Calpains

Calpains are a family of calcium-dependent, non-lysosomal cysteine proteases (proteolytic enzymes) that are expressed ubiquitously in mammols.

“At the lower temperature muscle proteins contract and squeeze out water far more slowly, which is crucial to keeping the meat moist. But it also needs to be tenderised, which at this temperature is done by enzymes, particularly calpains and cathepsins that weaken or break down collagen and other proteins. Calpains stop working at 40°C/105°F, cathepsins at 50°C/120°F, but below these cut-off points, the higher the temperature, the faster they work. Heating the meat slowly means these enzymes can perform their magic for several hours before denaturing, effectively ageing the meat during cooking. The result is the tenderest, tastiest meat imaginable.”

– Heston Blumenthal, In Search of Perfection, p. 166