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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Date
2013
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Massey University
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Abstract
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.
Description
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
Keywords
Fishery products, Fish as food, Smoked fish, Spoilage, Contamination, Toxigenic fungi, Mycotoxins, Maldives