SUMMARY The conception of Huxley concerning the structural basis of muscular contraction is universally accepted. It shows a close correlation between ATP and the fibrillar proteins, also existing during the first phase of the post‐mortem changes. Rigor development has been followed in whole fish and isolated beef muscles by measuring the torsion elasticity. Often there are great individual deviations in rigor development within a single species. Generally, at corresponding temperatures, rigor development lasts longer in mirror carp than in the gastrocnemius of beef; rosefish needs a longer time to reach maximum rigor than cod. Evidently, the rapid phase of ATP breakdown and increasing rigidity of muscles is initiated by inactivation of the Marsh‐Bendall factor in the post‐mortem period. Normally, contraction occurs when ATP is added to fiber fragments of aged meat. This implies that the aetomyosin complex formed during rigor development becomes dissociated, or at least may become dissociated easily, in aged meat, and that tenderness changes in the aging period are correlated to this process. ATP breakdown in fish muscle is highly activated by freezing and thawing (“biochemischer Verletzungseffekt”) and seems to be caused by inactivation of the relaxing factor. Fish (whole fish or fillets) frozen under normal commercial conditions immediately after death show an insignificant degree of thaw contracture. In cod and rosefish no significant difference has been found in the extractability of the actomyosin fraction, if the time passing between death and freezing was considered. In frozen muscle tissue stored below ‐18°C, ATPase activity and contractability decrease very slowly. This shows that the actin and myosin filaments are not subject to great structural changes by freezing and thawing. In the freezer‐burn area of muscle tissue the structure proteins lose the ability to contract on ATP addition more and more with increasing storage time; finally, even the plasticizing effect of ATP on the fibrillar proteins disappears, the fibrils scarcely change their original orientation.

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In the past decades many results on changes of food proteins caused by freezing and their relation to quality changes were found. Recently especially the nutrition physiological viewpoint of these alterations has aroused considerable interest. In the present paper the attempt is made to outline some aspects of these changes which are usually referred to as "denaturation" caused by freezing. After a description of the phenomena observed and their measurement, a survey of the main causes presumed for the "denaturation" of fish muscle proteins by freezing is given. With special consideration of the molecular range it can be said that generally aggregation phenomena prevail and that the participating molecules, or part of them, may still be native or more or less changed in their conformation before or during aggregation. Definite judgement of the nutritional properties of the altered proteins in frozen foods is possible only when convincing results of feeding experiments will be available.

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