Assessment of physio-chemical parameters in farmed mulloway (Argyrosomus japonicas) to establish optimal post-harvest practice.

Shiori Naka, Southern Cross University

Document Type Thesis

Abstract

Creating and maintaining an optimal ‘cold chain’ (temperature-controlled supply chain) is crucial in aquaculture operations. Two primary determinants of cold chain effectiveness are the ability to accurately measure degradative processes in seafood, and harvest methods that maximize product desirable characteristics. This thesis uses an emerging fish aquaculture industry, the production of mulloway (Argyrosomus japonicus) in brackish-water ponds in northern New South Wales, Australia.

The thesis begins with an overview of the aquaculture industry and provides the biochemical background underpinning the K-value and allied indices of degradative processes in seafood. These rely upon comparing adenosine triphosphate (ATP) concentrations with those of various ATP breakdown products. Accurate ATP quantification is therefore a fundamental precondition to the use of these indices. A new ATP extraction methodology is detailed and verified using farmed mulloway, farmed Atlantic salmon (Salmo salar), wild-caught redfish (Centroberyx affinis) and wild-caught silver trevally (Pseudocaranx georgianus). The ATP breakdown product inosine (HxR) was detected in silver trevally and Atlantic salmon, but not in mulloway or redfish, suggesting that ATP breakdown in the latter two species does proceed via an HxR-producing chemical pathway.

The effects of harvest methods on the quality of fish when stored at 0 ºC was assessed by comparing preparation of mulloway using ikijime (brain spiking) and ice-slurry procedures. Stored fish quality was assessed by physicochemical, rigor and pH measurements.

There was no significant difference between ice-slurry processed and ikijime-prepared mulloway muscle pH. However, ikijime-prepared mulloway reached rigor more slowly than ice-slurry-processed mulloway, particularly during summer. Statistically significant effects of harvest method on ATP concentration were apparent only on the day of harvest, with ikijime fish having the highest ATP concentrations in both summer (5.5μmol g-1) and winter (3.7 μmol g-1). Inosine monophosphate (IMP) was maximal in the first 24 hours post-harvest, regardless of season or harvest method. Inosine (Hx, responsible for ‘off’ flavours) in both ice-slurry-processed and ikijime-prepared fish increased slowly over the storage period, and did not differ significantly between method or seasons. Both ice-slurry processed and ikijime-prepared mulloway returned very low K, K1, G, P, H -values after 12 days. However, these values may have been affected by the absence of HxR from the ATP breakdown process in mulloway. Further research is required to develop new indices of degradative processes in seafood for mulloway and other fish species that do not produce all 6 ATP breakdown nucleotides.