Ecology of A.carbonarius and ochratoxin A production in vine fruits and control in the production chain

Abstract

This study examined black aspergilli, especially A. carbonarius and A. niger and ochratoxin A (OTA) contamination of grapes, during drying and industrial processing of dried vine fruits. This was complemented by studies on potential control using preservatives and physical factors such as modified atmospheres. Fungal population kinetics were determined in relation to grapes at harvest, and during drying at three different altitudes (sea level: 0-200 m; medium level: 250-500 m; high level: >500 m) in two seasons. At pre-harvest, A. niger aggregate species were the predominant fungal species while A. carbonarius was occasionally isolated, in both years studied. Both altitude and bunch position affected black aspergilli population dynamics. Overall, they were increased during drying. However, both black aspergilli groups were mostly isolated, at low and medium altitudes (<500 m). OTA contamination was influenced by bunch position, although altitude did not significantly influenced amounts. The fungal biodiversity was decreased during sun-drying of sultanas. The widest diversity of species occurred at the sea level. However, A. niger aggregate, were dominant during drying. Time of drying and altitude significantly influenced fungal loads of black aspergilli. In contrast, OTA production (ca 0.001 - 0.0025 μg g - 1 ) was not significantly influenced by altitude and drying time. Shannon Index of Biodiversity (H), for pre-harvest and pos-harvest studies, was determined for the first time. A. niger aggregate (ca 5.0 Log10 CFUs g -1 ) was predominant during industrial processing, while A. carbonarius was only isolated at low levels (1.5-2.0 Log10 CFUs g -1 ). Heat treatment (up to 90 o C) appeared to be the key-procedure for the elimination of fungal populations. In the contrary, SO2 treatment did not statistically alter fungal population dynamics. OTA contamination was not significantly affected by industrial processing. In vitro studies conducted on both White Grape Juice Medium (WGJM) and in sultanas with strains of A. carbonarius originated from Cretan sultanas and compared with a strain isolated from Italian wine grapes. They examined the impact of sodium metabisulphite (NaMBS), elevated CO2 (up to 50%) concentrations and aw levels, on black aspergilli spore germination, growth and OTA production. Moreover, fungal interactions in vitro and in situ were also investigated. In general, spore germination occurred over a wide range of sodium metabisulphite concentrations, although germ tube extension was significantly controlled. At ≥ 750 mg L -1 NaMBS, no spore germination was observed while both mycelial growth and OTA production were completely inhibited. Medium concentrations of NaMBS (≤ 250 mg L -1 ) enabled optimum spore germination, growth and OTA production (x 0.965 aw). The efficacy of controlled atmospheres x aw showed that there was very little inhibitory effect on spore germination. However, both germ tube extension and fungal growth were inhibited by 50% CO2. After 10 days, growth was not as effectively controlled. Aw had a bigger effect on OTA production than modified atmospheres. In situ experiments on sultanas confirmed these results. Competition and dominance of A. carbonarius over other fungal species showed that aw and temperature influenced Indices of Dominance and OTA production. In vitro and in situ, OTA production by A. carbonarius was significantly influenced by the fungal competitor used.