Water activity relations of spoilage fungi associated with smoke-dried catfish (Clarias gariepinus) sold in some open markets in Nigeria.

Spoilage fungi were isolated from the skin and the muscle of smoke-dried catfish samples, which were products of traditional and improved processing methods. The samples significantly different mean aw of 0.85 and 0.81 respectively [F (1, 3) = 0.014, P = 0.018], when they were checked immediately after purchase from selected open markets. The isolated spoilage fungi were identified by their phenotypic appearance and morphological features under microscope, with reference to standard identification guidelines. The isolates comprised of Aspergillus fumigatus, A. niger, A. flavus and Penicillium species. Effects of water activity (aw) on growth and sporulation of these species at ambient temperature (25 ± 5 °C) were studied on standard media (aw = 0.995) or media in which aw was modified using NaCl as follows: 0.98, 0.94, 0.86 and 0.80. All the isolates could grow in the range of the aw studied and there were statistically significant variabilities in the rates of growth among species [F (7, 50) = 63.34, P = 0.001] and in relation to media aw [F (28, 50) = 4.055, P = 0.001]. There was no limitant aw found in the studied conditions (0.995-0.80 aw x fluctuating ambient temperature, 25 ± 5 °C), as all the isolates were fast growing. The aw of the fish samples from the improved processing line was lower than those from the traditional processing lines. However, the aw of all the tested the samples was above the Codex Standard for smoked fish, smoke-flavoured fish and smoke-dried fish, which is 0.75 aw or less (10% moisture or less), as necessary to control bacteria pathogens and spoilage fungi. The results indicated that the open market fish samples may pose serious health risks if they are consumed after a short-term storage. Research Highlights Tolerance of smoke-dried fish spoilage fungi to water stress Effect of aw on growth characteristics of smoke-dried fish spoilage fungi Water activity relations of some spoilage fungi from dried fish. © Association of Food Scientists & Technologists (India) 2021.