Potential of using uric acid fluorescence in eye fluid for freshness assessment on Red Sea bream (Pagrus major)

Fluorescence characteristics of uric acid in Red Sea bream (Pagrus major) eye fluid was explored as a potential rapid and simple assessment for fish freshness. To investigate this, eye fluid samples were collected during storage at 22 +/- 2 degrees C for standard uric acid measurements by high-performance liquid chromatography at 12h intervals up until 36h storage. Simultaneously, uric acid fluorescence spectroscopy measurements by fluorophotometer at 3h intervals up until 36h storage were made. High-performance liquid chromatography results showed the concentration of uric acid increased with storage time in the Red Sea bream eye fluid; changes similar to those observed in Japanese dace eye fluid, only differing in concentration and rate of accumulation. The fluorescence signals of uric acid in Red Sea bream eye fluid increased with storage time, which the high-performance liquid chromatography results confirmed. To further explore this, uric acid fluorescence signals were plotted against a standard fish freshness indicator K value, which is calculated from the concentration of adenosine triphosphate and its breakdown products using paper electrophoresis method. A good exponential relationship between these two parameters (determination coefficient of 0.94). A high linear correlation between the predicted K value from the uric acid fluorescence signals and the measured K value (determination coefficient of 0.94 and root mean square error of prediction of 6.37%) indicate uric acid fluorescence characteristics in fish eye fluid has a high potential to be employed as a new, fast and simple method to assess fish freshness.