## 2 Composition, Availability, and Technological and Safety Constraints of Coffee By-Products
### Silverskin  
Green coffee beans are shipped to coffee-consuming countries, where their quality is evaluated based on odor and taste tests, as well as by size, form, color, hardness, and existence of defects [20]. Once received, beans are checked to confirm their quality and stored until being roasted. The roasting process consists of three steps: drying, roasting, and cooling. During the drying process, water and volatile compounds are slowly released, and the color of the coffee beans changes from green to yellow. Roasting reactions alter both the beans' physical and chemical properties, resulting in a complex mixture of Maillard reaction products [21]. The silverskin, the thin tegument covering the bean, is detached at this point (Figure 6). The coffee silverskin is the only by-product of the coffee roasting process [17; 64]. This by-product contains low moisture (4-7%), lipids (1.2-3%), and mineral content (5-9%). Proteins (8-20%) and carbohydrates (62-67%), mainly forming dietary fiber (52-70%), are the main component of the coffee silverskin [14; 38; 39; 65]. The dietary fiber fraction comprises cellulose (~18-24%), hemicellulose (~13-17%), and lignin (~29%) [39]. Caffeine is the main phytochemical in the coffee silverskin, followed by the chlorogenic acids (caffeoylquinic acids, dicaffeoylquinic acids, lactones of caffeoylquinic acids, coumaroylquinic acids, feruvoliquinic acids, lactones of feruvoliquinic acids, and feruvolyl-caffeoylquinic acids) [38; 40]. Other phenolic compounds found in coffee silverskin include caffeic, protocatechuic, and vanillic acids [41]. The coffee silverskin represents about 4% of the green coffee beans [17; 64]. The roasting of 4 tons of coffee yields approximately 30 kg of coffee silverskin [66]. Considering the world coffee annual production (171.3 million 60 kg bags of coffee beans) [43], 76 thousand tons of the coffee silverskin are produced each year worldwide in coffee-transforming countries. Its fibrous and thin nature poses unique technological constraints for its processing and utilization in various applications [67]. Challenges arise due to its low density and high inflammability, requiring careful handling and storage to ensure safety and prevent potential fire hazards [68]. Moreover, the physical characteristics of the coffee silverskin, including low compactability and poor flowability, may require specialized equipment and processing techniques to achieve effective extraction, purification, and incorporation into value-added products [69]. To overcome these challenges and maximize the commercial viability of the coffee silverskin as a sustainable resource, further research and development efforts are needed to explore innovative solutions and optimize processing methods [70]. This includes the development of efficient and cost-effective technologies that enable the extraction and recovery of valuable components from the coffee silverskin while ensuring safety and scalability [71]. Exploring alternative uses and applications that potentiate the unique properties of the coffee silverskin, such as its antioxidant and antimicrobial potential, can further enhance its value as a functional food ingredient and nutraceutical resource [72]. The occurrence of chemical, biological, and processing contaminants has also been examined to guarantee the nutritional quality and safety of the ingredients derived from the coffee silverskin [14]. The absence of pesticides and mycotoxins was confirmed in the coffee silverskin, although it was close to the limit set by the European Commission for roasted coffee (400 \(\mathrm{\SIUnitSymbolMicro g}\)/kg) but less than the amount established for instant coffee (900 \(\mathrm{\SIUnitSymbolMicro g}\)/kg) [73]. Nevertheless, no genotoxicity and cytotoxicity were observed in human liver cells treated with various concentrations of coffee silverskin [74]. Acute and sub-acute oral administration studies in animals have also demonstrated the safety of the coffee silverskin extract at the tested doses [55].