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Ouattara, L. Y., Kouassi, E. K. A., Soro, D., Soro, Y., Yao, K. B., Adouby, K., Drogui, A. P., Tyagi, D. R., and Aina, P. M. (2021). "Cocoa pod husks as potential sources of renewable high-value-added products: A review of current valorizations and future prospects," BioResources, 16(1), 1988-2020.

Abstract

Cocoa is among the most cultivated and important tropical crops in the world, and it is economically viable in the agro-pastoral systems of tropical Africa. Further, the amount of cocoa residue is steadily increasing due to the strong worldwide demand for chocolate products. This review of cocoa residue found that an average of 18 publications per year were published in the last 10 years. The most common type of publication on cocoa pod husks (CPH) was newspaper articles, which comprised 50% of the publications. This review examines the use of CHP in sustainable development, agrochemical materials, and agro-materials through their potential valorizations into high value-added products. Indeed, CPH is an abundant, accessible, and renewable resource of bioproducts, dietary fibers, nutraceuticals, functional foods, pectin, antioxidant compounds, theobromine, and minerals. Potential food applications of CPH include the production of flavor compounds, gums, texturing agents, and others. The production of biomaterials for food and non-food use, biofuels, and organic acids, such as lactic acid (the polymerization of which produces the PLA used in bioplastic production), are several potential areas for the biotechnological development of CPH and its fractions.


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Cocoa Pod Husks as Potential Sources of Renewable High-Value-Added Products: A Review of Current Valorizations and Future Prospects

Leygnima Yaya Ouattara,a,* Esaie Kouadio Appiah Kouassi,a Doudjo Soro,a Yaya Soro,a Kouassi Benjamin Yao,a Kopoin Adouby,a Allali Patrick Drogui,b Dayal Rajeshwar Tyagi,b and Pepin Martin Aina c

Cocoa is among the most cultivated and important tropical crops in the world, and it is economically viable in the agro-pastoral systems of tropical Africa. Further, the amount of cocoa residue is steadily increasing due to the strong worldwide demand for chocolate products. This review of cocoa residue found that an average of 18 publications per year were published in the last 10 years. The most common type of publication on cocoa pod husks (CPH) was newspaper articles, which comprised 50% of the publications. This review examines the use of CHP in sustainable development, agrochemical materials, and agro-materials through their potential valorizations into high value-added products. Indeed, CPH is an abundant, accessible, and renewable resource of bioproducts, dietary fibers, nutraceuticals, functional foods, pectin, antioxidant compounds, theobromine, and minerals. Potential food applications of CPH include the production of flavor compounds, gums, texturing agents, and others. The production of biomaterials for food and non-food use, biofuels, and organic acids, such as lactic acid (the polymerization of which produces the PLA used in bioplastic production), are several potential areas for the biotechnological development of CPH and its fractions.

Keywords: Cocoa pod husks; Pretreatment; Conversion; High value-added products; Biocomposite

Contact information: a: Laboratoire des Procédés Industriels de Synthèse, de l’Environnement et des Energies Nouvelles (LAPISEN), Unité Mixte de Recherche et d’Innovation en Sciences des Procédés Chimiques, Alimentaires Environnementaux et énergétiques, Institut National Polytechnique Félix Houphouët-Boigny, Yamoussoukro, Côte d’Ivoire; b: Institut National de la Recherche Scientifique (INRS Eau Terre et Environnement), Université du Québec, 490 rue de la Couronne, Québec City, Canada; c: Laboratoire des Sciences et Techniques de l’Eau (LSTE)/ Institut National de l’Eau (INE), Université d’Abomey-Calavi (UAC), 01 BP 2009 Cotonou, Benin;

* Corresponding author: leygnima.ouattara18@inphb.ci

INTRODUCTION

Environmental pollution is among the most serious problems facing humanity today (Cho et al. 2020). A fundamental problem linked to pollution is the elimination of the large quantities of organic waste that are continuously produced (Cho et al. 2020). Solving the problem of large quantities of organic waste or secondary raw materials is a necessity (Balentić et al. 2018). Consequently, specific applications are needed to use these raw materials in the most efficient way possible in the production process. The concepts of “wealth-generating waste” and “recyclable materials” are important for building a sustainable and healthy society through the efficient use of these waste resources (Daud et al. 2013). As global production of Theobroma cocoa beans (Malvaceae) has decreased, the governments of several tropical countries (Vriesmann et al. 2011a) have strongly expressed their support for the expansion of the cocoa industry by encouraging cocoa farmers to plant additional trees. This initiative has led to an increase of more than 50% in the production of cocoa beans (Uy et al. 2019). However, the increase in production has also favored the proliferation of undesirable residues, such as cocoa pod husks, on cocoa farms and plantations. In addition, as petroleum resources continue to decrease and the environmental concerns caused by petroleum products grow, research (Vásquez et al. 2019) has focused on the capacity of cocoa pod husks to produce biofuels and other biopolymers. However, cocoa pod husk (CPH) remains under-exploited, as it is a renewable resource rich in dietary fiber, lignin, and bioactive antioxidants, such as polyphenols (Lu et al. 2018).

The recovery of lignocellulosic fractions and bioactive compounds from cocoa pod husks can lead to the development of profitable basic products. Consequently, these cocoa pod husks can generate income for farmers and promote economic development (Lu et al. 2018).

This review article first focuses on analyzing bibliometric data to determine the trends, research priorities, current topics, and important research areas of cocoa pod husks. Then, the stages in the development of CPH are presented to summarize the mineral and biochemical composition of CPH. The main pretreatment strategies and the different ways of converting cocoa pod husks are also reviewed. Finally, the current state of CPH valorization and future perspectives on CPH valorization are discussed.

Dynamics of the Production of Beans and Cocoa Pod Husks

Approximately 50 countries produce cocoa, and the Ivory Coast is the largest producer. Figure 1 (International Cocoa Organization (ICCO)) shows the evolution of cocoa production in west Africa. The African continent alone supplies approximately 74% of cocoa produced globally. Among various countries of Africa, Ivory Coast is the biggest producer of cocoa (42%), followed by Ghana (20%), Nigeria (6%), and Cameroon (5%). Figure 1 provides the statistics of cocoa production in cocoa-producing countries. However, Ivory Coast suffers many consequences due to its immense production.

Fig. 1. Quantity of cocoa beans per y in the main producing countries of West Africa (International Cocoa Organization (ICCO) 2019)

In fact, 10 tonnes of wet pods are generated for each tonne of dry cocoa beans (Campos-Vega et al. 2018). Therefore, a large area is necessary for disposal, and the wet pods represent a major challenge for waste management. Currently, in Ghana and Ivory Coast, approximately 1% of this biomass is used to make soap (Antwi et al. 2019).

Figure 2 shows the amounts of beans and cocoa pods husks produced in the main cocoa-bean producing countries during the 2018/2019 campaign. As Ivory Coast produced approximately 6,500,000 tonnes of CPH, it now must manage this waste, which is generally stored in fields, and its decomposition attracts flies and other insects harmful to cocoa. Furthermore, the decomposition of these residues is a potential source of pathogenic microorganisms, such as black pod disease (Mansur et al. 2014), which is caused by Phytophthora palmivora and P. megakarya fungi. The disease causes an estimated annual yield loss of 20% to 30% worldwide, whereas individual farms may experience an annual yield loss of 30% to 90% (Lu et al. 2018).

Fig. 2. Production of cocoa beans and residual pod by country during 2018 and 2019 (Statista 2019)

Cocoa pod husk is a lignocellulosic biomass that is rich in minerals (in particular, potassium), fibers (especially lignin, cellulose, hemicellulose, and pectin), and antioxidants (phenolic acids, etc.) (Kouakou et al. 2018). However, it is still largely under-exploited. Appropriate use of this lignocellulosic material could offer economic benefits and reduce its environmental impact (Adjin-Tetteh et al. 2018).

Description of CPH

The CPH is the outer part of the fruit (exocarp), which has an oval, rough, and relatively thick appearance (Fig. 3 and Fig. 4).

Cocoa pod husk comes in different colors depending on the variety, and its roughness protects it against the elements, plagues, and damage that could be caused by impact (Vásquez et al. 2019). It is obtained after husking and removing the beans, and it represents 70% to 80% of the dry weight of the whole fruit. This natural, layered material comprises three distinct layers (Fig. 4), which are the epicarp, the mesocarp, and the endocarp, which constitute the outer, middle, and inner pericarp, respectively (Campos-Vega et al. 2018). The endocarp is the innermost part and occurs as a soft, whitish tissue that protects the cocoa beans in a well-lubricated inner chamber (Campos-Vega et al. 2018). The mesocarp has a hard composite structure capable of holding the cocoa beans in place even in severe conditions.

Fig. 3. Cocoa pod husks

Fig. 4. Different parts of CPH (Reprinted with permission; Campos-Vega et al. 2018)

The relatively soft outermost layer is the yellow (or purplish red upon maturity) cover, and it is this part that is exposed to the sun. When it turns black, this indicates that the fruit is rotting and dry (Babatope 2005).

Bibliometric Study on the Cocoa Pod Husk

Bibliometric analysis is a useful tool for analyzing publications in several areas of research (Maassen 2016). It is a quantitative approach of studying the metadata of scientific publications (Maassen 2016). It is a useful method for detecting research trends in a given area (Maassen 2016). It concerns three elements of scientific activity, which are its inputs, its outputs, and its impacts. It allows an assessment of scientific activity and the place of actors in relation to a given theme. This new scientific approach has been increasingly used and considered before undertaking studies on a given theme.

In this study, several word combinations were searched to identify themes from 2005 to 2019, which included “cocoa pods,” “cocoa pod husks,” and “cocoa residues.” With this method, it was possible to overcome the weaknesses of individual keyword analysis and identify relevant points and current search trends in different countries (Maassen 2016) for a given theme. This bibliometry on CHP was produced on 05/07/2019 with the internet platform SCIFINDER that lists all forms of publication dealing with the topic to be addressed. This database allowed assessment of the scientific interest in the subject of CPH. A total of 268 publications met the selection criteria. The selected publications were then analyzed according to the characteristics of the articles (type of document, country of origin, and citations), publication models (main journals and journal category), the importance of research (number of publications per year), the number of publications per author, and the areas of valuation (Maassen 2016).

Number of Publications per Year

Interest in CPH began in 1905 with a single publication. However, an increased interest in CPH began in 2003 and has continued to grow (Fig. 5). Over the past 10 y, an average of 18 publications per year have been produced on CPH. Over the past 5 y, an average of 24.2 publications per year were produced. This growing interest in cocoa pod husks is probably due to the increasing amount of this residue in the fields, which is becoming an environmental problem. Further, the enormous losses caused to cocoa farmers due to cocoa diseases, such as brown and black pod rot, have likely driven this interest.

Fig. 5. Number of publications produced annually related to CPH

In addition, the demand for chocolate-based products has led cocoa-producing countries to increase their production, which has resulted in increased waste products, such as CPH, which constitutes approximately 76% of the weight of whole pods (Chun and Husseinsyah 2016). Several studies have shown that 10 tonnes of CPH are produced as waste for every tonne of cocoa beans (Mansur et al. 2014; Sanyang et al. 2017). Therefore, the theme related to this material was topical and relevant to the interest of researchers.

Work Density by Publishing Type

The most common type of publication on CPH was journal articles, which comprised 54% of the publications (Fig. 6). Therefore, CPH has great potential and requires further exploration based on the number of patents filed (12), which represented 15% of publications. A single bibliographic review article was produced on CPH, which looked at the biotechnological recovery of all residual biomass from the cocoa industry (Vásquez et al. 2019) and justified the present study. Figure 6 illustrates the distribution of publication types.

Fig. 6. Density of works by type of publication

Areas of Valorization for Cocoa Pod Husks

The valorization of CPH has aroused interest in several fields, including radial electrochemical agrochemical bioregulators, thermal energy technology, soil fertilization, plant nutrition, food and animal chemistry, plastic treatment, and waste treatment and disposal (Fig. 7).

The large numbers of publications in the fields of soil fertilization, plant nutrition, food and feed chemistry, plastics manufacturing and processing, and waste treatment and disposal were justified by the rich mineral and fibre composition of this biomass. Several of these areas (soil fertilization, plant nutrition, and food and feed chemistry) have already been widely exploited and others (plastic manufacturing and processing) are still under study and deserve special attention.

The recovery of CPH begins with the harvest of the material, followed by drying. Indeed, the moisture content of the fresh pod is approximately 90% (Vriesmann et al. 2011a).
Therefore, quick drying is essential to avoid deterioration. Finally, the CPH residues are ground. A complete characterization (physico-chemical, biochemical, and thermal) and an appropriate delignification process are necessary to better explore the lignocellulosic components of CPH to identify the appropriate pathway for the residue.

Fig. 7. Publications of the Index of Scientific quotes based on the areas of CPH valuation

Cocoa Pod Husks Valorization Process

Chemical, Physico-chemical, and Biochemical Characterization Process of Cocoa Pod Husks

Figure 8 schematically illustrates the valorization process of CPH in value-added products. Several works have focused on the physico-chemical (Vásquez et al. 2019), thermochemical (Adjin-Tetteh et al. 2018), and biochemical (Giwa et al. 2020), characterization of CPH to identify the appropriate conversion pathways. Cocoa pod husks were found to be rich in raw energy, cellulose, hemicellulose, potassium, and many other minerals useful for soil improvement.

Fig. 8. Schematic representation of the characterization procedures of cocoa pod husks

Table 1 summarizes the proximal composition of CPH obtained in several regions of the world after its characterization.

Table 1. Composition of Cocoa Pod Husks in Previous Studies

Knowing of the moisture content of a biomass is crucial for the choice of the energy conversion method available for processing this biomass. According to Tsai et al. (2017), moisture content has a direct impact on the calorific value of a biofuel. The values ​​obtained during various studies were all over 10%. Thus, the residue would be better suited for biochemical conversion than thermal conversion (Adjin-Tetteh et al. 2018). Further, moisture in the raw material can act as a binder (Adjin-Tetteh et al. 2018) and lubricant by improving the gelatinization of starch and facilitating the formation of van der Waals forces (Adjin-Tetteh et al. 2018) and the possible diffusion of water-soluble substances in the matrix of the raw material (Titiloye et al. 2013). The ash contents of the different samples were quite high, which reflected a higher level of inorganic substances that can act as catalysts for the whole thermal conversion process (Titiloye et al. 2013). Tables 2 and 3 present the lignocellulosic and mineralogical composition of CPH in different studies around the world.

Table 2. Biochemical Composition of CPH in Previous Studies

An analysis of Table 1, Table 2, and Table 3 illustrates the rich lignocellulosic and chemical composition of CPH, which could lead to its use as a potential substrate for anaerobic digestion (Rastegari et al. 2019) to produce bioactive compounds.

Table 3. Chemical Composition of CPH in Previous Studies

These bioactive compounds have many applications, which include renewable energy, biopolymers, formulating plasticized composites, and acting as a fertilizer element (Djeke et al. 2011; Kouakou et al. 2018).

The disparities observed between the physico-chemical and biochemical compositions of the CPH could have been due to geographic factors, the location of the materials collected, the different methods of analysis, the variety of biomass, the differences in the solvents used, the different collection periods (Sandesh et al. 2020), and the different climatic and storage conditions (Titiloye et al. 2013), among other factors.

Lignocellulosic Biomass Pretreatment

Lignocellulosic biomass is a renewable material that can be converted into fermentable sugars and then converted into ethanol (Rezania et al. 2017). However, this convention is hampered by the recalcitrant nature of this plant biomass (Woiciechowski et al. 2020) due to the presence of lignin and the consequent difficulty of obtaining complete enzymatic hydrolysis has led to the implementation of different pretreatment strategies (Beukes and Pletschke 2010). Pretreatment aims to modify the properties of raw materials, to remove or dissolve lignin and hemicellulose, and to reduce the crystallinity of cellulose (Thamsee et al. 2019). It is performed to increase the surface accessible to hydrolytic enzymes (Wang et al. 2015). The pretreatment process further alters the microstructure, macrostructure, and chemical composition of lignocellulose to improve the efficiency of the hydrolysis of sugars (Chandra et al. 2015) to make them accessible to microbial degradation (An et al. 2015). The pretreatment process improves the efficiency of and stimulates sugar hydrolysis (Chandra et al. 2015). As the separation of carbohydrates from lignocellulose is a key step in the process, the choice of pretreatment strategy is crucial to facilitate the transformation of lignocellulosic biomass into products with high added value (Arevalo-Gallegos et al. 2017). The four pretreatment strategies are physical, chemical, physico-chemical, and biological.

In general, chemical and physicochemical pretreatments give good results. However, excessive use of chemicals could lead to serious environmental problems (Chen et al. 2017). The biological method consumes less energy and is less polluting than other methods, but it is expensive and time consuming, and enzyme activity in the decomposition of lignocellulose is low (Chen et al. 2017).

The following section discusses the pretreatment technologies commonly used in the recovery of lignocellulosic biomass in general and those applied to valorize CPH in particular.

Pretreatment with acid solution

Acid pretreatment is a highly effective chemical technique used to break down the lignocellulosic matrix by cleavage of the glucosidic bonds (Woiciechowski et al. 2020). Acid pretreatment mainly solubilizes hemicelluloses and part of the lignin (Woiciechowski et al. 2020). Inorganic acids ((sulfuric acid (H2SO4), nitic acid (HNO3), and phosphoric acid (H3PO4)) and organic ones (formic acetic and propionic acid) (Aslanzadeh et al. 2014) are generally used. However, most concentrated acids are highly toxic and corrosive, so the acid must be recovered, and the equipment used must be resistant to acid corrosion (Manzoor et al. 2013; Laurens et al. 2015; Chen et al. 2017). This results in high operational and maintenance costs, and the process destroys hemicellulosic sugars and transforms them into toxic compounds, such as acetic acid, furfural, and 5-hydroxymethylfurfural (5-HMF), which inhibits microbial growth (Woiciechowski et al. 2020). However, the conversion rate to sugar is higher than pretreatments that use hydrochloric acid, phosphoric acid, or nitric acid (Mosier et al. 2005; Sandesh et al. 2020). The optimization of biogas production by applying acid (H2SO4) pretreatments to CPH has been achieved (Ward-Doria et al. 2016). Likewise, cocoa pod husks have been used to release reducing sugars using autoclave-assisted hydrochloric acid hydrolysis to obtain reducing sugars (Shet et al. 2019). The concentration of hydrochloric acid (HCl) and the duration of the autoclave were used to optimize the hydrolysis process. Under optimized conditions 21.11 g/L of reducing sugar were released (Shet et al. 2019). Otherwise, a study by Dahunsi et al. (2019a) showed that cocoa pods husks could produce up to 71% biogas via the mono-fermentation of CPH pretreated with sulfuric acid (H2SO4) and alkaline hydrogen peroxide (H2O2). Additionally, xylitol has been produced from xylose from cocoa pod husks using sulfuric acid pretreatment, optimized by the response surface methodology (Santana et al. 2018). Xylitol was obtained at concentrations of 11.34 g.L-1, corresponding to a yield (Y p/s) of 0.52 g.g-1 with a fermentation efficiency of 56.6 %.

Alkaline pretreatment

Alkaline pretreatment is an economical process that is carried out by soaking a ground biomass in a basic solution at relatively low temperatures and pressures (Beukes and Pletschke 2010; Umagiliyage et al. 2015). It is an interesting technology, in which the formation of few fermentation inhibitors occurs (Beukes and Pletschke 2010). This pretreatment increases the surface area exposed to enzymatic hydrolysis and the accessibility of cellulose by removing the acetyl and uronic acid substituents associated with hemicelluloses (Umagiliyage et al. 2015). The alkaline solutions of sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2, and ammonium hydroxide (NH4OH) are suitable for the pretreatment of lignocellulose (Mirmohamadsadeghi et al. 2016; Saratale et al. 2016). For example, Ca (OH)2 can remove the acetyl group from hemicellulose to increase the digestibility of cellulose and reduce the steric hindrance of enzymes (Chen et al. 2017). Then, Ca(OH)2 can increase the crystallinity index by eliminating amorphous substances (Chen et al. 2017). The optimal pretreatment conditions (Ca(OH)2 and NaOH) for sweet sorghum bagasse have been determined (Umagiliyage et al. 2015). Expected yields of 85.6% biomass to biofuel conversion and 35.5% lignin removal can be achieved.

Several upgrading processes that employ CPH as recoverable biomass have integrated alkaline pretreatment and led to the production of biogas from the anaerobic co-digestion of CPH and poultry manure after pretreatment with sulfuric acid and hydrogen peroxide (Dahunsi et al. 2019b). The use of alkaline hydrogen peroxide resulted in solubilization of up to 81% of the lignin from the CPH.

The optimization of biogas production by applying acid (H2SO4) and alkaline (NaOH) pretreatments to CPH has been achieved (Ward-Doria et al. 2016), and the alkaline pretreatment gave the best result of up to 43.8% delignification. The effect of reagent type (sodium hydroxide (NaOH), alkaline peroxide (H2O2), and sulfuric acid (H2SO4)) was also evaluated on the delignification of CPH using response surface methodology (Nazir et al. 2016). The alkaline pretreatment still had the best results independent of reagent type.

Steam-explosion pretreatment

The steam-explosion process is among the most widely used physico-chemical pretreatment processes. This process occurs when water vapor is subjected to a high temperature for a few seconds to several minutes and then rapidly expelled from the reactor (Chen et al. 2017). The flow of vapor and liquid material cools rapidly due to the reduced pressure (Chen and Liu 2015). The main mechanism of the process is the high-pressure vapor in the fibers that causes the mechanical breakdown of the fiber. It is an inexpensive option for the pretreatment of agricultural residues, and it has a considerably reduced environmental impact (López-Linares et al. 2015). It has a lower environmental impact, lower requirements in terms of reaction conditions and cost, and fewer risks linked to chemical reagents and complete recovery of sugar than other methods. Further, this pretreatment leads to the formation of degradation products of sugars, such as acetic acid formed by the self-hydrolysis of acetyl groups at elevated temperatures or many phenolic compounds (Chen et al. 2017). This could justify why no valuation study of CPH has mentioned the application of steam-explosion pretreatment.

Explosion of ammonia-pretreated fibers

The explosion-of-ammonia-fiber (AFEX) pretreatment is a combination of steam-explosion pretreatment and alkaline pretreatment in anhydrous ammonia at high temperature (90 °C to 100 °C) and high pressure (1 MPa to 5.2 MPa) (Chen et al. 2017). After a rapid release of pressure, the ammonia is vaporized, which leads to a sudden change in temperature. As a result, the structure of the biomass is severely damaged, thus exposing the cellulose surface to increased enzymatic hydrolysis (Chen et al. 2017). The main advantage of AFEX pretreatment is the absence of substances that inhibit microbial fermentation. This process also produces a residue of ammonium salt that can serve as a nutrient for microbial fermentation (Chen et al. 2017). Thus, the resulting hydrolyzate after pretreatment can be directly used without further specific treatment. Under these conditions, AFEX is therefore suitable for the pretreatment of agricultural waste and herbaceous plants that contain a high content of cellulose (Wyman et al. 2005). However, the ammonia used must be recycled due to its high cost and volatility (Taherzadeh and Karimi 2008). However, to our knowledge, no study in the literature has yet explored the use of pre-blast pretreatment of cocoa pods husks fibers with ammonia.

CO2-explosion pretreatment

The CO2-explosion pretreatment combines steam explosion and the addition of CO2 to form carbonic acid, the presence of which greatly improves the efficiency of hemicellulosic hydrolysis (Chen et al. 2017). This process has enormous advantages, including the absence of inhibitors during subsequent fermentation; further, it is non-toxic, non-flammable, and cost-effective (Chen et al. 2017). In addition, the surface of the resulting substrate is easily accessible to subsequent enzymatic attack. A Box-Behnken design was applied to supercritical CO2 extraction to develop an appropriate ecological process to obtain a soluble fraction rich in phenolic compounds from CPH (Alemawor et al. 2009).

Microwave pretreatment

Microwave technology makes cellulose more responsive and improves the accessibility and adaptability of the lignocellulosic feedstock to enzymes (Chen et al. 2017). It is an energy efficient technology that is applicable in chemical reactions (Chen et al. 2017). Several studies have focused on microwave pretreatment of fibrous raw materials (Manzoor et al. 2013; Lo et al. 2015). Microwave pretreatment allows a subsequent increase in enzyme activity (Chen et al. 2017), but it requires a high investment. Microwave-assisted pretreatment optimization of CPH using sulfuric acid (H2SO4) was also performed by comparing the efficiency of the response surface methodology to that of the network of artificial neurons to produce bioethanol (Shet et al. 2018b), and a maximum of 9.10 g/L of sugars was released.

Biological method

The degradation of lignin is facilitated by biological pretreatment, as it can generate enzymes that break down lignin during the process. Biological pretreatment usually uses wood-rotting fungi (Chen et al. 2017). Different lignocellulosic biomasses have been biologically pretreated using fungi, such as white rot, Ceriporiopsis subvermisporaPleurotus ostreausCeriporia lacerataCyathus stercolerusPycnoporus cinnarbarinus, and Phanerochaete chrysosporium, on different lignocellulosic biomasses (Chen et al. 2017). Thus, the biological pretreatment of CPH has been carried out via the use of fungal species chrysosporium of the genus Phanerochaete (Laconi and Jayanegara 2015), and this species has shown its effectiveness in improving the nutritional value of CPH. Similarly, Pleurotus ostreatus (Alemawor et al. 2009) reduced cellulose, hemicellulose, and lignin content by 6 wt%, 3 wt%, and 6 wt%, respectively, with manganese ion supplementation (Mn+2). These results show that the Pleurotus species has promising cellulolytic and hemicellulolytic activity on CPH (Alemawor et al. 2009). Table 4 summarizes past work on the pretreatment of CPH.

Table 4. Main Pretreatment Strategies Applied to Cocoa Pod Husk in Previous Studies

Cocoa Pod Husk Conversion Processes

Several conversion routes (physical, biochemical, and thermochemical) of biomass into products with high added value, such as biofuels and biochemicals, have been exploited (Tsai et al. 2017; Adjin-Tetteh et al. 2018). Volatile materials are suitable for combustion, gasification, and pyrolysis. As the volatility of a given material increases, the ignition speed of the resulting biofuel increases (Chan and Choo 2013). However, among thermochemical conversion technologies, pyrolysis has sizable advantages over other biomass and waste processing technologies (Adjin-Tetteh et al. 2018). Pyrolysis is the decomposition of matter under the influence of water vapor (Chen et al. 2017). During the pyrolysis process, cellulose decomposes rapidly when heated to above 300 °C, which results in the release of gaseous products and the production of coke-like residues (Chen et al. 2017). In addition, other polysaccharide degradation products, such as furfural and aldehydes, can form in the presence of acid and limit microbial fermentation (Laser et al. 2002). This treatment is applied only when the biomass has a solids content less than 20%, which results in high energy consumption and relatively low productivity (Laser et al. 2002). The high-value-added bio-oil it produces can compete with and potentially replace non-renewable fossil fuels (Adjin-Tetteh et al. 2018).

Cocoa pod husk has been shown to be an excellent raw material for pyrolysis reactions (Adjin-Tetteh et al. 2018). Moreover, CPH contains a mixture of cellulose, hemicellulose, lignin, pectin, and crude fibers in large proportions. Therefore, it is a potential source of biomass substrates for biochemical production (Adjin-Tetteh et al. 2018). Indeed, a study on the technical-economic evaluation of five lignocellulosic biomass-treatment technologies (Maleka 2016) concluded that the two most effective treatment technologies for the recovery of cocoa pod husks are hydrothermal carbonization and anaerobic fermentation (Maleka 2016). Others studies have used CPH as a renewable energy source and analyzed its characteristics in a combustion chamber (Syamsiro et al. 2012). The crushed bales were charred at 400 °C for 2 h, which resulted in a higher calorific rate (17 M × Kg -1) with a high ash content.

Table 5. Main Conversion Processes Applied to Cocoa Pod Husk