# Collagen-Based Bioactive Bromelain Hydrolysate from Salt-Cured Cod Skin
## 1 Introduction  
Collagen is a structural protein present in different animal tissues. Its technological properties have been fundamental to various industries, and currently, its interest has increased due to its potential bioactive properties. Partial hydrolysis of native collagen produces gelatine, which is the usual collagen prepared industrially. Collagen and gelatine have different industrial applications; among the most well-known are the food, cosmetic, and biomedical industries [1]. In recent years, the demand for collagen-based peptides has grown dramatically. This is mainly due to the beneficial effects of these compounds on the skin. Oral ingestion of collagen-based peptides (generally gelatine hydrolyses) has been seen to promote collagen synthesis in the skin, increasing the size of collagen fibrils in the dermis. This is believed to improve skin hydration and prevent wrinkles [2]. Collagen-derived peptides also have antihypertensive and antioxidant activities that are associated with low molecular weight peptide structures [3]. Collagen-rich hydrolyses can be produced by controlled thermal or enzymatic hydrolysis [4]. However, enzymatic hydrolysis is preferred because it produces the material in its native state and needs less time and milder conditions to obtain lower molecular weight peptides [5]. Several enzymes have been used for the hydrolysis process [6; 7; 8; 9]. However, the protein hydrolyses prepared using bromelain showed higher antioxidant capacity, desired functional properties, and excellent industrial applications [10; 11; 12]. The industrial production of collagen-derived peptides requires two coupled steps [13]. In the first, collagen is extracted and purified. In the second step, the peptides are obtained by enzymatic hydrolysis, they are sterilised, and finally they are dried [13]. Therefore, collagen extraction is time and energy consuming [2].  
Classically, collagen is obtained from skins of mammalian and filamentous tissues but with several negative aspects. In recent years, due to the outbreak of animal diseases such as Bovine Spongiform Encephalopathy, Transmissible Spongiform Encephalopathies, and Foot and Mouth Disease, the search for alternative sources of collagen has increased [14]. Theincrease in fish consumption per capita and the consequent increase in fisheries by-products in the past two decades led academia and the industry to explore marine by-products as a reliable and economical source of many healthy and biologically active compounds such as collagen [15]. It includes marine fish, starfish, sponges, jellyfish, squid, etc. [16; 17; 18; 19]. Fish residues may account for an average of 55% of the total fish weight; of this material, up to 30% may be skin and bone [17]. Fish skin has more than 80% of its total protein content as collagen. The work performed to date proves that marine fish may be a promising source.  
This study aims to develop a new integrated methodology to produce a collagen-rich enzymatic hydrolysate, taking advantage of the salt-cured cod skin a by-product from cod industry abundant in countries such as Portugal.