subject of current controversy). Recent studies have made it clear that a very low but measurable rate of oxygen transfer is needed for successful ageing of red wines. [We are referring here to incredibly small amounts, so it’s wrong to talk of wine ‘breathing’ through the cork.] This transfer occurs through the interface between the cork and the inside of the bottle neck, a fact that can be demonstrated by sealing the uppermost interface between the cork and glass with a ring of oxygen-impermeant Araldite (an epoxy resin). This transfer normally takes place by diffusion, which can work against a pressure gradient (hence, counter-intuitively, this oxygen transfer also occurs into the pressurized contents of a Champagne bottle). It’s worth pointing out here that the capsule (whether it is metal, plastic or wax) does not prevent oxygen transfer. As corks age over decades they lose their natural elasticity and although no data exist, it is almost certain that their seal becomes less tight. This makes the possibility of mass transfer of gas due to pressure changes caused by swings in temperature much more likely, and this may well cause ingress of harmful quantities of oxygen.

This leads to another subject relevant to this piece, which is ullage level. It’s considered by many to be a reliable indicator of the past history of fine wines, and hence the wine’s condition. Low ullage level by its very existence indicates that more has come in and gone out of the bottle than a higher ullage level, yielding information about the condition of the cork and the variation in temperature the wine has been exposed to.

But the signs of neglect aren’t limited to transport of the wine. A retail shop I visited last week in central London was uncomfortably warm on a modestly sunny day in May. Goodness knows what the temperatures are like in mid-summer. The staff cool themselves with fans – if it’s too hot for people, it’s too hot for wines.

Of course, consumers themselves do a good job in cooking wine. Storage at ambient conditions, such as in closets or under the stairs, isn’t ideal, but I suspect most of the time UK residents aren’t going to be killing their wine, as long as it isn’t kept this way for too long. But the prospects for any wine stored ambiently during the freakishly hot summer of 2003 aren’t great.

Humidity
Why is humidity important? As discussed above, when corks dry out they lose elasticity. When wines are stored lying down the cork is in contact with wine at one end. Storing wines in humid conditions is thought to be beneficial because they prevent the other end of the cork drying out. But herein lies a dilemma: these high levels of humidity are damaging to labels and packaging. If you are cellaring wine for your own consumption and you aren’t the sort of tart who likes to stroke labels, then you’ll be delighted with the damp conditions of underground cellars because they are optimum for maintaining the condition of the corks, which are the guardians of wine quality. Replicate these damp conditions in a commercial warehouse and you’ll have complaints from customers, in particular from fine wine merchants who have to sell fine wine to the American and far-East markets, where unblemished labels are demanded by most collectors. Interestingly, this aspect of fine wine storage – the preservation of the label – has recently gained greater significance as fine wine brokers are increasingly relying on detailed digital photography of bottles as a prelude to a sale: the client can see the ullage level and the condition of the label, removing a lot of the subjectivity from the transaction.  

Controlling wine storage conditions~
How do the various commercial warehouses and cellars control environmental variables? If you were under the impression that in bond warehouses were kept at a constant 11 ºC, thus closely replicating the conditions of a classic underground cellar, then it’s time for that illusion to be shattered. 

‘Warehousing companies often claim that their warehouses are temperature controlled’, says Jeremy Pearson of London City Bond (LCB), ‘but there are no temperature controlled warehouses around: they all rely on the fabric of the building to maintain a constant temperature.’ The reason is simple: the air conditioning needed to maintain a steady 11 ºC would be prohibitively expensive in a big warehouse. ‘Temperature in our warehouses moves seasonally’, says Pearson. ‘In some there is the best part of 6ºC variation.’ [It should be added here that the claim about the lack of temperature control is disputed by others. Marcus Titley of Seckford Wines points out that their warehouse is actively temperature controlled; Seckford however only store their own stock and that of their private customers.]

London City Bond have 21 warehouses in all, but there are three large London-based ones, in Barking, Silvertown, and Tilbury (the largest, at 420 000 square feet). Barking and Tilbury were originally built as chill stores, which means they are well insulated. They also still have functional cooling facilities. ‘We could haul down the temperature to whatever we wanted’, says Pearson, ‘but this would cost a lot, and the wine trade won’t pay for it.’ Although the temperature in these units is not actively controlled, the maximum last summer at Tilbury, for example, was 16 ºC. Long term reserves are usually kept at LCB’s Vinoteque warehouse in Burton-on-Trent, which was purchased a year ago from Lay & Wheeler: this is built on a grain store and shows the most static temperature of all LCB’s sites, with only a small seasonal variation.

Pearson points out that small day-to-day variations aren’t likely to be problematic because of the buffering effect of wine packaging. ‘We have put sensors inside cartons and wooden cases and found out that you have to work quite hard to move the temperature,’ he explained.

Octavian is one of the most well known warehousing facilities in the wine trade. It’s an underground facility formerly owned by the Ministry of Defence, and was originally used for storing and making armaments. Because the storage area is 90 feet underground, it sits at a steady 13 ºC all year. The problem here is the high humidity: great for cellaring unlabelled wines, but under these conditions labels would deteriorate rapidly. ‘We have had to spend a lot of money making it fit for purpose’, explains Octavian’s Jeff Stanton. ‘We have a computer system that monitors the temperature and humidity, both below ground and above ground’. He explained that when these parameters need adjusting below ground and the conditions are right above ground, air is then shifted in. Interestingly, Stanton thinks that the market for cellaring fine wine is likely to expand in the UK in the next few years. ‘More and more people are investing in wine, and from April 2006 the rules will change allowing people to put wine into their SIPPs [self-invested personal pensions]: we’re anticipating quite a demand for this.’  

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