Physicochemical changes of MTGase cross-linked surimi gels subjected to liquid nitrogen spray freezing

Physicochemical properties of microbial transglutaminase (MTGase) cross-linked surimi gels subjected to liquid nitrogen (LN) spray freezing with different temperatures and cross-linking degrees were investigated. Gels with lower LN spray temperature (-90 °C) were found taking less time in cooling down themselves to maximum-ice-crystal generating temperature. Microstructure images showed the pores of gels became smaller and the structure gradually became denser, as freezing temperature decreased and cross-linking degree increased. It also revealed T₂₂ relaxation time of gels decreased significantly with surimi gels cross-linking degree increasing, indicating the binding ability of gels to moisture was enhanced accordingly. Meanwhile, the proton density weighted image brightness declined along with the LN spray temperature decreasing, and the image brightness showed a decreasing trend from outside to inside, indicating that water migrated and permeated easier from the inside of the gel network to the outside undergone higher LN temperature and lower cross-linking degree. Besides, the L^⁎ and W values of LN groups decreased along with LN spray temperature and cross-linking degree increasing. Moreover, -90 °C LN group with 46.70% cross-linking degree presented the highest breaking force which ascribed to their synergistic efforts in maintaining a stable and dense structure of gels via controlling ice crystals and cross-linkages' generation.