Aszú on the vine
At least three conditions are necessary for the aszú berries, the botrytised, noble shrivelled grapes: damp weather to start the fungal infection on totally ripe, heathy, undamaged grapes, a few days sunny/rainy-damp weather, then a long dry period.
Botrytis cinerea partly overwinters in the form of sclerotia, infecting the flowers in a latent manner; partly as conidium it moves from one berry to another. The berry skin has micro-damage that cannot be seen with the naked eye. The fungal hyphae can easily adhere to and then get under the skin. The plant cell walls change due to the fungal activity; the berries become brownish-blue, chocolate colour.
The berry loses lots of water through the damaged berry skin which is loosened by enzymes, and thus valuable materials concentrate in the grape.
Photo: Zsolt Tátrai
Photo: Zsolt Tátrai
Noble rot results from the combination of the biochemical processes of Botrytis with the physical changes of drying and shrivelling of the grape on the vine.
Thick skin and open, loose bunches favour the development of aszú berries. Under these conditions the fungal growth is limited and the fungal metabolism is regulated.
Chemical changes and effects on winemaking of Botrytis noble rot
The metabolism of the fungus results in a 34-45 % loss in absolute sugar. Despite this, significant increase in sugar concentration can be seen. Since the Botrytis prefers glucose to fructose, the glucose:fructose ration drops under 1.
