Enzymatic protein hydrolysis is a method commonly used to extract proteins from fish side stream products. The main purpose of a hydrolysis process is to degrade proteins thus solubilizing them in water, increase protein recovery and the yield of valuable components. Protein hydrolysis can be achieved chemically or enzymatically, where the latter is preferred of products for human consumption. The hydrolysis process will yield a water-phase (the protein hydrolysate), a sludge phase and oil phase, if based on a fatty raw material. The fish protein hydrolysate (FPH) can be dried to a protein powder that is applicable within a variety of products.

The production of FPH includes the following steps:

• Preparation of raw material (mincing and dilution with water)
• Adjustment of reaction temperature
• Addition of enzyme
• Hydrolysis reaction for a predefined hydrolysis time
• Inactivation of enzyme
• Separation of the water, oil and semi-solid phases

Proper dilution with water, usually 1:1, can prevent product inhibition and maximize product yield, but added water is also a factor influencing processing costs. In industrial operations, final products are often dried and a compromise between desired product yield and water that needs to be removed is imperative.

The hydrolysis process and enzyme performance depend on reaction temperature and pH. Several commercial proteases working well under the substrate natural pH are available, avoiding the need for pH-adjustment. At the end of the reaction, the enzyme activity is terminated by irreversible denaturation of the enzyme by heating the slurry to temperatures above 85 ℃ for at least 10 minutes. Finally, the slurry is separated into three phases: an oil phase, a water phase and a sludge phase.

The enzymes responsible for cleaving peptide bonds belong to the family of proteases (also called peptidases or proteinases). Several types of proteases are known (Table 1.4.1) and can be classified based on critical amino acid required for the catalytic function, pH-optimum for their activity, their site of cleavage or the requirement of a free thiol group Proteases can be divided into endopeptidases and exopeptidase, based on their preference for cleavage. Endopeptidases catalyze the peptide bonds in the interior of the peptide chain, leaving two new, smaller peptides. The exopeptidases, on the other hand, require the presence of an unsubstituted N- or C-terminus, only releasing free amino acids or small di- and tripeptides. Enzymes are systematically classified by a European Commission (EC) number according to rules of nomenclature defined by the Nomenclature Committee of International Union of Biochemistry and Molecular Biology (NC-IUBMB 1992). Proteolytic enzymes are defined as number 3 (hydrolases) 4 (proteases) and 11-19 (exopeptidases) or 21-99 (endopeptidases). To read more about protease enzymes, see more.

There are a multitude of distributors of commercial enzymes, where Novozymes (Denmark), Biocatalysts (UK) ABEnzymes (Germany), DuPont (US) and Enzybel (BE) are some examples of distributors of enzymes applicable for valorization of marine side stream materials. Proper choice of enzyme is important in the hydrolysis of marine side stream materials, as this will influence the processing costs and sensory and physicochemical properties of the final products.