Analysis of off flavours in grapes infected with the fungal bunch rot pathogens, Aspergillus, Botrytis and Pencillium
Abstract
Fungal bunch rots of grapes cause major losses to grape yield worldwide, yet the impact these moulds have on grape and wine quality is not well characterised. We sought to investigate the formation of unwanted volatile compounds of fungal origin in both synthetic grape juice culture media and in inoculated grape berries. Botrytis cinerea, Aspergillus niger, Aspergillus carbonarius, or Pencillium expansum were grown in synthetic grape juice medium and the culture homogenates analysed 4 and 7 days post inoculation. HS-SPME-GC-MS analysis of the culture homogenates 4 days post inoculation demonstrated that each of the fungi examined produced varying quantities of the mushroom or fungus-like aroma compounds, 1-Octen-3-ol, 1-Octen-3-one and 3-Octanone with A. carbonarius producing up to ten times the amounts of all three metabolites per mg of dry mycelium. Geosmin, an off-flavour commonly associated with musty or mouldy aromas was only detected in the P. expansum culture 4 days post inoculation. Low levels of geosmin were also detected in the A. carbonarius culture 7 days post inoculation. Methylisoborneol (MIB), another metabolite associated with musty aromas was identified only in grape juice medium inoculated with B. cinerea. Detached surface-sterilised Vitis vinifera (cv. Chardonnay) berries (13.5 ⁰Bx) were inoculated by placing 104 fungal spores on to the apex of each berry. The concentration of 1-octen-3-ol was significantly higher in grapes inoculated with either A. carbonarius, or Penicillium expansum (range 204 – 850 ng/L) than in grapes inoculated with A. niger or B. cinerea (24.8 and 34.5 ng/L respectively) five days post-inoculation. Berries infected with A. carbonarius had the highest concentrations of methylisoborneol. Elevated levels of 1-octen-3-one were also observed in all inoculated berries. Berries inoculated with A. carbonarius had significantly higher levels of gluconic acid (16.3 g/L) compared to the other fungi (range 0.53 – 1.62 g/L). The results indicate that different fungal pathogens produce a similar range of off flavours but the relative proportions when expressed on a dry fungal mass basis when expressed with respect to ergosterol, v ary. This may in turn influence the sensory properties of wine made from different batches of bunch rot affected grapes.
Issue: Macrowine 2016
Type: Poster
Authors
*NWGIC