# Reuse of Spent Coffee Grounds: Alternative Applications, Challenges, and Prospects--A Review
## 2 Characteristics of Coffee  
Coffee is formed as a result of ripening and proper processing of the coffee fruit. It consists of several biological structures, which are arranged in layers (Figure 1). The coffee fruit is surrounded by a hard and thick skin, which changes color from green to brownish-red as it ripens. The next layer is the epicarp, which surrounds the pulp (mesocarp). The coffee bean also has a pectin layer (mucilage), a thin layer of endocarp, and a silver skin, which adheres to the beans located in the center of the coffee fruit [5].  
Coffee contains 1-2% caffeine, 3-5% tannins, 13% proteins, 10-15% solid oils, and about 60% carbohydrates, which include other sugars (Table 1). In addition, it includes tannin, thiamine, xanthine, spermidine, guaiacol, citric acid, chlorogenic acid, acetaldehyde, spermine, putrescine, scopoletin, and waxes [10; 15]. Based on this, it can be concluded that coffee beans have a complex chemical composition. Variety, origin, and harvest season affect the content of individual components. Different components of raw coffee beans can react chemically during the roasting process, creating unique flavors and colors in different coffee beans.  
In addition, coffee contains components, some of which are biologically active: alkaloid caffeine (stimulates the nervous system), trigonelline (under the influence of the smoking process is converted into nicotinic acid), diterpene alcohols--caffeoyl and caffestol (affect lipid metabolism), chlorogenic acid (has antioxidant properties), organic acids (malic, citric, phosphoric), phenolics (coffee acid and its esters with quinic acid), tannins, theobromine, carbohydrates, proteins, and lipids. Coffee also contains B vitamins and minerals such as magnesium and potassium. The energy density of coffee without any additives is about 0.5-2 kcal per 100 mL [9]. The bioactive compound found in coffee is caffeine. Its content is influenced by a number of factors, such as the strength of the beverage and the method of preparation, as well as the species of coffee. The average caffeine content in coffee is 1-4 g per 100 g [17].  
About 500 varieties of coffee are known, two of which are the most popular among buyers. These are Arabica and Canephora, better known as Robusta. Coffee arabica plants usually reach heights of 3 to 5 m and are grown at altitudes of 800 to 2000 m above sea level in warm climates of 15 \({}^{\circ}\)C to 24 \({}^{\circ}\)C. Robusta coffee grows at lower altitudes--200 to 900 m--tolerates high temperatures and higher precipitation, requires higher humus content in the soil than Arabica, and is more resistant to disease, allowing for the mechanization of its processing. For these reasons, growing Robusta coffee is more economical,  
\begin{table}
\begin{tabular}{c c} \hline
**Ingredient** & **Content [\%]** \\ \hline Reducing sugars & 1 \\ Sucrose & 7 \\ Pectin & 2 \\ Starch & 10 \\ Pentane & 5 \\ Hemicellulose & 12–15 \\ Cellulose & 15–18 \\ Lignin & 2 \\ Fats & 10–15 \\ Protein (N \(\times\) 6.25) & 13 \\ Ash & 4 \\ Tannic acid & 3–5 \\ N-methylnicotinic acid & 1 \\ Caffeine (soluble) & 1–2 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of raw coffee beans [10; 15; 16].  
Figure 1: Cross-section of a coffee fruit (own elaboration).  
leading to a lower price on the market [18; 19]. The two coffee varieties differ in their physicochemical properties and slightly in the chemical composition of the beans. Arabica beans have a higher content of lipids and trigonelline, while Robusta beans contain higher amounts of caffeine and chlorogenic acid. This has a major impact on the taste of the coffee. Arabica is the species that is most valued for its sensory and flavor characteristics. Its flavor is mild, light, and rich in contrast to the pungent, sharp, earthy taste of Robusta [19].