Effect of duration of electrical stimulation on meat quality and the calpain system : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Science (Honours) at Lincoln University

Abstract

Calpains are a family of calcium dependent cysteine proteases that are found to be widely expressed across mammalian cells. Ubiquitous calpains are one type of calpain, and are present in two main forms, µ­-calpain and m-calpain, which are a reflection of the relative calcium concentration required for activation. Both forms of calpain contain two distinct subunits, 1 an 80kDa subunit, which is unique to each isozyme of calpain, and 2 a 30kDa subunit, which has regulatory properties and is common to both forms. The addition of calcium to the calpain system results in an autolytic cleavage to the calpain molecule, whereby sections of the N terminal of each subunit become truncated, removing domains that are blocking activity. Calpastatin is an endogenously expressed protein inhibitor and the only known naturally occurring inhibitor of calpain. Calpastatin has a monomeric structure with a molecular weight of between 105 and 110kDa, and like the calpains it inhibits, namely µ and m, requires calcium for activation. The role of the calpain system lies within the breakdown and turnover of muscle tissues; it is believed by many that calpains due to their proteolytic nature may be involved in the tenderisation process of meat. There are a number of factors that influence the quality of a particular cut of meat. Tenderness is often considered to be the most important criterion of meat quality, yet is the most difficult to determine, frequently giving the most problems through a high degree of variability. The decline in both pH and temperature has been found to influence the final quality of meat. Consequently, muscles with a higher pH were found to tenderise more slowly, indicating that the rate of pH decline after slaughter is related to the level of meat tenderisation. High levels of electrical stimulation have been found to give a more rapid decline in pH, causing µ-calpain to autolyse earlier than usual, elevating the initial levels of activated µ-calpain to begin the process of tenderisation earlier. However, elevated post-mortem levels of µ-calpain and calpastatin were found to have declined rapidly by 24 hours post-mortem. The rate and extent to which the pH declines dictates the activation and subsequent autolysis of µ-calpain, which influences the amount of tenderisation. During rapid declines in pH, larger amounts of µ-calpain will activate during the incubation period, thus resulting in increased degradation of calpastatin and myofibrillar proteins, causing meat tenderisation. Learning more about how the calpain system operates, and how processing techniques effect overall meat tenderness will be of potential benefit to the consumer, as it will help to move one step closer in obtaining the goal of providing a product with a consistent level of tenderness.