From Waste to Resource: Valorization of Carambola (Averrhoa carambola) Residues in Sustainable Bioelectrochemical Technologies

Abstract

The underutilization of fruit waste in agroindustry—particularly star fruit—leads to leachate generation, emissions, and disposal costs, highlighting the need for circular alternatives that treat organic fractions while producing energy. This study evaluated the bioelectrochemical conversion of carambola (Averrhoa carambola) residues in single-chamber microbial fuel cells (MFCs). Three 1000 mL reactors were constructed using carbon anodes and zinc cathodes, operated for 35 days with continuous voltage recording and daily monitoring of pH, conductivity, and ORP. Polarization curves were obtained, and FTIR and SEM analyses were conducted to characterize substrate transformation and anode colonization. The anodic biofilm was also profiled using metagenomics. Measurements were performed using calibrated electrodes and a data logger with one minute intervals. The systems exhibited rapid startup and reached peak performance on day 22, with a voltage of 1.352 V, current of 3.489 mA, conductivity of 177.90 mS/cm, ORP of 202.01 mV, and pH of 4.89. The V–I curve indicated an internal resistance of 16.51 Ω, and the maximum power density reached 0.517 mW/cm². FTIR revealed a reduction in bands associated with carbohydrates and proteins, consistent with biodegradation, while SEM confirmed extensive biofilm formation and increased anode surface roughness. Metagenomic analysis showed dominance of Acetobacter (59.35%), with Bacteroides (12.93%) and lactobacilli contributing to fermentative and electrogenic synergies. Finally, the series connection of three MFCs generated 2.71 V, sufficient to power an LED, demonstrating the feasibility of low-power applications and the potential for system scalability.

1. Introduction

Fruit waste in agroindustry poses a rapidly escalating environmental and economic problem [1]. Globally, nearly one third of all food produced for human consumption—approximately 1.3 billion tons annually—is lost or wasted; fruits and vegetables account for some of the highest loss rates throughout the supply chain, from postharvest to commercialization [2,3]. This discard represents a compounded waste of water, energy, and soil, and when it ends up in landfills, it generates methane, a greenhouse gas significantly more potent than carbon dioxide, intensifying climate change [4]. In Peru, the organic fraction dominates the municipal waste stream; kitchen and food scraps represent around 47% of the total, highlighting the magnitude of the challenge for local management systems [5]. The combination of high volumes of fruit waste and deficient infrastructure for valorization leads to soil and water contamination from leachates, proliferation of vectors, and economic losses for producers and processors, closing a loop that demands circular economy and bioelectrochemical solutions [6,7]. Furthermore, export standards impose aesthetic requirements that worsen rejection rates and lead to large scale waste in both field and processing facilities [8].

Furthermore, the urgency of adopting renewable energy is supported by compelling data, such as the fact that fossil fuel combustion accounts for more than 75% of global greenhouse gas emissions and nearly 90% of CO₂, making their replacement key to mitigating pollution and warming [9,10]. Additionally, costs have dropped dramatically: between 2010 and 2020, solar electricity became ~85% cheaper and onshore wind ~56%, making them the most affordable option in much of the world [11]. By 2030, renewables could supply 65% of global electricity, scaling to 90% by 2050 with significant emission reductions [12].

Due to this, microbial fuel cell (MFC) technology emerges as an excellent option, as it converts, through electroactive microorganisms, the chemical energy of organic matter into electricity while removing pollutant loads; thus, it simultaneously addresses energy and treatment needs [13,14]. Its performance depends on reactor architecture, microbial community, and substrate, enabling the use of sugar and organic acid-rich residues from agroindustrial streams [15]. Operating at ambient temperature, with a low footprint and potential for decentralized integration, MFCs are candidates for valorizing biodegradable waste flows that currently generate disposal costs and environmental liabilities in small- and medium-scale agrifood facilities [16]. For example, Huilca Modumba (2025) used mandarin waste in a triple-anode MFC; it reached 1.203 V and 507.88 mW/cm² with acidic pH [17,18]. Likewise, Ruvalcaba B. et al. (2024) evaluated grape pomace as substrate; they obtained 0.48 V, 210 mW/m², and a final pH of 7.2 after continuous operation [18].

In Peru, carambola (Averrhoa carambola) represents a tropical fruit with high rejection levels in markets and collection centers due to aesthetic criteria, size, or ripeness [19,20]. These residues, rich in moisture and acidity, generate leachates, vectors, and emissions if not properly managed [21]. Although there is no specific data on carambola, the country produces about 23 thousand tons of solid waste daily, of which, only 1.8% is recycled [22]; 78% could be valorized, especially the organic fraction. Given its composition rich in sugars and organic compounds, starfruit waste can be efficiently used in microbial fuel cells, emerging technologies that convert waste into bioelectricity through microbial communities [23]. These MFCs can power environmental sensors, public lighting systems, or low-consumption rural applications. Furthermore, they promote the circular economy by revalorizing waste, reducing emissions, and supporting local entrepreneurship in agricultural areas [24]. The innovation lies in transforming carambola waste into bioelectricity through microbial fuel cells, offering a sustainable and decentralized energy source.

The general objective of this research is to design and evaluate a microbial fuel cell system that valorizes carambola waste generated in Peruvian markets, converting it into bioelectricity useful for low consumption applications and simultaneously contributing to the treatment of the organic fraction. To achieve this, monitoring the key operational parameters of the system, including pH, voltage, current, oxidation–reduction potential (ORP), electrical conductivity, and chemical oxygen demand (COD), for 30 days is proposed to characterize electrochemical stability and purification efficiency. Likewise, structural changes in the carambola substrate between the beginning and end of the test will be determined using FTIR spectroscopy (Massachusetts, USA) and identifying the transformation of functional groups associated with biodegradation, and the total internal resistance of the MFCs will be estimated and polarization curves constructed to quantify power density as a function of current density, identifying the maximum power point and the limiting regime. These objectives will allow a holistic performance evaluation, with waste translating into energy in a reproducible and scalable way for community contexts.

2. Methodology

2.1. Fabrication of Microbial Fuel Cells

Three single-chamber microbial fuel cells (MFCs) were constructed using Pyrex glass vessels (Massachusetts, USA) with a working volume of 1000 mL. Each MFC incorporated a carbon plate anode (3 × 5 cm, HM-1.4, Horse, Lima, Peru), which was pretreated with 1 M nitric acid (Merck, Darmstadt, Germany, pH < 1.0) to increase its active surface area, then rinsed with sterile distilled water. The cathode consisted of zinc plates (A285, Arequipa steels, Arequipa, Peru) of the same dimensions, cleaned with fine abrasive paper and disinfected with 70% ethanol (Sigma Aldrich, Darmstadt, Germany, >99.8%). The electrodes were positioned 4 cm apart and connected by copper wires to an adjustable 100 Ω external resistor (see Figure 1). The star fruit (carambola) residues used as substrate were collected from local markets and artisanal processing sites in Trujillo, Peru, where surplus non-commercial fruit is commonly generated. Only fresh organic waste was selected, including pulp, peel, and fruit remnants discarded due to over-ripeness or superficial damage. The material was transported under hygienic conditions, without additives or preservatives, and stored at ambient temperature for no more than 24 h prior to processing. Subsequently, the residues were mechanically ground to homogenize the matrix and facilitate dosing in MFCs.

2.2. Characterization of Microbial Fuel Cells

During monitoring, parameters such as pH, electrical conductivity (mS/cm), and oxidation–reduction potential (ORP) were recorded daily using electrodes calibrated with certified standards. Voltage was recorded continuously with a data logger connected to a fixed 100 Ω resistor, at one-minute intervals. Polarization curves were also obtained by varying the external load to calculate current (I = V/R), current density (mA/cm²), power density (mW/cm²), and internal resistance through analysis of the linear segment of the V–I curve. Chemical oxygen demand (COD) was measured by closed-reflux dichromate digestion at 150 °C for two hours. Fourier transform infrared spectroscopy (FTIR, Thermo Scientific IS50, Mumbai, India) using attenuated total reflectance (ATR) was applied to determine chemical changes in the polyethylene substrate over a 4000–400 cm⁻¹ range with 32 scans. Samples were also analyzed by scanning electron microscopy (SEM-EDX, JEOL-JSM, Thermionic, Medellín, Colombia), after gold sputter-coating, to observe fungal colonization and structural alterations of the substrate.

2.3. Operation of the Microbial Fuel Cell

The MFCs were operated for 35 days in closed circuit with continuous magnetic stirring at 100 rpm and the cathode exposed to ambient air, forming an air cathode configuration. After an initial 24 h open circuit acclimation, the circuit was closed. During operation, evaporated medium volume was aseptically replenished without complete medium replacement to maintain the original fungal conditions; pH corrections were made when deviations exceeded ±0.5 units, using phosphate buffer. On the day of peak electrical performance, the three MFCs were connected in series to power a practical load (standard LED bulb), and the total voltage generated and the system’s transient response were documented. At the end of the experiment, electrodes were recovered for final SEM analysis.

2.4. Recovery and Cultivation of Microorganisms from the Anode

Microbial samples were collected directly from the anode surface using sterile swabs, focusing on areas with evident biological growth. For bacterial isolation, nutrient agar and MacConkey agar (Merck, plate diameter 90 mm) plates were incubated at 30 °C for 24 h. Fungal isolates were cultured on Sabouraud agar (Merck) under similar conditions. Preliminary characterization included Gram staining for bacteria and lactophenol cotton blue staining for fungi. Pure cultures were then obtained on slant tubes for subsequent genetic analysis.

2.5. Metagenomic Analysis of Anodic Biofilm

Microbial characterization of the anodic biofilm was performed through 16S rRNA gene amplicon sequencing. Samples were collected at the end of the experimental period (day 35) and stored at −20°C until processing. Total DNA was extracted using the commercial DNeasy PowerBiofilm^® kit (Qiagen, Germany, Hamburg), strictly following the manufacturer’s protocol to maximize DNA recovery from biofilm matrices. Amplification of the hypervariable V3–V4 region of the 16S rRNA gene was carried out using universal primers 341F (5′-CCTACGGGNGGCWGCAG-3′) and 806R (5′-GACTACHVGGGTATCTAATCC-3′). Sequencing was performed on the Illumina MiSeq platform using a 2 × 300 bp paired-end kit, through a certified third party service provider(San Diego, CA, USA). Raw sequence data were processed using the QIIME2 pipeline (version 2023.2). The DADA2 algorithm was applied for dereplication, quality filtering, chimera removal, and generation of representative sequences (ASVs). Taxonomic assignment was conducted using a Naive Bayes classifier trained on the SILVA 138 database, tailored to the V3–V4 region. Taxonomic profiles were visualized using bar plots and relative abundance charts.

3. Results and Analysis

Figure 2a shows the voltage evolution over time, revealing a startup phase (day 1: 0.127 ± 0.001 V) during which the potential gradually rises as the electroactive biofilm establishes and activation resistance falls, followed by a stable maximum (peak on day 22: 1.352 ± 0.351 V) with a series of peaks reflecting substrate availability before declining toward the final day. In MFCs fed with starfruit residues, a relatively rapid voltage increase is expected due to the high fraction of fermentable organic acids; however, voltage drops may occur when substrate is depleted, volatile acid metabolites accumulate, or pH falls because of the fruit’s intrinsic acidity [25]. Studies using avocado waste in dual-chamber MFCs with zinc and copper electrodes reported a voltage generation of 0.74 ± 0.02 V, indicating that nutrient-rich, easily biodegradable substrates can yield higher voltages [26]. Reactor configuration also plays a critical role: setups using fruit leachate with added sucrose and alternative electrode materials (e.g., porous graphite felt) have produced voltages of approximately 260 mV, underscoring the interplay between substrate concentration, electrode surface area, and biofilm distribution [27].

Figure 2b presents current over time, which typically mirrors the voltage trend as it is governed by internal resistance and substrate condition. During startup (day 1: 0.315 ± 0.101 mA), current rises more slowly than voltage due to kinetic limitations; after biofilm consolidation, it climbs to 3.489 ± 0.298 mA on day 22, then decreases to 2.446 ± 0.351 mA by day 35. In acidic matrices like star fruit, current can oscillate if pH or alkalinity are uncontrolled, affecting cathodic overpotential and proton diffusion [28]. Sharp current drops with relatively stable voltage often signal temporary increases in ohmic resistance (e.g., cathode fouling or membrane/diaphragm drying) [18,27]. Joint analysis of both curves allows inference of the balance among microbial kinetics, charge transport, and operation [28]. A tight correlation between voltage and current peaks suggests substrate availability is the dominant limitation; if voltage remains steady but current falters, efforts should focus on reducing internal resistance (improving electrode–biofilm contact, electrolyte conductivity, or cathode aeration) [29]. Although the abundance of organic acids and sugars in carambola waste favors rapid startups, the low initial pH may require buffering (e.g., phosphates) to sustain high currents and prevent periodic drops. The gradual stabilization of higher performance plates over time typically indicates selection of exoelectrogenic populations and improved anodic redox coupling [30]. Zinc was used as the cathode due to its low cost and availability, although its dissolution as a sacrificial anode can lead to corrosion, compromising system stability [24]. However, from a sustainability standpoint, the use of zinc cathodes presents significant challenges. Unlike conventional air cathodes, which employ catalysts such as platinum or non-precious metals (e.g., MnO₂, Fe-N-C), zinc entails a higher environmental footprint due to its extraction and disposal and generates metallic waste that requires treatment. Air cathodes, although more expensive in their initial setup, offer greater durability, lower environmental impact, and better compatibility with long-term applications in rural or decentralized contexts [15,28].

Figure 3a shows the pH evolution in the MFCs fed with carambola waste over 35 days. The initial pH (day 1) was 3.62 ± 0.14, reflecting the natural acidity of the substrate, which is rich in organic acids such as oxalic acid. This acidity may limit the activity of electroactive microorganisms, which prefer more neutral conditions for efficient metabolism [31]. Over time, the pH progressively increased, reaching 4.89 ± 0.18 on day 22 due to microbial production of alkaline metabolites such as ammonium and bicarbonate, and the breakdown of organic acids [28,30]. By day 35, the pH reached 5.39 ± 0.24, indicating microbial adaptation to the acidic environment, which favors electron transfer and energy generation in the MFC [32]. Studies with papaya waste show a strong pH dependence: at pH 7 they achieved 1.02 V and 17.97 mA, whereas at pH 4 the voltage dropped to 0.72 V and the current to 5.22 mA [33]. Another study with sucrose-enriched papaya identified an optimal pH of 4.98, achieving 0.955 V and 5.079 mA on day 15 [34]. On the other hand, during the 35-day monitoring, electrical conductivity values in the MFC fed with carambola waste showed significant evolution (see Figure 3b). On day 1, an initial value of 31.07 ± 1.73 mS/cm was recorded, while on day 22 the maximum of 177.90 ± 2.08 mS/cm was reached. Finally, on day 35, conductivity decreased to 121.57 ± 2.65 mS/cm. This behavior reflects a biochemical dynamic associated with microbial metabolism, where the initial low value is due to the limited concentration of ionic species in the medium, typical of the organic acids and minerals present in carambola waste [35]. As microorganisms degrade the substrate, ions such as ammonium, phosphates, and other metabolites are released, increasing electrical conductivity and enhancing electron transfer efficiency [36]. The peak on day 22 coincides with a stage of high bioelectrochemical activity. Subsequently, the decrease in conductivity may be attributed to substrate depletion, solids sedimentation, or ion absorption by biofilms, which reduces the availability of ionic species in the medium and can affect the MFC’s energy efficiency in its final phase [37]. For example, pineapple substrate reached a maximum conductivity of 69.47 ± 0.91 mS/cm on day 23; the higher conductivity observed here is likely due to pineapple’s intrinsic chemical composition, which, when degraded, releases a relatively greater amount of ionic species that contribute to conduction. Measuring these conductivity values is crucial for understanding and optimizing MFC performance, as improved conductivity contributes to lower internal resistance and more efficient electron transfer through the electrodes [38]. Figure 3c shows the ORP values of the MFCs, exhibiting significant evolution. On day 1, ORP was approximately 8.17 mV, indicating a highly reducing environment typical of initial conditions with low microbial activity [29]. As microorganisms began to metabolize the substrate’s organic compounds, ORP progressively increased, reaching a maximum of 202.01 mV on day 22. This increase reflects intensified electron transfer and greater energy generation resulting from bioelectrochemical activity [39]. On day 35, ORP decreased slightly to 171.19 mV, suggesting system stabilization and possible substrate depletion. This behavior is characteristic of MFCs in the maturation phase, where energy efficiency is maintained but with lower metabolic intensity [40].

The FTIR spectrum of the carambola waste used as substrates in MFCs is shown in Figure 4, revealing a variety of functional groups key to bioelectrochemical activity. The peak at ~1025 cm⁻¹ corresponds to C–O stretching in alcohols, ethers, or carboxylic acids, indicative of carbohydrates and phenolic compounds present in the biomass [41]. The signal at ~1150 cm⁻¹ is associated with C–O of esters or C–N of amines, which may derive from pectins and proteins [42]. The band at ~1642 cm⁻¹ is interpreted as a composite signal primarily attributable to δ(H–O–H) bending of adsorbed water and/or C=C stretching of unsaturated compounds (e.g., lignin fractions), with a possible contribution from the C=O stretching of conjugated carboxylic/ester groups; the contribution of amides cannot be confirmed in the absence of an unambiguous Amide II band (~1540 cm⁻¹) [41,42]. Comparing the initial and final spectra, changes in the intensity of these peaks suggest degradation of carbohydrates, lipids, and proteins during the MFC process [43]. This chemical transformation indicates the conversion of organic matter into electrons and protons, promoting energy generation [44]. Thus, FTIR not only confirms the rich biodegradable composition of star fruit waste but also evidences the role of its degradation in the system’s electrochemical performance. Other studies describe FTIR characterization of phytochemical extracts, such as those from Carica papaya, indicating the presence of alcohol and amine groups correlated with antifungal effects [45]. Similarly, FTIR analyses of chitosan derived from Pleurotus eryngii confirmed the presence of functional groups (OH, C–H, C=O, and N–H bending) and structural modifications resulting from deacetylation, factors related to its antimicrobial efficacy [46].

Figure 5a shows the voltage–current curve, which displays a linear relationship with a slope of 16.51 ± 1.51 Ω, corresponding to the system’s internal resistance. This value indicates moderate resistance, suggesting good medium conductivity and adequate electron transfer between the electrodes. The Pearson correlation coefficient (r = 0.9028) supports the quality of the linear fit and the stability of the electrical behavior. The specific characteristics of the substrate used in the MFCs play a crucial role in determining internal resistance [35]. For example, fruit wastes such as lemon, avocado, and banana peels exhibit distinctive internal resistances due to variations in sugar content, organic matter degradability, moisture, and the presence of redox-active compounds [46,47]. Avocado residues, for instance, have been reported to have very low resistance values (approximately 71.48 Ω), resulting in higher electron mobility and improved voltage and current outputs [26]. Conversely, substrates with higher cellulose content or insufficient biofilm development often exhibit higher internal resistance, limiting performance [29].

Figure 5b shows that the power density reaches a maximum of approximately 0.517 ± 0.054 mW/cm², while the voltage decreases as current density increases, displaying a peak voltage of 1124.789 ± 101.051 mV. This behavior is typical in MFCs, where energy efficiency is optimal at moderate load ranges but is reduced by ohmic losses and transport limitations at higher currents [48]. In experiments using banana peel waste as a substrate in MFCs, it was observed that at an optimized concentration of 50 g/L, the MFC with banana pulp produced a power density of approximately 0.09 W/m². In comparison, a lower concentration of 25 g/L yielded only about 0.019 W/m². This improvement is attributed to increased microbial activity and the corresponding reduction in internal resistance, which enhances electron mobility and improves power density [49]. Cashew juice is another fruit waste substrate that has been evaluated in MFC applications. A dual-chamber MFC using cashew juice as the feedstock produced a maximum power density of 31.58 mW/m² with a corresponding current density of 350 mA/m² and an open circuit voltage of 0.4 V. Although this power density is lower compared to substrates such as pineapple and papaya waste, it highlights the fact that the choice of fruit waste and its intrinsic properties (such as acidity and sugar content), combined with a reduction in internal resistance, are crucial factors in MFC performance [50].

Figure 6 presents two micrographs of the anode electrode from the MFC fed with carambola waste, comparing its initial state (Figure 6a) and final state (Figure 6b) after the operation period. Both images, captured at a 100 µm scale, reveal significant morphological changes resulting from the bioelectrochemical process. The initial micrograph shows a relatively smooth surface, with minimal imperfections and scarce particle accumulation characteristic of an unused electrode, reflecting a high availability of clean active area for microbial colonization and electron transfer. After use in the MFC, the final micrograph displays a markedly rougher and more heterogeneous surface coated with an irregular layer comprised of microbial biofilms, reaction products, and possible mineral precipitates. This biofilm provides evidence of electrode colonization by electroactive microorganisms responsible for oxidizing the organic matter in the carambola waste and transferring electrons to the external circuit [51]. The increased roughness can enhance electrode–microorganism contact, boosting electron transfer efficiency, although excessive accumulation of products and precipitates may also introduce additional internal resistance [52]. These changes confirm the capacity of carambola substrate to promote active microbial growth and sustain the energy generation process. They also demonstrate that the substrate–microorganism–electrode interaction produces physical modifications affecting both performance and durability of the MFC system, key information for optimizing electrode design and materials [53].

Figure 7 presents the metagenomic profile of the anodic biofilm from a microbial fuel cell fed with carambola residue, showing the composition at the species and genus levels. In both cases, a strong dominance of Acetobacter (59.35%) is observed, a group associated with incomplete oxidation of ethanol and sugars, suggesting its key role in degrading compounds present in the substrate and transferring electrons to the anode [54]. Second, Bacteroides (12.93%), indicating fermentative activity and the ability to degrade complex polymers present in the fibrous fraction of carambola waste. The presence of Lactisaseibacillus (8.55%) and other lactobacilli such as Lapidilactobacillus, Loigolactobacillus, Levilactobacillus, and Schleiferilactobacillus (together ~7.7%) suggests metabolic pathways producing organic acids that could serve as intermediates for electrogenic bacteria [55]. The detection of Caproiciproducens (4.91%) is notable, as this genus is linked to medium-chain fatty acid production—compounds of interest for bioenergy and synthesis of value-added products [54]. The “Other” group (~6.6%) reflects residual microbial diversity, which may include minority species with complementary functions in the bioelectrochemical ecosystem. The profile suggests a microbial community structured around metabolic synergies: primary fermenters, acid producers, and oxidizing/electrogenic bacteria [54,55]. Treatment of MFCs with rhamnolipids demonstrated an increase in the relative abundance of signal-transduction genes in exoelectrogens from 4.56% to 5.86%, corresponding to a rise in coulombic efficiency from 19.10 ± 0.79% to 27.39 ± 1.07% [56]. Additionally, substrate-specific metagenomic evaluations comparing systems fed with molasses and acetate quantified PCR product concentrations of 30 ng/µL for MFC1, 58.6 ng/µL for MFC2, 69.1 ng/µL for MFC3, 53.2 ng/µL for MFC4, and 18.2 ng/µL for MFC5, indicating variable nucleic acid yields that may reflect differences in microbial community composition [57].

Figure 8 presents, in the form of a flow diagram, the complete process of bioelectricity generation in MFCs using carambola peels as an agro-industrial waste. The scheme begins with the raw material—carambola fruits—and their conversion into waste after the pulp is removed. These residues are fed into MFC-type reactors, where electroactive microorganisms degrade the organic matter, releasing electrons and protons. The next stage shows three MFCs, labeled MFC 1, MFC 2, and MFC 3, connected in series via conductors, with a multimeter recording a total voltage of 2.71 V. This configuration allows the individual potentials of each cell to be summed, increasing the output voltage and demonstrating the possibility of scaling the technology for higher energy demands. Finally, the “Bioelectricity generation” phase is highlighted, underscoring the system’s ultimate goal. The analysis shows that starfruit peels are a viable substrate due to their high carbohydrate content and easily biodegradable compounds, which promote microbial metabolic activity. Connecting the cells in series mitigates the low individual voltage characteristic of MFCs, making them more competitive for low-power applications. Furthermore, this conceptual flow integrates waste valorization with clean energy generation, in line with circular economy and sustainability principles. The figure not only illustrates the experimental setup but also clearly communicates the practical applicability of MFCs in converting agro-industrial waste into renewable energy.

4. Discussion

The results obtained in this study confirmed the hypothesis that starfruit (Averrhoa carambola) residues, due to their high content of fermentable sugars and organic acids, are an effective substrate for bioelectricity generation in single-chamber microbial fuel cells (MFCs). The rapid start-up and peak voltage of 1.352 V reached on day 22 exceed values reported for other fruit residues in similar configurations, such as papaya (0.955 V) [34], mandarin (1.203 V) [17], or grape (0.48 V) [19]. This advantage can be attributed to the chemical composition of starfruit, which promotes both microbial colonization and electron transfer. The pH evolution from 3.62 to 4.89 reflects a progressive adaptation of the microbial community, consistent with studies indicating that moderately acidic conditions can sustain high currents when an adapted electrogenic community is present [28,33]. The increase in electrical conductivity to 177.90 mS/cm at peak performance indicates sustained ion release during substrate degradation, reducing internal resistance (16.51 Ω) and improving conversion efficiency, in agreement with observations in MFCs fed with pineapple or banana residues [38,49].

FTIR analysis revealed the degradation of carbohydrates, proteins, and lipids, confirming the biochemical transformation of the residue into simpler compounds readily utilized by electrogenic bacteria. Scanning electron microscopy showed a dense and heterogeneous biofilm, supporting the hypothesis that the anode’s surface structure and the nutrient availability in starfruit facilitate microbial adhesion and efficient electron transfer. Metagenomic profiling revealed a dominance of Acetobacter (59.35%), a key group in the incomplete oxidation of sugars and alcohols, along with Bacteroides (12.93%) and lactobacilli (~7.7%), which contribute fermentative pathways and intermediate organic acid production. This community structure aligns with reports highlighting the importance of mixed microbial consortia in maximizing bioelectrochemical performance [54,55]. In a broader context, these findings reinforce the potential of MFCs as agro-industrial waste valorization technologies, simultaneously contributing to renewable energy generation and environmental impact mitigation. Demonstrating that three MFCs connected in series can power an LED validates their scalability for low-energy applications such as environmental sensors, rural lighting, or monitoring systems in off-grid areas.

When compared with the international literature, the performance achieved with starfruit is competitive with other widely studied fruit residues. For example, optimized banana peel MFCs have reached 0.09 W/m² [49], whereas our system achieved 0.517 mW/cm² (equivalent to 5.17 W/m²), underscoring the relevance of the substrate used. Furthermore, the high conductivity and low internal resistance observed suggest that starfruit could serve as a reference substrate for future comparative studies. The implications of this work extend beyond the laboratory: in tropical fruit-producing regions, integrating MFCs into markets, processing plants, or rural communities could transform an environmental liability into a local energy resource, aligning with circular economy principles and the energy transition agenda.

Future research should focus on optimizing pH and conductivity through the use of buffering agents and mineral additives, as well as evaluating more sustainable cathode materials than zinc, such as low-cost catalysts in air cathodes. Additional efforts should be directed toward quantifying coulombic efficiency and chemical oxygen demand (COD) removal over extended operational cycles, alongside implementing metagenomics-guided bioaugmentation strategies to enhance power density. Furthermore, scaling up the system in series and parallel configurations could enable its deployment for community-level applications. In summary, this study demonstrates that starfruit residues are not only a viable but also a competitive substrate at the international level, and that their use in MFCs represents a tangible opportunity to integrate waste management, clean energy generation, and sustainable development in tropical contexts.

5. Conclusions

This study highlights the feasibility of using carambola waste as a sustainable substrate in single-chamber microbial fuel cells (MFCs), demonstrating effective bioelectrochemical coupling and microbial adaptation. The system showed stable electrogenic performance, biochemical transformation of the substrate, and development of a functional anodic biofilm. Metagenomic analysis revealed a fermentative and electroactive microbial community, supporting efficient electron transfer and substrate degradation. Physicochemical and morphological evidence confirmed the system’s operational integrity and microbial activity. Although performance peaked mid-experiment, a gradual decline toward the end suggests substrate depletion and biofilm aging, indicating the need for long-term operational strategies. The successful powering of an external load through MFCs connected in series validates the system’s scalability. These findings support the potential of tropical agro-industrial residues for decentralized energy generation and environmental remediation, especially in a rural context.

For future work, pH buffering and ionic conductivity should be optimized, external resistance and cathode design refined, COD removal and coulombic efficiency quantified, and metagenomics-guided bioaugmentation explored to increase power density and stability in both single cells and larger series/parallel arrays.