Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 30, 2010
Publication Date: April 1, 2011
Citation: Palumbo, J.D., O Keeffe, T.L., Vasquez, S.J., Mahoney, N.E. 2011. Isolation and identification of ochratoxin A-producing Aspergillus section Nigri strains from California raisins. Letters in Applied Microbiology. 52:330-336. Interpretive Summary: Ochratoxin A is a mycotoxin produced by a variety of Aspergillus species, on a wide range of food crops, including raisins and other dried fruit. We sampled raisin vineyards from the San Joaquin Valley, and tested those samples for ochratoxin A. We found ochratoxin in 37 of 40 samples, at levels far below the regulatory limit in place in the European Union, with the exception of one sample that was just above the limit of 10 parts per billion. We isolated 400 strains of black-spored Aspergillus species from these raisins, and tested them for ochratoxin production on media. Twelve strains produced ochratoxin, and these strains were all identified as A. carbonarius, using microbiological and genetic techniques. These strains all produced ochratoxin on raisin agar, used as a substitute for whole raisins. This study indicates that ochratoxin and ochratoxin-producing Aspergillus are present in California raisins, and suggests that further ecological studies are necessary to more completely understand the potential for ochratoxin contamination of the crop.
Technical Abstract: Aims: To determine incidence and levels of ochratoxin A (OTA) in California raisins, and to isolate and characterize OTA-producing fungi from California raisin vineyards. Methods and Results: Raisin clusters sampled from four California vineyards in the San Joaquin Valley were analyzed for OTA content using immunoaffinity and HPLC methods. OTA was detected in 93% of the samples, at levels from 0.06 ng/g to 11.4 ng/g. From these raisin samples, a total of 400 strains of Aspergillus were isolated and analyzed for OTA production. Twelve isolates, from five raisin samples, produced OTA. These isolates were identified as A. carbonarius, based on morphological characteristics and multilocus sequence analysis. Levels of OTA produced by these isolates on raisin agar ranged from 0.9 µg/g to 15 µg/g. Conclusions: 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. Significance and Impact of the Study: 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 is warranted to monitor ochratoxigenic potential of the crop.