Effect of refrigeration on texture of meat 5 The majority of beef the error had been only partially tenderness would have been improved if the beef had been stored for a further week. Many retailers nowadays condition beef for longer periods, but economic factors often still dictate the time of conditioning. Mechanism of ageing mue major change, which takes place in meat during ageing, occurs in the uscle fibre. Little or no change which can be related to tenderness improvement takes place in the structures which hold the fibres together (the connective tissue, collagen)(Herring et aL, 1967) Conditioning is caused by the presence of proteolytic enzymes in the muscle which slowly catalyse the breakdown of some of the muscle pro- teins. This causes weakening of the muscle so that the meat is more readily pulled apart in the mouth and is therefore tenderer. Two groups of enzymes are thought mainly responsible: calpains, which are active at neutral pH shortly after slaughter, and cathepsins, which are active at acid pH after rigor(Offer et aL, 1988) Dransfield(1994)states that it is generally accepted that tenderisation results from proteolysis by endogenous enzymes. The major problem in identifying the specific enzymes has been that the enzyme activities cannot be measured in meat since they depend on local in situ concentrations of cofactors and inhibitors. However, modelling the activation of calpains shows how tenderness develops and points to methods of optimising its and then calpain II is activated as the concentration of calcium ions ee y development. Calpain I is activated first, at low calcium ion concentratio further. There are enough free calcium ions to activate all of calpain I but only about 30% of calpain II. Tenderisation therefore begins when calpain I starts to be activated, normally at about pH 6.3 or about 6h after slaughter in beef, and rapidly increases as more calpain is activated After about 16h in beef, calpain II becomes activated and causes a further tenderization The calpain-tenderness model shows that in beef longissimus dorsi, most of the tenderisation is caused by calpain I. Approximately 50% of the tenderisation occurs in the first 24h, after which the rate is exponential. The model clearly shows that the ultimate tenderness of the meat will depend on(1)the tenderisation that occurs during chilling and(2)further tenderisation during storage. In extreme cases, for example dark, firm and Iry (dfd) beef, all the tenderisation will occur in stage 1 and none during ageing. The incidence of DFd beef is markedly dependent on the sex of the animal. It occurs in about 1-5% of steers and heifers. 6-10% of cows and 11-15% of young bulls (Tarrant and Sherington, 1981). Rigor develop- ment is very rapid in DFD beef and during normal cooling to an ultimate oH of 7.0, all of the tenderisation occurs before 24 h and no ageing occurs (Dransfield, 1994The majority of beef therefore had been only partially conditioned and tenderness would have been improved if the beef had been stored for a further week. Many retailers nowadays condition beef for longer periods, but economic factors often still dictate the time of conditioning. 3.2.1 Mechanism of ageing The major change, which takes place in meat during ageing, occurs in the muscle fibre. Little or no change which can be related to tenderness improvement takes place in the structures which hold the fibres together (the connective tissue, collagen) (Herring et al., 1967). Conditioning is caused by the presence of proteolytic enzymes in the muscle which slowly catalyse the breakdown of some of the muscle pro￾teins. This causes weakening of the muscle so that the meat is more readily pulled apart in the mouth and is therefore tenderer.Two groups of enzymes are thought mainly responsible: calpains, which are active at neutral pH shortly after slaughter, and cathepsins, which are active at acid pH after rigor (Offer et al., 1988). Dransfield (1994) states that it is generally accepted that tenderisation results from proteolysis by endogenous enzymes. The major problem in identifying the specific enzymes has been that the enzyme activities cannot be measured in meat since they depend on local in situ concentrations of cofactors and inhibitors. However, modelling the activation of calpains shows how tenderness develops and points to methods of optimising its development. Calpain I is activated first, at low calcium ion concentrations, and then calpain II is activated as the concentration of calcium ions rises further. There are enough free calcium ions to activate all of calpain I but only about 30% of calpain II. Tenderisation therefore begins when calpain I starts to be activated, normally at about pH 6.3 or about 6 h after slaughter in beef, and rapidly increases as more calpain is activated. After about 16 h in beef, calpain II becomes activated and causes a further tenderisation. The calpain-tenderness model shows that in beef longissimus dorsi, most of the tenderisation is caused by calpain I. Approximately 50% of the tenderisation occurs in the first 24h, after which the rate is exponential. The model clearly shows that the ultimate tenderness of the meat will depend on (1) the tenderisation that occurs during chilling and (2) further tenderisation during storage. In extreme cases, for example dark, firm and dry (DFD) beef, all the tenderisation will occur in stage 1 and none during ageing. The incidence of DFD beef is markedly dependent on the sex of the animal. It occurs in about 1–5% of steers and heifers, 6–10% of cows and 11–15% of young bulls (Tarrant and Sherington, 1981). Rigor develop￾ment is very rapid in DFD beef and during normal cooling to an ultimate pH of 7.0, all of the tenderisation occurs before 24 h and no ageing occurs (Dransfield, 1994). Effect of refrigeration on texture of meat 51