Pacific hake (Merluccius productus) hydrolysates as cryoprotective agents in frozen pacific cod fillet mince

Fish protein hydrolysates produced by proteolysis of Pacific hake (Merluccius productus) with Alcalase (FPH-A) or Flavourzyme (FPH-F) were investigated as a potential alternative to the 1 : 1 blend of sucrose-sorbitol (SuSo) commonly used for cryoprotection of frozen fish mince. The physicochemical properties of cod mince samples in the absence (control) or presence of 8% FPH-A, FPH-F, or SuSo were evaluated before and after 6 freeze-thaw cycles, with differences noted at the 5% significance level. Freeze-thawing of control sample increased expressible moisture (from 22% to 33%) and cook loss (from 3% to 16%). These poor water retention properties were improved in samples containing FPH or SuSo. Differential scanning calorimetry showed higher proportion of unfrozen water in freeze-thawed samples containing FPH-F or FPH-A (0.36 g/g) compared to SuSo (0.33 g/g) and control (0.24 g/g) samples. Textural analysis of cooked mince from unfrozen samples indicated greater hardness for FPH than SuSo and control samples, while freeze-thawing resulted in decreased hardness for FPH and SuSo samples. Content and surface hydrophobicity of extractable natural actomyosin (NAM) were maintained after freeze-thawing of samples containing FPH-F or SuSo, compared to 50% decrease in extractable NAM and a significant increase in surface hydrophobicity for the control. The presence of oligopeptides in both hydrolysates and the high levels of free amino acids including Asp, Glu, Arg, and Lys in FPH-F might be responsible for their cryoprotective action. This study provides strong evidence to support development of FPH as a new generation cryoprotectant to maintain quality of frozen fish.