Dr Rainer Jung is a Professor at the research center of the University of Applied Science in Geisenheim, Germany – one of the world’s leading viticulture and oenology institutes. Dr Jung specializes in the sensorial evaluation of wine, prevention of faults in wine and oxygen management. Through several long term trials for the cork industry he has accumulated in-depth knowledge of the impact of storage conditions. Below we discuss optimum storage conditions for wine and what we know (and don’t know) about the science of wine ageing.
What factors have a negative impact on wine during storage?
The most problematic influence is temperature. The Van’t Hoff factor clearly shows that at an increase of 10°C the chemical and enzymatic processes are accelerated two or threefold. We have run trials where we stored the same wine at 15°C and 25°C, and results showed that higher temperatures have a negative impact on the wine – especially the SO2 content.
Light has less of an impact. Nevertheless, there are a few wines that can get damaged by light, and there are important factors to consider such as the colour of the glass, the length of exposure and the type of light. Modern neon lights have a far lower UV light content and are therefore less damaging to the wine.
I think that in most cases the effect of light gets confused with the effect of temperature as they often go hand in hand. When wine is exposed to sunlight for too long it will heat up. We have done several trials on the effect of light on wine in the laboratory, where we exposed wine to light while maintaining a consistent temperature. We’ve come to the conclusion that temperature has a much stronger impact on the wine quality rather than light, unless the wine is very sensitive and within a clear glass bottle.
What is the impact of high and low temperature on wine?
If wine is exposed to heat then its colour will change from yellow to brown. The aroma also changes. Our sensorial trials have shown that the wine loses fruit aroma. In some cases, this can be perceived as positive for young and unripe wines, because they get softer and develop mature aromas. After a while these develop into intense ageing notes that are less pleasant.
We have not found any reasons to think that low temperatures have a negative impact on wine. There is little evidence to suggest that there is a significant impact on wine if it is stored at 10°C or 7°C, but it would be interesting to see whether this could increase shelf life.
Does the ability of wine to take up more oxygen at much lower temperatures cause problems when it subsequently gets heated up?
It does, but how the wine reacts to oxygen depends very much on the way it has been treated in the winery. Wines that have been exposed to oxygen during the vinification process will be much more stable than reductively made wines. When talking about these topics it is very important to remember that wines vary significantly in their ability to age.
Are fast and strong changes to a wine’s environment more damaging than long term changes of temperature?
We have not done any trials on this but it would be interesting investigate whether a wine that gets exposed to 60°C for two weeks shows the same changes as a wine that is stored at 30°C for four weeks.
Physical damage on bottled wines is a different question. If a wine gets exposed to excessive heat it expands. It might then press out the cork and wine would leak out of the bottle. When it gets cooled down a vacuum develops and sucks the wine out of the cork and into the bottle. Further oxygen will come into the bottle, changing temperatures, and will create a different pressure (a “pumping” effect). This will generally have a negative impact on wine quality.
Bottles which have lost wine will also have a larger head space which might lead to faster oxidation, but this very much depends on the wine’s stability and its ability to cope with oxygen.
Humidity is often discussed when one talks about wine storage conditions: what is its impact?
This topic is frequently discussed with regards to keeping the cork moist, but high humidity from the outside is not required as the cork is moisturized by the wine inside the bottle. Even keeping bottles upright does not reduce the humidity of the cork too much. The humidity of corks in horizontally stored bottles is higher, but sometimes too high, and leakage can occur – this does not happen in vertically stored bottles. The cork’s elasticity is maintained by the wine from the inside and (when the wine is stored vertically) by the position of bottles.
High levels of humidity can damage the wine’s packaging. It can lead to the development of mould below the capsule, on the labels and on cartons. Once humidity is significantly higher than 60% the risk of damage to the packaging is increased.
What happens to wine as it ages?
The aromatic composition of the wine changes: fruity esters are reduced as other flavours develop. The SO2 content is reduced. In particular, the amount of free SO2 [the SO2 that actively reverses oxidation processes] gets less and less. This means that the wine is more susceptible to oxidation. It changes the aroma from fruity to caramel-like, then on to Sherry and a bread-like aroma.
The colour also changes. In red wines sedimentation occurs because molecules become too big and precipitate in the bottle.
Which wines are more likely to change in quality?
During our trials with several closure companies we have made the observation that Riesling is particularly robust during maturation! In long-term trials Riesling showed very good ageing ability. Sauvignon Blanc, on the other hand, is less robust. When it comes to red wines, it is those with higher concentration and more tannins that are robust and have an ability to retain their quality for longer.
In your opinion, what are the ideal storage conditions for wine?
In order to establish an optimum it is important to consider energy and environment: you don’t need to cool wines more than necessary to maintain their quality. In the experience of the research institute in Geisenheim, a temperature between 15 and 18°C and a humidity level of 60% can be an ideal compromise between storage quality and energy input. These are the conditions in which we store wine at the institute.
What are your experiences with the quality of storage and transport conditions in the wine trade?
We do come across issues when we do trials for claims, especially when it comes to sea freight transports without temperature-controlled conditions and when the wines aren’t kept under deck. In those cases corks have been pushed out or the wine has been negatively affected due to the wrong handling.
In general, my experience in the wine trade is rather positive. It seems to me that wine merchants make sure that conditions are adequate.
Why do you think are there so many misunderstandings when it comes to this topic?
You can see articles being written about this topic without any scientific background. A topic such as vibration is being discussed intensely. You can find many who say that it can negatively affect the wine but I am not aware that this has actually ever been scientifically researched. It’s not only amateurs that make this mistake: scientists sometimes tend to accept certain statements without second guessing them and this can obviously lead to confusion.
As a final question: Do you think that the 2012 vintage will be any good in Germany?
There are good but not excessive yields and the grapes look healthy. The next weeks will be crucial as winemakers will soon have to stop spraying fungicides. If it gets humid now we might be in trouble. For the moment it looks very good and the dryness and hot weather in Germany helps to get a good and healthy ripening for our grapes.