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Rot, Rot, Utter Rot!

by Andrew McNaughton

Rotten to the core, a bit of a rotter, rot in Hell, noble rot ... ‘noble’ rot? The term ‘noble rot’ sounds just plain wrong, there can’t be anything noble about rot - can there? If you’re an enthusiast of fine dessert wines, you may however appreciate the strange love/ hate relationship between grape growers and the fungus Botrytis cinerea.

On the other hand, if you’re a strawberry grower there’s no love for B. cinerea, it’s all just hate. On a strawberry B. cinerea is nothing more than the crop-rotting grey mould (botrytis), it thrives in damp or humid conditions and can reduce a succulent red strawberry to sooty grey mush in days. Those quaint layers of straw often seen under strawberries aren’t for decoration, they’re for improving air flow and reducing the chances of B. cinerea gaining a foothold. How then does the grape grower tame this savage grey beast, or indeed, why?

When B. cinerea attacks a grape it dehydrates them, the result is a more intense sweetness and characteristic honey aroma in the wine. Get it right and the wine can command a dizzying price as a complement to fine dessert or sharp cheese. Get it wrong and you’ll have a lot of sympathy with a strawberry grower’s anguish. The grape grower plays a dangerous game for any mis-step in judgement.

The line between adulation and disaster really comes down to timing. If B. cinerea finds humid conditions and enters the grapes through an area of damage it’s game on. Grey mould spores are on the grapes virtually all the time, like ourselves the grape is protected from many pathogens by its skin. If its skin is breached, through insect attack or wind damage for example, the equilibrium with B. cinerea is disrupted and the fungus gains a foothold within the grape. High humidity often tips the balance in favour of B. cinerea, the commonest outcome then being a rampant invasion of the grapes and the loss of a once valuable crop. However ... if a humid spell of weather is followed by a drier period, the fickle grey mould can establish itself within the grape, but not overwhelm it. What happens then is the stuff of legend. The odds are however against the stuff of legend. Humidity must be between 85-95% and temperature between 20 and 25 degrees for botrytis to establish in the grapes. Following infection, humidity must plunge to below 60%, this effectively retains botrytis within the grape. At higher humidities the grape skin ruptures and all hell breaks loose in the grape as other opportunist pathogens invade. Being entrapped within the grape, botrytis dehydrates it and sugar content gradually rises to the point where it limits further fungal growth. The result is a sugary raisin-like grape, with a uniquely altered biochemistry; something to cause a skilled vintner to skip and clap their hands like a maniac.

The magic of making noble rot wine involves juggling an entirely altered biochemistry compared to normal wine. Organic acids such as malic and tartaric are metabolised and result in a higher pH. Complex polysaccarides are produced, including high molecular weight glucans which can cause filtration problems. Heteroplysaccarides are also produced which can inhibit yeast activity and therefore alcohol formation. The usual aromatics, such as terpenes, are destroyed and other aromatics, characteristic of noble rot, are produced. Some wine makers have been known to nurse their vats through the night, as they struggle to produce a wine under conditions which fight to destroy it at every step.

If they lose the battle, they may instead utter words more suited to the common form of botrytis: ‘vulgar rot’.
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