Effects of mackerel cathepsins L and L-like, and calpain on the degradation of mackerel surimi

During surimi processing, the total cathepsins B+L+L-like activities in minced, leached and NaCl-ground meats were 2.45, 2.07 and 1.77 units/g, respectively. There was still 77% activity left even after 8 weeks storage at -20 degrees C. High calpain and calpastatin activities In the crude enzymes from frozen mackerel surimi suggested that they were difficult to remove during processing and very stable during frozen storage. The SDS-PAGE analysis indicated that the optimal conditions for the hydrolysis of myosin heavy chain (MHC) were 40-55 degrees C, pH 5.5-7.0 for cathepsins L and L-like and pH 7.0-7.5 for calpain. The gel strength of surimi treated with calpain did not decrease significantly, but the strength of those treated with purified cathepsins L and/or L-like decreased significantly (P<0.05) after 2h incubation at 55 degrees C. These results suggested that cathepsins L and L-like left in surimi had MHC-degrading ability which consequently caused gel softening during setting at 40-55 degrees C.