Abstract
Drying optimization, to mitigate fungal growth and Ochratoxin A (OTA) contamination is a key topic for raisin and currant production. Specific indicators of environmental conditions and drying properties were analyzed using two seedless grape varieties (Crimson—red and Thompson—white), artificially inoculated with Aspergillus carbonarius under open air and tunnel drying. The air temperature (T), relative humidity, grape surface temperature (Ts) and water activity throughout the drying experiment, the grapes’ moisture content and the fungal colonization and OTA contamination during the drying process and their interactions were recorded and critically analyzed. Drying properties such as the water diffusivity (Deff) and peel resistance to water transfer were estimated. The grapes Ts was 5–7 °C higher in tunnel vs. open air–drying; the infected grapes had higher maximum Ts vs. the control (around 4–6 °C). OTA contamination was higher in tunnel vs. open air–dried grapes, but fungal colonies showed the opposite trend. The Deff was higher in tunnel than in the open air–drying by 54%; the infected grapes had more than 70% higher Deff than the control, differences explained by factors affecting the water transport. This study highlighted CFU and OTA indicators that affect the water availability between red and white grapes during open air and tunnel drying, estimated by the Deff and peel resistance. This raises new issues for future research.
Highlights
Grapes are susceptible to insect attack and fungal diseases, especially black and grey rot, downy and powdery mildew
Damaged grapes are vulnerable to further diseases such as summer bunch rot, which may be caused by Aspergillus niger, Alternaria tenuis, Cladosporium herbarum, Rhizopus arrhizus, Penicillium spp. and other fungi
The infected grape berries had higher maximum surface temperatures compared to the control grape berries especially in the case of open air–drying (Table 1)
Summary
Grapes are susceptible to insect attack and fungal diseases, especially black and grey rot, downy and powdery mildew. Damaged grapes are vulnerable to further diseases such as summer bunch rot, which may be caused by Aspergillus niger, Alternaria tenuis, Cladosporium herbarum, Rhizopus arrhizus, Penicillium spp. and other fungi. The critical factors that affect fungal growth during farming, harvesting and storage are temperature, moisture content (water activity—aw) and time that product remains under favorable conditions for fungi (integral of time–temperature–water activity). Other factors are the presence of fungal spores, mechanical damages (the inner area in vegetables is more susceptible to fungal invasion than the external due to lack of protective peel), insect with piercing–sucking mouthparts (with their metabolism increase substrate’s moisture and temperature and break the protective external part of the plant), storm and rain damages, moisture stress, mineral nutrients deficiency, pH, O2 and CO2 levels, chemical and physical treatments and for some commodities, the product drying and re–wetting speed. The region of grape origin can greatly influence OTA contamination showing a positive gradient moving from west to east and from north to south in Europe [5]
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