Impact of varying degrees of powdery mildew infection (<em>Uncinula necator</em> (Schwein.) Burrill) on the volatile compounds of Chardonnay grapes, must and wine

Abstract

<p style="text-align: justify;"><strong>Aims</strong>: The aims of this study are (i) to trace the major components responsible for the unpleasant mushroom, earthy and geranium odour in the transformation from grapes to wine in response to powdery mildew infection intensities, (ii) to determine the impact of an aggressive infection on volatile compounds in grapes and wine, and (iii) to determine their effect on the olfactory-taste evaluation of the resulting Chardonnay wine.</p><p style="text-align: justify;"><strong>Methods and results</strong>: A GC-MS technique was used to investigate the impacts of different degrees of oidium infections (0, 10, 50 and 100% of infected grapes) on the presence of volatiles in grapes, musts and wines. The number of volatiles was higher in oidium-infected grapes than in noninfected grapes, especially for terpenes, aldehydes and alcohols, with 1- octen-3-ol, 2-octen-1-ol, 2-heptanol and 2-octenal, the volatiles reminiscent of mushroom-, geranium-, and earthy-like odours, being detected in oidiuminfected grapes only. After overnight clarification, 1-octen-3-ol and 2- heptanol but also methyl-2-furoate were the only mushroom reminiscent volatiles detected in the musts. The infection also caused higher amounts of acetic acid, furfural and some of its derivates (2,5-furandicarboxaldehyde, 5-hydroxymethyl furfural, furaneol), most ethyl esters (except ethyl caprylate), 1-hexanol, maltol and 1-decanol in musts. After alcoholic fermentation the number of esters in young wines increased, whereas that of aldehydes decreased, especially in the wines produced from severely infected grapes. Though it was not detected by sensory analysis, we assume that the intensification of the odours reminiscent of pineapple, banana and coconut in the aged wines produced from infected grapes was likely a consequence of the increased amount of esters.</p><p style="text-align: justify;"><strong>Conclusion</strong>: Powdery mildew infection statistically decreased the freshness but enhanced the malty, sweet mouthfeel of Chardonnay wine, and neither the mushroom odour nor taste were perceived, which contradict some previously published findings.</p><p style="text-align: justify;"><strong>Significance and impact of the study</strong>: This study gives an answer to the significant problem of wine production from oidium-infected grapes. It reports for the first time the traceability of volatiles highly related to mushroom, earthy and geranium odours from the Chardonnay grapes to the wines. The overnight must clarification decreased the impacts of unpleasant volatiles on must and wine odour. This suggested that the oidium infection of the grapes does not necessarily hinder the production of wine, as no unpleasant odour could be sensory evaluated, not even in the wines made from highly infected grapes.</p>