Determination of nucleotide and enzyme degradation in haddock (Melanogrammus aeglefinus) and herring (Clupea harengus) after high pressure processing
Authors: Karim Nurul Ulfah , Kennedy James Terence , Linton Mark , Patterson Margaret , Watson Sally , Gault Norman 
Published: August 2019

BackgroundThe degradation of nucleotides and their enzymes had been widely used to evaluate fish freshness. Immediately after fish death, adenosine triphosphate (ATP) degrades into inosine-5-monophosphate (IMP) via adenosine-5-diphosphate (ADP) and adenosine-5-monophosphate (AMP). IMP degradation continues to produce inosine (ino) and hypoxanthine (Hx) and further deteriorates the fish by producing xanthine and uric acid. The dephosphorylation of IMP to Ino is carried out by the enzyme 5′-nucleotidase (5′-NT), whereas the degradation of Ino to Hx is carried out by the enzyme nucleoside phosphorylase (NP). This study assesses the application of high pressure processing (HPP) in two species of fishes; haddock (Melanogrammus aeglefinus) and herring (Clupea harengus) as a means to extend the shelf-life by slowing down the rate of nucleotides degradation.MethodsHaddock(Melanogrammus aeglefinus)and herring(Clupea harengus)fillets were subjected to HPP at 200, 250 and 300 MPa for 1 and 3 min before being stored for 14 days. In addition, 5′-NT and NP enzyme activities were determined on both fish species that were subjected to 100–600 MPa for 1 and 3 min.ResultsAdenosine triphosphate, ADP and AMP in both haddock and herring were lower at higher pressure levels. Inosine (Ino) increased (p< 0.05) after treatment at higher pressures in both species. Hx in herring decreased significantly (p< 0.05) at higher pressures but not in haddock.Kvalues are the ratio of Ino and Hx to all nucleotides.Kvalues in haddock were not significantly (p> 0.05) affected by the pressure treatment.Hvalues are ratio of Hx to the sum of IMP, Ino and Hx.Hvalues in haddock were significantly decreased (p< 0.05) with increasing pressure level.Fvalues are ratio of IMP to the sum of IMP, Ino and Hx.Fvaluesshowed no significant effects (p> 0.05) after pressure treatment. Furthermore,Kvaluesin control herring were significantly higher (p< 0.05) than those of the pressure-treated samples.Hvalues in herring decreased significantly (p< 0.05) with increasing pressure level.Fvaluesin herring showed no significant effects (p> 0.05) after pressure treatment. Pressure treatment brought a significant decrease (p< 0.05) in protein content in both haddock and herring. 5′-NT activity was 20–35 fold higher compared to NP activity in haddock and 15–44 fold higher than NP activity in herring. 5′-NT and NP activities decreased significantly with increasing pressure level in both species.DiscussionHigh pressure processing effectively slows down the conversion of Ino to Hx, delaying the undesirable flavour that develops in spoiling fish. The autolytic conversion of IMP to Ino by endogenous 5′-NT predominates in the earliest stages of storage is an autolytic process. However, both bacterial and endogenous NP enzymes are probably responsible for the gradual accumulation of Hx in fish.Kvaluesare recommended as a useful measurement of fish freshness.