## 1 Introduction  
Annual global production of silver carp (_Hypophthalmichthys molitirix_) reaches about 4.9 million metric tons [1]. In China, silver carp is the most common carp species used in surimi processing, supporting an annual consumption of about 3.5 million metric tons. Due to the bony nature of silver carp [2], about 2.1 to 2.5 million metric tons of by-products, containing about 20-30% of protein, are produced during processing [3]. To maximize the potential values of the by-products and reduce environmental impact caused by wastes, various methods have been explored to efficiently recover valuable components from the by-products. For example, our previous studies have reported that protein hydrolyses of different structures and functional properties can be prepared by enzymatic hydrolysis of the by-products [4]. The resulting hydrolyses can be used to develop nutritional supplements, food additives, and oral liquids, etc.  
Recently, other studies have demonstrated that protein hydrolyses could reduce deterioration in quality of fish surimi or mince during frozen storage or repeated freeze-thaw treatments [5; 6; 7; 8; 9; 10; 11; 12; 13]. Although frozen storage is often used to preserve fish product, protein aggregation and denaturation could still occur and lead to potential decrease in product quality. Peptides in protein hydrolysate could slow the formation of ice crystals, thereby contributing to structural stabilization of proteins during frozen storage [6]. Carbohydrate-based cryoprotectants such as sucrose-sorbitol blend (SuSo) though are commonly used to maintain the quality of commercial fish product during frozen storage, impart a sensorially unfavorable sweetness to the product, along with the added caloric and glycemic values. These limitations of using carbohydrate-based cryoprotectants could render the end products unsuitable for many consumers and people with dietary restrictions (such as diabetics) [5; 14]. Therefore, protein hydrolysate is a preferred alternative cryoprotectant to SuSo, especially with its additional nutritional value (short peptides and free amino acids) in the absence of unwanted product sweetness and added caloric and glycemic value.  
Our previous studies have indicated that the enzyme type employed, and the degree of hydrolysis are factors that determine the structures and functionalities of protein hydrolyses prepared from by-products of silver carp. It is possible to alter the specific process of enzymatic hydrolysis of by-products to meet different application needs [4]. To this date, there is limited information about the effectiveness of protein hydrolyses, prepared from by-products of silver carp, as potential cryoprotectant used in surimi products. In the present study, protein hydrolyses were prepared using commercial proteinase (Protamex, an enzyme mixture containing exo- and endopeptidases; Alcalase, endoprotease) based on our previous studies [4]. Degree of hydrolysis (DH), zeta potential value, molecular weight distribution, total and free amino acid compositions of the hydrolyses were examined; and then the potential cryoprotective effects of the hydrolyses on actomyosin samples extracted from freeze-thawed surimi and textural properties of the freeze-thawed surimi gels were evaluated.