Influence of Freezing on Muscles of Rainbow Trout (Oncorhynchus mykiss): A Histological and Microbiological Study

Abstract

Background: Freezing is a common and ancient method for preservation of foods which is applicable both under household and industrial conditions. The objective of the study was to establish histological and microbiological changes in dorsal and abdominal muscles of rainbow trout (Oncorhynchus mykiss) after freezing once and twice.

Methods: Forty-five fresh rainbow trout specimens were distributed into three groups of 15 fish each. The first group was subjected to histological and microbiological analysis immediately after delivery at the laboratory. The second fish group was frozen at -18 °C for 15 days, while the third group of fish was frozen at -18 °C for 15 days, thawed and frozen again at -18 °C for 15 days. Data were analyzed using GraphPad InStat 3 software.

Results: After freezing once, muscle fibers with intracellular void spaces were observed and retained stable peripheral boundary. In some muscle fibers, the endomysium boundaries were visible and with retained integrity. After freezing twice, damages and deformities were observed resulting in completely destructured muscle fibers. Large void spaces among the muscle fibers and bundles were greatly the result of shrinking and grouping of fibers and the laceration of endomysium and perimysium internum. Total microbial count and Enterobacteriaceae count had no significant differences (p>0.05) between fresh, frozen once, and frozen twice trout.

Conclusion: Muscles of rainbow trout (O. mykiss) are histologically damaged to a greater extent after freezing twice and thawing. However, microbiological indicators had no change significantly after freezing once and twice.