Potential Use of Industrial Cocoa Waste in Biofuel Production

Abstract

Worldwide, the wastes derived from food production are generated in elevated volumes annually. In particular, the cocoa industrial wastes represent a source of usable biomass for the elaboration of new products such as food, livestock feed, cosmetics, and chemical products, and they can even be used for the generation of biofuels. The cocoa industrial wastes include cocoa pod husk, mucilage, and bean shells, which contain compounds of interest for different industries. However, the lignocellulose content of these by-products requires a pretreatment to fully utilize them; thus, different biofuels can be produced, depending on the conversion technology used to obtain the highest biomass yield. Recent studies reported the use of cocoa industrial wastes for the production of solid, liquid, and gaseous biofuels; nevertheless, the most common use reported is as a direct combustion source, which is used to supply the same production plants. Therefore, the objective of this work is to carry out a review on the uses of the by-products generated from cocoa for the generation of biofuels, as well as the technological concept applied for the transformation. In addition, the future trends indicate the relevance of using catalysts in production to increase reactions in the conversion of compounds, including the use of statistical models to optimize the processing variables.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The wastes resulting from the agriculture industry represent 50% of the fresh weight of the total harvested production; in energy terms, this is equivalent to the potential of 90 million tons of oil [1]. In addition, the growing world population demands a greater amount of energy; thus, the shortage of nonrenewable resources is imminent. In addition, the generation of industrial wastes and pollutants represents an ecological and socioeconomic challenge for the population [2]. Therefore, the use of wastes is essential to reduce the environmental impact and create an economic model based on the production of biocomposites [3].

In order to face this environmental problem, the application of the biorefinery concept is a promissory strategy; a biorefinery is defined as the sustainable synergistic processing of biomass on marketable food ingredients, products, and energy [4]. To carry out this process, it is necessary to guarantee the availability of wastes over time, the technical and economic capacity of their production, and the environmental impact produced during the generation cycle [5]. In this sense, residual biomass sources from industrial processing with the potential for the production of biofuels are explored.

In particular, cocoa (Theobroma cacao L.) is an umbrophilic crop of great importance in the world due to its applications in the food, cosmetic, and pharmaceutical industries. Cocoa is a tree native to Central and South America; however, the highest production is concentrated in areas with a tropical climate in Africa, Asia, and South America [6–8]. In 2017, the world production of cocoa pods was estimated at 4.2 million tons [9]. Meanwhile, cocoa industrial wastes were estimated at approximately 85% of world cocoa production; these wastes are cocoa pod husk (CPH), mucilage (CM), and bean shell (CBS) [10]. For the cocoa-producing industry, these wastes are a serious problem as they represent economic losses and environmental pollution [6]. In Figure 1, the wastes obtained from the cocoa fruit are shown.

[figure omitted; refer to PDF]

The whole cocoa pod is a rich source of chemical compounds; however, the beans (main raw material) are the source with the highest content of fat, phenolic compounds, and alkaloids such as theobromine and caffeine [11]. Nevertheless, when the wastes are discarded in the first instance without treatment, they preserve different compounds that can be used to obtain other products through the biorefinery processing scheme [5].

Cocoa wastes contain lignocellulose, which can be used for the production of biofuels; however, it is necessary to carry out a pretreatment to degrade the structure and the fermentable compounds that can be used in a higher proportion. There are different types of pretreatments classified by their nature into chemical, physical, biological, and physicochemical; the choice of pretreatment will depend on the type of biomass and technical and economic aspects in the production of biofuel [12].

In order to reduce the generation of waste in agribusiness, the circular economy seeks options to transform these by-products into sources of bioactive compounds or for the generation of other products [13], as ingredients for food, livestock feed, or as a source for the development of biochemicals, biomaterials, and biofuels [14]. Table 1 shows the chemical composition of cocoa wastes.

Table 1

Chemical composition of cocoa wastes.

Waste	Moisture (%)	Ash (g/100 g)	Protein (g/100 g)	Fat (g/100 g)	Carbohydrates (g/100 g)	References
CBS	7.05	7.09	15.59	3.00	65.58	[15]
	7.7–10.1	7.3	15–18.1	0.66	17.8	[16]
	7.7	7.3	15	2.02	17.8	[17]
	3.6–13	5.96–11.4	10.3–27.4	1.5–8.5	7.8–70.2	[18]
CPH	6.4–14.1	5.9–13.0	2.1–9.1	5.9–13.0	17.5–47	[16]
	—	6.4–8.4	7–10	1.5–2	32–47	[19]
	—	11.37	11.27	7.15	46.6	[20]
	80.2	9.1	5.9	1.2	57.6	[21]
CM	92.7	7.51–7.58	5.47–5.56	1.91–192	67.99–68.35	[16]
	—	3.7–7.68	0.41–5.56	1.91–354	10.7–68.35	[6]
	80.55	9.34	5.12	0.46	52.43	[22]
	86.38	0.36	0.62	1.45	52.02–70.01	[23]

CBS = cocoa bean shell; CPH = cocoa pod husk; CM = cocoa mucilage.

The development of technologies that allow an adequate recovery of these wastes has become an economic advantage since it allows achieving the objective of zero waste in the cocoa industry [13].

The amount of cocoa wastes generated each year by the chocolate industry represents a severe health problem in the plantations, as well as an ecological damage, due to the amount of residual organic matter. Thus, the use of wastes is a contribution to minimizing the damage caused in an agroecosystem. Therefore, the objective of this work is to carry out a review on the use of the wastes generated from cocoa for the generation of biofuels, as well as the technological concept applied for the transformation.

2. Basic Concepts

Lignocellulose is an important part of plants; it is mainly found in herbs, trees, stems, husks, and flowers. The main source of lignocellulosic materials is residues from the forestry and agricultural industries. This material is of great importance due to its potential for energy production, which is derived from the composition of the heteromatrix, made up of cellulose, hemicellulose, and lignin that vary in quantity depending on the biomass of origin. [24]. In Table 2, the lignocellulose composition of the cocoa wastes is shown.

Table 2

Lignocellulose and pectin content in CPH.

Compound	References
	[25]	[26]	[27]	[28]	[29]
Cellulose (%)	29.93	23.04	33.02	23.8	32
Hemicellulose (%)	10.94	38.08	37	8.2	9.9
Lignin (%)	11.64	18.19	23.75	33.4	29.5
Pectin (%)	—	—	3.71	6.9	—

Cellulose is the most abundant organic polymer in the world, and it is present in the cell wall of plant cells [30, 31]. However, the proportion depends on the type of biomass, and its range is between 30 and 60% [32]. Cellulose is a linear and crystalline structure formed by microfibers that are linked by hydrogen bonds [33]; this formation is due to the long chains of glucose with approximately 10,000 units, linked together by β-1,4-glucosidic bonds [34, 35].

On the other hand, a hemicellulose is a group of branched polysaccharides that connect cellulose with lignin [30]. Hemicellulose is made up of hexoses (glucose, galactose, and mannose) and pentoses (xylose and arabinose), along with other compounds in smaller quantities [34]. These compounds are linked by β-1,4-glucosidic and sometimes by β-1,3-glucosidic bonds [33]. The main differences between cellulose and hemicellulose are the degree of polymerization, the branching in the main chain in hemicellulose, and the weaker structure in the face of chemical agents [31]. The proportion of this material in the biomass ranges between 25 and 30% [36].

Finally, the lignin is a cross-linked aromatic phenolic polymer made up of phenylpropane units, responsible for the structural rigidity in the outer fibers in plants [30, 33]. This polymer is found in different proportions depending on its origin (softwood, hardwood, or agricultural residue), which can be between 10 and 35% [31]. The three main units that make up lignin are p-coumaryl, coniferyl, and sinapyl alcohol [34]. In Figure 2, the composition of lignocellulose with the structure and interaction of cellulose, hemicellulose, and lignin is shown.

[figure omitted; refer to PDF]

3. Cocoa Industrial Wastes

3.1. Cocoa Pod Husk

Cocoa pod husk (CPH) is the external part of the fruit; therefore, it is the main by-product generated from the processing of cocoa, and it represents 75% of the total weight [19]. For every ton of cocoa beans, an estimated 10 tons of wastes are generated, which are generally considered of none economic value [37]; this waste is generated after harvest, when pulping the fruit and recovering the grains covered by the mucilaginous layer [38].

CPH is made up of three layers, which are the endocarp, mesocarp, and epicarp; however, there are few specialized studies on the use of a part of the cocoa pod [39]. Normally, the most common use of the complete by-product is as fertilizer for the soil of the agroecosystem; nevertheless, there are phytosanitary problems such as pests and fungal diseases when using the CPH without treatment [19].

The chemical composition of CPH allows using this waste for different purposes. Biofuel production is possible using CPH, due to its lignocellulose content [19]. Other compounds present in CPH are fiber, phenols, and minerals such as Ca, K, P, Mg, Na, Zn, Fe, Cu, and Mn [21, 40, 41].

3.2. Cocoa Mucilage

Cocoa mucilage (CM) is the whitish substance that covers the grain; other names as it is known are sweating or pulp [16]. The cocoa pulp juice is a fraction of CM, with a cloudy whitish appearance that is obtained after processing cocoa by means of handling and exerted pressure; this part of CM is widely used in obtaining products such as alcohol, vinegar, soft drinks, citric acid, and cocoa jelly [42].

The generation of CM is the result of the fruit pulping process; after splitting the cocoa pod in half, the beans are extracted with the mucilaginous layer. Immediately, they undergo a controlled fermentation where biochemical reactions are generated that allow the separation of the grain and the mucilage [43]. The amount of mucilage depends on the type of cocoa, maturation, and physical integrity of the fruits, which is reported between 3 and 5% of the total weight of cocoa [44].

CM is composed of spongy cells that contain cellular sap rich in different chemical compounds, containing 10–14% fermentable sugars (such as sucrose, glucose, and fructose), minerals (such as potassium, sodium, calcium, and magnesium), pectin, pentosans, citric acid, salts, cellulose, hemicellulose, and lignin [16, 42, 45].

3.3. Cocoa Bean Shell

Cocoa bean shell (CBS) is a lignocellulosic material obtained from the bean roasting process, and it constitutes 10–17% of the total weight of cocoa beans, although it may vary due to the fermentation process [18]. The importance of this waste lies in the migration of valuable compounds from the grain to the husk during fermentation [46]. The nutritional quality of CBS allows the generation of new products in the industry; in particular, the food, pharmaceutical, cosmetic, and agricultural industries have developed functional products based on this waste [47]. Moreover, CBS contains polysaccharides, phenolic compounds, methylxanthines [14], catechins, epicatechins, procyanidins, caffeine, theophylline, theobromine, and fiber [48–51]. Regarding energy, the potential content has been reported as an ecological source of alternative energy due to its calorific value, slightly higher than wood [6].

4. Biomass Conversion Technologies

The interest in the use of residual biomass from industrial processing has increased in the last decade since the quality of chemical compounds that conserve wastes can be used for the generation of new products with the application of different treatments [52]. The use of biomass in the production of biofuels is essential to reduce the consumption of fossil resources; this determines the development of an economy based on products of biological origin. For this reason, the type of biorefinery process will obtain the highest waste management in the agricultural industry [53].

The biorefinery can be classified by the system or model that is divided into seven categories, according to the type of biomass and technology used, which are conventional biorefinery, green, complete cultures, lignocellulosic raw material, the concept of two platforms, thermochemical, and marine [54]. The biorefinery is a process used for the generation of different energy products with the application of different technologies for the transformation of biomass [55].

4.1. Physicochemical Conversion

Physicochemical conversion, also known as mechanical extraction, is used to produce liquid biofuels from the seeds of different foods [56]. The main sources used are sunflower and rapeseed; however, used cooking oil, animal fats, and seaweed oil are also used [57]. The oil can be subjected to an esterification or transesterification treatment to produce biodiesel, in which the intention is to transform the triglycerides present in the crude oil into fatty acids and glycerol [58]. In addition, solid waste is generated used for the manufacture of livestock feed [56].

4.2. Biological Conversion

Biological conversion involves microbial action for the decomposition of organic matter [59], and the objective is to produce liquid biofuels and biogas [57]. The most widely used microorganisms are yeasts; however, the use of fungal bacteria and specialized enzymes produces different products of value [58]. This type of technology is adaptable to different biomass sources; therefore, it is a widely used technique around the world [56]. The most relevant processes are fermentation and anaerobic digestion [59].

4.2.1. Anaerobic Digestion

Anaerobic digestion transforms the organic material into biogas, which consists of methane and carbon dioxide and small amounts of hydrogen sulfide [56]; the mechanism of action is activated in the presence of bacteria in a medium lacking oxygen [59]. This process is carried out in biomass with high moisture content (80–90%) [56]. A series of key actions in anaerobic digestion are identified where complex biomass compounds are first hydrolyzed into simpler compounds [57]. Then, acidogenesis occurs that transforms simple compounds into volatile fatty acids, H₂, and CO₂ [59]. Finally, methanogens convert organic acids into CH₄ gas and CO₂ [58].

4.2.2. Fermentation

Fermentation is an anaerobic biological process carried out by yeasts [56], where the goal is to convert sugars like glucose, fructose, and sucrose into ethanol and CO₂ [59]. The sources most used in the generation of bioethanol are sugar cane and corn; however, in Europe, wheat and beet are used more frequently [56]. Biomass can be divided into three types: sugars, starch, and lignocellulosic materials [57].

The most common application is the action of yeasts for the fermentation of starch and generation of ethanol; nevertheless, the use of enzymes for the degradation of cellulose increases the production of ethanol [59]. Recent studies focus on the possibility of applying pretreatment to break the lignocellulosic structure of biomass of forest and agricultural origin for the production of ethanol by fermentation [56]. In addition, glycerol and carboxylic acids can be obtained as by-products from this process. However, the quality and yield will depend on the type of biomass used and other process factors such as temperature, time, and pH [57].

The intention of the production of bioethanol is the gradual replacement of fuels of fossil origin; at present, mixtures of ethanol and gasoline are made (80–20%, respectively) [56]. On the other hand, solid waste after ethanol production can be used as boiler fuel or livestock feed [57]; generally, other treatments such as liquefaction, gasification, or pyrolysis can be used to produce other valuable products [58].

4.3. Thermochemical Conversion

Thermochemical conversion transforms the biomass by breaking bonds for the production of energy, biogas, and bio-oils [58]. The type of biomass must have low moisture content, but high values of organic matter [59]. In addition, the pyrolysis process is considered the basis of thermochemical treatments, due to the formation of solid, liquid, and gaseous residues that are generated [56].

The types of thermochemical treatments are combustion [59], gasification [56], pyrolysis [57], and liquefaction [60]. The choice of the type of treatment depends on different factors such as biomass quality and industry specifications, which include final application, environmental impact, and financial aspects [58]. Industrially, thermochemical processes are more efficient compared to biological processes, in relation to the reaction time and transformation of organic compounds [56].

4.3.1. Combustion

The combustion process is defined as the burning of fuel to release the energy contained in the form of heat of reaction [61]. In this context, combustion converts the chemical energy of biomass into thermal energy [59]; this process generates approximately 90% of the energy obtained through biomass [57]. Combustion transforms carbon into CO₂, while hydrogen into H₂O. Other compounds such as sulfur convert to SO₂ and nitrogen to NO_x [60].

Thermal degradation of biomass occurs in different steps [59]:

(i) Drying of biomass and release of highly volatile materials at a temperature between 25 and 125°C

(ii) Release and burning of volatile materials between 125 and 350°C

(iii) Residual carbon oxidation and burning of volatile materials between 350 and 525°C

4.3.2. Gasification

Gasification is an oxidation process that converts biomass into a gas through the action of a gasifier [59]. When the treatment is carried out by high temperatures above 1200°C, the result is known as synthesis gas composed of H₂ and CO; on the other hand, at low processing temperatures, the product includes CH₄ and CO₂ with impurities of other compounds [56]. The resulting biogas has a calorific value of 4.5–6 MJ/m³; however, this value depends on the type of biomass from which it comes. The calorific value of biogas represents 10–50% of the calorific value compared to natural gas [57].

The resulting biogas can be used directly; nevertheless, this product can be transformed, through the application of different processes, into other types of fuels and chemical products [61]. The thermochemical process is more efficient compared to combustion and pyrolysis; in addition, it has been reported that gasification is the best process to generate hydrogen gas from biomass [58].

4.3.3. Pyrolysis

Pyrolysis consists of the application of specific temperatures for the transformation of biomass into solid, liquid, and gaseous products [57]. The types of resulting products are H₂, CH₄, CO, and CO₂ as gaseous compounds, for liquids with tar and oils, as well as solid compounds [58, 61]. This process is carried out in a temperature range of 400–800°C without the presence of oxygen. The quality and performance of the resulting biofuel depend on factors such as time, temperature, biomass composition, and process specifications; forestry and agricultural residues are suitable biomass for the production of biofuels, as a product oil with a calorific value of 38 MJ/kg is obtained [59].

The main application of the pyrolysis process is to obtain charcoal; however, the production of liquid biofuels has increased in the last decade at temperatures of approximately 500°C [56]. The classification of the pyrolysis process is based on the applied temperature range; carbonization occurs between 300 and 500°C, slow pyrolysis around 500°C, fast pyrolysis between 500 and 650°C, and finally, flash pyrolysis at temperatures above 650°C [57].

4.3.4. Liquefaction

Liquefaction is also called hydrothermal liquefaction [56], and it is carried out in an aqueous medium with the application of temperature between 280 and 370°C and high pressure of 10–25 MPa [57]. This type of technology can be used for different types of biomass; nevertheless, algae residues are ideal because this technology omits the drying of the biomass, compared to the pyrolysis process, which means significant savings in energy consumption [61].

The main product obtained from the liquefaction process is bio-oil, which has a calorific value of 30–35 MJ/kg, which can be transformed into different biofuels [62]. In this sense, the liquefaction process involves repolymerization reactions for the transformation of bio-oil, chemical products, solid waste, and gas [58].

5. Biofuels from Cocoa Industrial Wastes

Biofuel is defined as a solid, liquid, or gaseous fuel mainly generated from biomass [63]. These can be classified into first-, second-, and third-generation biofuels. Those of the first generation are obtained from food sources such as sugar, vegetable oils, and starches, while those of the second are those of lignocellulosic biomass and those of the third are those that are produced from aquatic materials such as algae [63]. An advantage of biofuels is that they produce less emissions of SO₂, NO_X, and soot than conventional fossil fuels since they have a minimum content of sulfur, nitrogen, and ash, which represents an advantage when using them [64]. In Figure 3, the general cycle for obtaining biofuels from cocoa industrial wastes is shown.

[figure omitted; refer to PDF]

5.1. Biochar

Biochar is produced with the transformation of biomass by thermochemical treatments [65]. This change produces a solid residue, along with a bio-oil and a gas composed of hydrogen, carbon oxides, and hydrocarbons [66]. The use of CPH in the generation of biochar uses temperatures between 400 and 800°C; however, alternatives to the process with high temperatures are sought because the biochar produced at 500°C presents high pH and decreased performance in general [67].

5.2. Bioethanol

Bioethanol is obtained from the fermentation of sugars by the action of yeasts. In general, starch-rich raw materials are used. The use of lignocellulosic material is possible through a pretreatment that hydrolyzes cellulose into glucose to generate fermentation [68]. The demand for bioethanol has increased the development of technically and economically feasible procedures; these are based on the use of biomass or a fraction of it [69]. Obtaining bioethanol is possible using different raw materials; therefore, first-, second-, and third-generation bioethanol is obtained [70]. Cocoa residues are a good source for the production of bioethanol because the different by-products contain various chemical compounds.

5.3. Biogas

Biogas is a renewable fuel that is produced from the anaerobic digestion of organic material (such as waste and residues from agriculture and the food industry) [71]. During this metabolic pathway, organic carbon is converted through oxidation-reduction reactions into 30–50% carbon dioxide (CO₂, its most oxidized state) and 50–70% methane (CH₄, its lowest form) [72]. Biogas can also contain 0–3% nitrogen, 5–10% water vapor, 0-1% oxygen, and some others corresponding to less than 1%, such as hydrogen sulfide, ammonia, hydrocarbons, and siloxanes. [73]. This fuel can be used to produce electricity, heat, or as a fuel for vehicles, with environmental, climatic, and economic benefits [71].

5.4. Biohydrogen

Biohydrogen is a sustainable fuel produced in its economical form by microbes through fermentation [74]. However, this product can be obtained from different biomass and with various thermochemical processes [75]. There are some complications to producing biohydrogen on a large scale and commercially; the alternatives to increase production efficiency are based on the application of pretreatments to augment the bioavailability of simple sugars in biomass [76]. Biohydrogen is considered the most ecological fuel; the waste from its processing is water vapor; its calorific value (142 MJ/kg) is higher than the value of methane, natural gas, and even gasoline [77].

5.5. Bio-Oil

Bio-oil is a mixture of organic compounds obtained from the pyrolysis of biomass. Among these compounds are sugar monomers and oligomers, as well as sugar derivatives, such as carboxylic acids, aldehydes, ketones, esters, and alcohols [78]. The abundant presence of carboxylic acids makes the bio-oil corrosive. The water content ranges between 20 and 30% depending on the nature of the biomass, which reduces its calorific value and viscosity; the main applications of bio-oil are low-quality boiler fuel, high-quality car engine fuel after upgrade, sources of value-added chemicals, functional carbon materials, and binders [79]. Some researchers develop methods to upgrade bio-oil into biofuel through hydrodeoxygenation, as well as studying strategies based on biomass pretreatments such as dry torrefaction, wet torrefaction, acid and alkaline treatment, steam explosion, downstream treatment of bio-oil such as emulsification, solvent addition, or filtration [80].

5.6. Liquid Smoke

Liquid smoke is a liquid product that is generated from the decomposition process of organic waste or from the condensation of smoke generated in the burning of biomass [81]. This liquid can be extracted from biomass by rapid pyrolysis. This process is carried out at a medium temperature and with high biomass flows [82]. Rapid pyrolysis of biomass is a thermochemical conversion process to produce biofuels. During this process, heavy tar, light tar (liquid smoke), charcoal, and uncondensed gases are produced [83]. Liquid smoke is also known as wood vinegar, pyrolysis liquid, pyrolysis oil, bio-crude oil, biofuel oil, pyroligneous tar, pyroligneous acid, wood liquid, and wood oil [82]. Table 3 shows studies of the production of biofuels from cocoa by-products.

Table 3

Biofuels generated from cocoa industrial wastes.

Product	Biomass	Process	Results	Reference
Biochar	CPH	Low-temperature pyrolysis with residence times of 30–120 minutes in 30-minute intervals.	Biochar with a calorific value of 17.8 MJ/kg with high potassium content. The resulting biofuel has characteristics similar to lignite.	[40]
Biochar	CPH	Pyrolysis at temperatures of 250, 300, and 350°C. Activation process with HCl to reduce the values of free fatty acids to use cooking oil used in the production of biodiesel.	The activated carbon generated from CPH showed better retention of free fatty acids than the esterification treatment with H2SO4.	[84]
Bioethanol	CPH	Fermentation using Zymomonas mobilis in a period of 0–8 days. The percentages of microorganisms added were 8–16% v/v.	An alcohol graduation of 10.62% was obtained under 8 day conditions with a concentration of 14% v/v.	[85]
Bioethanol	CM	Fermentation using Pichia kudriavzevii for 5 days at a temperature of 30°C.	The result shows that the maximum ethanol concentration was 13.8 g/L.	[86]
Bioethanol	CM	Saccharomyces cerevisiae was used for fermentation for 12 days.	The bioethanol production was 4.85%; it is reported as a low concentration for the amount of mucilage.	[87]
Bioethanol	CBS	Fermentation with variation in the amount of yeasts during 6 days of the process; hydrolysis pretreatment with H2SO4 at temperatures of 30–80°C in periods of 50–150 minutes.	8.46% bioethanol was produced under pH 8 conditions, with a yeast concentration for fermentation of 0.05 mg/g in 6 days of the process. pH was determined to be the main influencing factor.	[88]
Biogas	CPH	Fermentation process with the application of four pretreatments, acid (H2SO4), alkaline (H2O2), ground without treatment, and the last one not ground without treatment.	The biogas yields by pretreatment were acid 162.8, alkaline 564.8, ground without treatment 220.8, and unground without treatment 243.3; the highest production was obtained on day 18 of the process.	[89]
Biogas	CPH	The aerobic digestion process together with a hydrothermal pretreatment was used at different temperatures (150–220°C) in times of 5–15 minutes.	The untreated CPH biogas production was estimated at 357l (N)/kgVS, while the biogas production from pretreated biomass was 526.38L(N)/kgVS at 150°C for 15 minutes.	[90]
Biogas	CM	Fermentation process for 25 days with a pretreatment of a NaOH and NaOH-H2O2 solution.	The biogas produced was obtained in a concentration of 66.07% with a yield of 0.734 m3 CH4/kgVS.	[45]
Biohydrogen	CM	Fermentation process at two temperatures 35°C and 55°C; the organic load of volatile compounds 4, 8, and 12 gVS/L was determined.	The amount of gas produced daily was measured. The reaction was faster at 55 °C, the generation of the product was from the fifth day. The load was determined in 12 gVS/L and the hydrogen production of 703 mL.	[91]
Biohydrogen	CM	Fermentation of the biomass in a load of 10 gVS/L at a temperature of 35 °C at a pH of 5.5 in a period of time of 12 days.	The amount of biohydrogen produced was 3674.021 mL, which is equivalent to 91.85 mL H2/gVS.	[92]
	Pig manure
	Coffee mucilage
Biomethane	CBS	Fermentation for the production of biomethane from lignocellulosic material with a pretreatment with N-methylmorpholine-N-oxide at a temperature of 120°C for 3 hours.	The effect of the pretreatment was compared against an untreated control sample, in which the biomethane production increased from 199 to 226 ml of CH4/gVS. This effect was more relevant for rice straw with an 82% increase.	[93]
	Rice straw
	Hazelnut skin
Bio-oil	CPH	Pyrolysis at different temperatures from 300°C–600°C in 100°C steps	Bio-oil with a calorific value of 36.23 MJ/kg with characteristics similar to diesel. Biogas with a calorific value of 35.24 MJ/kg presented high values of CO2, CO, CH4, H2S, and H2O.	[94]
Biogas
Liquid smoke	CBS	Pyrolysis with temperatures of 450, 500, and 550°C with an increase of 5–15°C/minute.	The concentration of liquid smoke obtained was 18–23%. The higher speed of heating produced a greater quantity of coal, ash, and water; the calorific power of the product was 22.97 MJ/kg.	[95]
Biochar

CPH = cocoa pod husk; MJ/kg = megajoule/kilogram; H₂SO₄ = sulfuric acid; HCl = hydrochloric acid; v/v = volume/volume; H₂O₂ = hydrogen peroxide; VS = volatile solids; CO₂ = carbon dioxide; CO = carbon monoxide; CH₄ = methane; H₂S = hydrogen sulfide; H₂O = water; CM = cocoa mucilage; NaOH = sodium hydroxide; L = liter; g = gram; mL = milliliter; pH = potential of hydrogen; mg = milligram; CBS = cocoa bean shell.

6. Future Trends of Biofuels from Cocoa By-Products

The trends for the generation of fuels from cocoa lignocellulosic biomass explore the alternatives to use catalysts that accelerate and increase the reactions in the conversion of compounds used in the process. This is because recent studies determine that the use of catalysts produced from agricultural residues is an economically and ecologically feasible alternative [96]. The most promising proposals are based on the use of alkaline, acid, and enzyme catalysts that simplify operations by easily recovering from the mixture [97]. Other aspects to consider in the future are the optimization of the pretreatments for the variables of the process; some investigations use the statistical method of response surface to determine the precise conditions in obtaining biofuels [98].

Some aspects to be resolved in the future are the modernization of the technology used for the conversion and treatments of biomass; these include aspects such as temperature control, industrial capacity, and specialized equipment [63, 99]. In addition, obtaining an adequate and continuous supply of material as a source for the production of biofuels is a particular issue; each time they gain greater relevance due to the development of economies based on this activity, which, in turn, generate new policies, minimize the effects negative to the environment, and avoid biological risks in agroecosystems [100].

The main problem in the production of lignocellulosic biomass biofuels is the staggering at the industrial and commercial level; this is due to the type of technology to be used for the different residues generated from the agroindustry. However, this problem has been addressed by different investigations in recent years with the implementation of strategies to optimize the generation of fuels from lignocellulosic biomass [101].

7. Conclusions

Food production entails a generation of important wastes for the industry, due to the nutritional quality that they possess even after being considered as nonvalue materials. The challenge is to minimize the negative effect it has on the environment; there are different perspectives to achieve this objective; recovering these wastes and transforming them into new products is an alternative that benefits the integral use of the fruit; this can generate a stable economy around the use of wastes.

The generation of biofuels from by-products of the industry responds to the need to reduce the consumption of fossil fuels and provide a source of sustainable fuels that has the least ecological impact. In this context, cocoa industrial wastes have the potential to be a source of biomass in the production of biofuels, due to their chemical composition, and the amount of waste generated per year worldwide.

Regarding the production of biofuels, CPH was the most versatile by-product for the generation of different biofuels; however, for its better use, it is necessary to apply a pretreatment that allows obtaining rich substrates in compounds for fermentation and subsequent production of liquid and gaseous biofuels. In addition, it contains a significant amount of compounds for the formation of solid biofuels. CBS is the waste that although it contains a higher content of valuable compounds, it contains fewer applications for obtaining biofuels; nevertheless, it is more relevant in other industries such as food, pharmaceuticals, and cosmetics.

Authors’ Contributions

All authors actively participate in the complete preparation of the manuscript.

Acknowledgments

Financial support provided by CONACyT through the scholarship of C. J. Mendoza-Meneses for the development of his graduate studies project is gratefully acknowledged.

References

[1] N. Gontard, U. Sonesson, M. Birkved, M. Majone, D. Bolzonella, A. Celli, H. Angellier-Coussy, G. W. Jang, A. Verniquet, J. Broeze, B. Schaer, A. P. Batista, A. Sebok, "A research challenge vision regarding management of agricultural waste in a circular bio-based economy," Critical Reviews in Environmental Science and Technology, vol. 48 no. 6, pp. 614-654, DOI: 10.1080/10643389.2018.1471957, 2018.

[2] Y. Duan, A. Pandey, Z. Zhang, M. K. Awasthi, S. K. Bhatia, M. J. Taherzadeh, "Organic solid waste biorefinery: sustainable strategy for emerging circular bioeconomy in China," Industrial Crops and Products, vol. 153,DOI: 10.1016/j.indcrop.2020.112568, 2020.

[3] E. Righetti, S. Nortilli, F. Fatone, N. Frison, D. Bolzonella, "A multiproduct biorefinery approach for the production of hydrogen, methane and volatile fatty acids from agricultural waste," Waste and Biomass Valorization, vol. 11 no. 10, pp. 5239-5246, DOI: 10.1007/s12649-020-01023-3, 2020.

[4] IEA, IEA Bioenergy Task 42 Biorefining, 2014.

[5] C. Caldeira, A. Vlysidis, G. Fiore, V. De Laurentiis, G. Vignali, S. Sala, "Sustainability of food waste biorefinery: a review on valorisation pathways, techno-economic constraints, and environmental assessment," Bioresource Technology, vol. 312,DOI: 10.1016/j.biortech.2020.123575, 2020.

[6] Z. S. Vásquez, D. P. de Carvalho Neto, G. V. M. Pereira, L. P. S. Vandenberghe, P. Z. de Oliveira, P. B. Tiburcio, H. L. G. Rogez, A. Góes Neto, C. R. Soccol, "Biotechnological approaches for cocoa waste management: a review," Waste Management (New York, NY), vol. 90, pp. 72-83, DOI: 10.1016/j.wasman.2019.04.030, 2019.

[7] E. Hipólito-Romero, M. G. Carcaño-Montiel, J. M. Ramos-Prado, E. A. Vázquez-Cabañas, L. López-Reyes, J. Ricaño-Rodríguez, "Efecto de inoculantes bacterianos edáficos mixtos en el desarrollo temprano de cultivares mejorados de cacao ( Theobroma cacao L.) en un sistema agroforestal tradicional del norte de Oaxaca, México," Revista Argentina de Microbiología, vol. 49, pp. 356-365, 2017.

[8] F. Lahive, P. Hadley, A. J. Daymond, "The physiological responses of cacao to the environment and the implications for climate change resilience. a review," Agronomy for Sustainable Development, vol. 39 no. 1,DOI: 10.1007/s13593-018-0552-0, 2018.

[9] M. S. Beg, S. Ahmad, K. Jan, K. Bashir, "Status, supply chain and processing of cocoa—a review," Trends in Food Science & Technology, vol. 66, pp. 108-116, DOI: 10.1016/j.tifs.2017.06.007, 2017.

[10] L. Handojo, H. Triharyogi, A. Indarto, "Cocoa bean shell waste as potential raw material for dietary fiber powder," International Journal of Recycling of Organic Waste in Agriculture, vol. 8 no. S1, pp. 485-491, DOI: 10.1007/s40093-019-0271-9, 2019.

[11] A. Loullis, E. Pinakoulaki, "Carob as cocoa substitute: a review on composition, health benefits and food applications," European Food Research and Technology, vol. 244 no. 6, pp. 959-977, DOI: 10.1007/s00217-017-3018-8, 2018.

[12] A. Arora, P. Nandal, J. Singh, M. L. Verma, "Nanobiotechnological advancements in lignocellulosic biomass pretreatment," Materials Science for Energy Technologies, vol. 3, pp. 308-318, DOI: 10.1016/j.mset.2019.12.003, 2020.

[13] G. Grillo, L. Boffa, A. Binello, S. Mantegna, G. Cravotto, F. Chemat, T. Dizhbite, L. Lauberte, G. Telysheva, "Cocoa bean shell waste valorisation; extraction from lab to pilot-scale cavitational reactors," Food Research International, vol. 115, pp. 200-208, DOI: 10.1016/j.foodres.2018.08.057, 2019.

[14] A. C. Mellinas, A. Jiménez, M. C. Garrigós, "Optimization of microwave-assisted extraction of cocoa bean shell waste and evaluation of its antioxidant, physicochemical and functional properties," LWT, vol. 127,DOI: 10.1016/j.lwt.2020.109361, 2020.

[15] C. Botella-Martínez, R. Lucas-Gonzalez, C. Ballester-Costa, J. Á. Pérez-Álvarez, J. Fernández-López, J. Delgado-Ospina, C. Chaves-Lopez, M. Viuda-Martos, "Ghanaian cocoa ( Theobroma cacao L.) bean shells coproducts: effect of particle size on chemical composition, bioactive compound content and antioxidant activity," Agronomy, vol. 11,DOI: 10.3390/agronomy11020401, 2021.

[16] K. H. N. Figueroa, N. V. M. García, R. C. Vega, "Cocoa by-products," Food Wastes and by-Products, pp. 373-411, DOI: 10.1002/9781119534167.ch13, 2020.

[17] D. C. G. Okiyama, S. L. B. Navarro, C. E. C. Rodrigues, "Cocoa shell and its compounds: applications in the food industry," Trends in Food Science & Technology, vol. 63, pp. 103-112, DOI: 10.1016/j.tifs.2017.03.007, 2017.

[18] O. Rojo-Poveda, L. Barbosa-Pereira, G. Zeppa, C. Stévigny, "Cocoa bean shell—a by-product with nutritional properties and biofunctional potential," Nutrients, vol. 12 no. 4,DOI: 10.3390/nu12041123, 2020.

[19] F. Lu, J. Rodriguez-Garcia, I. Van Damme, N. J. Westwood, L. Shaw, J. S. Robinson, G. Warren, A. Chatzifragkou, S. McQueen Mason, L. Gomez, L. Faas, K. Balcombe, C. Srinivasan, F. Picchioni, P. Hadley, D. Charalampopoulos, "Valorisation strategies for cocoa pod husk and its fractions," Current Opinion in Green and Sustainable Chemistry, vol. 14, pp. 80-88, DOI: 10.1016/j.cogsc.2018.07.007, 2018.

[20] O. A. Olugosi, J. O. Agbede, I. A. Adebayo, G. E. Onibi, O. A. Ayeni, "Nutritional enhancement of cocoa pod husk meal through fermentation using Rhizopus stolonifer," African Journal of Biotechnology, vol. 18 no. 30, pp. 901-908, DOI: 10.5897/ajb2019.16940, 2019.

[21] R. Campos-Vega, K. H. Nieto-Figueroa, B. D. Oomah, "Cocoa ( Theobroma cacao L.) pod husk: renewable source of bioactive compounds," Trends in Food Science & Technology, vol. 81, pp. 172-184, DOI: 10.1016/j.tifs.2018.09.022, 2018.

[22] K. A. Goude, K. M. D. Adingra, O. J. Gbotognon, E. J. P. Kouadio, "Biochemical characterization, nutritional and antioxidant potentials of cocoa placenta ( Theobroma Cacao L.)," Annals Food Science and Technology, vol. 20, pp. 603-613, 2019.

[23] C. S. O. Nunes, M. L. C. da Silva, G. P. Camilloto, B. A. S. Machado, K. V. S. Hodel, M. G. B. Koblitz, G. B. M. Carvalho, A. P. T. Uetanabaro, "Potential applicability of cocoa pulp ( Theobroma cacao L.) as an adjunct for beer production," The Scientific World Journal, vol. 2020,DOI: 10.1155/2020/3192585, 2020.

[24] K. Kucharska, P. Rybarczyk, I. Hołowacz, R. Łukajtis, M. Glinka, M. Kamiński, "Pretreatment of lignocellulosic materials as substrates for fermentation processes," Molecules, vol. 23 no. 11,DOI: 10.3390/molecules23112937, 2018.

[25] R. Darmayanti, H. W. Amini, M. Rizkiana, F. A. Setiawan, B. Palupi, A. Susanti, I. Rahmawati, B. A. Fachri, "Lignocellulosic material from main Indonesian plantation commodity as the feedstock for fermentable sugar in biofuel production," ARPN Journal of Engineering and Applied Sciences, vol. 14, pp. 3524-3534, 2019.

[26] N. Y. Asiedu, F. A. Neba, A. Addo, "Modeling the attainable regions for catalytic oxidation of renewable biomass to specialty chemicals: waste biomass to carboxylic acids," South African Journal of Chemical Engineering, vol. 30,DOI: 10.1016/j.sajce.2019.07.003, 2019.

[27] D. C. Meza-Sepúlveda, A. M. Castro, A. Zamora, J. W. Arboleda, A. M. Gallego, A. V. Camargo-Rodríguez, "Bio-based value chains potential in the management of cacao pod waste in Colombia, a case study," Agronomy, vol. 11, 2021.

[28] R. F. Susanti, S. Alvin, J. Kim, "Toward high-performance hard carbon as an anode for sodium-ion batteries: demineralization of biomass as a critical step," Journal of Industrial and Engineering Chemistry, vol. 91, pp. 317-329, DOI: 10.1016/j.jiec.2020.08.016, 2020.

[29] J. S. da Costa, M. G. da Silva, M. V. Scatolino, M. D. R. Lima, M. R. de Assis, L. Bufalino, S. Numazawa, P. F. Trugilho, T. de Paula Protasio, "Relating features and combustion behavior of biomasses from the amazonian agroforestry chain," Biomass Conversion and Biorefinery,DOI: 10.1007/s13399-020-01121-1, 2020.

[30] V. Dhyani, T. Bhaskar, "A comprehensive review on the pyrolysis of lignocellulosic biomass," Renewable Energy, vol. 129, pp. 695-716, DOI: 10.1016/j.renene.2017.04.035, 2018.

[31] S. Wang, G. Dai, H. Yang, Z. Luo, "Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review," Progress in Energy and Combustion Science, vol. 62, pp. 33-86, DOI: 10.1016/j.pecs.2017.05.004, 2017.

[32] J. A. Okolie, S. Nanda, A. K. Dalai, J. A. Kozinski, "Optimization and modeling of process parameters during hydrothermal gasification of biomass model compounds to generate hydrogen-rich gas products," International Journal of Hydrogen Energy, vol. 45 no. 36, pp. 18275-18288, DOI: 10.1016/j.ijhydene.2019.05.132, 2020.

[33] H. K. Sharma, C. Xu, W. Qin, "Biological pretreatment of lignocellulosic biomass for biofuels and bioproducts: an overview," Waste and Biomass Valorization, vol. 10 no. 2, pp. 235-251, DOI: 10.1007/s12649-017-0059-y, 2019.

[34] D. Kumari, R. Singh, "Pretreatment of lignocellulosic wastes for biofuel production: a critical review," Renewable and Sustainable Energy Reviews, vol. 90, pp. 877-891, DOI: 10.1016/j.rser.2018.03.111, 2018.

[35] B. Kumar, N. Bhardwaj, K. Agrawal, V. Chaturvedi, P. Verma, "Current perspective on pretreatment technologies using lignocellulosic biomass: an emerging biorefinery concept," Fuel Processing Technology, vol. 199,DOI: 10.1016/j.fuproc.2019.106244, 2020.

[36] Z. Zhou, F. Lei, P. Li, J. Jiang, "Lignocellulosic biomass to biofuels and biochemicals: a comprehensive review with a focus on ethanol organosolv pretreatment technology," Biotechnology and Bioengineering, vol. 115 no. 11, pp. 2683-2702, DOI: 10.1002/bit.26788, 2018.

[37] A. I. Akinjokun, L. F. Petrik, A. O. Ogunfowokan, J. Ajao, T. V. Ojumu, "Isolation and characterization of nanocrystalline cellulose from cocoa pod husk (CPH) biomass wastes," Heliyon, vol. 7 no. 4,DOI: 10.1016/j.heliyon.2021.e06680, 2021.

[38] O. A. A. Eletta, A. G. Adeniyi, J. O. Ighalo, D. V. Onifade, F. O. Ayandele, "Valorisation of cocoa ( Theobroma cacao ) pod husk as precursors for the production of adsorbents for water treatment," Environmental Technology Reviews, vol. 9 no. 1, pp. 20-36, DOI: 10.1080/21622515.2020.1730983, 2020.

[39] L. Grob, E. Ott, S. Schnell, E. J. Windhab, "Characterization of endocarp powder derived from cocoa pod," Journal of Food Engineering, vol. 305,DOI: 10.1016/j.jfoodeng.2021.110591, 2021.

[40] C. H. Tsai, W. T. Tsai, S. C. Liu, Y. Q. Lin, "Thermochemical characterization of biochar from cocoa pod husk prepared at low pyrolysis temperature," Biomass Conversion and Biorefinery, vol. 8 no. 2, pp. 237-243, DOI: 10.1007/s13399-017-0259-5, 2018.

[41] L. Hennessey-Ramos, W. Murillo-Arango, J. Vasco-Correa, I. C. Paz Astudillo, "Enzymatic extraction and characterization of pectin from cocoa pod husks ( Theobroma cacao L.) using celluclast® 1.5 L," Molecules, vol. 26 no. 5,DOI: 10.3390/molecules26051473, 2021.

[42] E. O. K. Oddoye, C. K. Agyente-Badu, E. Gyedu-Akoto, "Cocoa and its by-products: identification and utilization," Chocolate in Health and Nutrition, pp. 23-37, DOI: 10.1007/978-1-61779-803-0_3, 2013.

[43] C. P. Guirlanda, G. G. da Silva, J. A. Takahashi, "Cocoa honey: agro-industrial waste or underutilized cocoa by-product?," Future Foods, vol. 4,DOI: 10.1016/j.fufo.2021.100061, 2021.

[44] P. M. Scheuer, A. N. N. Southgate, M. F. Martelli, C. Dias, M. E. da Silva, A. A. Coelho, A. de Francisco, "Quality properties of a bread made with levain and cocoa waste," Journal of Culinary Science & Technology, vol. 0,DOI: 10.1080/15428052.2020.1848685, 2020.

[45] T. Widjaja, S. Nurkhamidah, A. Altway, A. A. Z. Rohmah, F. Saepulah, "Chemical pre-treatments effect for reducing lignin on cocoa pulp waste for biogas production," AIP Conference Proceedings, vol. 2349,DOI: 10.1063/5.0051903, 2021.

[46] S. Jokić, N. Pavlović, A. Jozinović, Đ. Ačkar, J. Babić, D. Šubarić, "High-voltage electric discharge extraction of bioactive compounds from the cocoa bean shell," Chemical and Biochemical Engineering Quarterly, vol. 33, pp. 271-280, DOI: 10.15255/CABEQ.2018.1525, 2019.

[47] V. Barišić, A. Jozinović, I. Flanjak, D. Šubarić, J. Babić, B. Miličević, K. Doko, D. Ackar, "Difficulties with use of cocoa bean shell in food production and high voltage electrical discharge as a possible solution," Sustainability, vol. 12,DOI: 10.3390/su12103981, 2020.

[48] D. C. G. Okiyama, I. D. Soares, M. S. Cuevas, E. J. Crevelin, L. A. B. Moraes, M. P. Melo, A. L. Oliveira, C. E. C. Rodrigues, "Pressurized liquid extraction of flavanols and alkaloids from cocoa bean shell using ethanol as solvent," Food Research International, vol. 114, pp. 20-29, DOI: 10.1016/j.foodres.2018.07.055, 2018.

[49] D. Rossin, L. Barbosa-Pereira, N. Iaia, B. Sottero, A. C. Danzero, G. Poli, G. Zeppa, F. Biasi, "Protective effect of cocoa bean shell against intestinal damage: an example of byproduct valorization," Antioxidants, vol. 10 no. 2,DOI: 10.3390/antiox10020280, 2021.

[50] C. Cantele, O. Rojo-Poveda, M. Bertolino, D. Ghirardello, V. Cardenia, L. Barbosa-Pereira, G. Zeppa, "In Vitro bioaccessibility and functional properties of phenolic compounds from enriched beverages based on cocoa bean shell," Foods, vol. 9 no. 6,DOI: 10.3390/foods9060715, 2020.

[51] B. Lyra Colombi, P. R. Silva Zanoni, L. Benathar Ballod Tavares, "Effect of phenolic compounds on bioconversion of glucose to ethanol by yeast Saccharomyces cerevisiae PE-2," The Canadian Journal of Chemical Engineering, vol. 96 no. 7, pp. 1444-1450, DOI: 10.1002/cjce.23114, 2018.

[52] K. W. Chew, S. R. Chia, H. W. Yen, S. Nomanbhay, Y. C. Ho, P. L. Show, "Transformation of biomass waste into sustainable organic fertilizers," Sustainability, vol. 11 no. 8,DOI: 10.3390/su11082266, 2019.

[53] C. K. Yamakawa, F. Qin, S. I. Mussatto, "Advances and opportunities in biomass conversion technologies and biorefineries for the development of a bio-based economy," Biomass and Bioenergy, vol. 119, pp. 54-60, DOI: 10.1016/j.biombioe.2018.09.007, 2018.

[54] N. Q. Diep, K. Sakanishi, N. Nakagoshi, S. Fujimoto, T. Minowa, X. D. Tran, "Biorefinery: concepts, current status and development trends," International Journal of Biomass and Renewables, vol. 1, 2021.

[55] A. T. Ubando, C. B. Felix, W.-H. Chen, "Biorefineries in circular bioeconomy: a comprehensive review," Bioresource Technology, vol. 299,DOI: 10.1016/j.biortech.2019.122585, 2020.

[56] P. Adams, T. Bridgwater, A. Lea-Langton, A. Ross, I. Watson, "Biomass conversion technologies," Greenhouse Gas Balances of Bioenergy Systems, pp. 107-139, DOI: 10.1016/b978-0-08-101036-5.00008-2, 2018.

[57] A. Tursi, "A review on biomass: importance, chemistry, classification, and conversion," Biofuel Research Journal, vol. 6 no. 2, pp. 962-979, DOI: 10.18331/brj2019.6.2.3, 2019.

[58] S. Y. Lee, R. Sankaran, K. W. Chew, C. H. Tan, R. Krishnamoorthy, D. T. Chu, P. L. Show, "Waste to bioenergy: a review on the recent conversion technologies," BMC Energy, vol. 1 no. 1,DOI: 10.1186/s42500-019-0004-7, 2019.

[59] D. Zinla, P. Gbaha, P. M. E. Koffi, B. K. Koua, "Characterization of rice, coffee and cocoa crops residues as fuel of thermal power plant in Côte d’Ivoire," Fuel, vol. 283,DOI: 10.1016/j.fuel.2020.119250, 2021.

[60] J. Cai, Y. He, X. Yu, S. W. Banks, Y. Yang, X. Zhang, Y. Yu, R. Liu, A. V. Bridgwater, "Review of physicochemical properties and analytical characterization of lignocellulosic biomass," Renewable and Sustainable Energy Reviews, vol. 76, pp. 309-322, DOI: 10.1016/j.rser.2017.03.072, 2017.

[61] M. Shahbaz, T. Al-Ansari, M. Aslam, Z. Khan, A. Inayat, M. Athar, S. R. Naqvi, M. A. Ahmed, G. McKay, "A state of the art review on biomass processing and conversion technologies to produce hydrogen and its recovery via membrane separation," International Journal of Hydrogen Energy, vol. 45 no. 30, pp. 15166-15195, DOI: 10.1016/j.ijhydene.2020.04.009, 2020.

[62] G. Kumar, J. Dharmaraja, S. Arvindnarayan, S. Shoban, P. Bakonyi, G. D. Saratale, N. Nemestóthy, K. Bélafi–Bakó, J. J. Yoon, S. H. Kim, "A comprehensive review on thermochemical, biological, biochemical and hybrid conversion methods of bio-derived lignocellulosic molecules into renewable fuels," Fuel, vol. 251, pp. 352-367, DOI: 10.1016/j.fuel.2019.04.049, 2019.

[63] T. E. Odetoye, E. O. Ajala, D. S. Ogunniyi, "A review of biofuels research in Nigeria," Arid Zone Journal of Engineering, Technology and Environment, vol. 15, pp. 153-162, 2019.

[64] M. A. Hernández, A. J. González, F. Suárez, C. Ochoa, A. M. Candela, I. Cabeza, "Assessment of the biohydrogen production potential of different organic residues in Colombia: cocoa waste, pig manure and coffee mucilage," Chemical Engineering Transactions, vol. 65,DOI: 10.3303/CET1865042, 2018.

[65] L. Leng, H. Huang, H. Li, J. Li, W. Zhou, "Biochar stability assessment methods: a review," Science of The Total Environment, vol. 647, pp. 210-222, DOI: 10.1016/j.scitotenv.2018.07.402, 2019.

[66] D. Wang, P. Jiang, H. Zhang, W. Yuan, "Biochar production and applications in agro and forestry systems: a review," Science of The Total Environment, vol. 723,DOI: 10.1016/j.scitotenv.2020.137775, 2020.

[67] M. O. Ogunlade, I. F. Odesola, O. A. Akappo, E. O. Ige, "Effects of temperature on some chemical properties and yield df biochar derived from cocoa pod husks for soil amendment," pp. 74-78, .

[68] M. S. Buckeridge, A. Grandis, E. Q. P. Tavares, "Disassembling the glycomic code of sugarcane cell walls to improve second-generation bioethanol production," Bioethanol Production from Food Crops, pp. 31-43, DOI: 10.1016/b978-0-12-813766-6.00002-3, 2019.

[69] B. Sharma, C. Larroche, C.-G. Dussap, "Comprehensive assessment of 2G bioethanol production," Bioresource Technology, vol. 313,DOI: 10.1016/j.biortech.2020.123630, 2020.

[70] S. H. Mohd Azhar, R. Abdulla, S. A. Jambo, H. Marbawi, J. A. Gansau, A. A. Mohd Faik, K. F. Rodrigues, "Yeasts in sustainable bioethanol production: a review," Biochemistry and Biophysics Reports, vol. 10, pp. 52-61, DOI: 10.1016/j.bbrep.2017.03.003, 2017.

[71] N. Scarlat, J. F. Dallemand, F. Fahl, "Biogas: developments and perspectives in Europe," Renewable Energy, vol. 129, pp. 457-472, DOI: 10.1016/j.renene.2018.03.006, 2018.

[72] P. G. Kougias, I. Angelidaki, "Biogas and its opportunities-a review," Frontiers of Environmental Science & Engineering, vol. 12 no. 3,DOI: 10.1007/s11783-018-1037-8, 2018.

[73] I. Angelidaki, L. Treu, P. Tsapekos, G. Luo, S. Campanaro, H. Wenzel, P. G. Kougias, "Biogas upgrading and utilization: current status and perspectives," Biotechnology Advances, vol. 36 no. 2, pp. 452-466, DOI: 10.1016/j.biotechadv.2018.01.011, 2018.

[74] S. Preethi, T. M. M. Usman, J. Rajesh Banu, M. Gunasekaran, G. Kumar, "Biohydrogen production from industrial wastewater: an overview," Bioresource Technology Reports, vol. 7,DOI: 10.1016/j.biteb.2019.100287, 2019.

[75] S. Mona, S. S. Kumar, V. Kumar, K. Parveen, N. Saini, B. Deepak, A. Pugazhendhi, "Green technology for sustainable biohydrogen production (waste to energy): a review," Science of The Total Environment, vol. 728,DOI: 10.1016/j.scitotenv.2020.138481, 2020.

[76] S. Shanmugam, A. Hari, A. Pandey, T. Mathimani, L. Felix, A. Pugazhendhi, "Comprehensive review on the application of inorganic and organic nanoparticles for enhancing biohydrogen production," Fuel, vol. 270,DOI: 10.1016/j.fuel.2020.117453, 2020.

[77] M. K. Lam, A. C. M. Loy, S. Yusup, K. T. Lee, "Biohydrogen production from algae," Biohydrogen, pp. 219-245, DOI: 10.1016/b978-0-444-64203-5.00009-5, 2019.

[78] X. Hu, Z. Zhang, M. Gholizadeh, S. Zhang, C. H. Lam, Z. Xiong, Y. Wang, "Coke formation during thermal treatment of bio-oil," Energy & Fuels, vol. 34 no. 7, pp. 7863-7914, DOI: 10.1021/acs.energyfuels.0c01323, 2020.

[79] X. Hu, M. Gholizadeh, "Progress of the applications of bio-oil," Renewable and Sustainable Energy Reviews, vol. 134,DOI: 10.1016/j.rser.2020.110124, 2020.

[80] R. Kumar, V. Strezov, "Thermochemical production of bio-oil: a review of downstream processing technologies for bio-oil upgrading, production of hydrogen and high value-added products," Renewable and Sustainable Energy Reviews, vol. 135,DOI: 10.1016/j.rser.2020.110152, 2021.

[81] M. D. C. Surboyo, I. Arundina, R. P. Rahayu, D. Mansur, T. Bramantoro, "Potential of distilled liquid smoke derived from coconut ( Cocos nucifera L.) shell for traumatic ulcer healing in diabetic rats," European Journal of Dentistry, vol. 13 no. 02, pp. 271-279, DOI: 10.1055/s-0039-1693527, 2019.

[82] A. Abdullah, N. Putra, I. I. Hakim, R. A. Koestoer, "A review of improvements to the liquid collection system used in the pyrolysis process for producing liquid smoke," International Journal of Technology, vol. 7 no. 7, pp. 1197-1206, DOI: 10.14716/ijtech.v8i7.745, 2017.

[83] X. Xin, K. Dell, I. A. Udugama, B. R. Young, S. Baroutian, "Economic performance of small-scale fast pyrolysis process of coproducing liquid smoke food flavoring and biofuels," ACS Sustainable Chemistry & Engineering, vol. 9 no. 4, pp. 1911-1919, DOI: 10.1021/acssuschemeng.0c08530, 2021.

[84] D. Rachmat, L. J. Mawarani, D. D. Risanti, "Utilization of cacao pod husk ( Theobroma cacao L.) as activated carbon and catalyst in biodiesel production process from waste cooking oil," IOP Conference Series: Materials Science and Engineering, vol. 299,DOI: 10.1088/1757-899x/299/1/012093, 2018.

[85] M. T. Billah, T. D. Agratiyan, D. Ayu, N. K. Erliyanti, E. A. Saputro, R. R. Yogaswara, "Synthesis of bioethanol from cocoa pod husk using zymomonas mobilis," International Journal of Eco-Innovation in Science and Engineering, vol. 1 no. 01, pp. 31-34, DOI: 10.33005/ijeise.v1i01.12, 2020.

[86] T. R. Cortes, J. A. Cuervo-Parra, V. J. Robles-Olvera, E. R. Cortes, P. A. L. Pérez, "Experimental and kinetic production of ethanol using mucilage juice residues from cocoa processing," International Journal of Chemical Reactor Engineering, vol. 16,DOI: 10.1515/ijcre-2017-0262, 2018.

[87] C. Roini, N. Asbirayani Limatahu, T. Mulya Hartati, A. Sundari, "Characterization of cocoa pulp ( Theobroma cacao L.) from south Halmahera as an alternative feedstock for bioethanol production," IOP Conference Series: Earth and Environmental Science, vol. 276 no. 1,DOI: 10.1088/1755-1315/276/1/012038, 2019.

[88] O. Awolu, S. O. Oyeyemi, "Optimization of bioethanol production from cocoa ( Theobroma cacao ) bean shell," International Journal of Current Microbiology and Applied Science, vol. 4, pp. 506-514, 2015.

[89] S. O. Dahunsi, A. T. Adesulu-Dahunsi, J. O. Izebere, "Cleaner energy through liquefaction of cocoa ( Theobroma cacao ) pod husk: pretreatment and process optimization," Journal of Cleaner Production, vol. 226, pp. 578-588, DOI: 10.1016/j.jclepro.2019.04.112, 2019.

[90] E. Antwi, N. Engler, M. Nelles, A. Schüch, "Anaerobic digestion and the effect of hydrothermal pretreatment on the biogas yield of cocoa pods residues," Waste Management, vol. 88, pp. 131-140, DOI: 10.1016/j.wasman.2019.03.034, 2019.

[91] J. C. Rojas, K. G. Ramírez, P. E. Velasquez, P. Acevedo, A. Santis, "Evaluation of bio-hydrogen production by dark fermentation from cocoa waste mucilage," Chemical Engineering Transactions, vol. 79, pp. 283-288, DOI: 10.3303/CET2079048, 2020.

[92] C. J. Rangel Villegas, J. Sastoque Guerrero, J. S. Calderón Salazar, J. D. Mosquera Tobar, I. O. Cabeza Rojas, P. A. Acevedo Pabón, "Pilot-scale assessment of biohydrogen and volatile fatty acids production via dark fermentation of residual biomass," Universidad Cooperativa de Colombia, vol. 3, pp. 47-51, 2020.

[93] G. Mancini, S. Papirio, P. N. L. Lens, G. Esposito, "Effect of N-methylmorpholine-N-oxide pretreatment on biogas production from rice straw, cocoa shell, and hazelnut skin," Environmental Engineering Science, vol. 33 no. 11, pp. 843-850, DOI: 10.1089/ees.2016.0138, 2016.

[94] A. O. Akinola, J. F. Eiche, P. O. Owolabi, A. P. Elegbeleye, "Pyrolytic analysis of cocoa pod for biofuel production," Nigerian Journal of Technology, vol. 37 no. 4, pp. 1026-1031, DOI: 10.4314/njt.v37i4.23, 2018.

[95] L. Handojo, A. Indarto, "Cocoa bean skin waste as potential raw material for liquid smoke production," Environmental Technology, vol. 41 no. 8, pp. 1044-1053, DOI: 10.1080/09593330.2018.1520306, 2020.

[96] O. Awogbemi, D. V. V. Kallon, V. S. Aigbodion, "Trends in the development and utilization of agricultural wastes as heterogeneous catalyst for biodiesel production," Journal of the Energy Institute, vol. 98, pp. 244-258, DOI: 10.1016/j.joei.2021.06.017, 2021.

[97] H. C. Nguyen, M. L. Nguyen, C. H. Su, H. C. Ong, H. Y. Juan, S. J. Wu, "Bio-derived catalysts: a current trend of catalysts used in biodiesel production," Catalysts, vol. 11 no. 7,DOI: 10.3390/catal11070812, 2021.

[98] Y. Ouattara, E. Kouadio, E. Kouassi, S. Doudjo, Y. Soro, K. Yao, B. Abouby, K. Drogui, A. P. Tyagi, P. M. Aina, "Cocoa pod husks as potential sources of renewable high-value-added products: a review of current valorizations and future prospects," Bioresources, vol. 16,DOI: 10.15376/biores.16.1.ouattara, 2020.

[99] K. Sandesh, P. Ujwal, "Trends and perspectives of liquid biofuel—process and industrial viability," Energy Conversion and Management: X, vol. 10,DOI: 10.1016/j.ecmx.2020.100075, 2021.

[100] K. Aboudi, L. A. Fernández-Güelfo, C. J. Álvarez-Gallego, L. I. Romero-García, "Biogas, biohydrogen, and polyhydroxyalkanoates production from organic waste in the circular economy context," Sustainable Biofuels, pp. 305-343, DOI: 10.1016/b978-0-12-820297-5.00002-5, 2021.

[101] D. Carrillo-Nieves, M. J. Rostro Alanís, R. de la Cruz Quiroz, H. A. Ruiz, H. M. N. Iqbal, R. Parra-Saldívar, "Current status and future trends of bioethanol production from agro-industrial wastes in Mexico," Renewable and Sustainable Energy Reviews, vol. 102, pp. 63-74, DOI: 10.1016/j.rser.2018.11.031, 2019.