## 1 Introduction  
In the European Union (EU), nearly 59 million tons of food waste (corresponding to 131 kg/inhabitant) from households, retail establishments, and the food service industry are generated annually with an associated market value estimated at 132 billion euros [1]. Food waste (FW) is addressed in the EU Plan for the Circular Economy, including a FW reduction target under the Strategy of Farm-to-Fork within the European Green Deal [2]. The EU and EU countries are committed to meeting the Sustainable Development Goal 12.3 target [3] to halve per capita food waste at the retail and consumer stages by 2030 [4]. At the retail period, the main food waste produced is meat waste: around 70% of retail waste generated every day corresponds to meat waste [5]. This enormous amount of waste, currently mostly landfilled, is a valuable resource which, for both economic and ecological reasons, should be kept within the production cycle. Retail meat waste is constituted by the products for sale in the butcher shops that was originally destined for human consumption but that has not reached this purpose either because they have not been sold, or because they are remains from cleaning and conditioning the pieces sold. Meat waste is included in Category 3 of Regulation (EC) No 1069/2009 of the European Parliament and of the Council laying down health rules as regards to animal by-products and derived products [6]. Products in Category 3 are low risk. They include carcasses or parts of slaughtered animals that are suitable for human consumption, but are not intended for this purpose, such as bones, hides, skins, heads of poultry, etc. ; that is, food products from animal origins not intended for human consumption due to commercial motives, manufacturing or packaging defects, or other defects which do not imply any health risk. Category 3 products can be processed for feeding purposes, for the manufacture of cosmetics, for the manufacture of pharmaceutical products, etc. [6], so there is no impediment to using retail meat waste as raw material for obtaining products with a higher added value. Meat waste is mainly composed of proteins and lipids [7,8]. These essential nutrients can be recovered from meat waste via enzymatic hydrolysis. Enzymatic hydrolysis is regarded as a green technology [9,10]. The use of enzymes in this process is considered environmentally friendly because it can be performed under mild conditions, reducing the need for harsh chemicals (such as strong acids or strong alkalis) and high temperatures that can harm the environment [11,12]. The use of hydrolases enzymes to hydrolyze proteins (proteases) or lipids (lipases) is considered a gentler alternative method to alkaline or acidic approaches to protein and lipid solubilization and extraction [13]. During enzymatic hydrolysis with proteases, proteins are cleaved into smaller peptides and free amino acids [14], thus increasing their solubility [15]. Protein hydrolyses containing mainly di- and tripeptides has been proved to be superior to whole proteins and free amino acids in terms of being applied to different applications such as nutrition [16], biotechnology [17], and cosmetic industries [18]. The native structure of proteins is crucial in its recognition via proteases [19]. In meat waste, two types of native structures of proteins can be found: globular-type and fibrous-type [20]. The main protein in retail meat waste is collagen, an insoluble fibrous-type protein [21]. Collagen is less susceptible to hydrolysis than the globular-type proteins found in this animal waste [22]. But the reduced hydrolysis of collagen has the advantage of making collagen recovery possible for commercial use, including biotechnological and biomedical uses, cosmetics manufacturing, etc. [23]. Meat lipids are mainly composed of triglycerides, corresponding to about 95% of meat lipids [24]. In lipid hydrolysis, the triglyceride molecule is degraded into three separate fatty acid chains [25].