# Characterization of Coffee Silver Skin as Potential Food-Safe Ingredient
## 1 Introduction  
A real and effective solution to ensure access to safe and secure food is to limit food waste at the industry level and in home consumption [1]. Whatever the supply chain, a significant number of by-products, that are difficult to reuse and potentially responsible for environmental pollution, are produced [2, 3]. Furthermore, the valorization and reuse of food by-products fulfills sustainable development goals, specifically the 12 th (responsible consumption and production) and the 13 th (climate action) goals, established by the Food and Agriculture Organization (FAO agenda 2030). Coffee silver skin (CSS) is a thin layer that covers coffee seeds inside the coffee \"cherries\" of the genus _Coffea_ of Rubiaceae plants (_Coffea arabica_ and _Coffea caenphora_). CSS is the unique by-product discarded after the roasting process; coffee production gives rise to other by-products, such as coffee pulp and coffee husks, before the roasting step and spent coffee grounds after coffee preparation [4]. Since coffee is one of the most consumed beverages worldwide and one of the most traded items globally, its industry generates a lot of residues [5, 6]. CSS represents 4.2% (_w/w_) of seed total weight, with values of 1 ton of CSS for every 120 tons of roasted coffee [7]. Despite being a less abundant coffee by-product, the availability of CSS justifies the interest in its recovery and use in several applications in different sectors, not just for the food industry. CSS has been proposed as a material for renewable energy sources, as an electrode material, and as a biofuel source [7]. Due to its content of valuable compounds like fibers, sugars, and polyphenols, it would be interesting and useful to exploit it as an innovative and functional food ingredient [8]. [PERSON] and co-authors reported that different cultivars (_C. arabica_ and _C. canenphora_) have comparable phenolic profiles and compositional traits, and they were successfully utilized as food antioxidants and fiber integrators [9]. Coffee's provenience greatly influences its composition; thus, it is difficult to exactly determine it in each variety cultivated. This substrate contains a lot of dietary fiber (up to 55%), including both insoluble (\(\approx\)45%) and soluble (\(\approx\)10%) fractions [10]. Cellulose is present at a high level (\(\approx\)60%), which is helpful for the growth of many types of bacteria and molds to produce enzymes like \(\alpha\)-amylase and fructooligosaccharides. According to [PERSON] and [PERSON] [11], CSS can be easily incorporated into preparations due to its low content in lipids and carbohydrates. Recently, some authors found that CSS is also a good source of relevant amino acids [12]. Some studies explored the possibility to use CSS as a food ingredient in several matrices, despite little information about levels of ochratoxin A and other contaminants being available. In bread, CSS alkalinization was proposed to increase water-holding capacity and the content of fibers [13]. More recently, CSS was added to yogurt [14], to biscuits as a source of dietary fibers and prebiotics [15], and was also blended with instant coffee for new coffee-based beverages [16]. Due to its content of melanoidins, high molecular weight compounds mainly constituting polysaccharides, proteins, and chlorogenic acid molecules [17], CSS can find application as a food dye. Moreover, melanoidin contributes to the antioxidant activity of CSS, showing anti-inflammatory effects on different organs [18]. For marketing within the European Union (EU), it is very important to define whether products, like CSS, need to obtain approval as an ingredient for novel food [4], with special reference to its legal status within the EU and potential options for producers to obtain approval according to the Novel Food Regulation (EU) No 2015/2283 and the two implementing regulations (EU) No. 2017/2468 and No. 2017/2469) [19; 20].