Ochratoxin A in overripe grapes, raisings and special wines in vitro and in vivo studies on fungi isolated from grapes and raisins affected by physical, chemical and biotic agents.

Ochratoxin A (OTA) is a fungal secondary metabolite produced by Aspergillus and Penicilliumspecies. It has been detected in a wide range of commodities, including cereals, coffee, grapes, raisins, must and wine. Within grape derivative products, the raisins, red wine and sweet wines have reported to contain the highest OTA levels. Aspergillus section Nigri (A. niger and A. carbonarius) are considered the OTA source in these commodities and they are commonly isolated among other fungi from grapes and raisins. Starting from this basis the objectives of this thesis were focused into three main aspects: (1) Evaluation of the food products: vine dried fruits and special wines, concerning the mycobiota and OTA occurrence and incidence; (2) Ecophysiological studies of the ochratoxigenic fungi and accompanying mycobiota as affected by environmental conditions; (3) Control and preventive methods such as the evaluation of residual activity of pre-harvest fungicides during grape dehydration and the use of modified atmospheres. Wine origin and winemaking procedure showed to be determinant for the final OTA content. All special wines analysed from northern European regions were negative for OTA while more than 50% of wines from warmer regions were positive for OTA contamination. The wines with higher OTA levels were fortified musts followed by those made from dried grapes. Acoholic and malo-lactic fermentations, biological ‘crianza’ (Flor yeast) and the action of Botrytis cinerea in noble rot of grapes may diminish the OTA levels in wine. In grapes, the presence of Aspergillus section Nigri became predominant at harvest and mainly during sun-drying. Prevalence of Aspergillus section Nigri can be explained by their adaptation to environmental conditions of sun-drying, and by their ability to dominate other fungal species involved when coming into contact with them. Among the Aspergillus section Nigri, A. nigeraggregate was dominant, although A. carbonarius increased its incidence in dehydrated grapes and showed a higher potential for OTA production, close to 100% of strains were ochratoxigenic, it being optimal at 20ºC and high water activity. The complex fungal interactions studied in vitro and during grape dehydration may act as a control factor in some cases while other few fungal species stimulated OTA production. Integration of these findings on the effects of temperature, water activity and competitive abilities on the fungal spore resistance to ultraviolet radiation (UVC) provides a good explanation to the incidence of fungi found on both fresh and sun dried grapes. The ultraviolet light could also be considered as an alternative control treatment given that 10 min of UVC irradiation on fungal spores were enough to reduce almost the 100% of their viability. Regarding the residual effect of pre-harvest fungicides, it was observed that they remained active in grapes during further dehydration, so they could be effective in reducing OTA production by black aspergilli. The application of modified atmospheres packaging showed differential results in controlling fungal growth and OTA production, combination of 15% CO2with 1% O2 being the most effective treatment. However, to minimise the risk of OTA contamination during grape storage, modified atmospheres should be used in combination with low temperatures or with other preventive measures. Finally, the application of chemical or biological treatments to grape products, other than those intrinsic to the winemaking process, should be considered as the last option. Attention should focus in preventive rather than in corrective actions. A suitable crop management followed by a smart post-harvest processing of grapes and also during winemaking should comprise enough cautions to obtain a safe final product.

Full-text available at: http://hdl.handle.net/10803/8384