Effect of high pressure processing on the quality of herring (Clupea harengus) and haddock (Melanogrammus aeglefinus) stored on ice

Fresh herring and haddock were vacuum-packed and subjected to high pressure processing (200, 250 and 300 MPa at 10 °C for both 1 and 3 min) or left untreated as controls. The samples were stored in ice at 2 °C for up to 14 days, during which time the changes in microbial quality, total volatile base nitrogen (TVBN) and trimethylamine (TMA) production were monitored. Microbial shelf-life was determined as the storage time required for psychrotrophic counts to reach 10⁶ CFU g^?1, while chemical shelf-life was estimated as the time taken for TVBN and TMA values to reach 35 mg N 100 g^?1 and 15 mg N 100 g^?1 respectively.High pressure significantly delayed microbial growth (p < 0.05) in both herring and haddock. In the case of herring the microbiological shelf-life was extended from ～4 days in controls to ?13 days in fish pressure-treated at 200 MPa/3 min. Microbial numbers in all the haddock samples tended to be lower throughout the storage period, compared to herring, irrespective of pressure treatment, and none of the pressure-treated samples reached unacceptable psychrotrophic numbers until 14 days in ice compared to 10 days in the controls. The microflora of both the control and pressure-treated herring and haddock at day 0 were predominantly Gram-positive cocci (Micrococcus spp. and Staphylococcus spp.) and spore-forming rods (Clostridium spp. and Bacillus spp.). This microflora did not change significantly during storage of the fish in ice at 2 °C for 10 days.Pressure treatment also delayed TVBN and TMA production in the fish. In the case of herring, predicted values did not reach unacceptable levels until at least 18 days (200 MPa/3 min) compared to 5.5 days in controls. With haddock, TMA and TVBN values in control samples reached unacceptable limits between 6 and 10 days of storage. However, the levels remained below the limits of acceptability in all pressure-treated samples during storage, apart from on day 14, when the TVBN value in samples treated at 200 MPa/1 min exceeded the level of acceptability.Overall, taking account of the various spoilage indicators, the minimum treatment required to increase shelf-life in herring and haddock to ～13 days on ice was found to be 200 MPa for 3 min.