A comparative study on stability and functional properties of the proteins isolated from yellowfin tuna (Thunnus albacares) dark muscle by acid-aided and alkaline-aided processes

Introduction: At least 60% of the estimated 300,000 metric tons of tuna that are processed in Iran areby-products which arebeingwasted and converted to non-human products as fish meal or fertilizers. Therefore, a major challenge facing the tuna canning industry is to find the new processes to utilize tuna processing by-products (mainly dark muscle) into valuable foods. The characteristics of tuna dark meat (TDM) make it not acceptable for these industries. Therefore, the isolation of proteins from TDM for food application would be a more responsible way of using a nutritious and abundant rest raw material. The pH-shift technology for recovering fish proteins involves the solubilisation of chopped and homogenized fish flesh either in an aqueous acidic or alkaline solution. The protein rich solution is separated from solids (insoluble proteins, skin, bones, and scales) and neutral lipids by centrifugation. The soluble proteins are then recovered by isoelectric precipitation by adjusting the pH to 5.5 and the precipitated proteins are removed by centrifugation. This method can be potentially applied with any white/ dark muscle fish or fish by-products. No evidence can be foundon isolation of protein from TDM. Therefore, this study was carried out to investigate stability and functional properties of proteins recovered from TDM. Materials and methods: The ground TDM was homogenized for 1 min (speed 50) with 9 volumes of ice-cold distilled water. The proteins in the homogenate were solubilized by dropwise addition of 1 N HCl or 1 N NaOH until the intended pH (2.5, 3.0 and 3.5or10.5,11.0 and 11.5) was reached. The protein suspension was centrifuged. The soluble proteins were precipitated by adjusting the pHs to 5.5 using 1 N NaOH or 1 N HCl. Precipitated proteins were collected via a second centrifugation. Proximate analysis of tuna protein isolates (TPI) was carried out. TBARS, pH, viscosity, water holding capacity (WHC), gel strength, biting and folding tests, texture profile analyses (TPA), and color were measured. Qualitative protein analysis was carried out using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Results and discussion: The protein, fat and moisture contents of the acid-aided protein isolates were found to be 28.65, 5.35 and 74.36% respectively. While alkaline-aided protein isolates contained 29.57% protein, 4.17% fat and 71.23% moisture. A significant difference was found in TBARS level between the isolated products. The lowest TBARS value was found in acid-aided isolate and isolate treated at pH 11.5. The TBARS value of isolates extracted at pH 10.5 and 11 was 0.15 mg malondialdehyde / kg, which was below the border line recommended for fish products. Lipid oxidation in fish protein isolates has been reported during pH-shift process. The lipid content of TPI samples and activating of haem proteins as prooxidants at different pH may describe lipid oxidation in TPI samples. The average viscosity of TPIs was 3.81 cP (Centipoise). The highest viscosity scores were observed for the isolates prepared at pH 11.0 followed by the isolates made at pH 3.5 and 11.5. The isolates treated at pH of 2.5, 3.0 and 10.5 had the same level of viscosity. Low viscosity might be due to low cross linking degree of protein molecules. The low viscosity of the prototypes may possibly be explained by decreasing interaction between proteins and the surrounding medium. Therefore, denaturation and modification of protein conformation in tuna protein samples may have affected the viscosity. The WHC of the samples (12-16%) was similar to the proteins isolated from the other fish by-products. The highest value of WHC among TPIs was found for the isolates prepared at pH 3.5. The rest samples had the same value of viscosity. The WHC can be defined as the ability of a protein gel to retain water against a gravitational force. The level of water retained in a gel is affected by the same factors that affect the formation of a good protein gel ‘i.e.’ moisture, pH and salt. Furthermore, the WHC usually reflects the extent of denaturation of the protein and water contents. It has been reported that WHC is closely related to fish species, amount of salt, different processing method and the interaction between these factors. The highest scores for gel strength, biting and folding tests and TPA (hardness, cohesiveness, springiness and resilience) were observed in TPIs treated at alkaline pH. The muscle proteins being particularly responsible for gelation are myosin and actomyosin. It has been reported that alkali-aided protein extraction caused less denaturation than an acid-aided process. This lower denaturation of proteins leads to products with enhanced texture. Hardness and cohesiveness were found to be maximum for samples prepared at pH of 11.5. The increase in hardness may also be due to the stronger gel network formed by the concentrated myofibrillar proteins in the protein isolates. The difference between TPA parameters of the recovered proteins andthe TDM mightbe due to the difference in lipid and collagen content. The alkali-aided process recovered proteins of higher whiteness than the acid-aided process possibly due to high removal amount of myoglobin and haemoglobin during leaching. The electrophoretic patterns revealed the stability of proteins in alkaline pH. The lowest reduction in band intensity of myosin (myosin heavy chain) and actin was found when the alkaline-aided process was applied. Accordingly the highest band intensity of myosin and actin proteins was observed at the high pH (11). The weak bands of protein among acid-aided samples have possibly been due to the hydrolysis effect of enzyme activity.