Studies On The Changes Of Mackerel Muscle And Properties Of Stickwater Proteins From Fishmeal Production During Storage

Fresh stickwater (SW) is renowned for its good flavour. It is rich in nutrients and is anexcellent edible resource. However, a large number of SW generated in fish meal productionhas not been exploited. In this study, a number of things were investigated to understandbetter various aspects of SW. Firstly, changes in quality and physicochemical properties ofchub mackerel muscle stored at4oC and20oC were investigated. Secondly, the effect ofmicrowave on quality changes of SW was investigated. Lastly, the solubility and fractionationof SW protein was also investigated.Results showed that the shelf life of chub mackerel was very short. The total volatilebasic nitrogen (TVBN) achieved the initial decay at4days for the storage temperature of4oCwhile at20oC storage temperature, the decline was even faster and after storage period of15-20hour, it was not edible. Trimethylamine (TMA) increased from the initial5.85mg/100gto9-10mg/100g. At4oC storage temperature, histamine did not increase significantly whilefor the20oC storage temperature, the increase was three times within25hours. Furthermore,TBARS increased linearly at both temperatures, and peroxide value fluctuated upwards andthis was accompanied by rancid flavour implying fat oxidation.Results on salt soluble myofibrillar proteins stored at4oC and20℃on6thday and15thhour respectively showed the proteins decreased by78.1%and75.4%, the active sulfhydrylfell from60.14%to61.86%, while the total sulfhydryl decreased by63.72%to71.89%. Itwas observed that the total sulfhydryl declined faster than the active sulfhydryl. The activityof Ca2+-ATPase decreased and correlated to the decrease of active sulfhydryl content(R24℃=0.834; R220℃=0.584). The SDS-PAGE of myofibrillar protein showed that the myosinheavy chain, tropomyosin and troponin obviously reduced, while actin barely changed. Thesolubility of Sarcoplasmic protein in water during both storage did not reduce significantly,but the sulfhydryl content decreased. SDS-PAGE showed no obvious changes of sarcoplasmicprotein.Results on microwave treated SW showed that the homemade SW had similarcomposition with the one provided by factory. It was observed that with the decline infreshness of the raw materials, the histamine content of SW increased, although notstatistically significant. The total amino acid in SW accounted for more than60%of crudeprotein. Those essential amino acids accounted for about30%of total amino acids and thehydrophobic amino acids accounted for40%of total amino acids. Raw material freshnessdecline caused no decrease of any single amino acid in total amino acid content. The relativecontent of some free amine acids decreased, such as Arg, Tyr, His, Phe and Met, Lys and thesemight presumably degrade to biogenic amines and H2S.Results on solubility and fractionation of the SW protein showed that the protein contentof water soluble chub mackerel SW was55%, and increased slightly with the fresh degreeincreased, the content of protein soluble in low salt content was about20%, and decreasedwith the freshness drop; high salt soluble protein was about10%, affected a little by thefreshness of raw material. Water soluble and low salt soluble protein was mainly around37kDa. Water soluble protein of SW from grade one or two also contained a small amount ofproteins of60-120kDa, while these proteins disappeared in SW from grade three or four,instead, some small molecular protein appeared, and37kDa reduced. The salt soluble proteinconcentrated in30-50kDa, while there was also a small amount of protein of85-100kDa inSW from grade one or two, but this part of protein was not found in grade three or four. Thecontent of TCA-soluble peptide of different grade of SW was also found to increase as theraw material’freshness declined.Finally, the main suspended solids in the SW were found to be the aggregates of somelow molecular weight protein and the main force among them was the hydrophobicinteraction. It was further observed that when the suspended protein ratio was less than5%,there were little effects on solubility of supernatant protein and when the proportion of the suspended protein was increased, the solubility of supernatant protein decreased significantly.