On Food and Cooking

...And Diminish Toughness Uncontrolled
enzyme activity also tenderizes meat.
Enzymes called calpains mainly weaken the
supporting proteins that hold the contracting
filaments in place. Others called cathepsins
break apart a variety of proteins, including the
contracting filaments and the supporting
molecules. The cathepsins also weaken the
collagen in connective tissue, by breaking
some of the strong cross-links between mature
collagen fibers. This has two important
effects: it causes more collagen to dissolve
into gelatin during cooking, thus making the
meat more tender and succulent; and it
reduces the squeezing pressure that the
connective tissue exerts during heating (p.
150), which means that the meat loses less
moisture during cooking.
Enzyme activity depends on temperature.
The calpains begin to denature and lose
activity around 105ºF/40ºC, the cathepsins
around 122ºF/50ºC. But below this critical