Toxigenic fungi and mycotoxin production in Maldive fish (smoked dried tuna fish) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology, Massey University, Palmerston North, New Zealand
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This is the first study on the mycological safety of “Maldive fish”, a smoked dried tuna product that is both economically and nutritionally important to the Maldives. The most obvious concern with this product is the effect of fungal contamination. The initial aim of the current study was therefore to determine if Maldive fish supports the growth of toxigenic fungi and production of mycotoxins. The uncontrolled mycoflora on the product were characterised and related to the physiological parameters of the Maldive fish. Ninety six percent of the samples (n=25) were contaminated with one or more mycotoxigenic fungi with Aspergillus flavus (92%), A. tamarii (96%), A. niger (40%), A. ochraceus (12%) and Penicillium citrinum (60%) identified as the significant species. Subsequently, the potentially toxigenic isolates were screened for their corresponding mycotoxins aflatoxins, ochratoxin A (OTA), cyclopiazonic acid (CPA) and citrinin. A high proportion (72%) of isolates was able to produce toxic metabolites in vitro indicating possible contamination of the product with mycotoxins. Almost half (46%) of the A. flavus isolates were able to produce the potent carcinogen, aflatoxin B. All species on the surface were also found invading the product. The huge variability in aw levels (0.951 to 0.720) of the samples would support growth of a wide range of species. Furthermore, the slightly acidic pH (5.65 to 6.68) and low salt content (1.48 to 4.29%) together with the high ambient temperatures of the Maldives were eminently suitable for fungal growth and mycotoxin production. Quantification of aflatoxins from the product revealed two of the 25 samples to be contaminated above the legal limits and confirms potential exposure to significant levels of this toxin from Maldive fish infected with fungi. These results led to a new question: can fungal growth and mycotoxin production in Maldive fish be eliminated or reduced to safe levels? The most practical approach would be to reduce the aw to sufficiently low levels that inhibit fungal growth and mycotoxin production. The limiting aw levels for the most important species were therefore evaluated. The limiting aw for growth of A. tamarii was between 0.82 and 0.85 on NaCl media and between 0.79 and 0.75, on media containing sugars at ambient storage temperatures (25 to 35°C). The aw of Maldive fish should be maintained below 0.75 to prevent the growth of A. tamarii. The physiology of A. flavus has been extensively studied previously but the limiting values are dependent on the food matrix. A smoked fish agar was used to simulate Maldive fish for fungal growth (A. flavus) and mycotoxin production (aflatoxin and CPA) under varying conditions. No growth occurred at an aw of 0.75 while the toxin production was limited at an aw 0.80 under all incubation conditions (25°C to 40°C). Hence, control of A. flavus can be achieved by rapid drying of Maldive fish to an aw of 0.75 or below. This study has provided scientific evidence that the mycoflora on Maldive fish produce aflatoxins and other mycotoxins that are a food safety risk. Hence, control of toxigenic fungi is imperative and can be achieved through adequate drying. This information is crucial for the Maldives as well as other developing countries that consume hot smoked dried fish while it potentially has a broader application for other food products.
Appendix G removed for copyright reasons. First page of: Mohamed, S., Mo, L., Flint, S., Palmer, J., & Fletcher, G. C. (2012). Effect of water activity and temperature on the germination and growth of Aspergillus tamarii isolated from "Maldive fish". International Journal of Food Microbiology, 160(2), 119-123. http://dx.doi.org/10.1016/j.ijfoodmicro.1012.09.022
Fishery products, Fish as food, Smoked fish, Spoilage, Contamination, Toxigenic fungi, Mycotoxins, Maldives