Ochratoxin A production by Aspergillus section Nigri strains isolated from currants of Greek origin

Ochratoxin A (OTA) is a polyketide mycotoxin. The chemical structure of OTA consists of a 7-carboxy-5-chloro-8-hydroxy3,4-dihydro-3-methylcoumarin, linked through the 7-carboxy-group to l-β-phenylalanine by an amide bond (van der Merwe et al. 1965) and is produced by some Penicillium and Aspergillus species in several agricultural products (Pitt, 1987). Ochratoxin A has been classified as "possibly carcinogenic to humans" (group B) by the International Agency for Research on Cancer (IARC, 1993), because of its teratogenic, nephrotoxic, immunosuppressive, carcinogenic and cytotoxic properties. It is produced mainly by Aspergillus ochraceus, A. carbonarius and A. niger in warm climates and by Penicillium verrucosum and P. nordicum in temperate climates (Magnoli et al. 2006, Pitt et al. 2000).

 Strains of Aspergillus section Nigri are distributed worldwide growing on a great variety of substrates and they are considered as common food spoilage fungi (Pitt and Hocking, 1997, Raper and Fennell, 1965). Moreover, some of them are source of extracellular enzymes and organic acids applied in food industry. For this reason, black aspergilli have always been useful and important in biotechnology and food manufacturing (Samson, 1994). 

Many studies showed that OTA contaminates a variety of food commodities such as dried fruits, cereals, coffee etc. Kaya and Tosun (2013) analyzed organic foods produced in Turkey, for aflatoxins, OTA and fumonisin contamination. From a total of 221 samples it was found that 43.43 % of these samples were contaminated with OTA. The concentrations of OTA in dried fruits ranged from zero to 34.35 μg kg-1. Moreover in a survey conducted in United States it was found that from 665 samples (dried fruits, tree nuts, bulk raisins, dates, figs, prunes, almonds, pistachios, walnuts) analyzed, OTA was detected in 48 raisin samples, 4 fig samples, 4 pistachio samples, and 1 date sample (Palumbo et al. 2015). In addition from a total of 56 dried vine fruits samples, collected from Chinese markets analyzed for OTA presence, it was found that 58.9% of the samples examined were contaminated (<0.07 to 12.83 μg kg-1 OTA) (Zhang et al. 2014). Another survey showed that OTA occurred in 22% of a total of 228 samples of dates and dried fruits from Tunisian and Spanish markets (Azaiez et al. 2015). Meanwhile in Iran OTA was detected in 10% of dried vine fruit analyzed (Feizy et al. 2012). Ponsone et al. (2010) studied the natural occurrence of OTA in musts, wines and grape vine fruits from grapes harvested in Argentina and found that 60% of the dried vine fruit samples were contaminated with OTA, ranging from 0.26 to 20.28 ng g-1 while red must and red wine samples showed low levels of mean OTA contamination (0.12 ng mL-1 and 0.37 ng mL-1, respectively). A survey of aflatoxins and ΟΤΑ contamination in food products imported in Italy showed that dried vine fruits were the commodities mainly contaminated with ΟΤΑ, ranged from 2.76 to 9.87 μg kg-1 while green coffee was found to be contaminated with OTA 23.7 μg kg-1 (Imperato et al. 2011).

 Currants (Corinthian type raisins) are dried seedless grapes of Vitis vinifera L. cultivated and processed for hundreds of years. This type of dried vine fruits is produced almost exclusively in Southern Greece. Currants together with the other types of dried vine fruits (raisins and sultanas) are important constituents of the Mediterranean diet. They are considered as a rich source of antioxidant compounds (ascorbic acid) and polyphenolic compounds (anthocyanins) (Chiou et al. 2014). Moreover currants contain essential minerals, such as potassium, calcium and magnesium (Nour et al. 2011).

 The aim of this work was to investigate the potential of four new strains of Aspergilli section Nigri, which have been isolated from black currants originating from Crete and Corinth (ATHUM 6998, 6999, 7000 and ATHUM 6997), to produce OTA in black currants. The OTA production by the four strains was also studied in a synthetic microbiological medium, yeast extract sucrose (YES). It is worth mentioning that the same strains of Aspergillus section Nigri have been previously studied in our laboratory for their ability to produce AFB (Kostarelou et al. 2014, Tavoulari et al. 2014).