INFLUENCE OF WATER ACTIVITY, TEMPERATURE, OIL CONTENT AND PROBIOTIC BACTERIA ON GROWTH AND OCHRATXOIN A PRODUCTION BY ASPERGILLUS FRESENII AND ASPERGILLUS SULPHUREUS
thesisposted on 12.12.2019 by Yung-Chen Hsu
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Ochratoxin A (OTA) is a ubiquitous mycotoxin produced by some species of Aspergillus and Penicillium. It has been detected in a variety of foods such as cereals, coffee, grapes, cocoa, wine, and spices. Consumption of OTA has been linked to kidney and liver diseases. The aims of this study were to determine the effects of (1) water activity and temperature (2) oil content and grinding and (3) probiotic bacteria on fungal growth and OTA production by Aspergillus fresenii and A. sulphureus. In the first study, the two fungi were grown on ground Niger seeds with 0.82, 0.86, 0.90, 0.94 or 0.98 aw and incubated at 20, 30 or 37°C individually. The two species showed similar growth patterns on Niger seeds under all of the testing conditions. There was no fungal growth on ground Niger seeds with 0.82 aw and the optimal growth condition for the two species on ground Niger seeds was 0.94 aw at 30°C. However, the optimal conditions for OTA production by A. fresenii and A. sulphureus were different. The optimal conditions for A. fresenii to produce OTA on ground Niger seeds was 0.90-0.94 aw at 37°C; whereas, A. sulphureus produced OTA optimally with 0.90-0.94 aw at 30°C as well as 0.94-0.98 aw at 20°C. Overall, A. sulphureus produced higher levels of OTA than did A. fresenii. The highest concentration of OTA (643 μg/kg) produced by A. fresenii was detected on seed samples with 0.90 aw incubated at 37°C for 15 days, while the highest concentration of OTA (724 μg/kg) produced by A. sulphureus was detected on samples with 0.98 aw incubated at 20°C for 10 days.
In the second study, growth and OTA production by the two fungi on ground Niger seeds with different oil content (10, 25 and 35%) and on whole Niger seeds at 30°C were compared. All seed samples were adjusted to 0.94 aw in this study. The two fungi grew most rapidly on ground seeds with 35% oil content, producing high concentrations of OTA (229-453 μg/kg). On whole seeds, A. sulphureus and A. fresenii displayed slow growth until day 5 or 10, respectively, growing rapidly after that. The two species produced either non-detectable or below the limit of quantitation (<4 μg/kg) of OTA in ground seeds with 10 or 25% oil or in whole seeds during the 30-day incubation at 30°C.
In the third study, growth inhibition of A. fresenii and A. sulphureus by probiotic bacteria Bacillus coagulans, B. coagulans (strains unique IS2TM and GBI-306086), Lactobacillus acidophilus (strains LA-5 and LA-14), L. plantarum (strains 299V and LP115), and L. rhamnosus
was evaluated. Results of co-cultured method revealed that L. plantarum 299V had the highest levels of inhibition against the two fungal species; whereas, L. plantarum LP115, and L. rhamnosus showed only some inhibition effect against A. sulphureus and very little inhibition against A. fresenii. The two fungal species were not inhibited by L. acidophilus or B. coagulans.
Results from double-layer testing showed that the two L. plantarum strains and L. rhamnosus inhibited fungal growth completely when there were as few as 40-70 CFU probiotic bacterial colonies in the bottom layer of MRS agar; whereas, L. acidophilus inhibited fungal growth completely when the probiotic colonies were >125 CFU/plate. The three B. coagulans strains showed only partial growth inhibition against A. fresenii with 103 CFU/plate. Bacillus coagulans (unique IS2TM and GBI-306086) completely inhibited growth of A. sulphureus when there were as few as 40-70 CFU/plate; while B. coagulans completely inhibited the growth of A. sulphureus but only when there were >103 CFU plate. Even though the two fungal species were inhibited by some probiotic bacteria on MRS plates, the OTA production was not influenced.