The effects of various cryoprotective additives separately and in combination were studied on the myofibrillar protein integrity, biochemical enzyme activity levels and muscle ultrastructure in the freshwater teleost fish Rohu carp (Labeo rohita). Fish muscle samples were divided into eight groups and immersed in different mixtures of cryoprotective additives (S1-S8), then frozen at -20 or -30 degrees C for 24 months. Electrophoretic studies revealed early (within 6 months) alteration of the myofibrillar proteins myosin light chain, a-actinin and tropomyosin. Reduction of the storage temperature from -20 to -30 degrees C slowed down the degradative processes. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated that fish muscle treated with cryoprotective mixture SS (40 g L-1 sorbitol/3 g L-1 sodium tripolyphosphate/4 g L-1 sodium alginate) showed minimal post mortem changes in myofibrillar proteins. Ultrastructural results also revealed post mortem damage to the muscle, seen earliest (within 6 months) in the sample frozen-stored without additives (S2), as compared with the normal, unfrozen muscle (S1). The influence of cryoprotectants alone and in combination on fish muscle structural proteins, myosin and actin filaments (A and I bands), during prolonged frozen storage was investigated. After 12 months, samples frozen-stored with various cryoprotective additives (S2-S7), except S8, showed signs of myofibrillar disintegration. Beyond that time the degradative processes started showing up in all samples, with minimal muscle ultrastructural damage in sample S8. Again, reducing the storage temperature from -20 to -30 degrees C slowed down the degradative processes. Ultrastructural results correlated well with levels of biochemical enzymes (Ca2+ myofibrillar ATPase and succinic dehydrogenase) during frozen storage. This is the first report of the cryoprotective effects of these additives on this popular edible fish species. Of the various combinations of additives tested, cryoprotective mixture SS was found to preserve the muscle structure longest under frozen storage conditions. However, even this mixture was only effective for 18 months at -30 degrees C. Beyond that time the myofibrillar degradative processes were apparent with correlative electrophoretic, biochemical and ultrastructural studies. (c) 2006 Society of Chemical Industry.
