Two-dimensional electrophoretic analyses of cod (Gadus morhua, L.) whole muscle proteins, water-soluble fraction and surimi. Effect of the addition of CaCl2 and MgCl2 during the washing procedure.

Samples from pre- and post-rigor cod mince, surimi (a concentrate of fish myofibrillar proteins obtained after washing and dewatering the fish mince) and water from the first wash in the surimi manufacture, processed with and without the addition of 7.5 mM CaCl2 and 15 mM MgCl2, were analyzed by two-dimensional electrophoresis. The results showed that the main myofibrillar proteins, including myosin, actin and tropomyosin, remained in the surimi. Several other proteins were selectively removed during the washing procedure. Some additional major spots were detected in the two-dimensional gels containing samples of the wash water and surimi processed with the addition of Ca2+ and Mg2+ salts. These spots were either absent or present in minor amounts in the samples of post-rigor cod mince, wash water and surimi processed without Ca2+ and Mg2+ salts and in all the pre-rigor samples. This induced us to suggest that the new additional spots may constitute fragments of proteins originated by increased proteolytic activity during the surimi manufacture upon the addition of the Ca2+ and Mg2+ salts. Two-dimensional electrophoresis has proved to be a valuable tool to quickly and easily assess the effect of different processing conditions on the protein content of the products.