Pineapple Waste Biorefinery: An Integrated System for Production of Biogas and Marketable Products in South Africa

Abstract

Pineapple (Ananas comosus) is one of the most economically important fruit cultivars in South Africa. The fruit is locally consumed, processed into various industrial products or exported to foreign markets. Approximately 115,106 metric tons of pineapple fruit are harvested in South Africa. The pineapple value chain generates significant amounts of waste, in the form of pomace, peel, crown, stem, core and base. If not properly treated, pineapple waste (PAW) could have a profound detrimental impact on the environment. This calls for advanced technological platforms to transform PAW into useful bio-based products. A biorefinery is a potent strategy to convert PAW into multiple food and non-food products while effectively disposing of the waste. The objective of this review is to explore possible pathways for the valorization of PAW into energy and material products in a biorefinery. The paper looks at 10 products including biogas, biohythane, bioethanol, biobutanol, biohydrogen, pyrolytic products, single-cell proteins, animal feed, vermicompost and bioactive compounds. Several platforms (i.e., biochemical, chemical, physical and thermochemical) are available to convert PAW into valuable goods. Amongst them, the biochemical route appears to be the most favorable option for the valorization of PAW. Anaerobic digestion and fermentation are well-established biochemical technologies for PAW valorization. These methods are simple, low-cost, eco-friendly and sustainable. The focal point of emerging research is the enhanced efficacy of biorefinery platforms. The commercialization of PAW biorefining is a potential gamechanger that could revitalize the entire South African economy.

1. Introduction

Pineapple (Ananas comosus) is a perennial crop grown for its succulent and edible fruits in tropical and sub-tropical regions of the world. The plant is a member of the family, Bromeliaceae [1,2,3,4]. It is native to Brazil and Paraguay, where it was first domesticated in the Amazon Basin [1]. Pineapple is now naturized in several tropical areas of South America, Asia and Africa [5]. World pineapple production was estimated at 29.4 million metric tons in 2022 [6]. Africa accounts for about 17.3% of the world’s pineapple production [7]. South Africa is one of the leading commercial producers of pineapple in Africa. At present, a total of 18,646 ha of land is under pineapple cultivation in South Africa [8]. This produces around 115,106 metric tons of pineapple fruit [9]. The popularity of pineapple has been derived from its abundance, high nutrient content, juiciness, sweet taste, nice flavor and other benefits [3,10]. Pineapple is enriched with carbohydrates, proteins, vitamins, minerals, fibers, organic acids and antioxidants. It has a moisture content of about 85% [4,10]. Pineapple contains bromelain. Bromelain is a cluster of enzymes with high proteolytic activity [11]. Pineapple fruits can be freshly consumed or processed via juice extraction, canning, cooking and drying [3,10].

Large amounts of waste are generated from the value chain management of pineapple in South Africa. It is assumed that processing a ton of pineapple fruit yields more than 0.5 ton of pineapple waste (PAW) [12]. The main waste by-product streams from the pineapple include pomace, peels, crown, stem, core and base [1,3,13,14]. PAW can impose serious threats to the environment as its traditional disposal strategies are costly due to high transport and landfill requirements [15,16]. Currently, PAW is either burnt, used as animal feed or buried in landfills. While South Africa is ranked 15th in terms of greenhouse gas emissions, the country is still naive in embracing bioenergy technologies regardless of large market demand [10]. PAW contains huge amounts of organic compounds such as carbohydrates and proteins. It is regarded as a renewable bioresource with multiple applications. PAW is an excellent source of carbon (C) to produce high-value products in a biorefinery [16]. The multiplicity of the products falls into three main groups, namely food and feed, bioenergy and pharmaceuticals. Converting PAW into value-added products is a sustainable way to mitigate global climate change and ameliorate the environment [10].

In this context, a biorefinery facility couples the conversion of PAW into biofuels, power and other high-value biomaterials [17,18,19]. A wide range of platforms including biological, chemical, physical and thermochemical methods are widely available to convert PAW into a variety of bio-based goods [3,17]. PAW is a promising substrate for producing juice, wine, vinegar, lactic acid, biofuels, paper, animal feed, bioactive compounds, bromelain, etc. Predominant biofuels that can be obtained from PAW comprise biogas, bioethanol, biohythane, biohydrogen, biobutanol and biomethanol [3,10]. In addition, PAW biorefining releases significant amounts of nutrients that can be recovered for use in agriculture.

There is limited information on the conversion of PAW into useful products given that exorbitant amounts of the waste are generated in South Africa. PAW is underrated as a renewable resource for bio-based refinery projects in South Africa. The major problem of PAW valorization in a biorefinery is its hard-to-digest structure. It is believed that pretreatment is a sine qua non for appropriate utilization of PAW in a biorefinery [1,20]. This paper reviews the possible routes for utilizing PAW to produce biogas and other valuable products in a biorefinery. The paper focuses on 10 products consisting of biogas, biohythane, bioethanol, biobutanol, biohydrogen, pyrolytic fuels, single-cell protein (SCP), animal feed, vermicompost and bioactive compounds. These products are grouped into two broad categories: energy products and biomaterials. Energy products are generally utilized for their high energy content, whereas biomaterials are non-energy products manufactured for their chemical or physical characteristics [17]. The objective is to provide aggregated data that may be beneficial to many researchers concerned about biorefineries in the era of global warming and depleting fossil resources. This could proffer a cheap and sustainable route for managing PAW and probably revive the South African agroindustry. Regardless of all the socio-economic benefits, commercialization of biorefinery facilities in developing countries, and in particular South Africa, is very limited [21]. Most biorefineries are still in laboratory, pilot and demonstration phases across the globe. This work will provide knowledge-based decisions for scientists, policy makers, industrialists and other multi-stakeholders to shift their interest and investments towards advanced biorefineries.

2. Pineapple Waste Production in South Africa

Pineapple production is rapidly growing due to increased demand in the food and beverage industries. In 2022, the pineapple has a world cultivation of 29.4 million metric tons [6]. According to the Food and Agriculture Organization (FAO), global pineapple production is projected to rise at the rate of 2% per annum [7]. By 2030, world pineapple production is expected to reach 37 million metric tons [7]. Figure 1 illustrates the regions that are suitable for pineapple production in the world. Asia, the Americas, Africa and Oceania are the most active pineapple producers. They contribute around 46.6, 35.1, 17.5 and 0.7%, respectively [22]. Costa Rica, Philippines, Brazil, Indonesia, China, India, Thailand, Nigeria, Mexico and Colombia are notably the top 10 pineapple-producing countries [22]. Nigeria is the leading producer of pineapple in Africa. It produces nearly 1.42 million metric tons [10]. South Africa, with an annual yield of 115,106 metric tons, is ranked among Africa’s largest pineapple producers [9].

Pineapple cultivation is quite rampant in KwaZulu Natal Province (Hluhluwe Region) and Eastern Cape Province (Bathurst Region) of South Africa. The two provinces contribute 33 and 66% of the country’s pineapple production, respectively. Warm and humid climatic conditions in these areas are more favorable for pineapple production. Other provinces like Mpumalanga and Limpopo provide around 1% of the total pineapple production [5,8]. An estimated 70–80% of the pineapple is locally canned, concentrated into syrup or converted into juice, wine, dried fruit and fiber [8]. Only 20% of these finished products are distributed to local markets. The rest is exported to the global market [5,8]. Pineapple production generates huge quantities of PAW in South Africa. The actual production statistics of PAW in the country are not yet fully understood. Theoretically, a ton of pineapple can produce around 0.5 ton of PAW [12]. This points to a need for integrated management of PAW in a sustainable and eco-friendly manner.

PAW represents about 60% (w/w) of the total pineapple weight [3]. It falls within two broad groups: pineapple harvesting waste (i.e., leaves, roots, stems and other on-field residues) and post-harvest handling or processing waste. Post-harvest handling waste comes from industrial processes, mostly pulping, juicing and canning [11]. It consists of peels, core, stem, crown and pomace. Pomace refers to solid organic matter that is left after pineapple pressing for juice or oil extraction. It forms the bulk of pineapple post-harvest handling waste. The seed, skin and pulp constitute pineapple pomace. More than 95% of pineapple pomace is made up of insoluble organic matter [14]. Core is a by-product of pineapple preprocessing into sugars, bromelain, organic acids and bioactive compounds. It accounts for about 15% of the total PAW [14]. The peel and crown are main by-products of fiber extraction and bromelain extraction, respectively [11]. Around 30 and 25–30% of the pineapple processing waste is composed of peels and crown, respectively [14]. PAW has the potential to revitalize the South African industry and economy through the production of high-value products such as food, energy, biochemicals and pharmaceuticals.

3. Composition of Pineapple Waste

3.1. Lignocellulosic Composition

Table 1. Composition of polymeric substances in pineapple waste.

Component	Cellulose (%)	Hemicelluloses (%)	Lignin (%)	Reference
Peel	19.8	11.7	-	[3]
	11.2	7.0	11.52	[15]
	10.9	22.8	7.1	[24]
	22.9	36.8	5.12	[25]
	14.0	20.2	1.5	[26]
	35–50	20–35	5–30	[27]
	35	19	16	[28]
Mixed pulp and peel	2.43	1.69	0.42	[29]
Pulp	14.3	22.1	2.3	[3,26]
Core	24.53	28.53	5.78	[30]
	17.2	29.5	1.82	[25]
Cores and peelings	11.2–19.8	7.0–11.7	-	[31]
Shell	40.55	28.69	10.01	[30]
Crown	41.15	21.02	13.05	[32]
	43.53	21.88	13.88	[30]
	29.6	23.2	4.5	[26]
Whole	19.4	22.4	4.7	[26]

shows the lignocellulosic composition of PAW expressed on a percentage dry weight basis. PAW is made up of approximately 10% dry matter. Of this dry matter, organic matter constitutes about 96% with the remainder being mineral material [23]. The exact composition of PAW varies from one waste component to another. It is a function of the species, geographical location, environmental conditions and maturity of the pineapple [10]. PAW is mainly made up of cellulosic fibrous material. Cellulosic fiber provides extra-specific strength and rigidity to the plant material. Cellulose and hemicelluloses form a large portion of PAW. According to Paz-Arteaga et al. [23], PAW typically comprises 32.4% cellulose, 23.2% hemicelluloses and 19.4% lignin. Degradation of the carbohydrate compounds in PAW releases fermentable sugars that can be converted into a wide range of high-value products [1,3].

3.2. Proximate Composition

Proximate composition refers to the nutritional content of a substrate. It is generally estimated in the form of moisture, protein, fiber, ash, fat and carbohydrate content [10]. As shown in

Table 2. Proximate composition of pineapple waste (% dry weight).

Component	Moisture (%)	Protein (%)	Fiber (%)	Fat (%)	Ash (%)	Carbohydrate (%)	Reference
Peel	82–88	5–9	1–6	2–3	4–6	50–80	[10]
	6.22	5.14	17.4	1.14	2.42	85.07	[4]
Pomace	7.79	6.34	19.3	1.09	2.04	82.75	[4]
Stalk	8.5	4.25	19.73	-	8.54	-	[33]
Pulp	-	1.58	24.14	3.19	3.0	-	[30]
Crown	-	0.7	62.5	3.5	7.37		[30]
Core	84.9	3.6	9.14	2.35	1.7	83.03	[34]
	-	0.85	47.6	3.17	1.3	-	[30]
Cores and peels	71.1–92.2	3.1–5.0	5.0–42.0	2.4–4.8	-	35.0–83.0	[31]
Shell	-	0.75	65	2.0	1.5	-	[30]

, PAW contains large quantities of carbohydrates and high moisture content. Therefore, proximate composition is a critical parameter in designing prospective routes for the valorization of PAW into multiple products.

3.3. Ultimate Analysis

Ultimate analysis is a laboratory test that enables us to assess the suitability of a feedstock for product recovery. Ultimate analysis is also known as elementary analysis. It refers to a standardized measure of the exact composition of chemical elements like oxygen (O), hydrogen (H), C, nitrogen (N) and sulfur (S) present in different portions of PAW [7]. As given in

Table 3. Ultimate composition of pineapple waste fractions.

Fraction	Carbon (%)	Hydrogen (%)	Oxygen (%)	Nitrogen (%)	Sulfur (%)	Reference
Peel	49.3	5.7	-	0.6	-	[35]
Crown leaf	44.1	5.8	49.3	0.8	-	[7]
Crown and peel	44.95	5.5	47.65	1.68	0.22	[36]
Peel	47.39	6.13	40.64	1.08	-	[7]
Root	38.7	5.4	75.4	1.0	0.23	[7]
Stem	37.6	6.69	52.7	1.89	0.97	[37]
Leaf	40.5	6.91	50.3	1.78	0.36	[37]
Crown leaf	44.05	5.81	49.27	0.87	-	[7]

, the elementary composition of PAW can differ depending on the portion from which the test sample is extracted. According to Banerjee et al. [35], the peel was found to contain 43.9% C, 5.7% H and 0.6% N. Crown and peel waste comprised 44.95, 5.5, 47.65, 1.68 and 0.22% of C, H, O, N and S, respectively [36]. Mathew et al. [37] reported 37.6% C, 6.69% H, 52.7% O, 1.89% N and 0.97% S from pineapple stem. Ultimate analysis provides information on overall organic matter content and potential for product extraction from a feedstock. Results of this analysis enable appropriate mixing to achieve an optimal C/N ratio and nutrient balance in reactor medium [38]. Ran et al. [15] posited that a C/N ratio of 20:1 is suitable for the bioconversion of ensiled pineapple. A high N level indicates the presence of protein and acid amine in PAW [7]. Deficiency of N could probably restrict the growth and performance of microorganisms in the system [15]. A CHNS analyzer is used to determine the elementary composition of biomass waste.

4. Pineapple Waste Biorefinery in South Africa

The concept of biorefining is steadily rising as a tool for processing biowaste in South Arica towards a circularized economy. In 2018, the government of South Africa launched the Council for Scientific and Industrial Research’s (CSIR’s) Biorefinery Industry Development Facility (BIDF). This top-notch research facility was tasked to revive biomass industries and generate income by creating novel technologies or products from local biowaste [21]. The key remit was to tackle challenges encountered in the disposal of wood, pulp and paper waste that is currently being landfilled, burnt, stockpiled or dumped into the ocean [21]. Earlier on, the South African Biorefinery Research Platform was initiated by the Department of Science and Innovation (DSI) of South Africa. The task was to foster a culture of innovation and entrepreneurship in biomass utilization among universities, research councils and research institutions [21]. Given the state-of-the-art biorefinery facility in South Africa, it has a substantial impact on the local economy and job market.

Biorefinery refers to a facility or an interconnected system that integrates the sustainable transformation of biomass into energy and other marketable products [17,39]. It is comparable to a traditional petroleum refinery that fractionates petroleum into various fuels and other oil products [17,40]. As such, PAW valorization is symbolized by many biofuel products (biogas, bioethanol, biohythane, biobutanol, etc.) and other high-value products (food, pharmaceuticals, enzymes, etc.). In this scenario, a biorefinery is not a whole systems approach. However, PAW is converted into numerous products via separate routes and a network of facilities to minimize waste or achieve zero waste. Several technologies are integrated into a biorefinery to convert biomass into valuable goods. They are broadly classified into thermochemical, chemical, mechanical/physical and biochemical methods [17]. Mechanical (i.e., fractionation, particle size reduction and pressing) and chemical (i.e., hydrolysis, oxidation and esterification) platforms are used for the pretreatment of biomass prior to further processing into final products via thermochemical and biochemical pathways [18]. Like any other plant material, PAW is extremely resistant to biodegradation. Emerging research should focus on screening low-cost strategies for sustainable pretreatment of crop waste.

A schematic presentation of a PAW biorefinery for the production of energy and biochemicals is illustrated in Figure 2. Thermochemical conversion is one of the most convenient routes for processing PAW into biofuels. It involves repeated exposure of biomass to high temperature coupled with high pressure, with or without the addition of a solvent or catalyst. Thermochemical technology entails thermal cracking of organic matter to release bioenergy using direct combustion, gasification, pyrolysis and liquefaction. Resultant products are in the form of solid, liquid and gaseous fuels [18,41]. Gasification occurs under low oxygen and elevated temperature (800–900 °C) conditions to yield a blend of combustible gases [41]. Direct combustion is an ancient technology, in which biomass is burnt in the presence of oxygen to generate heat [17]. Biochemical processing, especially fermentation, is the most favorable technology for value-addition to PAW biomass. This route is characterized by low temperature and reduced reaction rates. Fermentation is an enzyme-assisted or microorganism driven platform that transforms biomass into high-value biochemicals [17,18]. Food, organic acids and pharmaceuticals have mainly been produced from fermentation. It is possible to convert PAW into biofuels like biohydrogen, biohythane, bioethanol and biobutanol using fermentation. Anaerobic digestion (AD) is another prominent biological route to transform PAW into energy, especially biogas and bioelectricity [17,18]. High-value products including bromelain, carbohydrates and polyphenols have been extracted and purified from PAW for potential application in food and therapeutic industries [21,35].

5. Energy Products from Pineapple Waste Biorefinery

5.1. Biogas

AD is currently being touted as the panacea to manage waste and convert it into energy. It is a biochemical process that transforms organic matter into biogas using a consortia of bacteria under anaerobic conditions. Temperatures ranging from 30 to 65 °C are adequate for the AD of organic matter [17,41]. Biogas is a mixture of 60–70% methane (CH₄), 30–40% carbon dioxide (CO₂) and other gases in trace amounts [42]. It can substitute fossil fuels for heating, power generation and transportation. The basic stoichiometric reaction of the AD process is shown in Equation (1) [41].

$$C_6{H}_{12}{O}_6\to 3{CO}_2+3{CH}_4$$

(1)

Biogas production by AD is a multi-step process involving hydrolysis, fermentation and methanogenesis. Hydrolysis converts complex molecules into monomeric units, while monomeric units are transformed into alcohols, acetic acids, fatty acids, hydrogen (H₂) and CO₂ during fermentation [43]. Biogas is generated from the products of fermentation during methanogenesis [42,43]. PAW is mainly composed of organic matter, hence it can be considered a potential feedstock for AD. It can be processed through solid-state AD due to its high total solid (TS) content. The TS content of fresh and dry PAW is estimated at 29 and 73%, respectively [15]. Authors have established that peels, pulp, core and crown from pineapple are the main sources of biogas [13,44]. According to Chulalaksananukul et al. [45], biogas production from PAW was reported in the range of 0.3 to 0.5 L g⁻¹ dry substrate and achieved a yield of 53% CH₄ content. Pineapple peels generated biogas yield varying from 0.41 m³ to 0.67 mL g⁻¹ VS with 41–65% CH₄ content [3]. These conversion rates can be surpassed by deploying suitable innovative strategies. It is astounding that the hydrolysis step is often rate-limiting in the AD of PAW. This is due to its lignocellulosic structure that is recalcitrant in nature. Pretreatment is an essential step to accelerate the AD and biogas potential of PAW. It is apparent that pretreatment of PAW can significantly improve CH₄ production. For example, Hamzah et al. [46] increased biogas and CH₄ yield by 23 and 44%, respectively, from subcritical water pretreatment of PAW. Alkali-pretreated pineapple peel was found to enhance biogas production by 36 and 91% compared to non-treated and acid-pretreated pineapple peels, respectively [47].

Low N content is also a technological hurdle to the AD efficiency of PAW. As a result, the AD of PAW as a sole substrate is insufficient to generate large volumes of biogas [13,44]. Researchers have proposed codigestion of PAW with other biodegradable substrates to improve biogas production. Animal manure is an organic waste that is highly rich in N and contains a diverse range of functional rumen flora. For instance, around 10 pounds of N have been reported from a ton of cow dung [44]. Therefore, it is a suitable cosubstrate to optimize the AD of PAW. Codigestion has been suggested to promote synergism, ensure nutrient balance, and provide buffering in biodigesters. This will ultimately enhance the overall digestion process and biogas yield of PAW [43,44]. As an example, a higher CH₄ yield of 17.19 mL g⁻¹ VS than control was obtained from codigestion of cow dung and PAW at a ratio of 1:2 and 12% TS content [44]. Anaerobic codigestion of pig slurry and PAW (20:80 v/v) increased CH₄ production by 87% compared to pig slurry as sole substrate [24]. On the other hand, codigestion of PAW with food waste was found to raise biogas production by 46% over monodigestion. Surprisingly, pig manure did not increase the biogas yield of PAW due to its inhibitory effect [48].

Digestate slurry is released as a by-product of the AD of PAW. It is about 90–95% of the feedstock fed into the bioreactor. This waste is rich in plant nutrients, thus appropriate disposal strategies are required to avoid environmental pollution. Using digestate waste as a bioresource for fertilizer production is one of the most sustainable strategies to promote agriculture and effectively manage waste. Anaerobic digestate can substitute mineral fertilizers in plant production systems [40,49]. Furthermore, digestate waste is a potent substrate for edible mushroom cultivation [40]. No studies were found in the literature regarding the use of PAW digestate for mushroom production. However, the use of digestate from the codigestion of dairy manure and food waste for Pleurotus djamor farming produced a harvest of 196.5 g bag⁻¹ with a biological efficiency of 39.9% [50]. Supplementing wheat straw with corn digestate yielded 5.1% more fruiting bodies during full-scale cultivation of P. ostreatus [51].

Mass balance is a crucial aspect that allows a complete assessment to ensure accurate mass flow at each step of a bioprocess [52]. Published research to establish the mass balance for conversion of solid PAW into biogas could not be retrieved from the literature. However, Castro et al. [53] estimated the mass balance for the AD of juice extracted from 250 tons d⁻¹ of pineapple leaves. Processing 250 tons d⁻¹ of pineapple leaves produced 192 tons d⁻¹ of juice together with by-products including wet fiber (46 tons d⁻¹) and solid leaf matter (13 tons d⁻¹). Codigestion of the pineapple juice with 48 tons d⁻¹ of chicken manure generated 22,000 m³ d⁻¹ of biogas.

5.2. Biohythane

A gaseous mixture of H₂ and CH₄ can be produced from the fermentation of PAW. The primary product of this reaction is biohythane. Figure 3 represents key process steps for the biosynthesis of biohythane from PAW. Typically, biohythane consists of 20% H₂ and 80% CH₄ by volume. It is a combustible gas that can replace petroleum fuels in automobile engines [54]. Using biohythane could possibly diminish the release of toxic emissions, mostly nitrogen oxides and carbon monoxide (CO) into the environment. Biohythane is produced by coupling dark fermentation with AD in a conventional two-stage reactor system [54,55]. The process is facilitated by H₂- and CH₄-producing thermophilic microbes. Hydraulic retention time (HRT) is a critical operational factor in biohythane production. It affects the bioconversion of volatile solids into a gaseous mixture. Hydrogenic bacteria favor short HRT and H₂ synthesis. Chu et al. [54] optimized the HRT for a two-stage fermentation of pineapple peel waste juice for biohythane production. The highest H₂ and CH₄ production rates of 599.0 and 174.6 mL (L-d)⁻¹ were observed at optimum HRT of 4 h and 3 d at a substrate concentration of 66.15 and 37.7 g COD L⁻¹, respectively.

The invention of a single-stage bioreactor system marked a paradigm shift away from traditional two-stage biohythane fermentation. Single-stage fermentation eliminates challenges including capital, energy and labor demands associated with the two-stage system [56]. Nguyen et al. [56] demonstrated the ability to produce biohythane from a combination of swine manure and PAW. A novel single-step anaerobic fermentation process containing hydrogenic and methanogenic bacteria in a two-chambered system was used. Results showed maximum gas production after 6 h HRT, achieving H₂ and CH₄ production rates of 1240 and 812 mL (L-d)⁻¹, respectively. In another single-stage AD process, biohythane synthesis from pineapple peel supplemented with palm oil mill effluent sludge was performed using Metharnosarcina mazei. It was found that a peak H₂ production rate of 1.93 L (L-d)⁻¹ and a CH₄ production rate of 0.67 L (L-d)⁻¹ could be achieved with a biohythane yield and CH₄ yield of 1.18 and 0.55 mL (L-substrate)⁻¹, respectively [57]. It is important to understand and control the parameters, including C/N ratio, HRT, pH, temperature, nutrient availability, substrate type and substrate concentration, that affect process performance during biohythane fermentation [54,56].

5.3. Biohydrogen

H₂ is a gaseous fuel produced from the fermentation of organic waste using algae, protists, photosynthetic bacteria and archeal species [42]. PAW can be envisaged as a low-cost, clean and sustainable feedstock for H₂ production. H₂ is a multifunctional energy carrier. Its energy density is approximately 142 kg g⁻¹ [58]. H₂ is a suitable biofuel for fuel cells, heat, power and electricity generation. Biologically, H₂ production is divided into two processes: dark fermentation and photo fermentation. Dark fermentation is preferred over photo fermentation because it can synthesize H₂ from several organic feedstocks with little to no interruptions [59]. Dark fermentation directly converts solar energy into H₂. It uses microbes to break down organic matter into volatile fatty acids (VFAs) coupled with electron transfer to hydrogenase which converts a proton into H₂ [60,61]. Dark fermentation involves the use of dark fermentative bacteria such as Clostridium and Enterobacter sp. The overall reaction for H₂ production is shown in Equation (2).

$$C_6{H}_{12}{H}_6+{6H}_{2}O \to {12H}_2+6{CO}_2$$

(2)

Integration of dark fermentation with photo fermentation in a single reactor offers an alternative route for H₂ synthesis. In this mixed culture fermentation, photo fermentation is performed by Rhodobacter sp. [62]. A simplified process flow diagram for combined biohydrogen production is depicted in Figure 4. H₂ production is severely affected by substrate type, substrate concentration, pressure, pH, temperature, inhibitors, organic loading rate, HRT and reactor configuration [60]. Maximum H₂ yields of 3.5 and 2.4 mL STP g⁻¹ VS were achieved at a substrate concentration of 26.4 g L⁻¹ VS from dark fermentation of the peel and core, respectively [60]. Pretreatment is an essential step to enhance H₂ production from PAW. Exposure of pineapple fruit waste to bacteria strains screened from wastewater produced maximum H₂ yield (836.33 mL) from alkaline-pretreated hydrolysate after fermentation by Bacillus altitudinis [58]. H₂ production from PAW could be a viable enterprise at large-scale in the foreseeable future. For example, a feasibility study on H₂ production from liquid PAW at full-scale was investigated [62]. It was established that an investment of 3000 metric tons per annum of H₂ could last for 20 yr with a return on investment of 68%. Recycling intermediary and waste products of H₂ production can help to improve the economic value of the process. These products can be further processed into useful bioproducts such as organic acids, VFAs and alcohols. VFAs are a source of biofuels and bioplastics [42].

5.4. Bioethanol

Bioethanol is one of the world’s leading biofuels that can replace gasoline in transportation. It is ethanol or ethyl alcohol manufactured using biochemical processes. Bioethanol is blended with unleaded gasoline to form a fuel called gasohol. E10 produced by mixing 10% ethanol and 90% gasoline is the most common type of gasohol. Nowadays, internal combustion engines operating on pure or anhydrous ethanol are becoming increasingly important. Its prospective use as a precursor or solvent in the production of biochemicals and biocomposites cannot be overlooked [42]. Brazil and USA are the primary producers of ethanol in the world [40]. Other producers include Ukraine, Pakistan, South Africa, and some European Union members and other Asian countries [26]. Feedstocks for bioethanol production are categorized into three main groups: sugary feedstocks (sugar cane, sugar beet, sweet sorghum), starch-containing feedstocks (maize, potatoes, wheat, etc.) and lignocellulosic feedstocks (wood, grass and other biomass) [63]. Current research trends focus on second generation feedstocks as a poverty alleviation strategy. PAW is a renewable second-generation feedstock for bioethanol synthesis. Its lignocellulosic structure calls for pretreatment to convert polymeric carbohydrates into simple sugars. Monomeric sugar units are fermented to ethanol using yeast. Yeast contains an enzyme called zymase, which acts as a catalyst to convert glucose into ethanol and CO₂ [64]. Chemical Equation (3) summarizes the bioconversion process for bioethanol production. The most popular yeast strain for ethanolic fermentation is Saccharomyces cerevisiae. S. cerevisiae multiplies fast under anoxic conditions with improved fermentative capacity, and resistance to high alcoholic and inhibitor concentrations [64]. Hydrolysis is a critical step in bioethanol production from cellulosic material. However, the recent market prices of industrial cellulases favor saccharification using acid catalysts [65].

$$\begin{array}{c}{2C}_6{H}_{12}{O}_6 \left(aq\right)\\ Glucose\end{array} \stackrel{zymase}{\to } \begin{array}{c}{2C}_2{H}_{5}OH \left(aq\right)\\ Ethanol\end{array} + \begin{array}{c}{2CO}_2\left(g\right)\\ Carbon dioxide\end{array}$$

(3)

There are three main pathways of ethanolic fermentation. These include direct fermentation (DF), consecutive hydrolysis and fermentation (CHF), and simultaneous saccharification and fermentation (SSF) [64,66]. Advantages of SSF over CHF are well-established in the literature. SSF averts the accumulation of hydrolytic products (e.g., cellobiose and glucose), thus limiting the rate of continuous hydrolysis of the feedstock [64,66]. Ethanolic fermentation is followed by distillation to separate ethanol from fermentation broth and purification via dehydration. Tropea et al. [66] obtained maximum ethanol production of 3.9% (v/v) through SSF of PAW for 30 d. This was equivalent to 96% of the calculated theoretical yield. Fermentation of PAW cell wall sugar by S. cerevisiae produced maximum ethanol concentration of 3.9%, corresponding to 85% of the theoretical value [64]. Casabar et al. [26] evaluated the effect of alkaline pretreatment and hydrolysis with Trichoderma harzianum on total and reducing sugar concentrations, and a bioethanol yield of pineapple fruit peel. Samples pretreated with 0% NaOH were found to produce the highest total and reducing sugar concentrations of 458.44 and 279.67 g L⁻¹, respectively. Microbial hydrolysis and 48 h fermentation of these samples produced a bioethanol yield of 5.98 g L⁻¹. In another study, pineapple peelings were subjected to pretreatment with 5% H₂SO₄ producing 0.3% (w/v) reducing sugar. Fermentation of pretreated hydrolysates generated the highest ethanol concentration of 9.13% [67]. The highest ethanol yield of 9.69 g L⁻¹ was obtained after 72 h of pineapple peel fermentation by S. cerevisiae [68]. Fermentation of sugar syrup recovered from hydrothermally pretreated, and enzyme hydrolyzed pineapple leaves at 30 °C for 72 h using S. cerevisiae resulted in ethanol production with higher than 91% fermentation efficiency [69]. Gil and Maupoey [63] found ethanol concentrations of 5.4, 4.7 and 4.9% v/v from SSF, DF and CHF of PAW, respectively.

According to a mass balance analysis conducted by Mund et al. [70], a total of 100 g dry pineapple leaf can be converted into 21.2 mL of ethanol (Figure 5). The premise of this theoretical yield was based on 100% conversion efficiency of glucose to ethanol. Biomass was pretreated using cellulose solvent and organic solvent lignocellulose fraction (COSLIF) to produce 51.48 g of pretreated biomass. Glucose released from COSLIF pretreatment was degraded with an enzymatic mixture of 10 FPU cellulase and 10 IU β-glucosidase [70]. Similarly, the mass balance calculated based on 1 g of pretreated pineapple leaf revealed an enhanced ethanol yield. Pretreatment of 1 g of leaf biomass using ultrasound-assisted, choline chloride: ethylene glycol (ChCl-EG) generated a solid fraction that contained 526.7 mg g⁻¹ cellulose, 13.51 mg g⁻¹ hemicellulose and 51.7 mg g⁻¹ lignin. Saccharification of the solid fraction using 20 U g⁻¹ cellulase produced 566.1 mg g⁻¹ of total reducing sugars. A total yield of 229.4 mg g⁻¹ ethanol was obtained after the sugars were subjected to fermentation by S. cerevisiae and Scheffersomyces stipitis [52].

5.5. Biobutanol

Biobutanol is one of the contemporary biofuels and biochemicals that can be derived from plant biomass [72]. Its unique properties such as high energy content, reduced volatility and hygroscopicity, high energy density, low corrosiveness and the aptitude to formulate around 85% blend with gasoline make it a sustainable biofuel for the next generation. More so, biobutanol is postulated to substitute for petrol in engines without any operational adjustments [73]. Conventionally, biobutanol is synthesized from plant biomass via the acetone–butanol–ethanol (ABE) fermentation pathway using Clostridium species [28,72,73]. However, it has been observed that elevated concentrations of biobutanol are lethal to microbial cells. As such, the titer volumes of biobutanol rarely exceed 13 g L⁻¹. Recent studies seek to genetically engineer microbes so that they can tolerate high biobutanol concentrations with improved bioconversion rate [74,75]. Several biotechniques to improve biobutanol fermentation strains are widely reported in the literature. These include mutation breeding, genome design, evolutionary engineering and synthetic biology [75]. Clostridium spp. can exploit monomeric and polymeric sugars (pentoses and hexoses), organic matter of agricultural and industrial origin, algae, C1 gases, etc., as fermentation feedstocks [73]. The ABE route is a multi-step process that involves the selection of feedstock, pretreatment, detoxification and fermentation [28,72,73]. It is important to recognize that the choice of feedstock is an expensive criterion that can consume about 60–70% of the total biobutanol production cost. Investment in low-cost raw materials like cellulosic biomass could provide a solution to this challenge [28,72,73]. PAW can be scrutinized as an alternative feedstock for bioproduct development. Undoubtedly, its high cellulose, hemicelluloses and lignin content makes it an ideal feedstock for biobutanol production. Nonetheless, the economic viability of PAW as a biobutanol feedstock is hampered by the high processing cost associated with biomass pretreatment. Non-selective pretreatment has been cited as the primary source of inhibitory compounds such as furan derivatives, aliphatic acids and phenolic compounds [73]. Several pretreatment mechanisms have been proposed to detoxify the ABE fermentation broth [73].

It is noteworthy that PAW is scantly reported as a feedstock for biobutanol production. As an example, Khedkar et al. [28] demonstrated the ability of pineapple peel to be metabolized by C. acetobutylicum B 527 via the ABE fermentation route. The highest ABE titer of 5.23 g L⁻¹ along with 55.6% substrate utilization was achieved in this study. Imaji et al. [76] evaluated the potential of biobutanol production from pineapple peel using sole and mixed strains of C. acetobutylicum and C. beijerinckii. It was discovered that biobutanol yields in the range 1.2–5.6 and 5.8–6.5 g L⁻¹ can be recovered from single and co-culture broth, respectively. The most common bioreactor design for the ABE fermentation pathway is the batch mode operation [75]. The type, composition and concentration of a substrate should be closely monitored and optimized as they can negatively impact the yield, productivity and efficiency of biobutanol [73].

5.6. Pyrolytic Fuels

Thermochemical conversion uses a catalyst in the presence of heat to produce fuel products such as syngas, bio-oil and biochar. PAW is one of the waste biomasses that can be used as fuel or feedstock in thermochemical conversions such as pyrolysis, gasification, combustion and hydrothermal liquefaction. The roles of catalysis in the thermochemical degradation of biomass waste are discussed in various studies [77]. Pyrolysis is a technology that can be used to transform PAW into valuable biomaterials such as biochar, activated carbon, bio-oil and gases [77,78,79]. In this process, a combustible portion of organic waste is thermally degraded at a temperature range of 400–700 °C in the absence of air [78]. The high moisture content of PAW is a challenge to pyrolysis as more energy is required to remove the moisture [77]. Many components of PAW (e.g., peels and core) can be utilized via pyrolysis to produce biochar. Biochar is a potent absorbent of many organic substances in the environment. It has been widely used to abate industrial pollution through the removal of dyes from wastewater [79]. For example, biochar produced from the pyrolysis of PAW at 300–600 °C for 0.5 to 1 h attained a maximum dye removal of 83.59% [79].

During liquefaction, the thermal depolymerization of PAW in the presence of water at moderate temperature (250–370 °C) and elevated pressure (10–25 MPa) releases crude-like oil (bio-oil) [77,78]. Bio-oil is a liquid fuel with widespread industrial applications. It is an alternative fuel for heat and power generation in engines or boilers. Biocrude is enriched in many essential chemicals. Upgrading and refining biocrude is a source of many value-added industrial products including acetic acid, glycolaldehyde, acetol, phenols, organic binder and polyurethane foams [77,78]. However, hydrothermal liquefaction is often associated with low conversion efficiency and poor bio-oil quality [77]. Gasification is a renowned technology to convert PAW into syngas under high pressure and temperature (≥374 °C) conditions. Unlike biocrude, syngas is nitrogen-free intermediary fuel for internal combustion engines. Compounds containing nickel and C-supported platinum-ruthenium are frequently used as catalysts for gasification [77]. Syngas is mainly a mixture of H₂ and CO [80]. It can be used as an intermediate for the synthesis of industrial chemicals and other fuels. Syngas acts as the main backbone for many chemical processes such as the Fischer–Tropsch process, and the synthesis of methanol and dimethyl ether. The Fischer–Tropsch route synthesizes higher hydrocarbons [80]. High pressure maintenance, equivalence ratio and energy demand have been regarded as the major problems in adopting PAW gasification [80].

6. High-Value Products from Pineapple Waste Biorefining

6.1. Bioactive Compounds

The demand for natural products in the food, health and pharmaceutical sectors is gaining interest in a circularized economy. PAW is rich in phytochemicals; hence it can be a potential source of nutraceuticals and other functional ingredients with pharmacological benefits [14]. Bioactive compounds with pharmacological applications can be derived from value-added compounds such as dietary fiber, pectin, organic acids, starch, enzymes, vitamins, minerals and phenolic compounds [14]. The availability of bioactive compounds may vary depending on the component of PAW. For instance, citric and ferulic acids are concentrated in leaves, ascorbic acid in the core, bromelain in leaves, stem and peels [81]

PAW is an important source of insoluble dietary fiber (DF) with industrial and health benefits [2,11]. DF is made up of a mixture of polymeric non-digestible carbohydrates such as polysaccharides and oligosaccharides. The gamut includes cellulose, hemicelluloses, pectic compounds, gums, starch and inulin linked to non-carbohydrate substances [11]. DF helps to lower type 2 diabetes and heart problems in humans. It is also an essential ingredient for the production of bioplastics, hydrogels and nanofiber materials [11]. Multiple methods including mechanical (milling and grinding), chemical (washing with water, alkali, acidic and alcoholic) and biological (enzymatic) have been used to extract DF from different fractions (pomace, peels and core) of PAW [2,11].

Pectin is a useful by-product that can be extracted from PAW, especially peels, pomace and core. It is a natural carbohydrate polymer that is widely utilized as a functional additive in the food and beverage industry. It is generally used as a thickener, emulsifier, gelling agent and stabilizer in various food industries [82,83,84]. Pectin can be utilized as a bioresource for production of edible films in the pharmaceutical sector. It has been observed to possess some medicinal properties like anticancer, cholesterol-lowering properties and regulation of blood glucose levels [84]. Shivamathi et al. [84] extracted pectin from pineapple peels using an ultrasound-assisted approach. Optimizing the process using response surface methodology achieved the highest pectin yield of 16.24% at 70.83 °C, pH 1.0 and liquid-to-solid ratio of 15.20 mL g⁻¹ after 21.88 min. A purity index of 89.5–90.0% was attained upon purifying extracted pectin using anion exchange chromatography (diethylaminoethyl cellulose) [84]. A pectin yield in the range of 3.88 to 13.06% was obtained from pineapple peels using acid extraction and ethanol precipitation. The yield of 13.78% was achieved under optimal conditions of 82.63 °C, pH 1.83 and 65 min of extraction [82]. A pectin yield of 1.02–2.12% was reported from pineapple peels on exposure to microwave heat extraction. The extracted pectin proved to be a high-quality biomaterial that could be used as a natural solvent and plasticizer in edible film production for the food industry [83].

Phenolic compounds refer to a collection of secondary metabolites that are extracted from plant materials. These compounds are some of the most crucial biologically active phytochemicals. They have been identified as possessing biological and health properties such as antioxidant, antiallergenic, antiatherogenic, anti-inflammatory, antimicrobial, antithrombotic, cardioprotective, anticancer and vasodilator activities [21,85]. Tannic acid, sinapic acid, caffeic acid, p-hydrobenzoic aldehyde, p-hydroxybenzoic acid, syringic acid, salicylic acid, trans-cinnamic acid and myricetin have been reported to be the most prominent individual phenolics from pineapple fruit [85]. Gallic acid, epicatechin, catechin and ferulic acid were reported to be common phenolic compounds in pineapple peel [86]. Extraction is a fundamental step in collecting PAW phenolic compounds. The choice of extraction technique depends on factors such as temperature, plant portion, pressure and the type of solvent [87]. A total of 2117.65 mg (100 g)⁻¹ caffeic acid was extracted as the main phenolic compound in pineapple peel using an ultrasound-assisted method [88]. Using Soxhlet extraction with methanol as solvent, the highest ferulic acid concentration, phenolic content, antioxidant activity and percentage yields of 0.7696 g (100 g)⁻¹, 2.365 mg g/GAE, 45% and 90.5% mg, respectively, were observed from pineapple peel powder [86]. Reflux extraction of pineapple peel produced gallic acid (31.76 mg (100 g)⁻¹), catechin (58.51 mg (100 g)⁻¹), epicatechin (50.00 mg (100 g)⁻¹) and ferulic acid (19.50 mg (100 g)⁻¹) as the major polyphenolic compounds [89]. Solid-state fermentation of PAW using Aspergillus niger was found to release free phenolic acids. Phenolic content and antioxidant activity were increased by 72 and 25%, respectively [23].

Bromelain is one of the most beneficial enzymes that can be extracted from different parts of pineapples. The economic potential value of the enzyme is estimated at USD 2400 kg⁻¹ [23]. Bromelain is a proteolytic group of enzymes well-known for its therapeutic and pharmaceutical properties, with a lack of known side effects on the human body. It is believed to have some antithrombotic and fibrinolytic properties, and therapeutic effects on the reverse inhibition of platelet aggregation, bronchitis and surgical trauma. It also promotes drug absorption during treatment. Bromelain can degrade collagen in meat; thus, it is an excellent meat tenderizer in the food processing industry [11]. It is also used as an anti-browning agent in the manufacture of infant formulas and for proteolytic degradation in textile industries [81]. Some physical and chemical techniques are widely available to extract and purify bromelain from PAW biomass. However, the most appropriate approach to generate a highly active and pure form of bromelain has not yet been discovered [90]. Optimization of bromelain extraction using a polysaccharide complex formation revealed a high recovery yield of 80–90% active bromelain from pineapple stems and peel [91]. The extraction of bromelain from pineapple peel using sodium phosphate buffer yielded maximum bromelain activity and a protein concentration of 0.75 and 1.60 mg mL⁻¹, respectively [92]. A study was performed to determine the most favorable conditions for bromelain extraction from pineapple stems. Maximum bromelain specific activities of 1.45, 10.38 and 12.05 CDU mg⁻¹ were reported from crude extract, purified extract and dried extract, respectively [93]. In addition, microbial enzymes such as cellulases, xylanases and pectinases with commercial value can be produced via fermentation using PAW substrate [11].

PAW is a potential ingredient for probiotic production and may possess some prebiotic effects. Pineapple peel and pomace were investigated as media to promote the growth of probiotic bacteria such as lactic acid bacteria and bifidobacteria. These wastes were observed to support the growth of Lactobacillus paracasei and L. acidophilus and can be utilized as functional food elements [4]. The mechanical extraction of starch from pineapple stems provides a source of carbohydrates for food and non-food applications [94]. Fermentation of pineapple peel, crown and pulp can produce a variety of organic acids such as citric acid, lactic acid and ferulic acid. Citric acid is the most prevalent organic acid that can come from PAW [95]. It is mostly synthesized from PAW substrate in solid-state fermenters using A. niger [96]. Organic acids are used as natural acidifiers in food and beverages, preservatives in the cosmetic industry and precursors for many chemical synthetic processes [11,95,96].

6.2. Single-Cell Protein

SCP production is an innovative and sustainable technology that can provide a protein-rich source. SCP is described as dead, dried microbial cell biomass or total protein originating from single strain or coculture fermentation [97]. While the worldwide demand for proteins is continuously rising, the market share of SCP is expected to be more than USD 18.5 billion by 2030 [98]. The growth of microorganisms has been preferred due to their rapid growth rate on different media and high yield. A wide range of fermentation substrates such as industrial waste, methanol, oil, food waste, agricultural and fruit waste, etc., have been utilized for SCP production. Amongst them, the most widely accepted substrates for SCP production are sugar-rich substrates. PAW is an excellent source of C that can support fermentation by a plethora of microorganisms like yeasts, bacteria, filamentous fungi and algae to produce SCP. The suitability of PAW is premised on the fact that it is an abundant, non-toxic, regenerable, non-exotic and low-cost material [97,98,99]. Most of the microorganisms utilized for the SCP technology are generally regarded as safe (GRAS) [98]. Harvesting and drying microbial cells produce a protein-rich powder that can be utilized in animal feed or as food supplements. Nowadays, many yeasts have been used to substitute conventional proteins such as fish meal, soybean meal and plant-derived protein. In addition, dry cell biomass has been proposed for use in the paper and leather industries, and as a foam-stabilizing agent and packaging material [98]. SCP may provide a promising solution to problems of malnutrition and environmental degradation. SCP offers many advantages over conventional protein. These include lower cultivation cost, minimal resource use, higher nutritional profile, safer protein source and a wider range of amino acids [98,99]. SCP comprises large amounts of proteins, essential amino acids (lysine and methionine), fats, carbohydrates, minerals and vitamins. Proteins represent 60–82% of SCP on a dry weight basis. However, SCP has limited scalability for human food due to the presence of significant amounts of nucleic acids (~10%) and poor cell wall digestibility. Low cell wall digestibility could result in limited bioavailability of proteins, allergic reactions, intestinal and skin diseases [97,98].

Studies have been performed to demonstrate the ability of microbial organisms to produce SCP from PAW. Yeast is the most extensively studied microorganism for SCP production. Important strains include S. cerevisiae, Candida utilis, C. arborea and C. pulcherrima [100]. As an example, the bioconversion of PAW into SCP was investigated using two yeast strains, S. cerevisiae and C. tropicalis. The biomass composition was found to be directly proportional to the concentration of PAW [100]. In another study, pineapple peel was evaluated for its ability to produce SCP under liquid-state fermentation using palmyrah toddy (Borassus flabellifer) harboring a natural mixture of yeasts and bacteria. Findings reported dry biomass yield and crude protein content of 9.4 g L⁻¹ and 49.7%, respectively [97]. Growing of C. utilis on pineapple cannery effluent as a sole C and energy source yielded freeze-dried yeast cells enriched with 55.3% crude protein, 51.2% true protein and 27.4% carbohydrate [101]. Using extracts of pineapple waste as a substrate for SCP production, the results showed a high yield and doubling time of 3.01 kg m⁻³ and 1.108 h, respectively, at 60% (v/v) substrate concentration [102]. The production of SCP from hydrolyzed pineapple peel was studied using fungi (Trichoderma viride). The results showed the highest dry cell weight of 0.66 g (100 mL)⁻¹ after product recovery [99].

6.3. Animal Feed

PAW contains sugars, fiber and organic acids; thus, it can be transformed into shelf-stable products with high economic value, such as animal fodder [3,10]. It can be used to boost the nutritional value of feedstuff for ruminant, poultry and fish diets. PAW may be incorporated in animal feed in a variety of ways depending on the type of livestock. It can be compressed into feed blocks, densified into pellets or directly fed as a source of roughage for ruminant animals [10,103]. A feed block with a moisture content of 8.7% and stable shelf-life was developed from compacting PAW (i.e., peels, crown and pomace) and paddy straw using molasses as a binding agent [104]. The high fiber content in pineapple leaf pellets may help to enhance the quality and quantity of milk in dairy feedlots [105]. The densification of pineapple residue into pellets with a moisture content of 35% showed the pellets’ friability, bulk density, true density and porosity in the ranges of 0.85–1.22%, 303.31–345.34 kg m⁻³, 1505.65–1520.35 kg m⁻³ and 77.022–80.05%, respectively [106]. Fermentation has been proposed as a strategy to enrich the nutritional value of PAW for poultry and cattle feeding [10,103]. In this context, the solid-state fermentation of pineapple peel was performed using a mixed culture of lactic acid bacteria (L. plantarum) and yeast (S. cerevisiae) for fattening bulls. Feeding fermented pineapple residue to Simmental bulls showed a significant difference in growth performance, meat quality and bacteria rumen characteristics compared to control units [107]. Some authors suggested ensiling to be a more attractive option to improve the nutritional content and digestibility of PAW for sustainable stock feed production [3]. A positive linear correlation was found between the quantity of pineapple peel, and the nutritional characteristics of forage sorghum, such as dry matter, crude protein, mineral matter, extract, soluble carbohydrate, cellulose, lignin and digestibility of dry matter on exposure to ensilaging for 65 d in experimental silos [108].

6.4. Vermicompost

Vermicomposting is a mature technology used to recover nutrients from PAW (peel, core, crown and leaves) [109]. It is considered a sustainable option to replace chemical fertilizers that can be detrimental to the environment. In this process, the combined action of microbial organisms and earth worms is deployed to degrade biomass waste into an organic fertilizer (vermicompost) [110]. The application of vermicompost to soil creates an excellent soil conditioner that can improve soil productivity and boost agricultural production [3]. Vermicomposting of PAW was reported to increase bulk density, ash content, pH, electrical conductivity, total phosphorous (P) and total potassium (K) by 92, 25, 10, 14, 21 and 28%, respectively. Notable was the reduction in moisture content (1%), volatile organic matter (12%), organic C (81%) and C/N ratio (76.4%). Results indicated a newly stabilized and mineralized organic product [111]. Zziwa et al. [109] demonstrated the ability to recover nutrients from pineapple peel using batch and continuous vermidigesters. Biomass waste was observed to decrease by 60 and 54% in batch and continuous setups, respectively. Earthworm biomass was higher by 57% for batch and 129% for continuous setups than control. Both setups lowered N and K content, whereas P content was significantly enhanced. It was concluded that the continuous system could be a better option for vermicomposting PAW, compared to batch setup [109]. In a pilot study, vermicomposting of PAW was performed using local earthworms (Eudrilus eugeniae Kinberg) in Ghana. The resultant product attained a pH range of 7.2–9.2, which is generally recommended for a high-quality vermicompost. The vermicompost was nutrient-rich with total N, total P, total K and C/N ratio of 0.4, 0.4, 0.9% and 9–10, respectively. Microbial contaminant loads of bacteria (Escherichia coli and Salmonella) and fungi (Aspergillus) decreased by 31–70 and 78–88%, respectively [110].

7. Pretreatment of Pineapple Waste for Biochemical Platforms

The structural complexity of PAW is an obstacle to the successful implementation of full-scale PAW-based biorefinery programs [112]. Generally, biochemical processes are often slow due to lack of access for microbes or enzymes to easily degradable matter. Therefore, pretreatment is quite common practice in many biochemical conversions. A few pretreatment mechanisms have been developed as a remedy to overcome steric hinderances in PAW biorefineries. PAW can be pretreated using physical, chemical and biological methods [70,113]. Pretreatment disrupts the amorphous structure of lignin enabling cellulose and hemicelluloses to be more prone to enzymatic and microbial degradation [46]. Numerous studies have demonstrated that pretreatment can significantly enhance the efficiency of enzymatic hydrolysis and microbial decomposition of PAW, making it a promising approach to valorize this abundant biowaste [46,112,113,114].

Physical methods comprising milling, microwave irradiation [112,113], hydrothermal treatment [46,68], steam explosion [115] and ultrasonication [116] have been examined. Low-pressure steam heating (LPSH) at 60 kPa and biomass loading of 2.5% for 20 min has been shown to be more efficient for 60.9% lignin removal from PAW. Around 77.6% cellulose and 91.0% hemicelluloses were successfully recovered from the biomass [114]. Optimal conditions for LPSH were observed at a biomass loading of 5% (w/v) and 80 kPa after 30 min of PAW pretreatment. About 46.9% of the lignin was removed from the biowaste, underscoring the feasibility of this method for sustainable biomass treatment [117]. Arifan et al. [20] obtained an optimum biogas yield of 357.2 mL g⁻¹ VS after thermal pretreatment of PAW at 60 °C for 25 min. Highest methane content of 67.3% was achieved at 100 °C after 45 min of pretreatment [20]. A total of 23.7% xylo-oligosaccharide and 18.3% gluco-oligosaccharide were derived from hydrothermally pretreated pineapple leaves at 160 °C for 60 min [118]. The production of cellulase inhibitors constrains physical methods to hydrolyze lignocellulose [112].

PAW can be pretreated with chemicals such as acid, alkali, organosolv and ozonation to fractionate holocellulose, making more cellulose available to biodegradation [71]. Dahunsi [119] reported 51% higher biogas yield than control from alkali-pretreated (H₂O₂) pineapple peel. Compared to acid pretreatment with H₂SO₄, alkali pretreatment produced 67% more biogas [119]. Combining organosolv (mixture of water and ethanol) with H₂SO₄ to pretreat pineapple peel produced the highest amount of xylose at 9.13 g L⁻¹. Other sugars like cellobiose, arabinose and glucose were found in the pretreated residue at concentrations of 1.38, 3.11 and 3.76 g L⁻¹, respectively [120]. Maximum sugar yields of 2.68 g L⁻¹ xylose and 8.84 g L⁻¹ glucose were obtained after pretreatment of pineapple leaves with 12% (v/v) H₂SO₄ for 80 min [121]. Meanwhile, pretreatment of pineapple leaves with 0.3 N H₂SO₄ accomplished maximum H₂ yield of 0.8 mL g⁻¹ VS [58]. Chemical agents have pitfalls such as high cost and generation of toxic inhibitors [122]. This limits their scalability to full-scale operations.

Biological pretreatments use fungi, bacteria or enzymes to break down complex organic compounds to produce sugars for biogas, bioethanol, organic acids and other products. The method is ecofriendly, yet it is slow [122]. The dearth of research findings in published literature suggests that biological pretreatment of PAW is an infant technology. Pineapple leaf waste was subjected to laccase pretreatment at an enzyme concentration of 100–1000 IU mL⁻¹, temperature of 30–60 °C and pH of 3–10 for a duration of 2–12 h. The highest lignin removal of 78.6% (w/w) and reducing sugar concentration of 492.3 mg g⁻¹ was recorded after 5.3 h of treatment [123]. Hydrolysis of pineapple leaf waste using endo-1,4-xylanase at pH 5.5 and a temperature of 50 °C resulted in maximum reducing sugar content of 70.9 mg L⁻¹. Optimal enzyme loading (0.5%, w/v) and residence time (45 min) yielded reducing sugar concentrations of 60.1 and 72.0 mg L⁻¹, respectively [124]. Recently, a microbial consortium has been proposed to be more efficient than a sole microorganism in improving the hydrolysis of organic matter [42]. At present, the largest biorefinery facility set up in South Africa is dealing with value-addition to wood, pulp and paper waste [21]. It can be envisaged that wood material is one of the most recalcitrant substances on earth. Hence, the choice of pretreatment should be the epicenter of research and development towards upgrading the South Africa biorefinery revolution.

8. Conclusions

Enormous quantities of biomass waste are generated from pineapple horticulture in South Africa. PAW may lead to serious environmental threats if not managed by environmental protection regulations. This calls for advancements in PAW processing while providing sustainable solutions to environmental management. PAW contains a lot of organic matter which can be converted into biofuels and other high-value products in a biorefinery. There are several techniques reported in the literature to promote PAW biorefining. These include biochemical, chemical, physical and thermochemical technologies. The integration of biofuel and biochemical production in a biorefinery is considered a suitable strategy for PAW valorization. In this biorefinery, a network of facilities uses distinct routes to produce a diverse range of bioproducts to achieve little or zero waste.

This paper reports on 10 products that can be derived from PAW biorefinery. The products encapsulate biogas, bioethanol, biohythane, biobutanol, biohydrogen, pyrolytic products, bioactive compounds, SCP, animal feed and vermicompost. The products are divided into two broad categories: energy products and biomaterials. Bioactive compounds such as bromelain, phenolics, organic acids, pectin, DF, probiotics, starch, etc., are well-known biochemicals with industrial, pharmaceutical and health benefits.

The gist of this paper is not to find contrast between biorefinery routes nor to identify the best-fit option for resource recovery from PAW, but to provide all the viable pathways for its valorization. Taking all the options together, the biochemical platform offers more advantages than most conversion pathways, yet it is nascent and time consuming. Selecting the most appropriate pretreatment method is pertinent to successful development of full-scale biorefineries. Otherwise, pretreatment requirements could add an extra cost to the entire biochemical process. Thermochemical pretreatment needs high power and energy due to the use of physical catalysts and heat. Chemicals are costly and they can release toxic compounds into the environment. Fermentation and AD are considered to be the most favorable routes for converting PAW into valuable goods. These pathways are affordable, sustainable and environmentally friendly. AD is a mature technology. It produces biogas, an alternative clean fuel for heating, electricity generation and combustion engines. Most biorefinery technologies are still at different stages of development. Biorefinery systems need to be enhanced and promoted towards full-scale valorization in South Africa. It is feasible to revamp the country’s industry and replace chemical products with sustainable products using modern technological platforms.

9. Future Recommendations

The biorefinery concept is an interdisciplinary phenomenon, integrating innovative insights from biology, chemistry, engineering and environmental science. A multistakeholder approach is essential to fully adopt this technology. Considering that South Africa has established a world-class biorefinery research facility, it is a prerequisite to engage expertise from various fields and advance the knowledge. This could help to address research gaps and promote technology transfer to large-scale operations. The possible way to accomplish this goal is to draw up a research and development agenda. Recommended focus areas to promote PAW biorefinery technology in South Africa include the following:

• Formulate policies and regulations that influence the growth and development of biorefineries.
• Diversify feedstock streams for biorefinery systems.
• Develop scalable biorefinery platforms.
• Optimize process parameters and pretreatment conditions for bioconversion platforms.
• Life-cycle and environmental impact assessment of feedstocks for biorefineries.
• Establish competitive markets for bio-based products.
• Conduct pilot studies on scalable biorefinery technologies.
• Transform small-scale facilities into commercial enterprises.