## 3 Biorefinery Strategies for Upcycling Coffee By-Products
### Biorefinery and Coffee By-Products for Bioactive Food Ingredients Production  
#### 3.2.1 Husk  
The conventional use of the coffee husk is to prepare the \"Cascara Beverage\" traditionally consumed in Yemen and Ethiopia [29], and for composting in coffee-producing countries [92; 93]. Current uses for the coffee husk include mushroom cultivation [94], lignin extraction [95], recovery of biomolecules from the lignin alkali hydrolysate [96], bioethanol production [45; 97], biosorbents preparation [98; 99], and animal feed [46]. Other biotechnological applications proposed for this by-product are the production of enzymes: amylase [100], xylanase [101], endoglucanase [102], protease [103], peptidase [104], and tannase [105], among others [106]; and biomolecules such as carotenoids [107], gallic [108], citric [109], succinin [110], lactic [111], and gibberell acids [112], and fruity flavor compounds [113].  
However, recent studies have provided new evidence to support the use of the coffee husk as a food ingredient for human consumption [114; 115; 116]. Simple, low-cost, and eco-friendly aqueous extraction methods were used to produce extracts enriched in bioactive compounds with antioxidant, anti-inflammatory, antimicrobial, and anti-adipogenic properties [117; 118; 119; 120], demonstrating the feasibility of using water as the only extractant of the coffee husk's phenolic compounds. Aqueous extracts yielded around 7-29%, being composed of caffeine (1%), chlorogenic acids (0.3%), and protocatechuric acid (0.1%), among other phenolic compounds [115; 117; 118]. Soluble sugars (glucose, fructose, and sucrose) were also present (~12%), as well as soluble proteins (5-7%). Most aqueous extracts are composed of soluble dietary fiber (~68%), which is solubilized together with phenolic compounds and other non-identified phytochemicals [114; 115; 117].  
Therefore, the existing evidence concerning coffee husk aqueous extracts supports the potential use of these extracts to modulate metabolic syndrome biomarkers and, consequently, prevent cardiometabolic diseases. Likewise, the valorization of the whole coffee husk flour and the water-insoluble residue obtained after the aqueous extractions, which are rich in antioxidant dietary fiber [114; 115], provide a more sustainable approach for the complete revalorization of coffee husks. All ingredients from the coffee husk revealed health-promoting properties associated with the prevention of metabolic syndrome. In conclusion, the coffee husk has shown a high concentration of dietary fiber and phenolic compounds. Considering its chemical composition and its biological activities demonstrated in multiple investigations, the coffee husk could be converted into two ingredients: an aqueous extract that could be used as a health-promoting food ingredient and another fraction that could be implemented as a source of antioxidant dietary fiber. This mixed high-fiber and phenolic compound content makes the coffee husk very appealing as a nutraceutical and ingredient in functional foods since combined antioxidant and fiber contents are likely to positively impact health, from the gastrointestinal tract to peripheral tissues.