Skin waste from tuna processing needs to be utilized, such as extraction of its collagen and gelatin. Their functional properties can be improved by enzymatic hydrolysis for conversion to peptides. Thus, the research objectives were to examine the characteristics and antioxidant activity of collagen, gelatin, and the derived peptide from yellowfin tuna skin. Collagen was extracted using 0.75 M acetic acid at 4 °C, while gelatin was prepared using 0.25% citric acid and extracted at 65 °C. Hydrolysis was carried out with 2% Alcalase, followed by fractionation with a molecular weight cut off sieve for both collagen and gelatin. Collagen yield was 22.6% with pH value of 6.63 and whiteness of 96.7%. Gelatin yield was 20.0% with pH value of 4.94 and whiteness of 51.0%. Hydrolysis for three hours resulted in 52.7% and 45.2% degree of hydrolysis for collagen and gelatin, respectively. The molecular weights of collagen peptides ranged from 2.94 to 11.93 kDa, while those of gelatin peptides ranged from 3.54 to 16,620 kDa. Antioxidant activities of these peptides were higher than those before hydrolysis. The high antioxidant activity (IC₅₀) of collagen peptides were found in <3, 3–10, and 10–30 kDa fractions as well as in the gelatin peptides.

Around 40% of the total catch weight of fish is regarded as byproducts, consisting of skin, fins, bones, scales, viscera, etc. The utilization of these byproducts is important to increase their commercial values as well as to prevent environmental pollution. Meanwhile, nowadays, it is getting a global trend to provide foods and other industrial materials which have been accredited as halal products for Moslem communities. As a way of processing fish byproducts to meet the halal criteria, preparation of collagen and gelatin would be useful to fulfill the market demand. As a result of screening studies on fishery byproducts, fish skin has been found to be the good source for halal collagen and gelatin, which show satisfactory quality compared with those from bovine sources which could cause bovine spongiform encephalopathy (BSE).

Fish and marine invertebrates constitute an important part of the human diet worldwide, and their muscles are the major edible parts. The muscles are a rich source of proteins, which in other terms determines the nutritional value and the quality of seafood products. Though the fish skeletal muscle proteins share many similarities with their mammalian counterparts, there exist great differences in their biological activity and structural stability. While the muscles of shellfish or aquatic invertebrates share many properties and general structural features with their vertebrate counterparts, they have unique characteristics. Therefore, understanding the biochemical and physicochemical properties of their major protein components is needed from the viewpoint of effective utilization of aquatic bioresources. Thus, in this review, we aimed to assemble the basis of such differences and also to understand the benefits of these proteins as the targets for studies on the structure–stability relationship of proteins.

An expression system of recombinant myoglobins (Mb) of 3 scombridae fish species was constructed. The stability of these Mbs was compared with native Mbs purified from slow skeletal muscle. The addition of hemin during the cultivation of an Escherichia coli strain harboring a pGEX-2T expression vector was found to be necessary to prevent recombinant Mb from degrading and to attain its proper folding. The stabilities of recombinant Mbs were generally lower than those of native Mbs, partly due to the absence of post-translational modification. The alpha-Helical content of bullet tuna recombinant Mb at 10 degrees C was the lowest (29.0%) among the recombinant Mbs examined (the values for bluefin tuna and bigeye tuna Mbs being 34.8 and 35.5%, respectively). On the other hand, the stabilities of recombinant Mbs of bluefin tuna and bigeye tuna against denaturants (urea and guanidine hydrochloride) were found to be similar, whereas bullet tuna recombinant Mb exhibited the lowest stability among these Mbs. The pattern of temperature-dependent decrease in the alpha-helical content supported these results.