*1.1.1 Chemical composition*

Coffee waste, being lignocellulosic biomass, which is mainly composed of the essential life elements (C, H, O, and N), which are primarily forming cellulose (59.2–62.94 wt%), hemicellulose (5–10 wt%), and lignin (19.8–26.5 wt%) [5, 6]. Besides, these elements are present in the form of recoverable compounds, such as essential oils and flavonoids, among others. However, since this material has already been subjected to a hydrothermal extraction process, the presence of these compounds is usually low compared with lignocellulosic constituents (10 wt%) [6]. Moreover, this type of waste usually has some elements considered inorganic micronutrients such as calcium, magnesium, or sodium, but their concentrations are generally less than 5.0% dry weight [5–7].

The main component of plant biomass is cellulose, which is made up of linear chains of D-glucose linked by β-1,4 bonds, and it has a form of crystalline fibrillar aggregates, which are formed due to the hydrogen bonds among the HOS present in the D-glucose, as can be seen in **Figure 1**. On the other hand, hemicellulose forms an aggregate of simple sugars of different structures that are attached to cellulose microfibers. Several authors had reported the presence of xylose, arabinose, galactose, and mannose in coffee residues. These types of molecules usually present cyclic structures of 5 or 6 constituents, being abundant in alcohol groups. However, their heterogeneity makes impossible the formation of crystalline arrangements [7, 8]. On the other hand, lignin, whose molecular representation is illustrated in **Figure 2**, is a biopolymer, not a polysaccharide, which is considered the most abundant in plant biomass. This biopolymer has a high structural diversity originated from the enzymatic dehydrogenation of coumaryl, coniferyl, and sinapyl alcohols and subsequent radical polymerization. This heterostructure provides properties such as hardness, resistance to microbial attacks, and oxidative stress, complicating its biodegradation [9].

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**Figure 2.**

*Lignin chemical structure.*

*Revalorization of Coffee Waste*

*DOI: http://dx.doi.org/10.5772/intechopen.92303*

*1.1.2 Surface physicochemical properties*

Given the structural diversity of the constituents of the coffee residue, a heterogeneous presence of functional groups on the surface of the material is expected, which will provide this biomass with unique characteristics. Cellulose and hemicellulose have functional groups of the alcohol type (R▬OH), which can favor the functionalization of these materials, for example, through esterification processes [9]. On the other hand, given its formation process, lignin as a macromolecule has phenolic and aliphatic hydroxyl groups, in addition to methoxyl, carbonyl, and aldehyde groups, among others [8]. The concentration of these groups will depend on the variety and class of the starting material. The structure of lignin is shown in **Figure 2**, and the functional groups mentioned above are indicated; it is important to highlight that lignin has aromatic rings capable of promoting interactions π-π\* with other compounds, which could allow the use of coffee waste as an adsorbent for organic compounds [10]. Among the various analytical techniques used to characterize solid materials is infrared spectroscopy with Fourier transform, which allows identifying surface functional groups simply and effectively. The infrared spectrum of coffee waste is presented in **Figure 3**. In it, the wavelengths at which the various vibrational modes

**Figure 1.** *Cellulose structure showing the hydrogen bonds.*

*Revalorization of Coffee Waste DOI: http://dx.doi.org/10.5772/intechopen.92303*

*Coffee - Production and Research*

were briefly analyzed.

*1.1.1 Chemical composition*

**1.1 Physico-chemical properties of coffee waste**

are generally less than 5.0% dry weight [5–7].

stress, complicating its biodegradation [9].

*Cellulose structure showing the hydrogen bonds.*

of valuable compounds and energy using mono-process extraction and biorefinery from coffee waste will be reviewed. Finally, the experimental design methods to optimize the different processes of coffee waste revalorization are analyzed.

The biomass revalorization, such as coffee waste, depends primarily on their physicochemical properties, such as chemical composition, presence of extractable compounds, and diversity of functional groups. These properties are altered according to the type and plant variety; in the case of coffee, the most commonly used is the so-called Arabica coffee, so their main physicochemical characteristics

Coffee waste, being lignocellulosic biomass, which is mainly composed of the essential life elements (C, H, O, and N), which are primarily forming cellulose (59.2–62.94 wt%), hemicellulose (5–10 wt%), and lignin (19.8–26.5 wt%) [5, 6]. Besides, these elements are present in the form of recoverable compounds, such as essential oils and flavonoids, among others. However, since this material has already been subjected to a hydrothermal extraction process, the presence of these compounds is usually low compared with lignocellulosic constituents (10 wt%) [6]. Moreover, this type of waste usually has some elements considered inorganic micronutrients such as calcium, magnesium, or sodium, but their concentrations

The main component of plant biomass is cellulose, which is made up of linear chains of D-glucose linked by β-1,4 bonds, and it has a form of crystalline fibrillar aggregates, which are formed due to the hydrogen bonds among the HOS present in the D-glucose, as can be seen in **Figure 1**. On the other hand, hemicellulose forms an aggregate of simple sugars of different structures that are attached to cellulose microfibers. Several authors had reported the presence of xylose, arabinose, galactose, and mannose in coffee residues. These types of molecules usually present cyclic structures of 5 or 6 constituents, being abundant in alcohol groups. However, their heterogeneity makes impossible the formation of crystalline arrangements [7, 8]. On the other hand, lignin, whose molecular representation is illustrated in **Figure 2**, is a biopolymer, not a polysaccharide, which is considered the most abundant in plant biomass. This biopolymer has a high structural diversity originated from the enzymatic dehydrogenation of coumaryl, coniferyl, and sinapyl alcohols and subsequent radical polymerization. This heterostructure provides properties such as hardness, resistance to microbial attacks, and oxidative

**134**

**Figure 1.**

**Figure 2.** *Lignin chemical structure.*

#### *1.1.2 Surface physicochemical properties*

Given the structural diversity of the constituents of the coffee residue, a heterogeneous presence of functional groups on the surface of the material is expected, which will provide this biomass with unique characteristics. Cellulose and hemicellulose have functional groups of the alcohol type (R▬OH), which can favor the functionalization of these materials, for example, through esterification processes [9]. On the other hand, given its formation process, lignin as a macromolecule has phenolic and aliphatic hydroxyl groups, in addition to methoxyl, carbonyl, and aldehyde groups, among others [8]. The concentration of these groups will depend on the variety and class of the starting material. The structure of lignin is shown in **Figure 2**, and the functional groups mentioned above are indicated; it is important to highlight that lignin has aromatic rings capable of promoting interactions π-π\* with other compounds, which could allow the use of coffee waste as an adsorbent for organic compounds [10].

Among the various analytical techniques used to characterize solid materials is infrared spectroscopy with Fourier transform, which allows identifying surface functional groups simply and effectively. The infrared spectrum of coffee waste is presented in **Figure 3**. In it, the wavelengths at which the various vibrational modes

**Figure 3.** *FTIR spectrum of a sample of coffee waste.*

of the surface groups can be detected are indicated. The absorption bands found are similar to those reported by multiple authors for coffee residues of the Arabica variety [5, 6, 8]. In the spectrum, two absorption regions can be evidenced, the first one from 3800 to 2700 cm<sup>−</sup><sup>1</sup> , finding signals around 3340 cm<sup>−</sup><sup>1</sup> corresponding to the vibrations of the OH bonds present in the alcohol groups, followed by a doublet of bands at 2920 and 2860 cm<sup>−</sup><sup>1</sup> of the CH interactions, present in all lignocellulosic structures. The second region, between 1900 and 750 cm<sup>−</sup><sup>1</sup> , has a higher number of corresponding bands with links C〓O of the carbonyl groups present in the aldehydes (1740 cm<sup>−</sup><sup>1</sup> ); C〓C of the double bonds of the aromatic structures of lignin (1640, 1525, and 1475 cm<sup>−</sup><sup>1</sup> ); CH of the methyl and methylene groups of the polymer chains of the constituents (1440 and 1380 cm<sup>−</sup><sup>1</sup> ); CO of the groups of the ester type (1320, 1240, and 1160 cm<sup>−</sup><sup>1</sup> ) and alcohol (1030 cm<sup>−</sup><sup>1</sup> ); and finally, the bands located at 870 and 810 cm<sup>−</sup><sup>1</sup> are characteristic signs of substitutions in aromatic structures. Together these bands corroborate the polymeric nature of the coffee residue and make it possible to elucidate, at least qualitatively, the type of surface structures it possesses. The functional groups detected on the surface of the material are primarily acidic, which means that they are capable of yielding the proton and therefore can grant a negative charge density to the biomass surface depending on the pH of the medium. Volesky [11] and Ahsan et al. [12] reported that this type of functional group acts as active sites in the processes of pollutant removal. Several studies have quantified the presence of this type of active sites, indicating in a general way the predominance of phenolic, carbonyl, and carboxylic sites [5, 9, 13].
