Useful Extracts from Coffee By-Products: A Brief Review

Abstract

The waste materials generated from the processing of coffee cherries are still rich in several bioactive compounds. Several studies highlight coffee by-products as a valuable source for diverse applications, such as biofuels, biopolymers, biocomposites, and natural antioxidants in food, pharmaceuticals, and cosmetics. The development of prospective strategies for the valorization of coffee wastes is a goal of a sustainable and circular bioeconomy, increasing the added value of these wastes and reducing environmental pollution. This brief overview describes the recently proposed procedures for the extraction and recovery of functional ingredients from the diversity of coffee by-products. The comparison between conventional and alternative extraction methods enables one to choose the most suitable techniques for valorizing a given coffee by-product.

1. Introduction

Coffee is one of the most consumed non–alcoholic drinks worldwide, and its production for the 2023/24 period is projected to reach 174.3 million 60 kg bags [1]. It belongs to the botanical family Rubiaceae, genus Coffea. The two main coffee tree species cultivated on a worldwide scale are Coffea arabica (arabica) and Coffea canephora (robusta). Most of the world’s coffee is produced in Brazil, Vietnam, and Columbia. Both the processing of the coffee cherries and their green seeds (often called beans) generate large amounts of waste. Depending on worldwide coffee production, at least 6–8 million tons of trash are created every year [2]. Such a large amount causes storage and disposal problems. Moreover, discarded in the environment, coffee waste may risk human and environmental health [3,4]. The growing need for various biobased products and ecological policy has forced material recycling and reuse. Thus, the valorization of coffee by-products has received more and more attention in recent years [5,6,7,8,9,10,11,12,13]. The main challenge in sustainability terms is to maximize the reuse of generated by-products by developing appropriate procedures to recover the bioactive compounds from the produced wastes [14,15,16,17].

Each coffee cherry comprises several layers—the outer skin (husk), pulp, sticky mucilage, thin parchment, and the coffee silverskin covering the green coffee seeds. A schematic representation of coffee processing is shown in Figure 1. The first step of coffee cherry processing starts with the hull and pulp removal using either a dry or wet method. These processes affect the composition of coffee seeds and their by-products. In the first method, harvested coffee cherries are dried in the sun or using a mechanical dryer. Then, they are hulled using a mechanical system to obtain dried skins of coffee named coffee husk. It is also known under the Spanish name cascara. More steps are involved in the wet method of production, and a greater variety of by-products are produced, such as pulp, cascara, mucilage, parchment, and partially silverskin. During the fermentation process, bacteria, yeast, and fungi break down sugars and pectins and produce alcohols, organic acids, and enzymes. Green coffee seeds are the final product of both processing methods. They are then roasted, and the main by-product of this process is silverskin, a tin tegument located directly around the two seeds of the coffee cherry. The spent coffee grounds (SCGs) are obtained after preparing coffee at the end of the coffee processing as a beverage. During all stages of production, significant amounts of waste are generated, and only about 10% of the coffee cherry ends up in the beverage. Approximately 650 kg of SCG is obtained from 1 ton of coffee seeds [18]. Iriondo-DeHond et al. reported that in Columbia, from 100 kg of mature Arabica coffee cherries during wet processing, as many as 39 kg correspond to skin and pulp, 22 kg to mucilage, and 39 kg to parchment [19].

The attractive compositions of coffee processing wastes, high availability, and low cost represent a good source of several bioactive compounds with nutritional value. Spent coffee cherries contain phenolic compounds, cellulose and hemicellulose polymers, lignin, carbohydrates, proteins, sugars, and caffeine [11,17,18,20,21]. In addition to this, they consist of a mixture of high amounts of fatty acids. Some inorganic micronutrients, such as calcium, magnesium, sodium, phosphorous, iron, manganese, and copper, are also determined [22]. These ingredients have health benefits, mainly exhibited by their antioxidant, anti-inflammatory, antimicrobial, and anti-ageing activity. The European Food Safety Authority (EFSA) has considered cascara as a traditional food that can be commercialized for human consumption [23].

Literature data indicate that the valorization of coffee by-products arouses increasing interest in the scientific world, and several review papers have been published regarding this topic. Many of them described the potential of coffee waste in various applications [3,7,8,11,17,18,21,22,23,24,25,26,27]. Another group of these papers is devoted just to spent coffee grounds, probably because it represents the most abundant waste in preparing coffee beverages [2,7,8,10,11,12,28,29,30]. Less attention is paid to the conditions applied in the extraction processes to separate a given group of phytochemicals [31,32]. This brief overview describes the recently proposed procedures for the extraction and recovery of functional ingredients from the diversity of coffee by-products. The comparison between conventional and alternative extraction methods enables one to choose the most suitable techniques for valorizing a given coffee by-product.

2. Chemical Composition of Coffee By-Products

The valorization of coffee by-products and diverse applications depends on their chemical composition. Genetic variation, bean maturation, agricultural practices, harvesting, and processing methods affect the content of individual ingredients. The observed discrepancies reported by different authors can also be attributed to the coffee variety and the techniques used for the extraction.

Dried coffee husk (cascara), a major waste generated in dry processing, comprises outer skin, dry pulp, and parchment. It is mainly composed of polysaccharides of cellulose (58% w/w) and hemicellulose (28% w/w), as well as lignin, a noncarbohydrate component of dietary fibre (25% w/w) [16,17,33,34]. Blanching of coffee husk at 90 °C for 1 min increased the glucose content but decreased the xylose molar percentage [34]. The most common phenolic compounds found in coffee husks are chlorogenic, protocatechuic, ferulic, and gallic acids [35].

On a dry weight basis, coffee pulp is mostly constituted of cellulose (36%), pectic polysaccharides (21%), hemicelluloses (9%), and free sugars (5%) [36]. Phenolic compounds are present in large quantities from four different major classes of polyphenols, including flavan-3-ols (monomers and procyanidins), hydroxycinnamic acids, flavanols, and anthocyanidins. The total phenolic content determined by the Folin–Ciocalteu assay in arabica variety coffee pulp was 21.99 mg of gallic acid equivalent (GAE) per 100 g of sample, while for robusta coffee pulp it was 80 mg GAE/100 g per 100 g sample [37].

Coffee mucilage is mainly composed of carbohydrates with highly branched structures consisting of the monomer units of arabinose, xylose, galactose, rhamnose, and galacturonic acid [38,39,40]. Potassium was the most abundant element, followed by phosphorus, calcium, sulfur, and magnesium.

Coffee parchment is one of the least explored coffee wastes, representing 5.8% of berry dry weight [41]. Compared to other coffee by-products, it has a high food fiber content [42,43,44]. Coffee parchment also contains phenolic compounds (2 mg/g GAE), such as gallic acid, chlorogenic acids, p-coumaric acid, and synapic acid [43]. Extruded coffee parchment showed higher phenolic compounds (6.5 mg GAE/g) and antioxidant capacity (32.2 mg of Trolox equivalents (TE)/g) in the ABTS assay [45].

Coffee silverskin is the only waste product from the coffee bean roasting process. Its composition is similar to coffee parchment [18,46,47,48]. Xylose, arabinose, galactose, and mannose are the main monosaccharides. Its content of dietary fiber (315 g/kg) was higher than in cascara (88 g/kg) [16]. Linoleic acid was the most abundant fatty acid in 40 studied Arabica coffees [16]. Besada et al. found that the geographical origins significantly affect the fatty acid composition content and silverskin’s antioxidant profile [49]. Among the essential elements, it is worth paying attention to the determined iron level (349 mg/kg) [47].

Spent coffee grounds, the residue obtained after brewing coffee seeds, are a rich source of carbohydrates, mainly polysaccharides of cellulose (8–15%) and hemicellulose (30–40%), proteins (13–17%), lignins (20–30%), lipids (7–21%), and oil [2,7,8,13,30]. The main fatty acids in the SCG oil fraction are linoleic, palmitic, oleic, and stearic [13,50]. Chlorogenic and caffeic acids are the main phenolic compounds [30]. Some flavonoids, such as quercetin, rutin, and cyanidin-3-glucoside, have also been reported [51]. Trigonelline is a plant hormone that is converted under thermal treatment to nicotinic acid [52,53]. Micronutrients in spent coffee grounds include vitamins C, E, and B₁₂ [2].

It should be noted that the thermal processes during the drying and roasting of coffee seeds lead to the production of melanoidins, the brown pigment, by non-enzymatic Maillard reaction [8,54]. They are responsible for coffee beverages’ flavor, aroma, and color. Their content in spent coffee grounds is 13–25% of the dry weight and mainly depends on the roasting process [55]. Alves et al. reported that the antioxidant capacity of melanoidins is strongly correlated with their bound phenolic compounds [56].

The potential of coffee by-products as a bioactive food ingredient is not only conditioned on its nutritional value but also on its safety for consumption. The coffee production process conducted at high temperatures can lead to the formation of some toxic substances, such as polycyclic aromatic hydrocarbons or acrylamide [57]. Moreover, under warm and humid conditions, coffee seeds can be exposed to various mycotoxins like Ochratoxin A and Aflatoxin B1 [57,58]. Thus, it is recommended to quantify the levels of toxic compounds to avoid cancer risk for coffee consumers and other foods based on coffee by-products.

3. Extraction of Phytochemicals from Coffee By-Products

The main goal of the extraction process is to choose the appropriate solvent and conditions to isolate a given compound or its group with the highest possible efficiency. Detailed knowledge of bioactive compound contents is important to understand their potential application in foods and pharmaceuticals. Despite some disadvantages, conventional solid–liquid extraction (SLE) methods, such as maceration, infusion, decoction, and percolation, are still commonly used due to their simplicity and wide range of applications. However, they are increasingly being replaced by advanced techniques to increase efficiency and selectivity. Several methodologies based on ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), pressurized liquid extraction (PLE), supercritical fluid extraction (SFE), enzyme-assisted extraction (EAE), and pulsed electric field extraction (PEFE) have been used for the isolation of phytochemicals from plant materials [32,59,60]. Additionally, the extraction process, a pretreatment step in some cases is also important to achieve higher efficiency for the separation of the appropriate group of bioactive compounds from coffee wastes [45,61,62,63].

Mathematical and statistical methods applied to chemical data analysis, like experimental design, response surface methodology, and principal component analysis, have often been used for determining the optimum extraction conditions from coffee spent samples, as several factors, like solvent type, temperature, extraction time, and liquid–solid ratio, influence this process [32].

Solvents commonly used for the recovery of bioactive compounds from coffee by-products are polar (e.g., water, alcohols, acetonitrile), intermediate polar (e.g., acetone, dichloromethane), and nonpolar (e.g., n-hexane, ether, chloroform). As an alternative to toxic and volatile organic solvents, ionic liquids (ILs), deep eutectic solvents (DESs), and supercritical and subcritical fluids have been proposed [64].

An example of an integrated biorefinery approach for the valorization of spent coffee grounds is depicted in Figure 2. The main separated fractions for the extraction of the bioactive compounds from coffee wastes are presented in the following sections.

3.1. Phenolic Fraction

The isolation of the phenolic fraction, which exhibits high antioxidant activity, was mostly performed by extraction with organic solvents such as methanol, ethanol, or acetone, mixed with water in different proportions. Although methanol exhibits high polarity and high extraction yields, ethanol is recommended when the extracted compounds are later used as food ingredients and in pharmaceutical manufacturing. Methanol turned out to be more effective for polyphenols with lower molecular weight, whereas for the extraction of higher molecular weight flavanols, aqueous acetone has been generally better [65].

Polyphenol contents and simultaneous extraction yields are usually calculated as the total phenolic content (TPC) using the Folin–Ciocalteu (FC) assay. However, the FC reagent is not specific only for phenolic compounds, and the presence of reducing compounds such as ascorbic acid, sugars, and some metal ions could increase the determined TPC values [66,67]. Advanced chromatographic techniques (e.g., HPLC-MS/MS, NMR) can help obtain accurate insights into the composition.

Several researchers have examined the antioxidant activity of phenolic extracts using different in vitro chemical assays. The antioxidant activities of phenolics are mainly expressed by direct free radical scavenging or indirectly by increasing the activity of antioxidant enzymes, such as superoxide dismutase, catalase, or glutathione peroxidase [68]. In a living organism, reactive oxygen species (ROS) can contribute to oxidative stress, i.e., an imbalance between ROS production and the ability of cells to detoxify the reactive intermediates [69]. The increase in ROS is positively related to the pathology of many diseases [70].

Reported studies revealed that alcohol/water mixtures are more effective in extracting phenolic compounds and exhibit a higher antioxidant activity than pure alcohol extracts [29,71,72,73]. Water might enhance the efficiency of the extraction process by helping the diffusion of extractable components like polyphenols through plant tissues. As was studied by Zengin et al., the total phenolic content in the SCG extract was decreased in the following order for methanolic aqueous solvents: 50% methanol (93.26 mg GAE/g) > 100% methanol (63.25 mg GAE/g) > water (56.86 mg GAE/g), but a different order was obtained for the coffee silverskin extracts under the same conditions: 100% methanol (35.68 mg GAE/g) > 50% methanol (25.02 mg GAE/g) > water (20.49 mg GAE/g) [72].

To enhance the efficiency of the isolation of phenolic compounds from coffee by-products, classical extraction is often replaced by advanced extraction techniques using elevated pressure and temperature. They are generally faster, which is an important advantage [31,73]. Especially since a long time of extraction at elevated temperatures may cause undesirable degradation of phenolic compounds, e.g., chlorogenic acid [73,74]. Ultrasonic treatment leads to a cavitation process favoring mass transfer at lower temperatures and causing a reduction of extraction time. In microwave-assisted extraction, the synergistic combination of heat and mass transfer working in the same direction accelerated the process. Pressurized liquid extraction involves a sample treatment with high pressure (usually up to 600 MPa) for short periods (5–10 min) [29].

Delgado et al. proposed a 1% HCl aqueous solution for the extraction of antioxidant metabolites from Coffea arabica var. Colombia pulps [75]. For the process lasting 5 min at room temperature, the value of TCP equals 424.0 mg GAE/100 g. A high extraction yield (85%) from coffee pulp waste was reported using USE (with a power of 396 W) and water as solvent [76]. A TCP value of 164.9 mg GAE/L was obtained compared to conventional extraction (128.3 mg GAE/L) at 75 °C, also lasting 5.5 min. Protocatechuic acid was found to be the main phenolic acid (90.0 mg/L), followed by gallic acid (25.1 mg/L), and chlorogenic acid (13.0 mg/L).

The better results for extracting compounds with antioxidant properties from coffee husk (cascara) were obtained with 50% ethanol, both using a water bath at 60 °C and ultrasounds at 35 °C, compared to pure water and alcohol [77]. For these methods, the values of total phenolics decreased in the orderof 50% EtOH (95.0 mg of catechin equivalent/g) > water (42.5 mg CE/g) > 100% EtOH (31.4 mg CE/g). Similar orders were observed for the antioxidant activity of the prepared extracts measured by DPPH, ABTS, and FRAP assays. UAE with propylene glycol has been reported to extract some antioxidants (chlorogenic acid, caffeine, and trigonelline) from coffee pulp [78]. The optimal conditions obtained using surface response methodology were as follows: liquid-to-solid ratio of 22.22 mL/g, a solvent concentration of 46.7%, and an extraction time of 7.65 min. Total phenolic content (9.29 mg GA/g) was higher than UAE extraction with ethanol (7.49 mg GAE/g).

Okur et al. compared UAE and PLE techniques with classical solvent extraction for spent coffee grounds according to total phenolic content and antioxidant activity [51]. Both UAE (25 kHz for 15 min) and PLE (500 MPa) using 80% MeOH for 15 min gave higher TPC values (9.5 GAE/100 g), compared to the classical method performed at 50 °C for 30 min (6.4 GAE/100 g). Similarly, Soxhlet extraction gave the lowest DPPH assay activity. A high Pearson correlation (0.816) between antioxidant activity (evaluated by DPPH and FRAP assays) and TPC values for all extraction methods was found. The extract after ultrasound-assisted extraction had a higher content of chlorogenic acid (85.0 ± 0.6 mg/kg) as compared to PLE (81.2 ± 1.1 mg/kg) [51].

Yoo et al. proposed the USE method and deep eutectic solvent, consisting of 1,6-hexanediol/choline chloride at a molar ratio of 7:1, for the recovery of phenolics from SCG [79]. The extraction was conducted at 60 °C for 10 min, yielding a TPC value of 17.0 mg GAE/g. The same solvent and method were used for the extraction of phenolic compounds from coffee silverskin, but at 85 °C for 90 min [80]. The TPC value of 22.39 mg GAE/g was obtained. DESs are obtained by mixing a hydrogen-bond acceptor (in this case choline chloride) and a hydrogen-bond donor (1,6-hexanediol) in specific molar ratios to form a homogenous liquid. They are formed through self-associated intermolecular interactions, most likely caused by van der Waals interactions, hydrogen bonding, and ionic bonding [81]. DESs were also evaluated as extraction media for coffee pulp [82]. Among studied extractants with different compositions, lactic acid-choline chloride (1:2 molar ratio) was the most effective in the heat stirring process (1 h, 60 °C) given in the FC assay TPC value of 44.2 g GAE/100 g dry pulp. Except for chlorogenic acid, caffeine, and theobromine, furfural was identified in the extract by UPLC-ESI-MS analysis.

In microwave-assisted extraction, the type of solvent used is an important factor, as it affects the solubility of the given compounds and the efficiency of the method [29]. The solvent must be selected considering the capability for microwave radiation energy adsorption and its affinity to diffuse adequately. Solvents such as ethanol, methanol, and water are ideal for this technique. Successful examples of phenolic extraction from spent coffee grounds with the MAE method were done by Coelho et al. [83]. A design of experiments was used to optimize the process. Good agreement was obtained between the predicted optimal response by the proposed models and the experimental values. It was found that the increase in water percentage in the mixture with ethanol diminishes the yield of the MAE process while increasing the TPC and DPPH values. Under the optimum extraction conditions (solvent/SCG ratio = 16.7, 69% EtOH), the content of total phenolics was 117.7 ± 6.1 mg GAE/g, and antioxidant activity in the DPPH assay was equal to 143.8 ± 8.6 (μmol TE/g). Pettinato et al. underlined the role of heating temperature and time on the extract composition using microwaves and 54% ethanol [84]. A short time of heating (1 min) produced an increase in extracted phenolic compounds when the temperature was increased from 20 °C to 150 °C, while for heating times longer than 20 min, the maximum was observed at 135 °C.

Supercritical fluid extraction (SFE) is another alternative to conventional extraction techniques. Carbon dioxide in a supercritical state has high diffusivity, low density, and low viscosity and allows the extraction of thermally unstable compounds [85]. The higher cost of SFE implementation on an industrial scale is its disadvantage, but this extraction method has no significant impact on the environment. The extraction with pure carbon dioxide provides the final product with no solvent, as it can be removed by changing pressure. For the extraction of polar compounds, like phenolics, a small amount of co-solvent (water, methanol, ethanol, or propanol) was added [85,86,87]. The use of co-solvents gives better results for the antioxidation capacity of the extract [87]. The pressure and temperature are the important parameters in the SFE method, indicating the effect of the solubility of the solvent and consequently the influence on the process yield. As was studied by Andrade et al., the temperature of 50 °C and the pressure of 200 bar for 4.3 h resulted in the highest content of phenolic compounds in spent coffee ground extract (57 mg GAE/g) [88]. The results were similar for pure carbon dioxide and with the addition of 8% EtOH. However, they were much lower compared to the obtained one in the Soxhlet extraction with ethanol for 6 h (151 mg GAE/g) or USE (220 V, 2 h at room temperature), also using the same solvent (588 mg GAE/g). All extracts found chlorogenic acid at the highest concentration among phenolic compounds.

Phenolic compounds can also occur in coffee by-products in the bound form. They are connected with sugars, alcohols, and other biomolecules from cell walls by ester or ether bonds. They can also be linked to macromolecules such as melanoidins, mainly by noncovalent interactions [89,90]. After isolating soluble phenolics with a 70% methanol solution, acidic hydrolysis was used for their insoluble fraction [28]. The value of TPC in the soluble fraction of coffee silverskin was 538 ± 64 mg GAE/100 g, while for the insoluble fraction it was 467 ± 29 mg GAE/100 g. Thus, this insoluble phenolic fraction can be a rich source of bioactive components with antioxidant properties. Alkaline hydrolysis was also applied to assess the total phenolic acid contents of spent coffee extracts [91]. Recently, Çelik and Gocmen noted the possible interactions between free and bound antioxidants in prepared foodstuffs [92]. They reviewed several factors affecting these interactions with the availability and extraction of soluble and bound phenolics.

The extraction yields of phenolic fractions reported in the recent literature data are shown in

Table 1. Recent procedures for the extraction of phenolic fractions from different coffee by-products.

Coffee By-Products	Extraction Conditions	Total Phenolic Content (mg GAE/g)	Antioxidant Activity	Ref.
Spent coffee ground	Water, 20 mL/g, 80 °C, 30 min	61.49 ± 1.36 mg GAE/g	324.51 ± 13.58 µmol TE/g (DPPH) 735.47 ± 0.60 µmol TE/g (ABTS)	[32]
	MeOH, 100 mL/30 g, room temp., 24 h	109–181 mg GAE/g of extracts	EC50 8.5–44.9 µg/mL (DPPH)	[50]
	80% MeOH, water bath, 50 °C, 30 min	6.40 ± 0.18 mg GAE/g	DPPH inhibition: ~58%	[51]
	UAE (25 kHz) 80% MeOH, 15 min	9.51 ± 0.12 mg GAE/g	~89%
	PLE (50 MPa, 60% amplitude), 15 min	9.42 ± 0.10 mg GAE/g	~87%
	Room temp., 1 min—water	3.83 ± 0.19 mg GAE/g
	20% ETOH	3.93 ± 0.15 mg GAE/g		[93]
	40% EtOH	3.93 ± 0.16 mg GAE/g
	20% acetone	4.40 ± 0.19 mg GAE/g
	40% acetone	4.37 ± 0.14 mg GAE/g
	UAE, 10 g/50 mL, 20 °C, 120 min.
	water	56.86 ± 0.16 mg GAE/g	ABTS: 164.70 ± 1.56 mg TE/g	[72]
	MeOH	62.25 ± 0.10 mg GAE/g	136.11 ± 13.35 mg TE/g
	50% MeOH	93.26 ± 0.14 mg GAE/g	218.75 ± 6.88 mg TE/g
	70% EtOH	93.35 ± 0.65 mg GAE/g	276.19 ± 9.65 mg TE/g
	5 g/150 mL, 6 h, boiling temp. EtOH	119.5 ± 2.1 mg GAE/g	Inhibition DPPH: 46.5%	[88]
	EtOAc	182.6 ± 28.2 mg GAE/g	93.5%
	7 g/210 mL, 2 h, room temp., UAE—EtOH	587.7 ± 46.6 mg GAE/g	Inhibition DPPH: 32.2%
	EtOAc	553.4 ± 59.8 mg GAE/g	29.1%
	SFE (200 bars), 50 °C, 4.3 h: SFE-CO2	24.1 ± 0.8 mg GAE/g	Inhibition DPPH: 11.7%
	SFE-CO2+ 4% EtOH	57 ± 3 mg GAE/g	47.9%
	Hydrothermal method, 5 g/35 mL, 150 °C, 3 h	9.44 ± 0.90 mg GAE/g		[94]
	DES (lactic acid with choline chloride, 1:2 molar ratio), 1 h, 60 °C	44.21 ± 1.09 g GAE/100 g		[82]
Cascara	Water bath, 60 °C, 1 h:		Inhibition:	[77]
	water	42.51 ± 0.72 mg CE/g	37.2% (DPPH); 76.1% (ABTS)
	100% EtOH	31.35 ± 1.90 mg CE/g	14.2% (DPPH) 52.6% (ABTS)
	50% EtOH	95.00 ± 1.39 mg CE/g	67.5% (DPPH) 91.5% (ABTS)
	USE, 35 °C, 1 h: water	34.10 ± 3.78 mg CE/g
	100% EtOH	16.54 ± 2.18 mg CE/g
	50% EtOH	77.57 ± 0.44 mg CE/g
Pulp	Water, 25 g/250 mL 75 °C, 5.5 min	128.3 ± 4.3 mg GAE/L		[76]
	Water, 25 g/250 mL, USE, 75 °C, 5.5 min,	164.9 ± 1.2 mg GAE/L
	Water, 25 g/60 mL, 5 min, room temp.	284.1 ± 6.5 mg GAE/100 g	38.1 mmol TE/100 g (ORAC)	[75]
		424.0 ± 3.2 mg GAE/100 g	57.1 mmol TE/100 g (ORAC)
	MeOH, 5 g/350 mL, room temp. 10 h	4.84 ± 0.17 mg GAE/g		[95]
	Soxhlet, MeOH, 65 °C, 10 h	16.49 ± 0.75 mg GAE/g
	MeOH, MAE (700 W), 3 min	12.94 ± 2.25 mg GAE/g
	46.7% propylene glycol—maceration 24 h	8.50 ± 0.02 mg GAE/g	5.63 ± 0.10 g TE/g (DPPH)	[78]
	USE 7.65 min	9.20 ± 0.14 mg GAE/g	7.56 ± 0.27 g TE/g (DPPH)
Silverskin	70% MeOH, 1 g/10 mL 1 min (soluble fraction)	538 ± 64 mg GAE/100 g	1.57 mmol TE/100 g (ABTS)	[28]
	Residue with 10 mL of MeOH:/water/HCl (70:29.1:0.9), 1 min (insoluble fraction)	467 ± 29 mg GAE/100 g	1.11 mmol TE/100 g (ABTS)
	10 g/50 mL, USE, 20 °C, 120 min:			[72]
	water	20.49 ± 0.27 mg GAE/g	73.66 ± 1.43 mg TE/g (ABTS)
	MeOH	35.68 ± 1.80 mg GAE/g	95.05 ± 0.04 mg TE/g (ABTS)
	50% MeOH	25.02 ± 0.37 mg GAE/g	63.50 ± 1.65 mg TE/g (ABTS)
	70% EtOH	25.34 ± 0.44 mg GAE/g	54.41 ± 0.76 mg TE/g (ABTS)
	DES (1,6-hexanediol/choline chloride, 7:1 molar ratio), UAE, 85 °C, 90 min	22.29 mg GAE/g	24.06 ± 1.78 mg GAE/g (DPPH) 59.13 ±4.55 mg Fe(II)/g (FRAP)	[80]
	Water, USE (42 kHz), 25 °C, 30 min:	266.17 ± 2.90 mg GAE/g	Inhibition: 96.1% (DPPH) 90.3% ABTS	[96]

MeOH—methanol, EtOH—ethanol, EtOAc—ethyl acetate; DES—deep eutectic solvent; GAE—gallic acid equivalent; CE—catechin equivalent; DPPH—2,2-diphenyl-1-picrylhydrazyl; ABTS—(2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) assay; FRAP—ferric ion reducing antioxidant power; ORAC—Oxygen Radical Absorbance Capacity; EC₅₀—concentration of the sample necessary to decrease the initial concentration by 50%; UAE—ultrasound-assisted extraction; MAE—microwave-assisted extraction.

, where their wide range of values can be observed. This fact can be attributed to the differences in raw materials and the type of solvent and extraction method. The results are often expressed in different units, making it difficult to compare them.

Using polar solvents, except phenolic compounds, also other compounds are extracted, e.g., caffeine. This xanthine alkaloid stimulates the central nervous system and exhibits hepatoprotective and neuroprotective properties [96]. Some studies reported that caffeine also has the potential to eliminate ROS like hydroxyl radicals and superoxide anions, thus contributing to the overall antioxidant activity of a given sample [97,98]. The effective caffeine extraction from SCG was obtained with 20–40% ethanol (1.45–1.50 mg/g), respectively, and 20% acetone (1.47 mg/g) at room temperature [93].

Generally, the conditions for extraction of phenolic compounds from coffee wastes, like a type of solvent, solvent-to-solid ratio, contact time, and temperature, were studied to determine the optimized values that result in a high extraction yield. In addition, the type of coffee by-products should be taken into consideration. The influence of the same type of coffee waste is best illustrated by the results of the research conducted by Bouhzam et al. [99]. Two samples of spent coffee grounds (SCG1 and SCG2) after the coffee brewing with an espresso machine were provided by two restaurants. Both were derived from mixtures of Arabica and Robusta coffee varieties, but the roasted coffee seeds were from different producers. All three extractions were conducted with 0.7 g of dry SCG and 4 mL of appropriate solvents: (i) water and vortex, (ii) water assisted by ultrasound (195 W), and (iii) supramolecular solvent (mixture of 1-hexanol, ethanol, and water) for 1 min duration. The determined contents of chlorogenic acid are presented in Figure 3. The highest extraction yield was obtained for sample SCG1, which was attributed to the characteristics of the coffee variety (Robusta is richer in chlorogenic acid), the roasting procedure, and the brewing method. Significant differences were detected among all the extraction methods. Extraction with water and UAE at room temperature yielded the highest chlorogenic acid content, followed by ultrasounds at 50 °C, extraction with water using a vortex, and then supramolecular solvents. The authors attributed this last solvent’s low efficiency to the analyte distribution between supra and infra phases.

The work by Rodrigues da Silva et al. expanded the knowledge about the chemical compositions of other, not yet tested coffee by-products [100]. Fifteen samples were collected from coffee trees on farms to factory coffee processing lines. They included coffee roasted silverskin released in the roasting stage, roasted coffee powder returned to the company, roasted coffee powder which did not pass the final quality control of the factory, silverskin pelleted by a brambati drum or by a probate drum, too small green coffee seeds, green coffee bean powder residue obtained during the separation of grains by size, coffee husk and parchment mix collected in a farm, green coffee bagasse collected after an industrial extraction, mature coffee leaves collected right after trimming, aged coffee leaf collected from the ground in a crop, coffee tree dry branches, coffee pulp, wastewater used in a semi-humid coffee processing and spent coffee ground after brewing coffee. Extraction was done with bath ultrasounds (160 W) for 10 min using 70% ethanol as solvent. Additionally, two-liquid phase extraction with heptane was conducted. The extracted and identified 127 components belong to several classes. Overall predominance of phenolic acids was observed for 13 out of 15 coffee by-products. Only in the husk with parchment and mature coffee leaf samples were flavonoids and sterols determined in higher numbers. The highest content of chlorogenic acid (as a 5-CQA isomer) was detected in defective green coffee seeds (72.94 mg/g), followed by mature coffee leaves (38.92 mg/g) and green coffee bean powder (29.80 mg/g). Thus, these by-products seem to be a rich source of that antioxidant. The content of mangiferin, a natural phenolic xanthonoid, in mature coffee leaves (19.42 mg/g) is worth attention as it treats neurological disorders [101]. The reported health-promoting properties of mangiferin include antioxidant, antitumor, anti-allergic, anti-inflammatory, and antibacterial activities [102]. Moreover, mature and aged coffee leaves contained a significant content of caffeine, 16.46 and 5.66 mg/g, respectively. Much higher than was determined in spent coffee grounds (1.33 mg/g). Thus, caffeine from these sources could be an active ingredient in energetic beverages and various medications.

Due to their high antioxidant activity, the extracts containing phenolic compounds found applications as additives to functional foods [8,30,103,104], nutritional supplements [105,106], and cosmetic formulations [107,108]. However, it should be remembered that phenolic compounds might interact with other components present in the food matrix. These interactions might increase or decrease their biological activities [109,110]. Thus, the synergistic and antagonistic effects of coffee phenolic compounds with food matrix should be considered for designing novel functional foods.

3.2. Lipid Fraction

The lipid fraction of coffee by-products is composed mainly of triacylglycerols, free fatty acids, sterols, tocopherols, esterified diterpenes, and phospholipids. Extraction of that fraction was mainly conducted by n-hexane due to its hydrophobic nature. Still, other non-polar solvents such as heptane [27], chloroform [111], or tetrahydrofuran [112] were also studied.

Different extraction methods for treating SCG, such as maceration, Soxhlet, UAE, MAE, and SFE, were reported and compared to show those with higher lipid yield and lower acid value. As in any extraction procedure, time, temperature, pressure, and solvent-to-sample ratio affect its efficiency. According to Kanlayavattanakul et al. [27], maceration with heptane without heat for 1 and 3 h was optimum as cost-effective and more feasible in industrial production. The oils mainly contained palmitic (38–49%), linoleic (22–33%), oleic (14–21%), and stearic (5–6%) acids (Figure 4). Maceration gave a higher linoleic acid content than Soxhlet and reflux methods. This unsaturated fatty acid is an important component of cosmetics to treat hyperpigmentation and reduce inflammation [113]. It was reported that SGC heptane extract from the instant coffee industry contained a higher oil fraction (24.3 wt.%) than from the expresso machine (14.8 wt.%) [114]. The pretreatment of SCG by non-thermal plasma (30 W, 10 min) or ultrasounds (40 kHz, 10 min) before Soxhlet extraction with n-hexane improved the extraction efficiency with a yield of 19.25% and 14.38%, respectively, compared to the control (9.41%) [115]. The use of natural deep eutectic solvents (NADES) in the pretreatment stage, followed by n-hexane extraction using UAE, also increased the yield of coffee oil [116].

To intensify the classical SLE using ethanol as a solvent, PLE and UAE were evaluated in terms of extraction yield [117]. The application of pressurized solvent resulted in a higher relative extraction yield (70.6%) than the experiments performed with the same 1:4 solid-to-solvent ratio, with (66.9%) or without (62.7%) the use of ultrasounds (600 W). However, using a higher ratio of ingredients (1:15) leads to the opposite situation, where the relative recovery of oil with ultrasounds (81.1%) and in their absence (82.4%) was higher than the results for PLE. Thus, increasing solvent volume acted significantly in producing more coffee oil.

The studies performed by Leow et al. showed generally similar yields of extraction (~10%) using n-hexane, tetrahydrofuran, acetone, and ethanol solvents [112]. Acetone extract contained the lowest amount of fatty acids, followed by tetrahydrofuran and acetone. Among antioxidant compounds, oleic acid, butylated hydroxytoluene (BHT), and β-tocopherol (vitamin E) were determined in these extracts. The antioxidant inhibition value of these extracts in the DPPH assay decreased in the order: ethanol > acetone > n-hexane ~tetrahydrofuran.

Dry and aqueous 2-methyloxolane used in a conventional Soxhlet system as an alternative solvent to n-hexane has been proposed by Chemat et al. [118]. The yields of lipid fraction were 13.67% and 15.84% for these solvents, respectively, at a solid-to-solvent ratio of 1:10 and extraction time of 6 h. SCG extraction with n-hexane gave 12.47%. The higher extraction efficiency obtained for aqueous 2-methyloxolane was attributed to better solubilization of polar lipids such as phospholipids in the solvent system.

To eliminate the organic solvent use, supercritical carbon dioxide (SF-CO₂) has also been employed to extract lipid fractions from coffee by-products [84,87,119,120,121,122,123]. Adding ethanol as a co-solvent significantly decreased extraction time compared to the Soxhlet method [87,122]. However, polar co-solvent makes simultaneous extraction of hydrophilic compounds (mainly caffeine), which may modify the mass of SF-CO₂ extract and the oil yield values [87,119,121]. Vandeponseele et al. recommended the separation of raw extract obtained after SF-CO₂ extraction into lipids and polar molecule fractions using liquid–liquid extraction with n-hexane to clarify the influence of the ethanol co-solvent [87]. This group also proposed selective extraction of lipid fraction using the SF-CO₂ method with the optimal moisture content of 6.4 wt.%; under these conditions, caffeine content is only 0.06 mg/100 g of SCG. For simultaneous recovery of lipids and caffeine, the optimal moisture content was 55 wt.% with 5.69 mg/100 g of caffeine [124]. The sequential procedure for extracting coffee oil enriched with phenolic compounds was described by Bitencourt et al. [125]. The first extraction step uses SF-CO₂ (60 °C, 400 bars) to separate the lipid fraction from SCG. Then, pressurized ethanol under the same conditions was used to obtain an extract from the obtained residue. The overall extraction yield of the first step was 19.1 g/100 g of SCG with a total phenolic content of 0.24 mg GA/g of extract, while the second step presented a yield of 5 g/g of SCG and TCP equal to 17 mg GAE/g extract. Both extracts, differing in chemical composition, could be used in different industrial applications. Enriched coffee oil with higher antioxidant activity is used in the cosmetic and food sectors [87,124]. The oil fraction with triglycerides could find application for biodiesel production. Supercritical antisolvent fractionation of SCG extracts using CO₂ with ethanol was also evaluated in four separators aligned in series at 60 °C at different pressures and ethanol concentrations to obtain partitioning of lipid and phenolic fractions [87].

The literature data show that pressure is the most important parameter for lipid extraction due to its influence on the density of CO₂ and the solvation power of lipids [121,122,123,126]. Low temperature and high pressure resulted in higher yields [121,124]. The mathematical modelling and several strategies for the SF-CO₂ process were recently discussed [127].

A comparative study of oil extraction from spent coffee grounds was conducted by Muangrat and Pongsinkul using different methods [120]. Soxhlet and pressurized liquid extraction (PLE) methods with propanol as solvent were compared with SF-CO₂. The PLE procedure had the highest oil yields (14.02%), followed by Soxhlet with propanol (13.75%) and SFE (12.11%). The authors decided to expand supercritical CO₂ extraction on a larger industrial scale using a 15 kg SCG sample and recirculated carbon dioxide flow. Under the optimal extraction conditions (50 °C, 200 bar, 2 h), obtained using response surface methodology, a yield of 12.14% was achieved. Recovery of a lipid fraction from silverskin coffee using SF-CO₂ has also been proposed by Nasti et al. [121]. The extraction time was reduced from 6 h to 2 h compared to conventional Soxhlet extraction with n-hexane, although similar yield values were obtained, 3.1 ± 0.1 and 3.0 ± 0.1%, respectively. Linoleic and palmitic acids were the main fatty acids. This study also indicated the presence of long-chain arachidic, behenic, and lignoceric acids, not common in other vegetable oils. Adding ethanol as a co-solvent (5%, v/v) contributed to an increase in caffeine content but did not significantly affect the extraction yield of oil (3.3 ± 0.3%). Liquid and supercritical CO₂ extractions with 5% ethanol as a cosolvent showed yields (16% and 15%, respectively) similar to those obtained with control methods, where ethanol and n-hexane were used [120]. They allowed time savings because they lasted 1 h versus 5 h and provided oils containing high contents of total phenolic compounds (970 and 857 mg GAE/100 g oil). Microwave-assisted oil extraction from SCG achieved a significant decrease in extraction time compared with the classical liquid procedure and used less solvent per gram of oil produced [128].

Table 2. Recent studies on extraction of lipid fraction from spent coffee grounds.

Condition of Extraction	Maximum Oil Yield (%, goil/100 gdry SCG)	Ref.
Solid/solvent = 1:17, hexane		[27]
Maceration, room temp. 1 h/3 h	12.9/13.1
Maceration, 60 °C, 1 h/3 h	8.6/10.7
Soxhlet, 1 h/3 h	12.1/15.2
Reflux 1 h/3 h	10.1/10.3
Soxhlet, chloroform, 1 g/12.5 mL, 7 h	12.3 ± 0.33	[111]
Soxhlet, hexane, 1 g/12.5 mL, 3 h	12.5 ± 0.15
Soxhlet, methanol, 1 g/12.5 mL, 12 h	11.9 ± 0.47
UAE (30% amplitude), hexane, 1 g/4 mL, 0.5 h	14.55
Soxhlet, hexane 1 g/9 mL, 1 h	17.3	[114]
Soxhlet, hexane 1 g/9 mL, 8 h	24.6
Soxhlet, hexane, 4 h,	9.41	[115]
UAE pretreatment (10 min)	14.38
NTP pretreatment (30 W, 10 min)	19.25
UAE (37 kHz), hexane, 10 g, 60 °C, 45 min with NADES pretreatment (1:10 ratio, 55 °C, 6 min)	13.55	[116]
Soxhlet, ethanol, 5 g/150 mL, 40 °C, 145 min	15.64 ± 0.52	[117]
Soxhlet, ethyl acetate, 5 g/150 mL, 40 °C, 145 min	15.04 ± 0.60
Soxhlet, hexane, 5 g150 mL, 40 °C, 145 min	14.52 ± 0.52
PLE, ethanol, 80 °C, 200 bars, 35 min	15.29
SC-CO2, 40 °C, 200 bars, 295 min	12.19 ± 0.21
SC-CO2 + ethanol, SCG/solvent ratio = 1:2, 80 °C, 25 min	15.9
Soxhlet, propanol, 10 g/180 mL, 6 h	13.75 ± 0.35	[120]
PLE, 103 bars, propanol, 120 °C, 5 min	14.02 ± 0.31
SC-CO2, flow rate 110–170 L/h, 200 bars, 50 °C, 2 h	12.11 ± 1.33
Soxhlet, hexane, 5 g, 5 h	16.35 ± 1.71	[122]
Soxhlet, ethanol, 5 g, 5 h	15.95 ± 0.11
SC-CO2 + 5% ethanol, 14 g, flow rate 10 mL/min,	15.45± 0.09
300 bars, 60 °C, 1 h
Soxhlet, hexane, 6 g/260 mL, 4 h	8.6	[126]
MAE (190 W), hexane, 6 g/40 mL, 95 °C, 10 min	11.54

UAE—ultrasound = assisted extraction; NTP—non-thermal plasma; PLE—pressurized liquid extraction.

shows recent examples involving the use of different extraction procedures for the separation of lipid fractions from spent coffee grounds. The reported maximum yields are generally in the range of 10–20% of dry SCG mass. Higher values of oil extraction yields with pressurized ethanol were reported than those obtained by Soxhlet extraction using n-hexane, considered a reference method for the extraction of lipids [117,118]. Similar results were achieved by Soxhlet extraction and PLE with propanol [119]. Araujo et al. recently reviewed the studies in the field of pressurized fluid extraction of SCG oil [129]. The extraction of coffee oil with the assistance of microwave or ultrasound systems, although requiring more energy compared to the Soxhlet method, uses less solvent with higher efficiency. One of the main advantages of these systems is a significant reduction in extraction time.

3.3. Biofuels

Due to their characteristics and properties, coffee by-products were also studied for incorporation of their compounds in the manufacturing of different biofuels through a biorefinery concept [124,125,126,127,128,129,130,131,132]. They could be transformed through thermochemical processes (such as pyrolysis, hydrothermal carbonization, and torrefaction) and biochemical processes into biodiesel, bioethanol, biofuel, and biogas, or could be used for combustion. Several approaches can be used to valorize solid coffee residues for the production of fuels and energy, which will be described in the next chapters. Schematic examples of these methodologies are earlier depicted in Figure 1. Waste or residue generated during biofuel generation can be used further as a feedstock for the production of fuel pellets and compost [133,134].

3.3.1. Biodiesel Production

Coffee oil presents a high concentration of triglycerides and long fatty acids, which are suitable substrates for biodiesel production, which involves transesterification reactions to esters and glycerol. The preliminary drying process of SCG is necessary because of the lower biodiesel yield during the transesterification reaction, as the high water content leads to soap formation [135,136]. The drying process also prevents the growth of microorganisms. Thus, the moisture content of SCG less than 10% is recommended [135].

The efficiency of biodiesel production depends on several parameters, such as free fatty acid content, alcohol-to-oil molar ratio, catalyst concentration, reaction temperature, and time [111,137,138]. For example, Bui et al. reported that transesterification reached the best yield of 76.23% at MeOH of 6:1, with sodium hydroxide as the catalyst of 0.75 wt.% and a temperature of 55 °C [138]. The transesterification reaction can be realized in one step with alcohol and an alkali catalyst, particularly when the content of free fatty acids in extracted coffee oil is below 1% (corresponding to an acid value of 2 mg KOH/g), [133,139]. Suasnaba et al. achieved a biodiesel conversion of 92.48% in a one-step transesterification reaction at 60 °C within 90 min stirring methanol and oil at a molar ratio of 6:1 and NaOH (1% wt) [140]. Glycerol, created as a by-product, must be separated from the biodiesel. Different catalytic processes can convert this compound into several value-added products [139]. An alternative way for biodiesel production without removing the extractant and using a catalyst has been proposed by Supang et al. [141]. Coffee oil was first extracted from SCG using ethanol, and then the EtOH-oil mixture (30:1) was transesterified in supercritical conditions (275 °C, 40 min), achieving an ester content of 88.37% wt.

The two-step catalytic process represents the appropriate production of biodiesel when coffee oil contains more than 1% of free fatty acids, [137,142,143]. First, the pre-heated oil at 60 °C is converted to esters using 50% methanol (at a molar ratio of 1:12) and 1% of the acid catalyst H₂SO₄ for 3 h [140]. The target of this step is to decrease the free fatty acid content in oils to reduce the soap formation in the second step of transesterification. Soap formation prevents the separation of biodiesel and glycerol phases and also needs a higher amount of water for washing. In the second step, alkali-catalyzed transesterification with KOH and 25% MeOH for 2 h at 60 °C to produce methyl esters [142].

Biodiesel can also be produced by direct transesterification when oil extraction from SCG is integrated with esterification and transesterification, which allows for saving production time and cost [144,145,146,147]. Kim et al. explored a direct transesterification process of SCG with ultrasound assistance [144]. Up to 97.2% of high fatty acid methyl ester content was achieved in the presence of KOH with an irradiation time of 10 min. The use of ultrasounds allows for a shorter reaction time in comparison to a conventional mechanical stirring method. Tarigan et al. additionally impregnated SCG with H₂SO₄ before the direct transesterification [145]. The coffee biodiesel yield reached a value of 17.08% of dried SCGs at 70 °C and a 12 h reaction time, which was equivalent to an oil-to-biodiesel conversion rate of 98.6 wt.%.

The direct conversion of spent coffee oil to biodiesel could also be conducted in the presence of enzymes as catalysts [148,149,150]. Enzymes offer several advantages, such as increased efficiency of biomass conversion, enhanced biodiesel production, and reduced energy consumption. Lipase-B (Novozyme-435) was proposed as a catalyst for biodiesel synthesis from spent coffee oil. Around 96% of oil-to-biodiesel conversion was obtained after 6 h at a 1:5 oil-to-methanol molar ratio and 40 °C [148]. The decrease in biodiesel yield (~26%) was observed after the tenth reaction cycle, probably due to the glycerol deposition, which blocks the enzyme’s active sites. Alonazi et al. used a mixture of two microbial lipases immobilized on calcium carbonate support [149]. Maximum conversion to esters up to 99.33% was obtained within 12 h. Immobilization of an enzyme ensures a large surface area, reusability, and easy detachment from the end product.

Yang et al. compared two different thermochemical SCG valorization routes for biodiesel production [151]. In the first method, microwave-assisted oil extraction of defatted SCG using n-hexane for 5 min was used, followed by hydrothermal liquefaction (HTL) at 295 °C for 12.5 min. The second approach was conducted by the direct HTL process of raw SCG. The compositions of the obtained biocrudes were different from each other. HTL of defatted SCG generated fewer biocrudes (18.9 wt.%) and more nitrogen-containing aromatic heterocyclic compounds (21.7 wt.%) than of raw SCG (28.1 and 3.6 wt.%, respectively).

Recently, electrolytic transesterification was performed using SCG biomass [152]. Under the optimal parameters, i.e., 0.75 wt.% NaOH catalyst loading, 2 h electrolysis time, and 40 V DC voltage, the highest biodiesel production of 98.32% was achieved.

3.3.2. Bioethanol Production

Solids that remained after coffee oil extraction contain lignocellulosic components (cellulose, hemicellulose, and lignin) and could be used to produce bioethanol. The production of bioethanol from lignocellulosic biomass involves several steps, such as its pretreatment, followed by enzymatic hydrolysis to generate sugar monomers, then microbial fermentation of sugars and distillation or pervaporation to obtain the final product [24,153,154]. The direct conversion of SCG to bioethanol is problematic due to the presence of triglycerides and free fatty acids, which inhibit the activity of the enzymatic saccharification process [155].

The pretreatment step destroys the dense structure of lignocellulose and separates carbohydrates from lignin to achieve successful cellulose enzymatic hydrolysis [24]. Several thermal, chemical, and biological methods were used for this purpose and also their combination to improve the utilization of the lignocellulosic material. The most applicable pretreatment technologies of lignocellulosic biomass have been discussed, showing their advantages and disadvantages [155,156,157]. Enzymatic hydrolysis is carried out to obtain fermentable sugars (pentoses and hexoses) by cellulase enzymes at mild conditions (pH of 4.5–5, 40–50 °C) [158]. Hemicellulose has a faster hydrolysis rate than cellulose [24]. The yeast Saccharomyces cerevisiae is mainly employed during fermentation as it tolerates a wide range of pH and its role in transforming furfural and hydroxymethylfurfural into non-inhibitory substances during fermentation. The fermentation process depends on the yeast strain, type of sugar and its concentration, and fermentation conditions. Mussatto et a. evaluated the sugar metabolism of three different yeast strains (Saccharomyces cerevisiae, Pichia stipitis and Kluyveromyces fragilis) in hydrolysates produced by acid hydrolysis of SCG [159]. S. cerevisiae provided the best ethanol production with 11.7 g/L and 50.2% efficiency. Under the same experimental conditions, from coffee silverskin hydrolysate, insignificant (≤1.0 g/L) production of ethanol was obtained for all the evaluated yeast strains, probably due to the low concentration of sugars. Some yeast species, e.g., Saccharomyces cerevisiae, ferment only hexoses, whereas others (e.g., Pichia stipitis) consume hexoses and pentoses [160]. The treatment of lignocellulosic biomass for hydrolysis and fermentation after the pretreatment process could be guided in separate steps or be conducted independently in different units for degradation to monosugars in a hydrolysis reactor and subsequently converted to ethanol in a fermentation unit, but the yield of ethanol production is lower [159,161,162,163].

Liquid hot water pretreatment at 180 °C for 20 min with a solid-to-liquid ratio of 1:6 (w/v) was combined with separate enzymatic hydrolysis (50 °C for 48 h, utilizing various β-mannase) and a fermentation process at 30 °C for 12 h [164]. Based on these optimal conditions, a bioethanol concentration of 101.79 ± 0.23 mg/g SCG was achieved. The study reported by Syahruddin et al. revealed that 10 g of spent coffee grounds, subjected to acidic hydrolysis at 55 °C for 60 min and 4 days of fermentation, produced 40% bioethanol [165]. An optimization study of acidic pretreatment using response surface methodology for ethanol production from coffee husk waste was conducted by Morales-Martínez et al. [166]. The fermentation process generated an ethanol concentration of 48.19 g/L. Alkaline pretreatment with NaOH of coffee pulp was utilized to facilitate its enzymatic hydrolysis [167]. The subsequent fermentation process with yeast S. cerevisiae produced 13.66 g/L of ethanol after 48 h at 30 °C. Anh Nguyen et al. integrated bioethanol production from SCG with additional separation of D-mannose [168]. This monosaccharide is active against metabolic syndrome, diabetes, intestinal diseases, and urinary tract infections. Pretreatment with 60% ethanol (150 °C for 2 h) effectively enhanced the hydrolysis run at 30 °C with cellulase and pectinase enzymes. The proposed fermentation step allowed yeasts to use glucose and galactose to produce ethanol while retaining D-mannose in the fermented broth [168].

Orrego et al. reported the production of bioethanol using coffee mucilage without any pretreatment before fermentation with an S. cerevisiae Y-1546 strain [169]. The optimal condition for microbial fermentation was achieved at 28 °C and pH 4.0, which resulted in 0.47 g ethanol/g of fermentable sugars.

During ethanol production from coffee wastes, other bioactive compounds and value-added products have also been obtained, like furfural and hydroxymethylfurfural during the pre-treatment process and lignin residues in enzymatic hydrolysis [169,170,171].

4. Lignocellulose Fraction

The pretreatment step of coffee by-products in bioethanol production allows for obtaining a lignocellulosic fraction whose composition of complex polysaccharides depends on the used conditions [154,155]. Its main component is lignin, a natural phenolic polymer, which binds carbohydrate polymers cellulose and hemicellulose. This fraction could be used directly or after a fermentation step to produce biopolymers and composites [7,26,46,172,173]. The removal of fats is recommended before alkali extraction [7]. The diversity of components in lignocellulosic biomass provides high resistance to chemical, enzymatic, and microbial attacks; thus, a pretreatment step is needed to destroy the interaction between components [174,175]. This step increases biomass surface area, improves raw materials’ porosity, and separates hemicellulose and lignin. Figure 5 presents the content of coffee pulp components after acidic pretreatment (6% H₂SO₄, 140 °C for 45 min) and a combination of acid-alkaline-microwave pretreatment [176]. The alkaline step was performed after washing and drying the previously obtained solids using a 2% NaOH solution at 120 °C for 20 min and was followed by microwave treating at 300 W for 20 min. Based on the fibre composition, the combined pretreatment gave the best results, as this procedure reduced hemicellulose and lignin contents by 71.4% and 79.2%, respectively, while the remaining solid consisted of 77.5% cellulose.

The other techniques used for the separation of lignin from coffee by-products include the use of ionic liquids and deep eutectic solvents [177,178,179]. Disopropylammonium acetate showed, after 4 h of treatment of coffee husk at 120 °C the yield value of 71.2% [178]. DES involving choline chloride and lactic acid mixed at a 1:4 molar ratio resulted in only a small increase in cellulose (from 39.9% to 43.2%) and lignin (from 21.5% to 33.2%), while hemicellulose was completely removed from coffee husk after the pretreatment step [179]. Procentese et al. reported that DES pretreatment of coffee silverskin was most effective with choline chloride-glycerol at a reaction time of 3 h at 150 °C [180].

The organosolvation pretreatment was also applied for the isolation of lignin from lignocellulosic biomass using organic solvents with the addition of an acid catalyst [181,182]. Lee et al. used sequential extraction that enabled recovery, except for lignin and also methyl esters of fatty acids from the SCG wet sample [182]. Pretreatment was performed at 161 °C using a mixture of two solvents with different polarities, n-hexane and methanol, in the presence of sulfuric acid to facilitate both esterification reactions and lignin decomposition. The obtained liquid phase was separated into two layers. The upper layer was collected by n-hexane evaporation to recover esterified lipids, while lignin was isolated from the methanolic phase after water addition.

All coffee by-products contain cellulose, a complex carbohydrate containing several thousand glucose units. Usually, cellulose is obtained by dissolving lignin and hemicellulose along with low-molecular-weight compounds. The choice of the efficient method for its separation depends on the desired properties of the final product. For example, Singh and Murthy stated that the presence of hemicellulose and lignin improved the mechanical properties of cellulosic fiber compared to that derived from pure cellulose [183]. Depending on the applied extraction method, coffee wastes can lead to cellulose fibers or crystals [184,185]. The reported methods for the isolation of cellulose from coffee by-products involve the use of acidic, alkaline, or oxidizing treatments [186,187,188,189,190]. Sometimes an additional pretreatment is required to partially weaken the molecular framework of the matrix. Acid pretreatment directly attacks hemicellulose and a small fraction of lignin, enhancing the matrix surface for the alkaline step. Then, the solvating action of NaOH leads to cellulose release. According to cellulose fibers produced by acidic hydrolysis with H₃PO_4, from SCG, they showed greater thermal stability than those obtained with H₂SO₄ [191].

Collazo-Bigliardi et al. proposed initially pressurized hot water treatment of coffee husk at 180 °C for 60 min for separation of phenolics, followed by alkali treated with NaOH at 80 °C for 3 h [186]. The residual solid was bleached using sodium chlorite NaClO₂ at a reflux temperature for 4 h. Similar procedures for the separation of cellulose from coffee parchment have been reported with sodium hypochlorite NaClO for the bleaching process [187]. The catalytic oxidation using the radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was used for cellulose nanofibers from spent coffee grounds [191]. Ionic liquids and deep eutectic solvents have also been proposed for lignocellulosic biomass fractionation and isolation of cellulose [192,193,194].

Polysaccharides from the lignocellulose fraction can also be converted by appropriate microbial fermentation into some important industrial precursors, such as short-chain organic acids [195,196], pectolytic enzymes [197], or used for the biosynthesis of polyhydroxyalkanoates, a type of natural polymer [198]. Coffee by-products could also be a good source of prebiotic compounds, as lignocellulose fractions contain valuable dietary fibres with a beneficial health effect on the human gut [199,200,201]. Several applications for compounds from lignocellulosic fractions as precursors for biopolymers and composite production can be found in the literature [7,26,46,202].

5. Conclusions

More and more different waste materials are generated from the processing of the coffee cherries and the coffee beverage-making processes. They are often burned or discarded in landfills, but such proceedings cause environmental problems due to the potential release of residual caffeine, tannin, and methane by anaerobic waste decomposition. However, these wastes are still rich in several bioactive compounds that can find diverse potential applications. Thus, the development of prospective strategies for the valorization of coffee waste is a goal of a sustainable and circular bioeconomy, increasing the added value of these wastes and reducing environmental pollution [203].

Several extraction procedures using different solvents and techniques for coffee waste valorizing have been proposed to obtain products for specific applications. Some by-products, such as spent coffee grounds, are more suitable for biofuel production [149]. At the same time, coffee silverskin, due to its high dietary fiber content, found application as a food ingredient in new formulations [28,30].

The general scheme for the integrated biorefinery approach of coffee by-products depicted in Figure 2 shows many possibilities for obtaining compounds with high benefits for humans. Moreover, various fractionation schemes are possible even using one extraction method, only changing the extraction conditions. For example, SC-CO₂ extraction with ethanol as a cosolvent to remove the lipid fraction from SCG was followed by fractionation by carbon dioxide in supercritical conditions at different pressures, allowing three fractions containing: (i) polyphenols and caffeine, (ii) triglycerides, and (iii) terpenes, sterols, and tocopherols [88]. In another work, spent coffee ground was fractionated with N,N dimethylcyclohexylamine by switching the amine into its hydrophobic bicarbonate salt in the presence of CO₂ [204]. Three fractions were obtained: carbohydrates rich in dietary fiber; phenolics with chlorogenic acid as a main component, and lipid fractions predominantly made of linoleic and palmitic acid.

The proposed procedures have been mostly applied only at a laboratory scale, and only a few works have investigated larger-scale solvent extraction experiments [114,143,205]. The heterogeneity of coffee waste could present some difficulties in standardizing extraction conditions for industrial applications. It should be remembered that the composition of coffee by-products can vary depending on the coffee type, processing, and brewing conditions; an example is the results presented in Figure 2. Moreover, efficient collection and transportation of huge amounts of waste can be a challenge.

The choice of an appropriate extraction method should consider not only the yield and purity of a given fraction or compound but also the cost, operation techniques, process time, energy, and environmental impact [206,207,208]. The exergoeconomic analysis of oil extraction processes from SCG using Soxhlet and UAE extraction processes showed that using ultrasounds has more benefits from the exergy and economic viewpoints [206]. The exergetic efficiency was increased by 5.5%, and the operating cost was reduced by 78.2.

Continued research and development in this area should be directed towards low-cost and eco-friendly procedures for extracting bioactive compounds on a larger scale with detailed economic analysis. The new interesting and useful applications of coffee by-products can be a driving force to reach a sustainable economy.