Title: The effect of different cooling techniques on the microbial aquality and sensory shelf life of Icelandic golden redfish (Sebastus marinus) fillets

Author(s): Nathan Semwanga
Final project
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Redfish fillets; superchilling; slurry ice cooling; specific spoilage organisms; sensory shelf life; spoilage indicator.


Innovations in the application of different fish cooling techniques have been developed in order to delay spoilage, thus extending shelf life. In the study, effects of different in-process cooling techniques on microbial, chemical and sensory properties of redfish fillets during storage and distribution simulations were evaluated using five treatments. Cooling techniques included the use of slurry ice as a fillet cooling medium (SIC) and/or Combined Blast and Contact (CBC) technique to superchill the fillets. Storage temperature was monitored using I-button loggers. Growth of specific spoilage organisms (SSO), total viable psychrotrophic counts (TVC) and production of microbial metabolites were determined by conventional and molecular microbial and chemical (TVB-N, TMA, pH, salt content) methods. The study showed that superchilling by CBC for skin-on fillets efficiently reached a product temperature of -1°C but resulted in a reduced shelf life of 8-9 days and 10 days for CBC and SIC-CBC treatments, respectively, due to their poorer microbial quality. Skipping the slurry ice cooling step prior to CBC treatment proved undesirable to this fatty product as the sensory results suggested that other deteriorative changes, in addition to microbial changes, contributed to faster quality loss. A shelf life of 11- 12 days was obtained for the skinless fillets that were not CBC treated. SIC treatment did not provide sufficient cooling to achieve a measurable freshness and shelf life extension compared to untreated fillets. H2S-producing bacteria were the dominant SSO and were more resistant to superchilling than Photobacterium phosphoreum and pseudomonads. The level of TVB-N in differently treated redfish fillets reached 12-15 mgN/100 g at the end of shelf life, being lower than the EC limit of 25 mgN/100 g. Indeed, low SSO counts (<log 7 CFU/g) and TVB-N levels at sensory rejection indicated that redfish quality deterioration was not only caused by microbial spoilage, but most likely induced by oxidative changes. Finally, the study showed that spoilage was faster in CBC treated skin-on fillets than in non-CBC skinless fillets.

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