Freezing Technology

1.3.3.2 Autolysis and nucleotide catabolism

Autolysis and nucleotide catabolism

                 According to the present understanding, rigor mortis is caused by bonding of the myosin heads, extending radially from the thick microfibrils, to the active centers in actin units of the thin filaments. This leads to the formation of a rigid structure of interconnected myofilaments. This reaction between these major muscle proteins is made possible by changes in the regulatory proteins. These changes are in turn induced by an increase in the concentration of Ca²⁺ in the sarcoplasm. In a resting muscle, the concentration of free Ca²⁺ in the sarcoplasm is lower than 10^-7 M. This level is maintained because of the action of various calcium pumps, located in the cell membrane and in the sarcoplasmic reticulum, which drives the Ca²⁺ out of the sarcoplasm, against the concentration gradient. The calcium pumps work at the expense of energy from ATP hydrolysis. After the death of the animal, a gradual depletion of ATP in the muscles takes place, due to exhaustion of the creatine phosphate and glycogen reserves. This impairs the action of the calcium pumps and increases the concentration of Ca²⁺ in the sarcoplasm. At ATP concentrations below 10^-4 M and that of Ca²⁺ above 10^-6 M, rigor mortis sets in. As the individual muscle fibers contain different quantities of ATP, they do not enter rigor at the same time. Thus, stiffness in a muscle sets in gradually.

                 The resolution of rigor is a process still not completely understood but always results in the subsequent softening (relaxation) of the muscle tissue and is thought to be related to the activation of one or more of the naturally-occurring muscle enzymes, digesting away certain components of the rigor mortis complex. The softening of the muscle during resolution of rigor (and eventually spoilage processes) is coincidental with the autolytic changes. Among the changes, one of the first to be recognized was the degradation of ATP-related compounds in a more-or-less predictable manner after death.

 

 Degradation of ATP to hypoxanthine (Hx) and ribose

                Freshness is expressed by K-value. This K-value expresses the relationship between inosine and hypoxanthine and the total amount of ATP-related compounds:

                                                                 HxR + Hx

           K(%) = ------------------------------------------------------------------------- x 100

                                  ATP + ADP + AMP + IMP + HxR + Hx

                

                 Very fresh fish, have low K-values. K-values increase gradually which is species dependent. Inosine is said to be more or less flavourless, while hypoxanthine has been reported to impart a bitter flavour in spoiling fish (Spinelli, 1965). Hx is considered to have a direct effect on the perceived bitter off-flavour of spoiled fish (Hughes and Jones, 1966). It is now widely accepted that IMP is responsible for the desirable fresh fish flavour which is only present in top quality seafood.