## 3 Biorefinery Strategies for Upcycling Coffee By-Products
### Biorefinery and Coffee By-Products for Bioactive Food Ingredients Production
#### 3.2.4 Silverskin  
Studies have explored the aqueous extraction of the coffee silverskin to develop food ingredients with healthy properties. Sustainable aqueous extraction of phytochemicals from coffee silverskin has been proposed according to [PERSON] et al. (patent number WO 2013/004873) [136]. This extract is a sustainable source of bioactive compounds such as chlorogenic acid (2.8 mg/g) and caffeine (19.2 mg/g), but the major component in coffee silverskin extract is soluble dietary fiber (22%) [137]. Based on its chemical composition and observed biological activity, the green aqueous extract could be used as a healthy food ingredient. Coffee silverskin extract could be consumed as a nutraceuticaltical or added to the most appropriate food matrix based on sensory customer acceptance. The food applications of coffee silverskin extract in beverages [137], biscuits [138], and bread [114] have been described. Coffee silverskin extracts can also be applied to produce functional yogurts [67]. Other sustainable potential uses for this extract include cosmeceuticals production since compounds from coffee silverskin protect skin cells from UV damage and prevent skin aging. Moreover, the extract presents adequate properties to be included in a cosmetic formulation [139; 140]. The coffee silverskin flour and water-insoluble residue (from the phytochemicals green extraction) can also be used as a fiber-rich food ingredient, with the ultimate goal of recycling the whole by-product. The coffee silverskin water-insoluble residue contains 70% dietary fiber, with the majority being insoluble. This fraction also comprises 10-20% proteins and about 3% fat, depending on the species. This fraction also reported in vitro antioxidant and prebiotic properties [65]. Likewise, dietary fiber fermentation releases short-chain fatty acids, which modulate anti-inflammatory gene and protein expression [141]. OTA was detected in the coffee silverskin water-insoluble residue at lower levels (2.9 \(\upmu\)g/kg) than those permitted by the European Commission. However, no other mycotoxins or pesticides were found [62]. Furthermore, coffee silverskin dietary fiber has been used to enhance the nutritional quality of biscuits by improving the moisture, texture, thickness, and color of the novel biscuits [138]. The coffee silverskin has been used as a dietary fiber ingredient to formulate bread, reducing its caloric density. Moreover, cakes with up to 30% coffee silverskin water-insoluble residue as a flour substitute have been developed, improving moisture content, textural, and sensory attributes [142; 143]. Cakes with coffee silverskin water-insoluble residue exhibited physical and sensory properties similar to the control cake [142]. Alternative sustainable uses of coffee silverskin are polylactic acid production as a replacement for synthetic polymers [144], cellulose isolation and purification for the development of sustainable packaging [145] and biofilms [146], the production of butanol [147], succinic acid [148], bio-oil [149], composites [69], and adsorbent [150], as well as an energy source through pyrolysis [151]. Antioxidants and value-added bioactive compounds are produced from the coffee silverskin via pyrolysis [152]. More research is being conducted demonstrating the presence of a plethora of bioactive compounds and coffee bean roasting-derived phytochemicals that provide health benefits in biological systems [153]. According to circular economy strategies, the coffee should be fully revalorized either as flour or fractionated into two different ingredients (extract and water-insoluble residue), generating zero residues. Research has corroborated the coffee silverskin components' nutritional properties and biological activity. Thus, the different applications of coffee silverskin as a food ingredient and its safety validation have confirmed its viability as a sustainable food ingredient or nutraceutical for human consumption.