Isolation and identification of ochratoxin A-producing Aspergillus section Nigri strains from California raisins

To determine incidence and levels of ochratoxin A (OTA) in California raisins and to isolate and characterize OTA-producing fungi from California raisin vineyard populations.

Forty raisin clusters sampled from four California vineyards in the San Joaquin Valley were analysed for OTA content using immunoaffinity and HPLC methods. OTA was detected in 93% of the samples, at levels from 0·06 to 11·4 ng g⁻¹. From these raisin samples, a total of 400 strains of Aspergillus were isolated and analysed for OTA production. Twelve isolates (3%), from five raisin samples, produced OTA. These isolates were identified as Aspergillus carbonarius, based on morphological characteristics and multilocus sequence analysis. Levels of OTA produced by these isolates on raisin agar ranged from 0·9 to 15 μg g⁻¹.

OTA is a common contaminant of raisin vineyards, but average levels are much lower than EU regulatory limits for dried fruit. The primary species responsible for OTA contamination in California raisins is A. carbonarius.

This study illustrates that low-level OTA contamination of raisins occurs in California and that ecological studies of A. carbonarius within the Aspergillus section Nigri population on raisins are warranted to monitor ochratoxigenic potential of the crop.