## 2 Composition, Availability, and Technological and Safety Constraints of Coffee By-Products
### Husk  
The coffee husk is the first by-product produced during coffee processing. This by-product is generated in coffee-producing countries at 40 kg per 100 kg of green coffee beans [22]. The definition and composition of the coffee husk depend on the processing method employed. Dry coffee husks obtained through the dry processing method (Figure 3A), consist of skin, pulp, mucilage, and parchment and are rich in insoluble dietary fiber. They contain approximately 30% hemicelluloses, 25% cellulose, and 24% lignin (Table 1) [23; 24; 25; 26]. Coffee husks are also sources of phytochemicals, including phenolic compounds, which account for approximately 1.2% and mainly include anthocyanins and proanthocyanidins [27]. Figure 1: Coffee cherry anatomy from the outside to the inside (skin, pulp, mucilage, parchment—comprising the coffee husk, silverskin, and coffee bean). Figure 2: Dry (**A**) and wet (**B**) processing of freshly harvested coffee beans to produce green coffee beans. Coffee husks generated through the wet method (known as wet husk or pulp) (Figure 3B) are composed of skin and pulp and contain significant levels of phenolic compounds (1.5%) and caffeine (1.3%) [28]. The wet husk contains four major groups of phenolic compounds: hydroxcinnatic acids, flavanols, flavonols, and anthocyanins [28]. The primary phenolic compounds in coffee husks are chlorogenic and protocatechiu acids, which account for more than 80% of the overall phenolic compounds [29]. Other phenolic compounds identified in coffee husks are epicatechin, catechhin, rutin, ferulic acid, and cyanidin-3-glucose and 3-rutinoside [18]. In addition, the coffee pulp presents lower lipid (0.5-2%) and mineral (8-9.5%) content, and higher protein (8-17%), moisture (78%), and carbohydrates content (-58-85%) than the coffee parchment (Table 1). These differences in composition between the dry and the wet husk provide unique opportunities for their utilization in various applications, from dietary fiber enrichment to the extraction of bioactive compounds [42]. Exploring these distinctions and utilizing the specific properties of dry and wet husk can lead to the development of innovative products and contribute to the sustainable utilization of coffee by-products. The coffee husk is a substantial by-product generated during coffee processing, with a production rate of approximately 40 kg per 100 kg of green coffee beans in coffee-producing countries [22]. Coffee husks obtained from the dry processing method account for nearly 45% of the coffee cherry [19], while coffee husks obtained from the wet processing method represent around 29% of the dry weight of the entire cherry [21]. This indicates a significant \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Husk** & **Pulp** & **Mucilage** & **Parchment** & **Silverskin** \\ \hline Type of processing & Dry & Wet & Wet & Wet & Dry/wet \\ Processing step & Pulping & Pulping & Hulling & Pulping & Roasting \\ \hline
**Macronutrients (\%)** & & & & & \\ Moisture & 13.0–15.0 & 78.0–81.0 & 80.0–84.0 & 8.1–10.0 & 4.0–7.0 \\ Protein & 6.0–16.0 & 8.7–17.0 & 8.9–17 & 3.1–17.4 & 7.9–20.6 \\ Lipids & 1.0–4.1 & 0.5–2.0 & - & 0.3–4.1 & 1.2–3.0 \\ Total sugars & 26.0–30.1 & 10–15 & - & - & 0.4 \\ Dietary fiber & 32–43 & 21.5–28.0 & 30 & 64.3–92.6 & 52.0–69.8 \\ Ash & 1.4–6.2 & 8–9.5 & 0.7 & 0.5–1.0 & 5.8–9.5 \\ \hline
**Bioactive compounds (mg/g)** & & & & & \\ Total phenolics & 10.1–15.0 & 10.0–13.0 & - & 2.0–3.0 & 3.6–20.3 \\ Caffeine & 6.8–12.0 & 2.0–10.1 & - & 0.1–1.3 & 0.7–9.5 \\ Tannins & 18–93 & 1.8–8.6 & - & - & 2.5–7.66 \\ Chlorogenic acids & - & 1.8–3.37 & - & 0.05 & 1.6–2.5 \\ Caffeoylquinic acid & 0.8 & 1.7–1.8 & - & - & 0.1–3.1 \\ Hydroxymethylfurfurral & - & - & - & - & 0.2–3.2 \\ Melanoidins & 1.5 & - & - & - & 1.7–2.3 \\ \hline
**References** & [23; 24; 25; 26; 27] & [28; 29; 30] & [18; 25; 31; 32] & [14; 33; 34; 35; 36] & [24; 25; 37; 38; 39; 40; 41] \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of coffee by-products obtained through coffee processing. Values expressed in dry matter.