Effect of immersion freezing with the edible medium on protein structure, chemical bonding and particle size in grass carp (Ctenopharyngodon idellus) during frozen storage

One of the most worrisome problems with frozen fish is its undesirable softening texture, due in part to the alteration of fish protein structure. Therefore, a new immersion freezing (IF) technology with the edible medium, which was proven to improve food freezing speed and quality, was compared with air blast freezing (AF) and liquid nitrogen freezing (LNF) on the scope of protein structure. Protein fractions, structure, chemical bonds and particle size of the grass carp muscle were used as indicators. At the end of 24 weeks of frozen storage, the content of salt-soluble protein in IF (106.22 +/- 1.01 mg g(-1) meat) was significantly higher than that of AF (90.77 +/- 1.11 mg g(-1) meat) and LNF (97.44 +/- 1.40 mg g(-1) meat) (P < 0.05). Compared to AF, the alpha-helix content of IF and LNF was higher, the content of beta-turn and random coil was lower, while no statistical difference was detected in IF and LNF groups (P < 0.05). Meanwhile, the intrinsic fluorescence intensity, ionic bonding and hydrogen bonding in IF were significantly higher than in AF. The lowest value of D-[3,D-2] was observed in the IF group after 24 weeks storage. Therefore, IF is an effective approach to reducing protein denaturation and can be adopted as an alternative freezing method.