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H. O. HULTIN, Dept. of Food Science, Univ. of Massachusetts,Amherst, Marine Station, P.O. Box 7128, Gloucester, MA 01930 Sensitivity of cold water fish to lipid oxidation has been a major impediment in the production of protein isolates such as surimi. Recent evidence suggests that a major contributor to this oxidation are the heme proteins. Fish hemoglobins are unstable and readily de-oxygenate, dissociate into monomers, and release their heme groups as the pH is reduced below neutrality. The high variability in reactivities of hemoglobins from different fish species must be considered. Recently, processing methods have been developed for isolating functional proteins from fish muscle tissue by first solubilizing the proteins at low ( 3) or high (10.8) pH followed by isoelectric precipitation of the proteins at pH 5.3–5.5 after removing insoluble impurities. Thus hemoglobins in the fish tissue in either the acidic or akaline process would be subjected to pH values where increased pro-oxidative activity could be expected. Approaches to reducing lipid oxidation have included pH control, hemoglobin removal, limited exposure to low pH, selective solubilization and removal of cellular membranes. Exposure to pH values below neutrality for as short a time as possible has given promising results. One of the advantages of the solubilization process for isolating fish muscle proteins is the possibility of removing the oxidative-prone membrane lipids by centrifugation. While this can be done in a straightforward manner in the laboratory, it has been difficult to generate sufficient g force at economical cost commercially. However, treatment with calcium or magnesium salts in the presence of certain organic acids appears to break the linkages between cytoskeletal proteins and membranes so the membranes can be removed at low g force. This procedure works with both the alkaline and acid solubilization processes. It has been observed that the high or low pH treatment by itself can render the membrane lipids less susceptible to lipid oxidation.
Session 3, Fish protein recovery using pH shifts
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