## 1 Introduction  
Mussel cultivation for human consumption has been practiced for years through rope or raft cultivation systems, and the total global productivity of the common blue mussel (_Mytilus edulis_) is approximately 20,000 tons (9% of the total global mussel production) [1]. Of the total produced mussels, 27% is discarded as by-products [2]. By-product mussels can be edible mussels, including seed mussels, undersized mussels, mussels with broken shells or mussels that are fouled with barnacles, or they can be inedible due to the presence of toxins such as azaspiracids (AZA) and diarrhetic shellfish poisoning (DSP) toxin. However, mussel by-products are a rich source of protein, lipids and essential amino acids. Bioprocessing of by-product mussels for high-value commercial ingredient development could contribute positively towards the circular economy. However, characterisation of the raw material/by-products is necessary before initiating such work, as there is a high degree of variability in the composition of mussel by-products, which will result in variability in the end products. The harvesting season, by-product type, location and climate all have significant effects on the composition of mussel by-products. The spawning or the reproductive period for _Mytilus edilus_ is spring to summer, whereas gametogenesis occurs during the winter season when the mussels get bigger prior to spawning. Depending on when the mussels are harvested, the size, meat yield and composition vary. A high meat yield, protein, lipid and pigment content (e.g., carotenoid) are often associated with the gametogenesis phase of maturation in mussels [3,4]. Meat yield is the amount of meat obtained per known quantity of whole wet mussel and when calculated after cooking it is referred to as the cooked meat yield [5]. Cooked meat yield values lower than 30% are not viable for sale and mussels with a cooked meat yield less than 30% are categorised as by-products. The whole mussel is made up of the shell, meat, extrapallial fluid and byssus threads. Proximate composition analysis of mussel meat found that it contained 58.7% protein, 22.5% carbohydrates, 7% lipids and 11.8% ash on a dry weight basis, apart from other minor components [6]. Lipids and proteins are the main components studied for health-promoting benefits. A recent study revealed that consuming mussel meat three times a week for two weeks as the protein source in a personalised meal can moderately improve the omega-3 index and whole blood docosahexaenoicenoic acid (DHA) and eicosapentanoic acid (EPA) content in young healthy adults [7]. EPA and DHA are known for their anti-inflammatory benefits and mussel lipids rich in these polyunsaturated fatty acids (PUFAs) have been used previously in formulating bone supplements [8]. Proteins in mussels can be hydrolysed using proteolytic enzymes to yield hydrolyses containing bioactive peptides. Several enzymes such as trypsin (EC 3.4.21.4), papain (EC 3.4.22.2), Alcalase(EC 3.4.21.62), Protamex(EC 3.4.21.62 and EC 3.4.24.28) and others have been used previously to generate bioactive peptides [9,10]. The pH optima of the enzyme, temperature, batch time and enzyme:substrate ratio can influence the degree of hydrolysis and resulting bioactivity of the peptides and hydrolyses. Several proteases such as papain (EC 3.4.22.2), Flavourzyme(EC 3.4.11.1) and Protamex(EC 3.4.11.1) and Protamex(EC 3.4.2) have been used to cleave mussel meat proteins for the manufacture of bioactive peptides [11]. Protamex(EC 3.4.2), a serine protease, is known for its ability to produce non-bitter-tasting hydrolyses [12]. Bioactive peptides from _Mytilus edulis_ have previously shown bioactivities, including ACE-I inhibition and antioxidant potential, as well as antimicrobial, anticoagulant, anti-inflammatory, osteogenic, hepatotoxicity and anticancer activities [13, 14, 15, 16, 17, 18].