Botrytis - affected grapes produce some alluring wines!
- Friday 16 January 2009
Of all the moulds and rots that attack grapes hanging on the vine, Botrytis cinerea
is perhaps the most ghastly. The grape berries shrink and shrivel, and become covered with an ugly, ash coloured mould.
If someone handed you a bunch, you’d most likely toss it in the bin and quickly wash your hands. But you’d be better off tossing the infected bunches into a wine press instead. When you press them, it can take two or three hard pressings before a little golden liquid runs out.
But after a sluggish, difficult fermentation, the resulting nectar becomes the grapevine’s love song to the human beings who look after them. Sauternes, Tokaji, Trockenbeerenauslese
and other botrytised dessert wines all start out as grotty bunches of rotten grapes.
So how in the world do the aromas and flavours of honey, apricot, peach, caramel, toffee, dried flowers, passionfruit, mango, marmalade and quince – one could go on and on – arise from these disgusting-looking grapes?
Botrytis can be destructive or beneficial. When it ruins a crop of grapes in a protracted period of wet, cool weather, it’s called bunch rot, or grey rot (left). But when wet mists at night are followed by dry, sunny afternoons, it works its magic and utterly transforms ordinary white grapes into, as the Hungarians put it, ‘the king of wines and the wine of kings’.
When spores of botrytis fungus that land on grape berries germinate, they grow filaments, whose tips exude an enzyme that dissolves tiny holes in the berry’s skin (above) and allows the filaments to work their way inside (overleaf, top right).
They also wiggle their way through any cracks in the skins. Once inside the berry, the fun begins…
The filaments cover the skins and make millions of spores (right). More than half the water inside each berry is lost to the rot, and as the berries shrink, the sugar inside doubles in concentration. The rot metabolises the acids, along with a bit of the sugar. The result is very sweet juice with lip-smacking acidity.
As the grape rots (left), botrytis digests its sugar and acid, and excretes glycerol, which contributes to the silky mouthfeel of the wines. It also injects an enzyme – laccase – into the berries, that turns the juice a lovely golden colour, and reacts with tannins and other phenolics to reduce bitterness and astringency – with amazing results.
Laccase is also an oxidising enzyme, and as the sugars oxidise, they produce a stunning range of honey, apricot, and caramel flavours.
The process of botrytis leaves behind many other compounds in the juice, including acetic acid, gluconic acid, pectinase, and its own antibiotic, botryticine. In the past, botrytised wines were used as a remedy for all manner of ailments – and when you taste a Sauternes or other botrytised dessert wine, you can understand why.