## 2 Composition, Availability, and Technological and Safety Constraints of Coffee By-Products
### Mucilage  
The coffee mucilage is a pectin layer found between the pulp and the parchment (Figure 4). After de-pulping, the mucilage remains attached to the coffee bean, and being highly hydrated, is an impediment to further drying the beans. As a result, before the beans can be dried and processed, the mucilage must be degraded by wet fermentation, which is solubilized by yeast, bacteria, and fungi. The mucilage is an extremely hydrated sticky tissue, comprised of 1% pectins with a high degree of esterification with uronic acid being the major constituent, followed by arabinose, galactose, xylose, rhamnose, and glucose as neutral non-cellulosic monosaccharides. In terms of composition, this coffee by-product is mainly constituted of water (84%), protein (7%), soluble sugars (4%), total dietary fiber (1.5%), caffeine (1%), and ash (1%) [18]. The micronutrients found in the coffee mucilage include Ca, Fe, Mg, K, P, and Na [32]. Phenolic compounds, including flavonoids and tannins, have also been reported in the coffee mucilage [25].  
This by-product is generated in countries that produce coffee using the wet method [12]. The mucilage is produced at 20 kg per 100 kg of fresh coffee cherries [51]. Given its high moisture content, mucilage poses challenges for drying and processing, requiring wet fermentation to facilitate subsequent processing steps [52]. Technological constraints associated with mucilage include the need for specialized equipment and controlled fermentation conditions to ensure the appropriate degradation and removal of the mucilage layer [53]. The sticky nature of the coffee mucilage can complicate handling and processing, requiring specific techniques like washing (a method to rinse off the fermented mucilage) and mechanical de-mucliaging (where a machine is used to physically strip away the mucilage without damaging the coffee bean) for its removal [54]. However, advances in processing technology and optimization of fermentation parameters have been explored to overcome these constraints and enhance the utilization of the coffee mucilage in value-added applications [52].  
Nonetheless, it is important to consider potential contaminations in the coffee mucilage due to the fermentation process. Improper fermentation conditions or inadequate hygiene practices during wet fermentation may lead to microbial contamination, including the growth of undesirable microorganisms such as molds, bacteria, or yeasts [53]. These contaminants can affect the quality and safety of the coffee mucilage and its derived products. Proper hygiene practices, strict quality control measures, and adherence to food safety regulations are crucial during wet fermentation and subsequent processing steps to mitigate potential contaminations. Regular monitoring and testing for microbial contaminants should be implemented to ensure the safety of coffee mucilage-derived ingredients and products [52]. By implementing appropriate quality assurance protocols, the risk of contamination can be minimized, allowing for the safe utilization of the coffee mucilage in value-added applications.