Ochratoxin A Production by Some Fungal Species in Relation to Water Activity and Temperature

Abstract

The combined effects of water activity (llw) and temperature on growth and patulin production by strains of Penicillium expansum, Penicillium patulum, and Aspergillus clavatus were determined. Malt agar media were used, in which the aw was adjusted by addition of sucrose or glycerine. The minimum llw values for patulin production by P. expansum, P. patulum, and A. c!avatus were 0.99, 0.95, 0.99, respectively. The temperature ranges for patulin production by P. expansum. P. patulum. and A. clavatus were 0-24, 4-31, and 12-24 C, respectively. The optimum temperatures for patulin production by P. expansum and A. clavatus were low compared with those for growth. Optimum temperatures for patulin production at high llw by P. patulum varied with the strain tested and were 8 or 31 C. The temperature range for patulin production in apples by P. expansum was determined. The minimum temperatures for rotting and patulin production were 1 C in Cox Orange cv. and 4 C in Golden Delicious cv. The amount of patulin accumulating in rotten tissue of six apple varieties differed greatly. The invasiveness of and patulin production by various strains of four patulin-producing fungal species were tested. All P. expansum strains tested caused rot containing patulin. The increase of rot and patulin production by P. crustosum and A. clavatus depended on the strains tested. None of the P. patulum strains was able to invade Golden Delicious apples. Patulin is a metabolite of various Penicillium, Aspergillus, and Byssochlamys species. The toxicity of patulin to microorganisms, plants and animals, and its carcinogenicity to mice have been reviewed by Stott and Bullerman (23). Patulin has been found frequently in apples decayed by Penicillium expansum (2, 7). Results of Harwig et al. (7) indicate that where apples processed for juice include unsound fruit, patulin may be introduced into the end product. This has been confirmed by the demonstration of patulin in apple juice in Canada (17), the United States (21, 25) and Sweden (8). Patulin has also been detected in fruit other than apples (1,5,17). P. patulum, synonymous with P. urticae and P. griseofolvum (16), was frequently isolated by Graves and Hesseltine (6) from flour and refrigerated dough products and it could have been the dominant mold of 1 Labomtoryf(Jr Zoonoses and Food Microbiology. 2 Laboratoryfor ChemicalAna(vsis ofFoodstufjs. spontaneously molded bread containing patulin (13). Bullerman and Olivigni (3) isolated molds from commercial Cheddar cheese. Some of these molds produced patulin in laboratory media; one of the isolates was identified as P. patulum (22). Another patulin producing fungal species is Aspergillus clavatus. This species was involved in intoxication of calves fed moldy barley (11). P. crnstosum, which occurs only rarely on food products, is another patulin producer. P. verru-cosum var. eye/opium is proposed as its correct name (16). The most important factors determining growth and, therefore, production of patulin are temperature and moisture. The effect of temperature on growth and production of patulin has been determined by various investigators (5,15,20). The temperature range for production of patulin by P. expansum has been determined for tomatoes and bread (5, 15). but not for apples. The moisture requirement for germination of spores of P. expansum and P. patulum isolated from stored corn was determined by Mislivec and Tuite (10); however, the effect of moisture on growth and production of patulin received little attention. The range of temperature over which microorganisms can grow is influenced by the water activity (aw) (19). Therefore, in this investigation the effects oftemperature on fungal gro"th and patulin production were studied at different levels of aw. In addition, the temperature limits of patulin production in apples were determined. MATERIALS AND METHODS Organisms and spore suspensions Fungal strains were maintained as lyophilized cultures. P. expansum strains RIV SO and RIV 287, and P. crustosum strain RIV 58 had been isolated from a meat product, a naturally rotten apple, and a bakery product, respectively. A. clavatus strains RIV 612 and RIV 672 had been isolated from com post. Other strains of P. expansum, P. patulum. P. crustosum. and A. clavatus had been received from Dr. R. A. Samson, Centraal Bureau voor Schimmelcultures. Baarn, The Netherlands. Cultures grown for l week at 24 Con malt extract agar (Oxoid) were washed with an aqueous solution of 0.6o/o sodiumhepta-decyl-sulfate (Tergitol-7,BDH) to prepare spore suspensions.