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Review

Assessment of the Pretreatments and Bioconversion of Lignocellulosic Biomass Recovered from the Husk of the Cocoa Pod

by
Amílcar Díaz-González
1,
Magdalena Yeraldi Perez Luna
2,
Erik Ramírez Morales
1,
Sergio Saldaña-Trinidad
2,
Lizeth Rojas Blanco
1,
Sergio de la Cruz-Arreola
2,
Bianca Yadira Pérez-Sariñana
2,* and
José Billerman Robles-Ocampo
2,*
1
Unidad Chontalpa, División Académica de Ingeníeria y Arquitetura, Universidad Juarez Autónoma de Tabasco (UJAT), Av. Universidad, Manuel Sanchez Marmol, Cunduacán CP 86690, Tabasco, Mexico
2
Centro de Investigación y Desarrollo Tecnológico en Energías Renovables (CIDTER), Cuerpo Academico de Energía y Sustentabilidad, Universidad Politécnica de Chiapas (UPChiapas), Carretera Tuxtla Gutierrez.-Portillo Zaragoza Km 21+500, Col. Las Brisas, Suchiapa CP 29150, Chiapas, Mexico
*
Authors to whom correspondence should be addressed.
Energies 2022, 15(10), 3544; https://doi.org/10.3390/en15103544
Submission received: 13 April 2022 / Revised: 4 May 2022 / Accepted: 6 May 2022 / Published: 12 May 2022
(This article belongs to the Section A: Sustainable Energy)
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Versions Notes

Abstract

:
The production of biofuels (biogas, ethanol, methanol, biodiesel, and solid fuels, etc.), beginning with cocoa pod husk (CPH), is a way for obtaining a final product from the use of the principal waste product of the cocoa industry. However, there are limitations to the bioconversion of the material due to its structural components (cellulose, hemicellulose, and lignin). Currently, CPH pretreatment methods are considered a good approach towards the improvement of both the degradation process and the production of biogas or ethanol. The present document aims to set out the different methods for pretreating lignocellulosic material, which are: physical (grinding and extrusion, among others); chemical (acids and alkaline); thermochemical (pyrolysis); ionic liquid (salts); and biological (microorganism) to improve biofuel production. The use of CPH as a substrate in bioconversion processes is a viable and promising option, despite the limitations of each pretreatment method.

1. Introduction

The use of traditional fuels has a serious impact on both the environment and public health [1,2,3]. The use of fossil fuels has resulted in the rapid growth of the total primary energy supply by over 62%, which corresponds to 14,281.89 Mtoe, and a 65.35% or 33,513.3 Mton increase in total carbon dioxide emissions, prompting the search for alternative renewable energy sources with a lower environmental impact [4,5]. Traditional fuels such as diesel and gasoline cannot fully meet the requirements of homogeneous charge compression ignition (HCCI) combustion mode in practical wide-range engines [6]. One study reported the use of ethanol and methanol as good replacements for gasoline in an HCCI engine [7]. The use of butanol was also reported, resulting in optimal combustion phase and thermal efficiency levels comparable to conventional diesel consistently achieved [8]. Biofuels such as biobutanol, ethanol, and methanol have high concentrations of oxygen, which improves the efficiency of engines [9]. Therefore, alternative energy sources could be used in the internal combustion engine to partially replace conventional fuels and greatly decrease toxic exhaust emissions and greenhouse gases; thus, this assists to alleviate the world energy crisis and environmental pollution [6,9]. Biomass is the most abundant and versatile form of renewable energy in the world, with CPH identified as a promising alternative for the generation of bioenergy [10,11,12,13]. According to the International Cocoa Organization (ICCO), 4.78 million tons of cocoa beans were produced globally in 2018–2019 [14]. Constituting 70–75% of the wet weight of cocoa fruit, it is estimated that the harvesting of each ton of dry beans generates ten tons of CPH, which can be used to produce biofuels [15]. However, the three main components of CPH—cellulose, hemicellulose, and lignin—have limited the degradation of the material [16]. Cellulose and hemicellulose can be hydrolyzed into fermentable sugars [17], while the binding of cellulose, hemicellulose, and lignin results in a complex matrix [18,19]; herein, the lignin fiber exerts a recalcitrant effect, protecting carbohydrates from the degradation caused by microorganisms or enzymes [20]. Therefore, pretreatments are used in the hydrolysis of CPH in order to break down the recalcitrant structure [21]. Nowadays, a single pretreatment method for obtaining different biofuels is not available for all lignocellulosic residues [19,22]. Pretreatment methods, such as grinding [23], the use of ultrasound or hot water [24,25,26,27], and alkaline or acidic techniques [28,29], as well as the use of white rot fungi (a biological method), have shown favorable results in the use of lignocellulosic waste, a classification to which CPH pertains [19,30,31]. The present paper provides an overview of studies on the application of pretreatment methods on lignocellulosic waste such as CPH.

Main Components of the Shell

As shown in Figure 1, the main components of CPH are cellulose, hemicellulose, and lignin [32], which can be used as a carbon source to produce biofuels. Cellulose is neither soluble in water nor degradable by human beings due to its special structure, in which it comprises β-1,4-D-glucose glycosidic bonds [33].
Table 1 shows the components of the cell wall of different types of lignocellulosic waste. It is observed that most biomass has a cellulose content of between 30 and 35%, while the lignin content for each material analyzed ranges from 7 to 39%. Compared to the other types of biomass presented in Table 1, CPH contains the highest percentage of hemicellulose in its cell wall, while, in terms of its cellulose and lignin content, CPH falls within the average, with percentages of 35–35.8% and 14–15%, respectively. Figure 2 shows the structural components of the lignocellulosic biomass [34,35], with CPH containing 10% hemicellulose [36], while other authors put this at between 8.7 and 12.8%. With its presence in CPH causing microbial antagonism [32], lignin is an aromatic polymer formed by three primary units—guaiacyl (G), hydroxyphenyl (H), and syringyl (S)—which are bound by ether aryls or C–C bonds [37].
Like wood and other lignocellulosic residues, CPH has a complex structure, in which crystalline cellulose is protected by lignin and hemicellulose [33]. As the low accessibility of enzymes to cellulose remains limiting in using lignocellulosic residues for biofuel generation, pretreatment is necessary for the production of biofuels [30].

2. Pretreatments

Generally, lignocellulosic materials have a complex organic polymer crystal structure formed by the close physical and chemical association among cellulose, hemicellulose, and lignin [21]. These barriers, hemicellulose and lignin, limit the conversion of native cellulose into sugars by up to 20%, namely that which is neither rearranged nor destroyed by any pretreatment process [45]. In order to improve the efficiency of the use of CPH to obtain biofuels, such as ethanol and biogas, a delignification pretreatment is necessary to decompose the crystalline structure [46].

2.1. Physical Pretreatment

The physical pretreatment method is a technique used to change the appearance or structure of the lignocellulosic matter (husks, stubble, stems, and straw) by means of the mechanical principles of techniques such as grinding, extrusion, irradiation, ultrasonication, steam explosion, and immersion in hot water [24]. Grinding reduces the particle size and crystallinity of the fiber [34], where the surface area of the CPH increases along with the intensity with which the lignocellulosic waste (stubble, straw, or husk) is ground; thus, this generates more optimal conditions for the degradation of the material [47,48]. While this pretreatment can also shorten the initial fermentation time involved in the use of anaerobic digestion to obtain biogas [49], it consumes a large amount of energy [50,51,52]. Another pretreatment method used on lignocellulosic biomass, extrusion pretreatment, involves the use of a chamber fitted with temperature control devices and a rotating screw, which produces friction among the biomass, the screw, and the chamber walls. This pretreatment method, during which the CPH is cut, mixed, and heated, creates an active site that is easier to hydrolyze than the untreated material [45,53,54,55]. Heating via microwave irradiation and ultrasonic pretreatment can change the internal microstructure of the lignocellulosic biomass [35,56], enabling more than 80% of the hemicellulose and 90% of the lignin to be removed from the lignocellulosic waste without either excessive carbohydrate degradation or the solubilization of large amounts of cellulose [33]. However, the installation cost is too high to be suitable for industrial applications [56,57,58,59]. The physical pretreatment method has certain advantages, such as increasing the surface area of the substrate by decreasing the particle dimensions, improving the accessibility of the substrate, and increasing its susceptibility to microbial and enzyme attacks. The limitations include high-energy demand and scalability issues. The inside perspectives are the combination of pretreatments that could be applied to abstain the cost of such an intensive particle size reduction process, such as a combination with chemical pretreatment [19].

2.2. Chemical Pretreatment

The chemical pretreatment method is used to break the covalent bonds of lignocellulose in order to improve the degradation rate of the raw material, with many different alkaline and acidic pretreatment methods often adopted [24,60]. Table 2 shows studies that have applied chemical pretreatments on different types of biomass. The acids used in the studies presented in Table 2 are as follows: hydrochloric acid; formic acid; phosphoric acid; and sulfuric acid. The last example given is the most commonly used, at concentrations ranging from 0.5 to 5% (v/v), and has also been selected for use in combination with various acids. The most common pretreatment time is between 1 and 3 h, while the temperature is also a highly variable factor that ranges from room temperature to 200 °C.

2.3. Alkaline Pretreatment

Table 3 shows alkaline pretreatment methods applied to different types of lignocellulosic waste. There are reports of research on the production of bioethanol from the residual biomass of four species—Saccharum arundinaceum, Arundo donax, Typha angustifolia, and Ipomoea carnea—applying an ultrasound-assisted alkaline pretreatment with 0.5% sodium hydroxide (NaOH) and obtaining a maximum fermentation efficiency of 85.04% for Ipomoea carnea. A 5% NaOH seawater solution is reported as capable of delignifying empty palmoil bunches by up to 86.6%, thus improving the production of fermentable sugar [69,70].
Ionic liquid (1-ethyl-3 methylmidazolium acetate) pretreatments applied at an alkalinity of 7% along with NaOH are reported to show improvements in enzymatic hydrolysis [71]. Pretreatment with 1%, 3%, and 5% NaOH for 12 h at 20 °C increases the biochemical potential of the production of methane from garden waste by up to 70% [72]. An 87.7% delignification level was reported for the bamboo bark; and a total reducing sugar yield of 97.1% via the use of a combined alkaline salt pretreatment, consisting of 9% sodium potassium dodecahydrate (Na3PO4.12H2O) in the presence of hydrogen peroxide (0.3 g/g H2O2) and applied at a temperature of 80 °C for two hours. Moreover, evaluations were conducted on the following alkaline salts: sodium carbonate (Na2CO3), sodium acetate (CH3-COONa), sodium hypochlorite (NaClO), dehydrated trisodium citrate (Na3C6-H5O7.2H2O), and dehydrated sodium molybdate (Na2MoO2) [73].
Table
Table 3. Alkaline pretreatments used on different types of residues.
Table 3. Alkaline pretreatments used on different types of residues.
Raw MaterialPretreatmentAbstractRef.
Wheat strawNaOH 0.1 or 0.01 mol/L for 2 h at room temperature.Hemicellulose decreased from 23.0% to 16.1% after 2 h. Then, 21.8% of the lignin was removed, mostly on the surface of the lignin frame.[61]
ReedLiquid hot water (180 °C, 60 min) and 8% NaOH (w/w) at 160 °C for 1 h.Maximum ethanol concentration: 38.76 g/L, which can increase up to 50.6 g/L in the presence of pressurized oxygen.[74]
Rice strawIn total, 200 mL of 10% NaOH solution for 75 min.Biogas yield increase of around 25%, and faster co-digestion.[75]
Olive pomaceIn total, 8% NaOH (w/w) at 25 °C for one day.A 96% elimination of initial lipids and a 30% increase in methane production. Higher efficiency compared to microwave and ultrasound pretreatments.[76]
Grape pomaceIn total, 10% NaOH w/w at 20 °C for 24 h.A 36% increase in methane generation, 50% lignin elimination, and 22% cellulose. [60]
Pine foliageSurfactant-assisted NaOH.In total, 73.47 ± 1.03% of lignin was degraded, 0.477 g/g of reducing sugars were obtained, and there was a 6.01% improvement in fermentation efficiency.[67]
Corn straw and rice husk Immersion in 95% (v/v) of 1.4 M glycerol-NaOH and microwave radiation for 2 min at 180 °C.A 22.6% lignin decrease, losses of hemicellulose and cellulose for corn straw, and improvement in hydrolysis performance. Minimal effects on the rice husk.[77]
Oil palm residues (bunches and leaves)In total, 4.8 and 10% NaOH at 150 °C for 30 min.Reduction of the lignin content from 25.83 to 13.61% and from 30.92 to 19.23%, and of hemicellulose from 23.24 to 7.42% and from 13.95 to 8.10%, resulting in an increase in the percentage of cellulose.[78]
The advantages of pretreatment include: the hydrolysis of lignin and the alteration of cellulose structure. The limitations are the high costs of alkali and the formation of inhibitors. The exposure of biomass to active chemical reagents constitutes the chemical pretreatment method, which is more preferable than other physical or biological techniques; this is because of its high efficacy, and higher biogas yields that are based on the improvement of the depolymerization of complex biomass substrates and recalcitrant components [19].

2.4. Acid Pretreatment

Various acids have been used to improve the digestibility of lignocellulosic biomass. Research was conducted via a pretreatment method using both weak and strong acids, such as acetic acid, citric acid, and oxalic acid (C2H4O2, C6H8O7, and C2H2O4), on rice straw; the application of citric acid resulted in an increase in biogas production of up to 7.4 times higher than that obtained with the untreated biomass [79]. Sulfuric acid (H2SO4) has also been used as a substrate for a pretreatment for obtaining bioethanol from elephant grass [80]. Wheat straw pretreated with H2SO4 was also evaluated, with the dual purpose of ascertaining the raw material and bioethanol production yields [81], while rice and straw waste treated with H2SO4 and wood waste treated with phosphoric acid (H3PO4) were tested for the production of ethanol and xylose, respectively [82,83,84]. The effect of different H3PO4 concentrations (0%, 2%, 4%, 6%, and 8%) on a mixture of corn stubble and cow manure were compared, finding superior results for biogas production with a 6% H3PO4 concentration [85]. The acid pretreatment method has some advantages, including the hydrolysis of hemicelluloses and the alteration of structure. The limitations are the high cost for equipment and acids, and also, the formation of inhibitors. This pretreatment is reported as one of the most frequently applied conventional pretreatment practices of biomass substrate materials [19].

2.5. Thermochemical Pretreatment

Pyrolysis has been used as a pretreatment to break down cellulose, at high temperatures, into various products such as charcoal, gaseous substances, coke, and pyrolysis oil [86,87,88]. Levoglucosan, the main compound obtained from the rapid pyrolysis of biomass, can be directly fermented or hydrolyzed to glucose in order to increase its efficiency [89,90]. Oil rich in levoglucosan was obtained from pyrolyzed pine wood and used as a raw material for the production of biodiesel by means of oleaginous microorganisms [91], while Escherichia coli KO11 was used in a separate study to convert pyrolytic sugars into ethanol, achieving a fermentation level of 2% (w/v) [92]. The process for producing biofuels from pyrolysis remains challenging due to the presence of biocatalyst inhibitors and corrosive products in the pyrolyzed matter [87,90,93].

2.6. Ionic Liquid Pretreatment

As a salt that melts at room temperature, ionic liquid presents a strong polarity and is non-volatile, difficult to oxidize, easy to synthesize, and easy to recover. While it can effectively avoid the environmental pollution caused by the use of traditional organic solvents and is considered a green solvent as it replaces volatile organic solvents in many fields [94,95], one of the disadvantages of this method is that ionic liquids can become more viscous during the pretreatment process [96,97,98]. The ionic liquids most commonly used for the pretreatment of lignocellulosic material are as follows: 1-butyl-3-methylimidazolium chloride; 1-butyl-3-methylimidazolium hexafluorophosphate; 1-butyl-3-methylimidazolium acetate; 1-benzyl-3-methylimidazolium chloride; 1-butyl-1-methylpyrrolidinium chloride; 1-butyl-3-methylimidazolium methyl sulfate; N,N′-dimethyl ethanol-ammonium; 1-ethyl-3-methylimidazolium; and 1,3-dimethylimidazolium [99,100,101].

2.7. Biological Pretreatment

Enzymes found in bacteria and fungi have been studied to determine their potential for degrading lignocellulose into fermentable simple sugars [37]. Table 4 presents studies that used biological pretreatments on different types of lignocellulosic waste. The key to this technique is both to identify a species of microorganism with a strong lignin degradation capacity and accurately determine the fermentation conditions [102]. Pretreatments were performed on the lignocellulosic materials using the Pleurotus ostreatus strain for 28 days, with the results compared to those obtained via an acid pretreatment [103]. Another study demonstrated the utility of Pleurotus ostreatus, Phanerochaete chrysosposrium, and Ganoderma lucidum for the degradation of rice straw in order to compare methane yields, with Phanerochaete chrysosposrium the most efficient strain evaluated [104]. Research conducted on the pretreatment of wheat straw with Polyporus brumalis isolated and identified three fungal strains of the rice plant, Aspergillus niger, Aspergillus sojae, and Aspergillus terreus, which were used to treat rice straw and buffalo manure; the Aspergillus terreus pretreatment demonstrated the best levels of digestibility over a 15-day period. The positive effect on pH stability and increased methane yield presented by the strain, Trametes versicolor, was reported by a study which used corn silage as a substrate [105,106,107]. One study compared the effects of Trichoderma reesei and Pleurotus ostreatus and used different moisture contents and incubation times for the degradation of lignin, cellulose, and hemicellulose. The best-performing combination obtained was the P. ostreatus pretreatment conducted at 75% humidity for 20 days, which obtained 33.4% lignin removal and 120% more methane production than that obtained using untreated rice straw [108]. Other studies have evaluated the pretreatment of palm oil waste for saccharification and fermentation processes, using phosphoric acid and Pleurotus floridanus [109]. The successful use of the strain Phlebia spp. was also reported in an anaerobic culture used with the bacterium Clostridium saccharoperbutylacetonicum for the production of butanol from cellulose obtained from wood [110]. Other research groups used Phlebia spp. for the production of bioethanol [111,112,113]. The sugar transporter gene (Pdhxt1) of the white rot fungi (WRF) Phanerochaete sordida YK-624 was identified by a study which reported that it improves aerobic fermentation; further, it is capable of producing ethanol via saccharification and simultaneous fermentation in the presence of a low cellulase concentration [114]. A study conducted on the sequential pretreatment of wheat straw with the WRF Ganoderma lobatum and the black rot fungi Gloeophyllum trabeum obtained a glucose yield 2.8 times higher than that obtained via untreated straw and 150% higher than that obtained via an individual treatment [115]. With the same objective, a combined alkaline pretreatment was applied with four strains, P. chrysosporium, I. lacteus, P. eryngii, and P. ostreatus, finding that the Pleurotus eryngii strain was the most effective, obtaining a sugar yield of 329 mg/g. Meanwhile, the combined treatment obtained, during enzymatic hydrolysis, a sugar yield 1.1–1.2 times higher than a simple treatment (fungal or alkaline) [116]. Research conducted in situ found that in a membrane-aerated biofilm reactor, pretreatment with Irpex lacteus improved saccharification and simultaneous fermentation yields from beech wood pretreated with steam; this obtained improvements in the final ethanol yield (65 to 80%) [117]. The presence of different strains, such as Grifola frondosa, Hericium coralloides, Meripilus giganteus, and Trametes gibbose, has also been studied in terms of the effect of the strains on the calorific power of wood [118].
The biomass most commonly studied for fungal pretreatment is rice waste, followed by wheat straw and corn stubble. The vast majority of these studies aim to improve bioethanol production processes, focusing mainly on saccharification, as is the case with the S. griseorubens pretreatment, which has presented an efficiency of 97.8% [1]; however, an efficiency of 76.5% and a sugar concentration of 52.91 g/L were obtained using T. hirsuta for 24 h [2]. Similarly, studies have focused on the selective degradation of lignin, obtaining up to 34.7% effectiveness for the fungus Fusarium moniliforme, which is the best-performing strain among those tested [3]. Biological pretreatments have also been shown to improve the rapid pyrolysis of lignocellulosic compounds, by increasing the yield of aromatics from 10.03 to 11.49% and reducing the amount of coke obtained from 14.29 to 11.93% by weight [4].
The advantages of biological pretreatments include: the hydrolysis of lignin and hemicelluloses, the alteration of cellulose structure, no inhibitory compound formation, and low energy consumption. The limitations are: the process is slow, there is carbon loss, and the necessity of a sterile area. In relation to development and different perspectives, white rot fungi are able to form enzymes that acquire high hydrolytic action towards lignocellulosic substrates degradation, such as lignin peroxidase, lacasse, and manganese peroxidase. White rot fungi that degrade lignin have been used mainly for biological treatment [5].

3. Bioconversion Technology of the Cocoa Pod Husk

Cocoa pod husks are typically discarded to decompose in the plantations where they have been harvested, producing bad odors and spreading disease in plants [39]. Table 5 and Table 6 show the various studies that evaluated the reuse of cocoa waste.
The chemical components of CPH have been evaluated for various biotechnological applications, including the obtaining of pectins for use as additives in cosmetic or pharmaceutical products. Obtaining fertilizers by means of the waste’s structural characteristics, further to using waste in the area of healthcare, such as the use of waste-derived antioxidants, are some of the research areas that have drawn great interest in the exploitation of waste derived from the cocoa-harvesting process. The combustion potential of CPH was evaluated at 17 MJ/kg [23], while the highest calorific values of different types of biomass and cocoa shells were also evaluated [145]. Research has used CPH in the esterification process and as a source of potash for the production of biodiesel [146,147,148], while a study on the transesterification of neem seed oil used a catalyst prepared from cocoa shell ash; another used CPH as a substrate for anaerobic digestion, pretreating the waste with sulfuric acid [15,149]. Research was carried out on both the hydrolysis of CPH using hydrochloric acid followed by fermentation with Pichia stipites, and the characterization of CPH via proximal analysis and elemental analysis for the application of the pyrolysis process [150,151].
Table
Table 6. Bioenergetic strategies for the use of CPH.
Table 6. Bioenergetic strategies for the use of CPH.
Raw MaterialProcessObtainingResultRef.
Cocoa pod huskAnaerobic digestion.BiogasEvaluated biogas performance through hydrothermal pretreatment.[152]
Thermochemical and direct combustion.Solid fuelQuantified the amount of cocoa pod husk generated and evaluated the potential for power generation in Uganda.[153]
Anaerobic digestion.BiogasEvaluated the bioenergy potential of cocoa residue.[16]
Direct combustion; gasification; pyrolysis; anaerobic digestion; and hydrothermal carbonization.Solid fuel and gasInvestigated the feasibility of converting CMC into energy through five technological processes.[41]
Pelletization and carbonization.Solid fuelStudied the use of CMC as an energy source.[23]
Semisolid.Xanthan gumUsed the microorganism Xanthomonas campestris.[154]
Solid state fermentation.Cattle fodderDegradation by means of the fungus, Pleurotus ostreatus.[155]
Solid state fermentation.EnzymeObtaining Fructosyltransferase.[156]

4. Conclusions

The pretreatment technology used on CPH is an important issue in the application of biofuel (e.g., biogas and ethanol) generation processes in small- and large-scale cocoa-producing countries. Research shows that the degradation rate for CPH improves with the use of pretreatment methods (physical, chemical, and biological). Because CPH is hard to degrade using bacteria, it must be pretreated to enable its use as a substrate for the production of biogas fermentation via anaerobic decomposition, with pretreatment improving the digestion rate and obtaining a biogas yield of up to 71%.
While the physical pretreatment method is important for the biofuel (e.g., biogas and ethanol) generation process, the feasibility of its use is closely linked to costs and yields. Using the physical method as the basis for CPH pretreatment, combined with other pretreatment methods, is the most efficient approach.
Chemical pretreatment using sulfuric acid has positive effects, since it degrades lignocellulosic waste, such as rice straw, cane bagasse, and corn stubble, over 70% faster than other methods; thus, it could facilitate the degradation of CPH. Alkaline and acid pretreatments can cause secondary pollution, including: environmental damage or toxic effects for both human beings and the environment; and the chemical pretreatment acids and alkaline not being eco-friendly.
The biological pretreatment method was shown to perform better than the physical and chemical pretreatment methods, although it does require a biological bacterial agent that efficiently degrades CPH, where the use of Pleurotus ostreatus to degrade rice straw for methane production was 120% more efficient than the other methods evaluated. Among the various biological agents, the effectiveness of the following should be emphasized: WRFs, including Pleurotus ostreatus; Trametes versicolor; Phanerochaete chrysosporium; Irpex lactues; and, brown or black rot fungi, such as Coniophora puteana, Postia placenta, Gloephylum trabeum, and Laetoporeus sulphureus. The interest of being able to use the residual biomass of the cocoa pod husk lies in: the structural characteristics; and the contents of lignin, hemicellulose, and lignin being similar to other lignocellulosic residues (corn, rice, tomato, and citrus residues, among others). Within the different investigations, data were generated on the production of biofuels, such as biogas, ethanol or biodiesel; if it be taken into account that the residue obtained from the cocoa industry is the main one, an opportunity is perceived in the development of biofuels. The pretreatment methods discussed in the present paper have considerable potential for further development and application, while additional research is required on the suitability of CPH as a material to be subjected to biodegradation for energy production purposes, such as the generation of biogas.

Author Contributions

Conceptualization, L.R.B.; validation, S.S.-T.; review, J.B.R.-O. and S.d.l.C.-A.; resources, M.Y.P.L.; writing—original draft preparation, A.D.-G.; review, editing and supervision, B.Y.P.-S.; project administration, E.R.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The financial support of the National Council of Science and Technology is greatly appreciated (CONACYT).

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 15 03544 g001 550
Figure 1. Schematic role for the pretreatments of interest. “Reprinted with permission from [33]. 2018, Elsevier”.
Figure 1. Schematic role for the pretreatments of interest. “Reprinted with permission from [33]. 2018, Elsevier”.
Energies 15 03544 g001
Energies 15 03544 g002 550
Figure 2. Schematic structure of: (a) hemicellulose, (b) cellulose, and (c) lignin-forming units. “Reprinted with permission from [35]. 2016, Elsevier”.
Figure 2. Schematic structure of: (a) hemicellulose, (b) cellulose, and (c) lignin-forming units. “Reprinted with permission from [35]. 2016, Elsevier”.
Energies 15 03544 g002
Table
Table 1. Cell wall composition of different types of lignocellulosic waste.
Table 1. Cell wall composition of different types of lignocellulosic waste.
Biomass
(Dry Basis)		Cellulose
(% Mass)		Hemicellulose
(% Mass)		Lignin
(% Mass)		Ref.
Sunflower stem36.3210.0818.38[38]
Sunflower flower head23.196.9612.05
Cocoa pod husk35–35.836.5–37.514–15 [39,40,41]
Corn stover35177[42]
Sorghum husk32277[42]
Rice husk282312[43]
Citrus pulp12.8222.5[44]
Tomato pulp121239
Table
Table 2. Acid pretreatments applied on different types of biomass.
Table 2. Acid pretreatments applied on different types of biomass.
Raw MaterialPretreatmentAbstract Ref.
Wheat strawHydrochloric acid (HCl) at 0.1 or 0.01 mol/L for 2 h at room temperature.There was no significant effect with 0.01 mol/L of HCl. With 0.1 g/L, the hemicellulose content decreased from 23.0% to 17.4% at 0.5 h, and to 13.4 after 2 h; and the cellulose and lignin decreased from 38.7% to 36.2%, and from 11.9% to 11.4%, respectively.[61]
Poplar and fir Sulfuric acid (HS2O4) with a concentration of 0.2 a 2.5% (v/v), temperature 180−200 °C, 2−12 min. The conditions of the maximum release of glucose, mannose, and xylose are similar for poplar: a glucose conversion of 87%.[62]
Fir woodFormic acid (CH2O2); acetic acid (C2H4O2); y lactic acid (C3H6O3) at 130° for 3 h.Delignification 73.0–76.5%, increased cellulose content from 61.1 to 79.8–85.4%. High solids yield (75.5, 72.2 and 69.3%).[63]
Rice strawSulfuric acid solution (H2SO4) (0.5%, 1.0%, 1.5%, y 2.0%, v/v) of 5% (w/v) at 121 °C for 20−40 min, followed by a biological treatment.Reduction of the size of the rice straw, the formation of lignin droplets, and the removal of hemicellulose causing a percentage increase in the proportion of crystalline cellulose. Increase in digestibility of up to 70%.[64]
Empty bunches of palm fruits, rice husk and pine woodSulfuric acid (H2SO4) at 5% (v/v) at a solid/liquid ratio of 10% (w/v) and 121 °C for 30, 60, and 90 min.Maximum sugar yield at 60 min. The maximum H2 production rates of 2640, 3340, and 2565 mL of H2/L day.[65]
Cane bagasse and corn stoverSulfuric acid (H2SO4) at 0.5% (w/w) for 5 min at 190 °C, followed by N-methyl morpholine N-oxide (NMMO).Elimination of hemicellulose, a decrease in hydrolysis time (−48 h), and the conversion rates during the hydrolysis of 91.5 and 98.3%.[66]
Pine foliageSulfuric acid (H2SO4) assisted with con surfactant. Elimination of 59.53 ± 0.76% of lignin, 0.588 g/g of reducing sugars were obtained, and there was a 16.1% increase in fermentation efficiency.[67]
Goat willow (Salix caprea L.)Fosforic acid (H3PO4) at 85% at 30 °C for 2 h.Greater effectiveness in the bark than in the wood. Up to 30.6% cellulose and 59.7% xylan were removed, and the conversion of cellulose to glucose was tripled.[68]
Table
Table 4. Biological pretreatments used on different residues.
Table 4. Biological pretreatments used on different residues.
Raw MaterialStrainStudy ObjectiveRef.
Combined lignocellulosic matterPleurotus tuoliensis. Biogas production.[119]
Wheat strawPhanerochaete chrysosporium, Pleurotus ostreatus, Irpex lacteusDifference in chemical composition and in vitro gas production.[120]
Hardwoods from India (Pithecellobium dulce and Tamarindus indica)Pseudolagarobasidium acaciicola AGST3, and Tricholoma giganteum AGDR1.Degradation and study of pyrolysis kinetics.[121]
Wheat straw and oak shavingsCeriporiopsis subvermispora and Lentinula edodes.Chemical characterization and enzymatic hydrolysis.[122]
Rice paddy strawTrametes hirsute. Improvement of saccharification and sugar production.[123]
Corn stubble, barley straw, corn cob, and wheat strawIrpex lacteus. Bioethanol production.[124]
Rice strawTrametes hirsuta and Myrothecium roridumImprove enzymatic saccharification and hydrolysis.[125]
Wheat, rice, sugarcane, and pea strawTrichoderma longibrachiatum, Phanerochaete chrysosporium, Neosartorya fischeri, Myceliophthora thermophila, and others.Comparison of effectiveness between the various strains of fungi and bacteria for the improvement of saccharification.[126]
Rice straw and saucePholiota adiposa and Armillaria gemina.Simultaneous pretreatment and saccharification.[127]
Rice huskPhanerochete chrysosporium. Simultaneous pretreatment and saccharification.[128]
Parthenium spp.Trametes hirsuta ITCC136, Pycnosporus sanguineus ITCC 230, Trametes versicolor NCIM 1086, Pleurotus ostreatus ITCC 3047, and Sporotrichum sp. NCIM 1203.Ligninolytic enzymatic activity, structural changes, and solid state fermentation.[129]
Rice paddy strawStreptomyces griseorubens ssr38.Delignification; enhance enzymatic hydrolysis.[130]
Corn stoverIrpex lacteus CD2.Enhance fast pyrolysis.[131]
Rice paddy strawTrametes hirsute. Delignification; enhance enzymatic hydrolysis.[132]
Rice strawPhanerochaete chrysosporium H, Fusarium sp. 82, Fusarium sp. 89, and Fusarium moniliforme 812.Delignification; enhance solid-state fermentation.[133]
Wheat straw and banana stemPleurotus ostreatus HP−1.Efficient bioethanol production.[134]
Moso bamboo (Phyllostachys pubesescens)I. lacteus CD2 and E. taxodii 2538.Delignification and improvement of thermal decomposition.[135]
Corn stemIrpex lacteus. Enhance enzymatic hydrolysis.[136]
Wheat strawPhanerochaete chrysosporium. Detailed structural changes.[137]
Corn stoverEchinodontium taxodii 2538.Enhance the decomposition pyrolysis.[138]
Corn stubble, switchgrass, wheat straw, soybean straw, and hardwoodCeriporiopsis subvermispora. Delignification; enhance enzymatic hydrolysis.[139]
Table
Table 5. Obtaining CPH components by means of chemical extraction.
Table 5. Obtaining CPH components by means of chemical extraction.
WasteExtract/FractionApplicationResultRef.
Cocoa pod huskEthanol extractPotential antioxidant.Antioxidant activity.[140]
Dietary fiberAntioxidant activity.Polysaccharide without starch and total phenolic content.[141]
Organic extractFertilizer.Dry matter of aerial parts.[142]
PectinGel formation.Suggest the use of pectin from the cocoa pod shell as a gelling agent or thickening additive.[143]
NaOH extractAntiviral; antibacterial.Anti-HIV, anti-influenza activity, and vitamin C enhancement.[144]
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Díaz-González, A.; Perez Luna, M.Y.; Ramírez Morales, E.; Saldaña-Trinidad, S.; Rojas Blanco, L.; de la Cruz-Arreola, S.; Pérez-Sariñana, B.Y.; Robles-Ocampo, J.B. Assessment of the Pretreatments and Bioconversion of Lignocellulosic Biomass Recovered from the Husk of the Cocoa Pod. Energies 2022, 15, 3544. https://doi.org/10.3390/en15103544

AMA Style

Díaz-González A, Perez Luna MY, Ramírez Morales E, Saldaña-Trinidad S, Rojas Blanco L, de la Cruz-Arreola S, Pérez-Sariñana BY, Robles-Ocampo JB. Assessment of the Pretreatments and Bioconversion of Lignocellulosic Biomass Recovered from the Husk of the Cocoa Pod. Energies. 2022; 15(10):3544. https://doi.org/10.3390/en15103544

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Díaz-González, Amílcar, Magdalena Yeraldi Perez Luna, Erik Ramírez Morales, Sergio Saldaña-Trinidad, Lizeth Rojas Blanco, Sergio de la Cruz-Arreola, Bianca Yadira Pérez-Sariñana, and José Billerman Robles-Ocampo. 2022. "Assessment of the Pretreatments and Bioconversion of Lignocellulosic Biomass Recovered from the Husk of the Cocoa Pod" Energies 15, no. 10: 3544. https://doi.org/10.3390/en15103544

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