Freezing Technology

1.3.3.1 Cytoplasm anaerobic metabolism

 Aerobic and anaerobic breakdown of glycogen in vertebrate & invertebrate muscle tissue.

Under anaerobic conditions, ATP may be synthesized by two other important pathways from creatine phosphate or from arginine phosphate. The former source of energy is restricted to vertebrate muscle (teleost fish) while the latter is characteristic of some invertebrates such as the cephalopods (squid and octopus). In either case, ATP production ceases when the creatine or arginine phosphates are depleted. It is interesting to note that octopine is the end-product from the anaerobic metabolism of cephalopods and is not acidic (unlike lactate), thus any changes in post mortem pH in such animals are not related to the production of lactic acid from glycogen. For most teleost fish, glycolysis is the only possible pathway for the production of energy once the heart stops beating.

After death, when the regeneration ceases, the ATP is rapidly degraded i.e. after death, the anaerobic muscle cannot maintain its normal level of ATP, and when the intracellular ATP level declines from 7 - 10 µmoles/g to <1.0 µmoles/g tissue, the muscle enters rigor mortis. Post mortem glycolysis results in the accumulation of lactic acid which in turn lowers the pH of the muscle. In cod, the pH drops from 6.8 to an ultimate pH of 6.1- 6.5. In some species of fish, the final pH may be lower: in large mackerel, the ultimate rigor pH may be as low as 5.8 - 6.0 and as low as 5.4 - 5.6 in tuna and halibut, however such low pH levels are unusual in marine teleosts.  In general, fish muscle contains a relatively low level of glycogen compared to mammals, thus far less lactic acid is generated after death. Also, the nutritional status of the fish and the amount of stress and exercise encountered before death will have a significant effect on the levels of stored glycogen and consequently on the ultimate post mortem pH. As a rule, well-rested, well-fed fish contain more glycogen than exhausted fish. It is show that bleeding of fish significantly reduced the post mortem production of lactic acid.

The post mortem reduction in pH of fish muscle has an effect on the physical properties of the muscle. As the pH drops, the net surface charge on the muscle proteins is reduced, causing them to partially denature and lose some of their water-holding capacity. Muscle tissue in the state of rigor mortis loses its moisture when cooked and is particularly unsuitable for further processing which involves heating, since heat denaturation enhances the water loss. Loss of water has a detrimental effect on the texture of fish muscle and it has been shown by Love (1975) that there is an inverse relationship between muscle toughness and pH, unacceptable levels of toughness (and water-loss on cooking) occurs at lower pH levels.