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Frozen storage and cryoprotection of eggs from chum salmon (Oncorhynchus keta) Craig, Cheryl Lynne


This study was initiated to assess the influence of frozen storage of eggs from sexually mature female chum salmon (Oncorhynchus keta) on the physical properties of the membrane and the yolk. The effectiveness of several salts, sucrose, glycerol and ethylene glycol as cryoprotective agents for preventing any observed cryodamage to the eggs was determined. Electron microscopy was employed to gain an insight into the micro-structure of the chum egg envelope. In addition, whole eggs were subjected to flat plate compressive loading in order to determine the strength of the membrane. The membrane appeared to be a complex structure which enabled intact eggs to withstand compression forces of 0.47 to 1.22 Newtons. The membrane rupture energy ranged from 2.36 to 7.99 (x 10⁻⁴) Joules, and was independent of egg size. Upon freezing fresh eggs at -10°C for 7 days, then thawing in air at 25°C, a drop of 46% in membrane rupture energy was experienced. The flow behaviour of yolk expressed from fresh chum eggs, as examined with the aid of a Brabender rotational viscometer, displayed a non-Newtonian pseudoplastic nature. Analyses by differential scanning calorimetry and polyacrylamide gel electrophoresis indicated the presence of several major proteins. When the yolk material had been frozen at -10°C for 7 days, then thawed in air at 25°C, little variation was observed to occur by any of the techniques used. Immersion in a 2.5 M solution of sodium chloride for 30 min at 25°C was the most effective and consistent cryoprotective agent out of all of the potential agents tested. Caviar produced from previously frozen, then thawed eggs that had been so treated before freezing differed from that produced from both fresh and untreated frozen and thawed eggs in the rate of sodium chloride uptake and yolk apparent viscosity for the same chloride content. Membrane strength of such caviar was similar to that produced from fresh eggs, being greater than that produced from untreated previously frozen and thawed eggs. Sensory evaluation studies indicated that caviar from eggs that had been cryoprotected were ranked similar to caviar from untreated frozen and thawed eggs in the parameters of yolk viscosity and membrane strength. Both were significantly different from high quality caviar produced from fresh eggs. However, the hedonic ratings indicated a preference for caviar from cryoprotected eggs over that processed from untreated frozen and thawed eggs. Results of this study indicate that cryoprotection of chum eggs to reduce the defects caused by frozen storage is feasible. With further research, this technique holds potential advantages for the caviar industry.

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