Characterization of tuna dark muscle protein isolate.

Fish protein isolate (FPI) was extracted from tuna dark muscle (TDM) using the pH‐shift method that improves significantly its sensory quality and nutritional value. The amino‐acid composition of the FPI was close to that of TDM, but the total protein content was higher. The solubility of FPI in water was higher than 85% at pH ≥ 6 and pH ≤ 3, but it dropped to about 10% at pH values close to the isoelectric point pI ≈ 5.0. The soluble FPI was present in the form of aggregates that were characterized by light‐scattering. The microstructure of FPI solutions on larger length scales was probed by confocal microscopy, large protein flocs formed at pH close to pI. SDS‐PAGE indicated that a significant fraction of myosin heavy chains was broken down into smaller chains. The viscosity decreased with decreasing of pH, which correlated with the decreasing size of the aggregates. Novelty Impact Statement: Tuna dark muscle (TDM) represents a significant fraction of industrially processed tuna and it is currently considered a low value by‐product because of its poor sensory qualities, but TDM has a high protein content that does not suffer from sensory defects if it is extracted by dispersing TDM at pH 12 and precipitating the dissolved protein at pH 5.5. The protein isolate is highly soluble (> 85%) in water at pH > 6 and pH < 3, where it is present in the form of small aggregates with average radii between 90 and 250 nm, depending on the pH. The characterization of the composition of TDM protein isolate and its properties in water show that it is a viable product for use in industrial applications. [ABSTRACT FROM AUTHOR]