Quality changes in chum salmon (Oncorhynchus keta) ikura affected by salt, thermal pasteurization, and storage time

Chen Liu

Salmon caviar or ikura is a popular food produced from eggs of Pacific salmon principally chum salmon (Oncorhynchus keta) and pink salmon (O. gorbuscha). Salmon caviar should have 3.5-5% water phase salt (WPS) to inhibit the growth of Listeria monocytogenes, and for products that are pasteurized, the recommended pasteurization protocols (6 log reduction of L. monocytogenes) are equivalent to a heat treatment of 2 minutes at 70ºC. However, thermal processing may lead to undesirable texture changes, so a kinetic study of textural change was conducted to determine at what point changes occur so that improved pasteurization protocols could be developed. Besides thermal processing, brining also influences the eggs' quality and factors affecting protein properties. Protein denaturation temperature (PDT) is affected by pH, and water activity in ikura at different WPS. In this study, PDT at: 0, 3.23%, 3.76%, 3.91%, 4.10%, 4.23%, 5.24% and 7.17% WPS were determined by differential scanning calorimetry. The PDT correlated with textural changes in unsalted roes heated at 70ºC for different times (1, 6, 11, 15, 19, 23, 27 and 31 min). The PDT increased significantly (92 to 96.5ºC) (P<0.05) with an increase in WPS from v 0 to 7.17% which followed a polynomial trend (R 2 =0.99), but pH did not appear to be affected. Compression force of the unsalted roes decreased linearly (R 2 =0.89) with increasing heating time. In addition to measuring textural change, oxygen permeability of packaging was examined as part of this study. Two films with different oxygen transmission rates (OTR, 40 and 62 cc/(m 2·day) ) were used as packaging materials for ikura of 5.24% WPS, a common WPS for commercial ikura, over a 60-day-long shelf-life study. During storage, ikura exhibited significant changes (P<0.05) in pH, water activity and firmness of texture; however no significant differences in pH and water activity were observed between ikura packaged with the two film types (P>0.05) until the second month. The CIE L*a*b* color parameters of ikura in different films did not significantly change during storage (P>0.05). The TBARs (thiobarbituric acid reactive substance content as malondialdehyde) in both types of ikura packaging in the different films slightly increased (P>0.05) until the last 10 days of storage, at which point the oxidation of the ikura in the higher OTR was substantially higher.. These results indicate that with an appropriate increase in WPS, it could be possible to pasteurize ikura at a higher temperature for a shorter time, possibly reducing the undesirable softening that occurs to salmon caviar following pasteurization. Also, packaging ikura in materials with lower oxygen transmission rate could reduce the onset and rate of lipid oxidation during storage.

Quality changes in chum salmon (Oncorhynchus keta) ikura affected by salt, thermal pasteurization, and storage time

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