The quality of the finished product will depend on the methods used for stunning fish. In many freshwater species, the way a fish is stunned is crucial to its quality. Prolonged agony can cause undesirable compounds to accumulate in tissue and blood. Lack of oxygen in the blood results in an accumulation of lactic acid and reduced metabolic products in the muscle tissue. In addition, the fish’s nervous system becomes paralyzed, resulting in red spots on the surface of the skin and in the muscle tissue near the backbone. These spots reduce the quality of the final product.
The most common method is to use a grading machine. Grading machines usually consist of a series of compartments connected by slits of varying sizes. These compartments have conveyor belts or rotating rollers in a V-shape. The fish are sorted according to their maximum thickness, which is correlated with their length. They can be programmed to remove ribs according to the desired thickness. If you want to avoid the ribs, you can use a contoured cut.
The final products of preliminary processing can be sold or further processed. These products are a good source of protein, minerals, and other nutrients. Some of these by-products are recovered to a high degree. A meat-bone separator can recover up to 50% of the mass of processed backbone. In addition to that, it can also be used as a constituent in other industrial processes, such as making fertilizer. Various other types of fish by-products can be used to produce value-added products and commodities.