Physico-chemical properties of botrytised Chenin blanc grapes to assess the extent of noble rot

Abstract

The production of botrytised wines involves grapes with high sugar concentration and complex aromas caused by the presence of noble rot. The aim of this paper was to characterise botrytised grapes on a physico-chemical basis and therefore identify markers of its evolution. Colour analysis, compression and puncture tests and phenolic profiles were performed in order to identify the potential markers and discriminating tools for the presence of the fungus. Chenin blanc grapes from two parcels in Loire Valley (France) were sampled during a 4-week period. Significant changes of phenolic composition like the total polyphenol which decreases up to 40 % were observed as B. cinerea was more advanced on the berries. Myricetin a compound that has not been reported before in white grapes was found only in highly affected berries. At the same time as the level of infection increases, the presence of the fungus seems to induce, from one side, an increase of catechin, epicatechin and epicatechin gallate, and from the other side, the decrease in caftaric acid, coutaric acid, quercetin-3 glucoside and kaempferol-3 glucoside. Puncture tests appeared to be the more reliable mechanical tool to differentiate between infection levels by itself; however, the combination of puncture and compression tests with colour showed a more significant and clear differentiation. This combination appeared to be the most adequate for stablish markers of noble rot evolution in the conditions studied. A decrease in mechanical resistance of berries’ skin (measured by the maximum puncture force required for skin rupture) was observed during infection. These results on botrytised Chenin grapes are new and provide a novel approach for an objective measurement of noble rot in Chenin grapes.