Mechanism of sea cucumbers (Apostichopus japonicus) body wall changes under different thermal treatment at micro-scale

Thermal treatment was essential in sea cucumber (SC) processing, which can affect textural properties and storage stability of SC. TPA showed that the elastic gel of SC body wall was formed and then deteriorated. Hardness of SC was significantly increased from 2.33 N to 5.66 N (10-80 min) and then decreased into 1.51 N (640 min) when heated at 60 degrees C, while it was decreased from 2.73 N (10 min) to 0.15 N (160 min) when heated at 120 degrees C. The weight, volume and length of SC were decreased when temperature ranged from 60 to 120 degrees C. Scanning electron microscopy and transmission electron microscopy showed that collagen fibers and D-period were destroyed and disappeared gradually. Free hydroxyproline and ammonia nitrogen increased significantly during thermal treatment. The molecular weight of protein and polysaccharide decreased obviously along with the increase of temperature. SC collagen was degraded into peptide fragments during heating, and the cleavage sites were identified as G, P, A, L, Y, K and R. Therefore, changes of SC gelation and deterioration were related to unfolding and degradation of SC collagen and break of polysaccharide, accompanied by the break of hydrogen bond, glucopeptide bond and peptide bond during thermal treatment.