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Posted: Tuesday, 13 July 2021 10:36


TechTalk: Climate Change and Consequences of not Adapting

July 13: It has been widely known that the quality of grapes and the wine are related to climate yet a great majority of grow continue to carry on with their old practices as if nothing has changed and so the wine remains insufficiently stabilised and would not mature as it should, writes John Salvi MW, who explains why the finest red wines are rarely produced in the hottest and driest regions, which are becoming fewer and fewer and might include Bordeaux

There is nothing new about the notion that the quality of the grapes or wine is in relation to the climate.  As far back as 1946, the great Emile Peynaud (founder of modern oenology) postulated that the composition of the grapes depended upon the temperatures during the summer and the solar radiation (the totality of the electromagnetic waves emitted by the sun). 

He studied their effect upon the colloidal (a mixture or solution having particles in suspension) composition of the must (grape juice).  We now know for certain that many wine producing regions have become hotter during the summer vegetative season and dryer during the period of maturation and harvesting. Therefore, logically, the colloidal composition cannot be the same.  This is proved by the higher alcohol levels, the lower acidities and the greater concentration of polyphenols (tannins and colour).

In view of this it is both surprising and disappointing that a great majority of growers continue as though nothing has changed.  Alcohol is a strong solvent at above 20°C and more alcohol will extract more colloidal constituents, among them polyphenols and polysaccharides.  At high levels these constituents become very difficult to stabilise. 

Insufficiently stabilised, the wine will not develop and mature as it should.  It would be hard to clarify and will develop microbiological deviations, which will spoil the taste and smell of the wine.  All colloidal matter requires stabilisation and if not done the wine will precipitate tannins, colouring matter and polysaccharides into the lees, structurally and profoundly modifying the expression, structure, taste and smell of the wine.

Oxygen- Good, bad and ugly

We have a dichotomy regarding oxygen which is essential for phenolic stability, to fix the colour and allow the tannins to evolve and become silky and smooth (polymerisation).  BUT, oxygen is also the main cause of negative microbiological development leading to both taste and smell spoilage, particularly Brettanomyces (commonly referred to as Brett).  Naturally, the richer and more alcoholic the wine, the more oxygen it uses; so to avoid falling into this trap one must studiously avoid both over-ripeness and over-extraction.

Oxidation is one of the worst possible spoilages a wine can suffer.  It has 4 main results.

Loss of freshness of the fruit

Loss of elegance and complexity

Oxidation of some polyphenols with spoilage of colour

Microbiological spoilage

The result is abundantly clear on colour, taste and smell, but the process is extremely complex.  The higher the temperatures and the more persistent the sun, the greater the concentration of oxidases (any of a group of enzymes that bring about biological oxidation) and thus the greater the potential for oxidation, which leads to the above-mentioned microbiological degradation and thence to spoilage of taste, smell and colour as well as complicated enzymes like laccase and tyrosinase, which promote oxidation but are inhibited by CO2. Also, sotolon (a very powerful aromatic compound with a smell of fenugreek or burnt sugar at a lesser concentration) and quinones (oxidised derivatives of aromatic compounds).  Oxidised polyphenols produce quinones that degrade the colour.  Too much copper from successive treatments also adds an oxidative factor.

Water resources and evapotranspiration are becoming vital issues, particularly regarding the evolution of pH through the increasing concentration of potassium caused by very hot weather, leading to tartrate precipitation that is not easily stabilised and is almost systematically accompanied by phenolic precipitation and microbiological spoilage.  This is important because many of the above damaging factors are inhibited by CO2, and this cannot be kept at a sufficient level if the pH is too high (3.8).

All the above explains why the finest red wines are rarely produced in the hottest and driest regions, which are becoming fewer and fewer.  Bordeaux may become one of those regions eventually, but now is the time to adapt and there is a great deal that we can do to avoid all the above dangers. It may also explain the paradox in India.

John Salvi, Master of Wine


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