Quality of Frozen Rainbow Trout. Effects of different freezing and thawing treatments

Doktorsavhandling, 1994

The effects of different freezing, frozen storage and thawing treatments on the muscle tissue of farmed rainbow trout (Oncorhychus mykiss) were studied. Biochemical measurements were correlated to sensory estimated quality changes. The freezing process affects the proteins, the lipid fractions and the membrane structures in the fish muscle. To estimate the effect a marker enzyme method was used. The activity of lyso-somal enzymes in the pressed out muscle tissue fluid (CTF) giv an indirect estimate of the effects of the freezing treatment on the membrane structure. Three enzymes were chosen: acid phosphatase (AP), a-glucosidase (AG) and b-N-acetylglucosaminidase (NAG). During ice storage of up to 14 days, only low activity of AG and NAG could be detected in the CTF. An increase was seen for AP after 3 days on ice, with a further marked increase after 14 days. With an added freeze-thaw cycle, significantly higher le s of all three enzymes were detected than after ice storage, irrespective of the length of ice storage. AP seemed to be least appropriate of the three, since it was released from lysosomes already during ice storage . When varying the freezing parameters, keeping the thawing and storage conditions constant, the fastest freezing resulted in the least membrane disintegration. When varying the thawing conditions instead the fastest thawing also resulted in the least e ct on the membrane. Different steps during thawing were shown to have various effects on the enzyme leakage. Fast tempering and short duration in the latent temperature zone resulted in the least membrane disintegration. Four different freeze handling processes, similar to processes used in the fish industry were compared, focusing on sensory and biochemical changes in the fish. No significant differences could be measured before storage between treatment groups, exce in the case of single or double freezing. After storage, clear differences could be seen in enzyme leakage between fish first frozen individually and in blocks, where the individually frozen fish released less enzyme. In the sensory evaluation a pref nce for the individually frozen fish also was seen. I During industrial fish freezing, glazing is done. This results in an undesirable increase in fish temperature. Comparing the effect of a fast and a slow return to frozen storage temperature again after glazing, on liquid loss, resulted in least liq loss in the fish fast returned to storage temperature. The effect of frozen storage time did not have a significant major effect on the enzyme leakage. A lower storage temperature had less effect on the membrane integrity than a higher storage temperature. Different storage time and temperature in combin on with different thawing treatments showed that fast thawing was of greater importance both to sensory quality and biochemical measured effects, in the fish stored at lower temperature. Faster thawing resulted in less membrane disintegration irrespective of frozen storage time.