Conventional and Unconventional Transformation of Cocoa Pod Husks into Value-Added Products

Abstract

The drive for a sustainable society and a circular economy has motivated researchers around the globe to turn to the transformation of renewable raw materials like biomass into value-added products that are akin or superior to their fossil counterparts. Among these biomass raw materials, cocoa pod husks (CPH) which is the non-edible portion of cocoa (ca. 70–75% weight of the while cocoa fruit) remains a promising bio-resource raw material for the production high-value added chemicals but yet largely underexploited. Currently, the most popular applications of CPH involves its use as low-value application products such as animal feed, raw material for soap making, and activated carbon. However, the rich source of lignocellulosic content, pectin, and phenolic compounds of CPH means it could be used as raw materials for the production industrially relevant platform chemicals with high potential in the agrochemicals, pharmaceutical, and food industries, if efficient transformations routes are developed by scientists. In this chapter, we will shed light on some of the works related to the transformation of CPH into various value-added products. An economic evaluation of the transformation of cocoa pod husk into relevant chemicals and products is also discussed.

1. Introduction

Cocoa (Theobroma cacao L.) is one of the most cultivated and valuable crops in many developing tropical countries including Ivory Coast, Ghana, Nigeria, and Cameroon whose collective efforts alone accounts for about 74.5% of annual global cocoa beans. Three species of T. cacao L. (Sterculiaceae) namely Criollo, Forastero, and Trinitario are the dominant market produce and commercial cocoa species of all 22 species present in that genus [1].

Cocoa pod husks (CPH) are the non-edible part of the cocoa pod with a percentage composition of 67–76% of the total cocoa pod wet weight. This translates to every kilogram of dry cocoa bean produced generating 10 kg of wet cocoa pod husks [2]. For instance, it has been estimated that the annual world crop of 1 million tons of cocoa produces about 10 million tons of pod husks as by-product, and constitutes about 67% of the fresh pod weight. After removal of the cocoa beans, treated and exported abroad, CPH is usually discarded on the farm, which often is left to decompose as an organic fertilizer. However, CPH left on the soil surface also act as a source of inoculum for plant diseases such as black pod rot (BPR) due to the development of Phytophthora spp. bacteria. BPR causes an annual cocoa yield loss from 20 to 30% worldwide, while individual farms may suffer an annual cocoa yield loss from 30% up to 90%. A graph of the production of cocoa bean and cocoa pod husks generated from around the world by countries known to be among the leading producers of cocoa is shown in Figure 1.

Figure 1.

Generation of cocoa bean and cocoa pod husks by various countries.

The development of cutting-edge technologies that can efficiently transform these hitherto waste materials generated from cocoa into useful chemicals that could potentially improve the global value chain of cacao production, is crucial and highly sort after and concomitantly reduce the negative environmental impact. Many researchers have developed interests in this area of study because of the vast availability of CPH which poses a major waste management challenge confronting cocoa-producing nations. In light of this, there have been multiple reports on the valorization of CPH into value-added products in an attempt to contribute to our drive for a sustainable society and a circular economy. Nonetheless, CPH have been hugely underexploited even though there have been numerous published literatures on this subject matter. In fact, research interest in CPH valorization dates back 1905 with a single publication. The publications increased significantly from 2003 and has continued to grow ever since. Averagely, for the past decade there has been about 18 publications per year on CPH (Figure 2).

Figure 2.

Publications produced annually related to CPH transformations.

Majority of these publications related to CPH were journal articles, hugely representing over 50% of the global works related to CPH transformations to various value-added products. However, a number of patents have also been filed (representing about 15% of the global publications related to CPH transformations), signifying the importance of the works and results discovered in relation to CPH as a bio-resource raw material. Figure 3 below shows the work density by type of publication CPH.

Figure 3.

Publication work density on cocoa pod husks (reproduced and modified with permission from: Ref. [3]).

The renewed and increased interest in CPH can be attributed to the enormous quantities generated on the farm, the environmental challenge that rotten CPH poses as well as the concomitant spread of black pod diseases that has accounted for the huge losses recorded by cocoa farmers [1, 2, 3]. Besides CPH has been found to be a valuable bio-resource due to the myriad of value-added products such as activated carbon, soap, animal feed, soil manure and fertilizer, biofuels, paper, biofuels, and nutraceuticals that it can be transformed into. It has also been found to be a repository of base chemicals of high value such as aldehydes, ketones, theobromine, phenols, potash, and pectin [4, 5, 6, 7, 8, 9]. CPH applications in several areas including radial electrochemical agrochemical bio-regulators, thermal energy technology, soil fertilization, manure and fertilizer production, food and animal chemistry, plastic treatment and waste treatment, and disposal are still being explored. Whilst soil fertilization, plant nutrition, and food and feed chemistry aspects of CPH application have been extensively exploited, plastic manufacturing, and processing is still underexplored and deserve special attention [3]. Compositionally, CPH comprises of mesocarp, sclerotic part, and epicarp (Figure 4).

Figure 4.

Fresh cocoa pod fruit (a) and dried cocoa pod husk (b and c).

Primarily, CPH consists of fibrous materials that includes ~19–26% cellulose, 9–13% hemicellulose, 14–28% lignin, and 6–13% pectin. The mesocarp contains mainly (~50%) cellulose, while the epicarp is enriched with lignin and the endocarp on the other hand rich in pectic substances [9]. The hemicellulose fraction of CPH has been reported to consist of arabinan, arabinoxylan, and xylan, which have been deduced from the high amount of isolable arabinose and xylose [10], along with other hemicelluloses fractions such as xyloglucans, galactomannans, and glucomannans [11]. CPH is also a good source of phenolic acids, with quantities ranging from 4.6 to 6.9 g GAE/100 g.

Numerous technologies and transformation routes have been explored for the valorization of CPH into valuable products. Among these transformation routes are biochemical, physical, physicochemical, and thermochemical processes. Unconventional valorization routes such as supercritical carbon dioxide extraction, microwave, and ultrasound technologies have also been investigated and are still under exploration.

The main objective of this chapter is to shed light on some of the scientific efforts tailored at valorizing CPH either by conventional or unconventional approaches into valuable platform chemicals and products, as well as the challenges and future perspectives on the efficient use of CPH as a potential agro-waste resource and its economic viabilities.

2. CPH valorization routes and technological approaches into chemical platforms, fuels, and low value products

To date, conventional valorization routes for transforming CPH to specialty chemicals occur either via biochemical, thermochemical, or physicochemical techniques. A combination of these techniques is also possible, given that lignocellulosic biomass usually requires pretreatment especially before biochemical conversion.

2.1 Biochemical transformation of CPH into fuels

Biochemical transformation of renewable raw materials involves the use microorganisms as catalyst to transform biomass into valuable products. It is often regarded as a cheaper approach for converting biomass to chemical, energy, and fuels. However, due to the recalcitrant nature of lignin component in biomass, the use of microbes to transform crude biomass into valuable products is often challenging and difficult. In this context, it is imperative to pretreat the biomass raw material in order to render cellulose and hemicellulose susceptible to microbial action. The pretreatment processes may be physical, thermochemical, biological, or physicochemical. The nature of pretreatment approach dictates the types of the intermediate chemical that would be obtained for further conversion to final product. The main biochemical routes that have been investigated using CPH as raw material are fermentation and anaerobic digestion.

2.1.1 Anaerobic digestion of CPH

Anaerobic digestion (AD) is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. AD basically occurs in three steps: decomposition or hydrolysis of biomass, followed by conversion of treated biomass to organic acids, and finally conversion of acids into methane gas. The main product of AD is biogas which contains methane, carbon dioxide, and some traces of hydrogen sulfide which is one of the main sources of renewable energy. The process also produces an aqueous mixture consisting of microorganisms involved in the degradation. Large volumes of CPH generated and its composition makes it a viable candidate for AD biogas production.

In 2018, Acosta and co-workers [12] investigated the production of methane and biogas yields from CPH and compared it to other agricultural residues, to evaluate the quality of the biomass raw material as a new feedstock for biogas production. The authors concluded that 50% of organic matter from CPH was transformed to biogas with 60% yield of methane. Dry AD was the preferred process choice for the authors because it gave the highest yields of methane and also, the operating conditions were stable [12].

In another interesting work, Antwi et al. [13] investigated the potential of valorizing CPH via anaerobic digestion and the impact of hydrothermal pretreatment on biogas yield. They compared the biogas yield and methane content of untreated anaerobically digested CPH to those obtained from the hydrothermally pretreated CPH at different severity levels. Based on their results they concluded that AD is an effective process of converting CPH to fuels. Furthermore, the impact of the pretreatment is diverse in that biogas yield increased for CPH treated at low severity levels up to 3.0. Hydrothermal pretreatment at severity levels above 3.0 lead to inhibitions in the AD process that lowered the biogas yield.

Several reports on the valorization of CPH or cocoa related residue to biogas via AD has proven that to be an effective approach, however, a form of pretreatment (physical, thermochemical, biological, or physicochemical) of the biomass is required to separate lignin from cellulose and hemicellulose [5, 14, 15, 16, 17].

2.1.2 Fermentation of CPH

Fermentation is the conversion of sugars contained in biomass hydrosylate to specialty chemicals using microbes. The type of microbe used dictates the fermentation pathway as well as the end products. The conversion of CPH to bioethanol, bio-butatnol, and propanoic acid by fermentation reported in literature has been highlighted below.

Shet et al. [17] hydrolyzed CPH with HCl to release reducing sugars under optimized conditions (8.36% W/V of CPH, 3.6 N HCl, and 7.36 hours) using response surface methodology (RSM). The hydrosylate was neutralized using 5 N NaOH followed by fermentation to produce bioethanol. The inoculum was Pichia stipites at 2% V/V. After 72 hours of fermentation, bioethanol was distilled from the broth at a concentration of 2 g/L. They demonstrated that CPH to ethanol conversion is feasible and that CPH offers a cheaper and renewable feedstock for ethanol production. A similar work was done by Samah and co-workers [18], where CPH was hydrolyzed with HCl, H₂SO₄, and HNO₃ at different concentrations (0.25, 0.50, 0.75, 1.00, and 1.25 M). They were further heated to 75 and 90°C for 2 and 4 hours. The highest glucose content of 30.7% W/V was obtained for CPH samples treated with 1.00 M of HCL at 75°C and 4 hours. The hydrosylate was then fermented using Saccharomyces cerevisiae for 48 hours at room temperature to obtain a maximum ethanol concentration of 17.3% V/V after 26 hours of fermentation.

Hernández-Mendoza et al., 2021 on the other hand performed alkaline hydrolysis on CPH and examined the effect of NaOH concentration, residence time, and temperature using a central composite design (CDD). The solid fraction was examined with X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to investigate morphological changes. It was further subjected to enzymatic hydrolysis which optimized the enzyme and solid loadings to convert cellulose to reducing sugars. The yeast S. cerevisiae was applied to ferment the hydrosylate. The optimum condition for the alkaline hydrolysis process occurred at 5% W/V NaOH for 30 minutes at 120°C which led to an increased in the cellulose content of CPH to 57 ± 0.25% relative to that of the untreated sample of 27.68 ± 0.15%. SEM revealed changes in porosity and structure of CPH, whilst XRD showed increase in crystallinity. Enzymatic hydrolysis yielded 66.80 g/L of reducing sugars of which 80.74% were consumed during fermentation producing 18.06 g/L of ethanol in 24 hours. They concluded that CPH is a promising feedstock for bioethanol production [19].

Propionic acid production from CPH was reported for the first time by Sarmiento-Vásquez et al. [20]. In their work, alkaline and enzymatic treatment is conducted with 2.3% W/V NaOH and 2.4% V/V Cellic® CTec2, respectively to convert CPH to fermentable sugars such as glucose to a maximum yield of 275 mg glucose/g CPH. Subsequently 7.5 g/L CPH hydrosylate is fermented with Propionibacterium jensenii (DSM 20274) in the presence of 7.5 g/L of glycerol. A maximum propionic concentration of 10.28 ± 1.05 g/L after 75 hours of fermentation.

Sandesh et al. [21] successfully produced acetone, bio-butanol, and ethanol from inductive assisted H₂SO₄ hydrolyzed CPH using Clostridium acetobutylicum. A product distribution of 5.04 ± 0.32 g/L of acetone, 11.73 ± 0.84 g/L butanol, and 1.43 ± 0.04 ethanol is reported to have been obtained after 312 hours of fermentation.

These results are a demonstration of the potential of CPH as a cheap feedstock for the production of biochemicals via fermentation techniques after different pretreatment approaches have been applied to the CPH biomass to convert it to fermentable sugars.

2.2 Thermochemical approaches

Thermochemical biomass conversion approach involves all processes in which heat is used to transform biomass in the solid form to other states in the presence or absence of oxygen. Processes that fall under this category are direct combustion, gasification, pyrolysis, hydrothermal liquefaction, and torrefaction. This section examines how thermochemical conversion processes have been applied in CPH valorization.

2.2.1 Direct combustion

In direct combustion, biomass is burnt in ovens, kilns, fluidized bed combustors, furnaces with excess oxygen or air to obtain gases and ash. The combustion chambers are usually operated at temperature above 900°C. Gases and ash are the key products. The ash has been found to contain 40% potash which consists of 43% potassium carbonate and 27% potassium hydroxide. This is the process soap-makers in most West African countries harness potash from CPH to produce soft soap known locally as alata samina [22]. These CPH potash soap have been found to contain superior properties such as higher solubility, lathering capacity, cleansing power, and consistency compared with those produced with chemical KOH [23]. Furthermore, CPH ash has also been applied as fertilizer. Studies shows that replacing about 50% of conventional NPK fertilizer with CPH ash has had positive impact on nutrients uptake by maize plants and grain yield [24]. It has had similar effect on soil fertility, fruit growth, and yield in tomato production [25]. CPH ash obtained in a furnace at 650°C for 4 hours was evaluated by [26] as a heterogeneous catalyst for the transesterification of soya bean oil to biodiesel. Their results demonstrated that CPH ash is a superior catalyst for generating high yield of biodiesel with quality and engine performance close to that of diesel from petroleum.

2.2.2 Pyrolysis

In pyrolysis biomass is thermally decomposed in an inert atmosphere at elevated temperatures. The biomass is usually converted to volatile products with solid residue called char where the proportion of each fraction depends on the conditions of pyrolysis that the biomass was subjected to. The volatile fractions are usually condensed to obtain the liquid (bio-oil) and non-condensable gaseous fractions. Operating parameters such as reaction temperature, pressure, catalysts, hot vapor residence time, solid’s residence time, etc., affect the overall process performance. The conditions of pyrolysis fall into three categories namely slow pyrolysis, fast pyrolysis, and flash pyrolysis. In slow pyrolysis, the temperature of the biomass is raised to about 500°C at low heating rates with long residence times. The solid char is the main product and it is the main route of producing charcoal which can used as fuel, activated carbon, soil conditioners, and feedstock for producing chemicals. On the other hand, in fast and flash pyrolysis, the liquid fraction or bio-oil is the preferred product. In fast pyrolysis, temperatures of about 500°C and short vapor residence time of about 2 seconds are typical to generate bio-oil from biomass. Flash pyrolysis is similar to fast pyrolysis except that the residence time is shorter in the former [27].

Pyrolysis is the most widely exploited biomass to liquid (BTL) conversion route in that the crude bio-oil can be directly used in boilers and turbines to generate electricity and heat as well as feedstock for synthesizing fuels, base, and fine chemicals [28]. By this technology, bio-oils that of high value and substitute for fuels from non-renewable sources can be produced [29]. Tsai and co-worker [30] demonstrated that slow pyrolysis of CPH produces bio-chars of more than 60% carbon content and a calorific value greater than 25 MJ/kg, dry basis at temperatures between 190 and 370°C for 30–120 minutes. They concluded that though this type of biochar exhibited lignite-like feature, it is not suitable for use as fuel in boilers due to the high potassium content. Several researchers have applied this process to CPH and have generated similar products [31, 32, 33].

CPH was pyrolyzed under fast pyrolysis conditions at temperatures 550–600°C by [29] for 2–4 minutes to yield 58 wt.% bio-oil, 30 wt.% biochar, and 12 wt.% non-condensable gases. Analysis of the bio-oil shows it contained a complex mixture of carboxylic acids and ketones with 9,12-octadecadienoic acid being the most abundant.

In another work by Mansur et al., the authors [2] reported the possibility of upgrading pyrolysis oil obtained from CPH via the use of heterogeneous catalysis. Firstly, they pyrolyzed CPH at 500°C for 50 minutes to yield pyrolysis oil which contained several chemical compounds including benzenediols, ketones, carboxylic acids, aldehydes, furans, heterocyclic aromatics, alkyl benzenes, and phenols. This oil was subjected to catalytic upgrade using ZrO₂-FeO_x where ketonization, selective oxidation, and demethoxylation reactions occurred and selectively yielded acetone, 2-butanone, phenol, cresol, xylenol, and ethyl phenol.

Prior to pyrolysis, it is imperative to pretreat the biomass by sun drying, oven drying to avoid moisture saturation, and mechanical comminution to increase the surface area for effective pyrolysis.

2.2.3 Gasification

Gasification is a thermochemical biomass conversion process which occurs at elevated temperatures above 700°C in a limited amount of oxygen. Usually 70–80% is transformed to synthesis gas (CO and H₂) and the remainder is biochar. It is possible to obtain some amount of bio-oil if the condition is favorable. To maximize the yields of synthesis gas and improve on the overall efficiency of the process, supercritical water, and catalyst is used [27]. The synthesis gas can be transformed to fuels and myriad of chemicals via Fischer-Tropsch synthesis [34]. CPH has been converted to gaseous products of varying composition by gasification. For instance, Gunasekaran et al. [35] investigated the numerical and experimental potential of CPH gasification in an open-core gasifier. According to their results, the composition of CO, H₂, and CH₄ in the producer gas was found to be 20–24%, 12.0–16.5%, and 2.0–3.2%, respectively for the conditions that were tested. The conversion efficiency and cold gas efficiency were determined to be 82 and 38%, respectively. Further, the predicted performance parameters and temperature distribution were found to be at par. Thus, CPH was found to be a promising raw material for an open-core gasifiers.

The application of recycle system on a CPH gasification in a fixed-bed downdraft reactor was carried out by Pranolo and co-workers [36]. The aim was to produce low tar fuel gas from CPH using recycle stream consisting of CO, H₂, CO₂, and CH₄. They successfully reduced the tar content in the product gases up to 62% at temperatures ranging from 750 to 780°C. Therefore, the gas may be used as a substitute fuel for electricity generation.

2.2.4 Physicochemical routes

The valorization of CPH by physiochemical approach has mainly been by solid phase extraction or leaching in which solutes are removed from a solid by a liquid solvent [37]. Such processes have been applied in the extraction of phytochemicals and pectin from CPH. Phytochemicals are natural functional foods that possess a rich reservoir of bioactive components and nutraceuticals. Nutraceuticals was coined by Dr. Stephen De Felice and is a derivation from words “nutrition” and “pharmaceuticals.” Phytochemicals are mainly foods or parts of foods that provide medical or health benefits including the prevention and treatment of diseases. There has been rapid increase in the consumption of plant-derived bioactive. Plants produce these chemicals to protect themselves but recent studies have shown that these chemicals can protect humans, animals, and other plants against diseases compound [38].

Rachmawaty et al., 2018 studied the extraction of bioactive components from CPH and the in vitro antifungal activity assay against the pathogenic fungus Fusarium oxysporum. The F. oxysporum is a deadly fungus that can cause diseases in nearly every agriculturally important plant. In the study, CPH was dry milled using a grinder into powder. Two solvents, acetone-water (7:3) and 70% ethanol was used to extract the phytochemicals. A solvent to sample ratio of 10:1 was used such that 200 ml of solvent was used for 20 g of CPH sample. The extract was found to contain alkaloids, flavonoids, tannins, and saponins, and triterpenoids which indicates the antimicrobial potential of the CPH extract. GC-MS analysis revealed four major components in the acetone solvent namely isopropyl myristate, benzenedicarboxylic acid, 9-octadecenoic acid (Z)-, methyl ester and octadecanoic acid, methyl ester. For the ethanol solvent however, three main components were found namely octadeca methyl-9,12-dienoate; 9-octadecenoic acid (Z)-, methyl ester; hexadecanoic acid, 15-methyl-methyl ester.

The acetone extract recorded the highest phenolic content and also a higher anti-fungal activity than the ethanol extract. Agar diffusion method was employed for antifungal testing and it showed that the extract was able to inhibit the fungal growth therefore leading to the conclusion that the CPH extract has great potential as a natural fungicide.

Pectin, a family of complex, acid-rich polysaccharides found in plant cell wall have been recovered from CPH by this approach. They have been extensively applied as gelling and stabilizer in cosmetics, food, and pharmaceuticals. They have the ability to reduce serum cholesterol, glucose, cancer incidence, and improved immune response in humans [39].

Pectin recovery from CPH was studied by Valladares-Diestra et al. [40] using citric acid hydrothermal treatment of CPH with concomitant production of xylooligosaccarides via enzymatic hydrolysis of the solid fraction after extraction. An optimum condition of 120°C, 10 minutes, and 2% W/V was employed for the recovering pectin. An amount of 19.3% of the biomass was recovered as pectin. They concluded that the prospects of implementing this novel method for the extraction of valuable chemicals such as pectin is very high.

Vriesmann et al. [39] optimized the variables affecting the nitic acid extraction of pectins from CPH using RSM. The optimum extraction condition was determined as pH 1.5, a temperature of 100°C, and time of 30 minutes. By these conditions a yield and uronic acid (UA) content (representing pectin content) of about 9.5 and 80%, respectively were predicted. However, experimental results gave a yield of 9.0 ± 0.4% and UA content of 66%. The predicted and experimental yield values were in close agreement, on the contrary, experimental UA content value was 17.5% lower than the predicted. This disparity was attributed to the low quality of the model for used the prediction. They further characterized the pectin a homogalacturonan highly esterified and acetylated one with some rhamnogalacturonan insertions.

Recently, Vriesmann and de Oliveira Petkowicz [41] compared the use of nitric acid and boiling water for the extraction of pectin. The pectins obtained from both extraction process was similar and identified as low methoxyl type. Rheological analysis suggests that both formed gels at low pH in spite of their high acetyl content therefore, the pectin can be used in acidic products.

3. Unconventional CPH valorization processes

Recently, processes that are considered green have been utilized to extract bioactive chemical from biomass feedstocks. These processes are gaining popularity due to their inherent benefits such lower temperature, less activation time, and higher carbon yield. Microwave, ultrasound as well as super and subcritical fluid extraction have been applied to obtain valuable chemical from CPH and is discussed in this section.

3.1 Microwave-assisted valorization of CPH

Microwave has been utilized in recent times to extract biochemicals instead of conventional processes as uniform heating, time, and solvent savings [42, 43, 44] are the main advantages of this process. Additionally, it has been found to improve the accessibility and reactivity of cellulose when used to pretreat lignocellulosic biomass. Moreover, subsequent enzyme action is heightened [43]. This is the most widely applied unconventional process for CPH valorization.

In the work of Mashuni et al., 2020, microwave was used to assist the extraction of phenols from CPH using 85% V/V ethanol as solvent. The microwave heating power was varied from 100 to 300 W whilst the extraction time spanned 5–30 minutes. Using the Folin-Ciocalteu method with gallic acid as a standard, the total phenol content of CPH was determined. The highest amount of phenol content was found to be 853.67 mg/L after 20 minutes of extraction at 200 W of microwave power. Upon characterizing the extract with GC-MS, it was revealed that the phenols present are butylhydroxytoluene; 6,6′-methylenebis(2-(tert-butyl)-4-methyl-phenol); 3-methoxy-2-((2E,6E)-3,7,11-trimethyl-dodeca-2,6,10-trienyl) phenol; and 7-hydroxy-3-(1,1-dimethylprop-2-enyl) coumarin. They concluded that microwave assisted extraction (MAE) is a promising technique for the extracting phenols quickly and efficiently [45].

Novel research was conducted by Nguyen and co-workers [42] where they extracted saponin from CPH via MAE using methanol as solvent. They used RSM (CCD) to identify the optimum parameters for the process. According to their findings, the optimum MAE conditions for obtaining the maximum saponin content and extraction efficiency from dried CPH were 85% methanol concentration, 40 minutes extraction time, 600 W microwave power, 6 seconds/minute irradiation time, and 50 ml/g solvent to sample ratio. The saponin content and extraction efficiency determined under these conditions were 69.9 mg escin equivalents/g dried sample and 71.1%, respectively. Thus, the CPH has a huge potential as a source of bioactive compounds for used in the nutraceutical and functional foods industries and to harness these compounds the optimum MAE conditions should be applied for best results. MAE was applied to isolate pectin from CPH using oxalic acid by Pangestu and colleagues [46]. In their work, they used RSM to investigate how pH, liquid to solid ratio (L/S), and irradiation time interact to affect the quantity of pectin isolated. A pH of 1.16, L/S of 25, and 15 minutes of irradiation were found to give a maximum yield of 9.64%. They emphasized that this route reduced the extraction time by 2–6 times. Further the L/S ratio can be minimized without considerable impact on the results. MAE was concluded as a powerful technique for isolating pectin using a cheaper and safer acidifying agent such as oxalic acid. Villota et al. [47] carbonized H₃PO₄ and KOH activated CPH via microwave assisted pyrolysis at 450°C for 5 minutes. The effect of H₃PO₄ and KOH on the activation of char from CPH and in all cases, H₃PO₄ activated carbon was observed to have higher yield and better textural properties (BET surface area = 1237.47 m²/g, pore volume = 1.11 cm³/g, and mesoporous) relative to that activated with KOH which exhibited severe material loss as well as low strength. Microwave assisted pretreatment of CPH has been applied by several researchers. A summary is presented in Table 1.

Pretreatment method	Objective	Observation	Reference
Microwave assisted H2SO4 hydrolysis (15.65 g of CPH, 6% V/V acid, and 8 minutes of irradiation)	Release fermentable sugars for onward fermentation to bioethanol.	Hydrosylate (9.10 g/L max) containing glucose, galactose, cellobiose, xylulose, and arabinose.	[48]
Microwave assisted NaOH hydrolysis (3% NaOH, 100 W, 2.5 minutes, and 5 g CPH)	Delignification of CPH.	Increase in cellulose content especially when the microwave irradiation period was prolonged.	[49]
Microwave (300 W, 25 minutes)	Increase porosity in lignin covering cellulose and hemicellulose to facilitate enzymatic action.	The sugar yield CPH was low, the yield of ethanol was considered high (61 ml/kg).	[50]

Table 1.

Various microwave-assisted pretreatment of CPH.

3.2 Ultrasound-assisted valorization of CPH

This process was implemented in the work of Hennessey-Ramos and colleagues, 2021 to extract pectin from CPH. RSM was used to determine the optimum operating conditions that is 6.0% feedstock concentration, 40 μL/g enzyme, and 18.54 hours on stream. Experiments involving three processes for extracting pectin namely acid, ultrasound-assisted and enzymatic extraction were conducted and compared. The results are summarized in Table 2.

Parameter/process	Citric acid	Ultrasound-assisted citric acid extraction	Enzymatic extraction
Yield, g pectin/100 g CPHP	8.08	8.28	10.20
GA content, g GA/100 g pectin	60.97	42.77	52.06
GA yield, g GA/100 g CPHP	—	—	5.31

Table 2.

Comparative analysis of chemical, ultrasound assisted, and enzymatic pectin processes.

CPHP, cocoa pod husk powder; GA, galacturonic acid.

From the results enzymatic extraction of pectin gave the best results for pectin yield followed by ultrasound-assisted citric acid extraction. The low GA content was attributed to duration (45 minutes) and temperature (60°C) of the process. They asserted that industrial operations above 60°C for ultrasonic assisted citric acid pectin extraction with the aim of increasing GA content would not be feasible owing to the inherent advantage of low temperature operation for such technologies. In the extraction of microcrystalline cellulose from CPH, it was pretreated with alkali followed by ultrasonication. Ultrasound applied after alkaline pretreatment of the feedstock brought about cavitation action that helped to effectively remove fibril aggregates from the microcrystalline cellulose. A sonication time of 60 seconds and two cycles of the ultrasonication process considerably reduced the particle size of the microcrystalline cellulose to 280 nm [51].

3.3 Super and subcritical fluid extraction of biochemicals from CPH

Long extraction periods, low yield and quality of extracts, and loss of volatile compound are among many limitations of traditional extraction processes that has warranted the development of novel and green processes that overcome these limitations. Super critical and subcritical fluid extraction are among such processes that are considered efficient and time-economic [43, 52, 53, 54]. In a recent study on the extraction of phenols from CPH using supercritical CO₂, Valadez-Carmona et al. [7] employed a Box-Behnken design to maximize the process variables that is temperature, pressure, and co-solvent. The optimum conditions obtained were 60°C, 299 bar, and 13.7% ethanol. By this approach, the extraction time was lowered even though the yield was low (0.56%), the quality of the extracts was improved whilst the loss of volatile compound was minimized. The highest total phenolic compounds (TPC) were found to be 12.97 mg GAE/g extract whereas the total antioxidant capacity was 0.213 mmol TE/g extract. These findings demonstrates that supercritical CO₂ extraction is a promising technique that can be exploited for the isolation of natural antioxidants from CPH for use in food, cosmetic, or pharmaceutical products. Another interesting work was published by Muñoz-Almagro and co-workers [55] where they compared conventional and subcritical water extraction of pectin from CPH. The latter process is a technique in which water provides H⁺ and OH⁻ ions at high pressure and temperature for dissolving both polar and non-polar compounds. At high temperatures the hydrogen bonding in water is weakened thereby decreasing the dielectric constant value and water polarity which consequently lowers the energy required for dissociation of water molecules in solute-matrix interactions and extraction efficiency is increased [55]. In the subcritical water extraction process, a pectin yield of 10.9% as opposed to 8% obtained using conventional extraction with citric acid as solvent. Characterization of the pectin showed that high molecular weight pectin (750 kDa) was preferentially extracted during the subcritical operation. These green techniques have been shown to possess high selectivity towards targeted compounds and potential for CPH valorization.

4. Future perspective

Although several transformation techniques have been investigated for the conversion of CPH to valuable products, there is still a need to develop efficient and sustainable approaches for a holistic CPH biomass valorization process. In this context, the development of cutting-edge technologies that can efficiently transform these hitherto waste materials generated from cacao into useful chemicals that could potentially improve the global value chain of cacao production, is crucial and highly sort after. Although of interest, the uncontrolled co-production of char and gaseous products limits the overall yield of bio-chemicals so-obtained, and thus the overall efficiency of this approach. Being able to fractionate these lignocellulosic biomass waste into valuable chemicals in a selective fashion is highly desirable from economic and environmental considerations, but it remains a very important scientific challenging task due to scientific bottlenecks such as: (i) recalcitrance of lignocellulosic biomass to hydrolysis, often requiring high activation temperatures which are not compatible with the stability of sugars, the main components of lignocellulosic biomass waste, and (ii) high dilution ratio to prevent recombination reactions (for instance caramelization of monomeric sugars) leading to the unwanted formation of tar-like materials. In order to overcome such scientific hurdles, researchers should consider the coupling of mechano-catalytic technology to first release sugars contained in CPH, which can be achieved without the need of any solvent, translating into efficient and environmentally friendly synthesis approach, and a pyrolysis process to valorize lignin, the co-product of the CPH fractionation after the mechano-catalytic step.

5. Economic aspects of CPH valorization

Using pectin production as a valuable product case-study from CPH, an economic analysis using Aspen Process Economic Evaluator was modeled, and allowed the estimation of investment and return of the stimulated process with the possibility to obtain a considerable profitability with a 20 years operation plant life and a pectin production capacity of 108,127.4 Ib./year, annual interest rate of 20%, a salvage value (fraction of initial investment) of 20% and depreciation method straight line and an income tax of 40%. An Internal Rate of Return (IRR) of 33% was obtained over a capital cost of $5,509,000 (USD), operational cost of $2,135,300 (USD), 17 years durations of startup, and a 4 years payback period. These values indicated a positive suggestion that the implementation of pectin production process from cocoa pod husks as an investment project owing to its better long-term benefits compared to those generated by investing in banks [56]. An important aspect of the economic viability of CPH valorization that is often ignored is the cost of the raw material which is often considered waste and of low value. According to the findings of a study conducted in Indonesia on the need for economic and sustainability assessment of the valorization of CPH, farmers demand higher levels of compensation to collect or process the raw material than expected. Only a small section of farmers were willing to carry out collection and processing for 117GBP/t CPH. This could offer some explanation for the low patronage of CPH valorization innovations in that country [57].

6. Conclusions

CPH has been demonstrated to be an excellent source of phenolics, pectin, and lignocellulosic contents that can be used for the production of platform chemicals relevant in the agrochemical, pharmaceutical, and food industries. However, although cocoa remains a prime economic cash crop in developing countries like Ghana, Ivory Coast, Indonesia, etc., the efficient transformation of cocoa pod husks into valuable products in such countries other than leaving them on farm sites to rots are scarce. Therefore, it is paramount for such developing countries to develop end-user applications for CPH that will be beneficial for industries, consumers, researchers, and also serve as extra income for farmers. It is of no doubt that the development of processes that are easy to implement, less expensive, sustainable and environmentally friendly, to convert CPH into high high-value added products, such as biofuels could significantly prevent the excessive consumption and reliant on fuel/diesel and the production of greenhouse gas. Increased valorization techniques for CPH will concomitantly increase the overall sustainability of the cocoa agribusiness and open up new avenues for sustainable incomes for cocoa farmers.

Acknowledgments

The authors would like to thank the French National Centre for Scientific Research (CNRS) through its Support Action for Collaboration with sub-Saharan Africa Grant for supporting this work. The kind contribution and efforts of Jedidian A. Adjei (KNUST Ghana) are duly acknowledged.