# Marine Bioactives and Their Application in the Food Industry: A Review
## 2 Marine Sources of Bioactive Molecules
### Marine Fish
#### 2.4.2 Fish Proteins  
Fish protein is the dominating source of seafood-derived protein as fish is considered a rich source of easily digestible protein and essential amino acids. Much research has investigated the utilization of fish muscle protein and protein-derived products such as peptides, collagen, and gelatin, which are used as functional ingredients in the food, pharmaceutical, and nutraceutical sectors. Approximately 11-27% of seafood comprises crude proteins and non-protein nitrogen (NPN) compounds. NPN compounds include free amino acids, peptides, amines, amine oxides, guanidine, quaternary amino acids and ammonium compounds, polyamines, nucleotides, nucleic acids, and urea. In addition, all other muscle and seafood proteins are classified into three groups, namely myofibrillar, sarcoplasmic, and stroma types. Sarcoplasmic protein contains approximately 15-35% of the total muscle tissue protein, while myofibrillar represents 65-75%. Stroma types only represent approximately 3% of the total muscle tissue protein [31,35,136]. Sarcoplasmic proteins such as myoglobin and other albumins can be extracted to water or neutral solutions, whereas myofibrillar proteins like actin, myosin, tropomyosin, and troponins are soluble in high salt solutions. Stroma-type proteins (collagen and elastin from connective tissue) remain in the residue after extraction of myofibrillar and sarcoplasmic proteins [137]. On the other hand, the content of NPN compounds in seafood, including marine fish, is primarily influenced by factors like the specific species, freshness level, and habitat. Fish processing discards also play a key role as a source for obtaining high-quality protein and protein-derived products (e.g., hydrolyses/peptides). Generally, the percentages of proteins in processing by-products are cuts (15-20%), bones (9-15%), heads (9-12%), skin and fins (1-3%), viscera (12-18%), and scales (5%) [138,139]. The global demand for collagen and gelatin continues to grow steadily due to their extensive applications in the food, nutraceutical, pharmaceutical, and cosmeceutical industries. Gelatin, the partially hydrolyzed form of collagen, is a widely employed ingredient in the food sector. The functional properties of the extracted collagen and/or gelatin are closely tied to their quality and their suitability for specific uses. In particular, fish skin collagen and gelatin are gaining more attraction as an alternative to mammalian collagen because of their purity and low risk of possessing unknown pathogens [31,85,118,140]. Development of Fish Protein hydrolyses/Peptides  
Fish protein hydrolyses (FPH) have demonstrated excellent physicochemical and functional properties such as antioxidant, antihypertensive, mainly as inhibitors of ACE, antimicrobial, and immunomodulatory functions, which are often linked to peptide sequences. Hence, these proteins and their hydrolyses have been analyzed for their nutritional and functional properties related to the possibility of recovering biologically active peptides over the years. Bioactive peptides are specific protein fragments acting as a source of nitrogen and amino acids that have a positive impact on body functions or conditions and may ultimately influence health [5,118]. In particular, bioactive peptides with short sequences, typically containing 2-20 amino acids, showed greater biological activities. The nature of bioactive peptides produced from a specific protein is determined by two key factors: the selectivity of the enzyme employed for peptide generation and the primary sequence of the protein substrate [35,141]. Bioactive peptides can be produced using chemical (acid or alkaline) hydrolysis, enzymatic degradation, or microbial fermentation of proteins. When it comes to marine bioactive peptides, they are typically produced through enzymatic hydrolysis using suitable proteolytic enzymes derived from microbes, plants, and animals.