Comparison of chemical, microbiological and histological changes in fresh, frozen and double frozen rainbow trout (Oncorhynchus mykiss)

Abstract

The final quality of fish meat depends on the chemical and microbiological quality of fish at the time of freezing as well as on other factors including storage temperature and freezing rate. Analysis of chemical composition (water, protein and fat content), expressible drip, total volatile nitrogen levels, microbiological analyses (total viable counts, Enterobacteriaceae, psychrotrophic bacteria) and histological examinations on dorsal skeletal muscles were carried out to distinguish fresh, frozen and double frozen rainbow trout (Oncorhynchus mykiss). Significantly higher expressible drip and total volatile base nitrogen concentrations (P < 0.05) were observed in frozen and double frozen trout, whereas chemical composition of fresh fish muscles was not significantly affected by freezing. The highest total viable counts, counts of Enterobacteriaceae and psychrotrophic bacteria were determined in double frozen trout. The light microscopy of fresh trout muscles did not show any microstructural changes, whereas deformations of muscle fibres and optically empty areas were found in frozen trout. Remarkable defects of the muscle structure in double frozen trout were demonstrated and total disruption of muscle fibres was found. The freezing of trout resulted in various structural changes in the dorsal skeletal musculature. This is a first study comparing changes in fresh, frozen and repeatedly frozen trout. Chemical, microbiological and subsequent histological examinations can be used for revealing the foul practices confusing the consumer with offering thawed fish instead of fresh cooled fish.

Histological discrimination of fresh and frozen/thawed fish meat: European hake (Merluccius merluccius) as a possible model for white meat fish species

“…On the contrary scattered areas of swollen and shrunken-fragmented eosinophilic fibers were recorded. These alterations were similar to those described by Sigurgisladottir et al (2000) in frozen salmon (Salmo salar) and by Popelka et al (2014) in rainbow trout (Oncorhyncus mykiss), likely due to autolytic enzymes which are known to be the main responsible for the post mortem tissue softening (Ahmed, Donkor, Street, & Vasiljevic, 2015).…”

“…The possible onset of tissue modifications related to fish spoilage within the expected shelf life of the product, that are generally due to the combination of enzymatic autolysis oxidation and microbial growth (Ghaly et al, 2010;George, Van Wettere, Michaels, Crain, & Lewbart, 2016), was also considered in the preliminary assessment. Since myofiber vacuoles are reported as the main change associated with freezing, the analysis was focused on the detection of the possible presence of vacuoles in fresh samples at different time of conservation, since this aspect was not considered in the previous studies (Bozzetta et al, 2012;Popelka et al, 2014;Meistro et al, 2016).…”

“…An alternative method for discriminating fresh and frozen/thawed products is based on the recognition of histological structural changes (empty vacuolar spaces) induced by freezing (Love, 1958;Simeonidou, Govaris & Vareltzis, 1997;Sigurgisladóttir S, Ingvarsdóttir H, Torrissen OJ, Cardinal M, Hafsteinsson, 2000;Alizadeh, Chapleau, De Lamballerie, & Le-Bail, 2007;Alizadeh, Chapleau, De Lamballerie, & Le-Bail, 2009). This approach, firstly proposed as a discriminating method for the common carp (Cyprinus carpio L.) (Pavlov, Dimitrov, Penchev & Georgiev, 2008), has been recently applied to other species characterized by different muscular composition and fat content: gilthead (Sparus aurata), red mullet (Mullus barbatus), swordfish (Xiphias gladius), bonito (Sarda sarda), salmon (Salmo salar), turbot (Psetta maxima), albacore (Thunnus alalunga), little tunny (Euthynnus alletteratus), rainbow trout (Oncorhyncus mykiss) and anchovy (Engraulis encrasicolus) (Bozzetta et al, 2012;Richelmi et al, 2013;Popelka, Nagy, Pipová, Marcinčák, & Lenhardt, L., 2014, Meistro et al, 2016. Despite the high accuracy of these methods in the detection of the freezing process, the presence of unspecific microscopic alterations reduced the assay accuracy and precision leading to "non-conclusive results".…”

Assessment of food quality and microbial safety of brown macroalgae (Alaria esculenta and Saccharina latissima)

“…This suggests that the freezing method (block freezing) and storage temperature (−30 °C) used in this study resulted in a toughening of the kelp thallus. Toughening during freezing is a well documented phenomenon in fish muscle, and is correlated with crystal formation and disruption of cells, which can be minimized by optimization of freezing conditions . This result was so far only detected in heat‐treated specimens, and not in raw kelp ( A. esculenta only).…”

Emerging Techniques for Differentiation of Fresh and Frozen–Thawed Seafoods: Highlighting the Potential of Spectroscopic Techniques

“…Measurements of volatile composition by solid phase micro-extraction/gas chromatography/mass spectrometry analysis (SPME/GC/MS) was also suggested as markers to differentiate fresh from frozen–thawed products [ 63 ]. Microbiological parameters, including total viable counts, counts of Enterobacteriaceae, and psychrotrophic bacteria were measured, among other analyses, in order to distinguish fresh, frozen, and double frozen rainbow trout ( Oncorhynchus mykiss ); the double frozen fish exhibited the lowest microbiological quality [ 71 ].…”