3.1.1. Salt-Soluble Protein Content

MP is the dominant salt-soluble protein in the surimi, whose solubility variation is an important indicator to reflect its denaturation during frozen storage [34]. As shown in Figure 1a, the salt solubility of MP from all groups of surimi were about 86 mg/g before freezing and kept decreasing during the entire storage period. After 60 days, the solubility of the control group reached 58 mg/g. The decrease of protein solubility could be attributed to the freezing-induced denaturation/aggregation of MP in surimi as the storage time extended [12,35]. With the addition of cryoprotectants, the MP solubility of frozen surimi was significantly higher, whose values were 62, 70, and 72 mg/g, respectively, regarding to CMCO-A, -B and commercial group. Similar to most other saccharide cryoprotectants, CMCO could restrain the movement of water molecules and inhibit the growth of ice

in frozen surimi, which alleviated the MP denaturation. Moreover, the MP solubility of CMCO-B group was higher than that of CMCO-A at the same storage time. It has been shown that the complexity of hydrogen bonding enhanced in the food matrix as the DS of charged polysaccharide increased [20,36]; thus, water molecules became more bounded to the protein networks, which led to a better cryoprotective effect to the MP, i.e., CMCO-B group > CMCO-A group, whose content of salt-soluble protein was 70 and 62 mg/g, respectively.

**Figure 1.** Effects of CMCO on salt-soluble protein content (**a**) and Ca2+-ATPase activity (**b**) of MP at different storage time.
