The effect of the use of tropical fruit waste as a co-substrate for dairy cow feces on methane production

Ignatius Manguma¹, Rita Purwasih^2,3, Rama Mohan Poludasu², Endang Kusdiyantini⁴ and Sutaryo Sutaryo²

¹ Master of Energy, Graduate School, Diponegoro University, Semarang, Indonesia, 50275
soeta@lecturer.undip.ac.id
² Department of Animal Science, Faculty of Animal and Agricultural Sciences, Diponegoro University, Semarang, Indonesia, 50275
³ Department of Agroindustry, Subang State Polytechnic, Subang, Indonesia, 41285
⁴ Department of Biology, Faculty of Science and Mathematics, Diponegoro University, Semarang, Indonesia, 50275

Abstract

Anaerobic processing of organic waste for biogas production is one strategy to overcome environmental problems while creating renewable energy. This study aimed to investigate the effect of using various types of tropical fruit waste (TFW) combined with dairy cow feces (DCF) on methane production. The study was conducted using a batch-type digester. The process with a substrate of DCF alone was a control. The co-digestion process was conducted using DCF combined with 10 types of fruit waste with the highest production in Indonesia, namely pineapple, rambutan, banana, durian, avocado, jackfruit, snake fruit, orange, mango and papaya. Based on the results of the study, the use of TFW as a co-substrate for the DCF significantly (p<0.05) increased methane production compared to the control on the 30^th, 60^th and 90^th observation days. The increase in methane production resulted from the combination of DCF with durian, banana, snake fruit, papaya, avocado and rambutan fruit wastes (70.06-234.64%). The combination of DCF_Durian and DCF_Banana showed the highest methane production (619.08 ± 142.15 and 560.59 ± 157.70 mL/g-VSadded). Meanwhile, the use of mango, pineapple, orange and jackfruit wastes did not show significant differences (p<0.05) from the control. The addition of TFW as a co-substrate of DCF significantly (p<0.05) affected the concentration of volatile fatty acids (VFAs), the concentration of total ammonia nitrogen (TAN) and the pH value. The type of TFW had various effects on each of the observed variables. The combination of DCF and TFW is an effective strategy to increase biogas production, but the characteristics of the fruit wastes need to be considered. The use of a combination of DCF_durian and DCF_banana is most recommended.

Keywords: biogas, co-substrate, dairy cow feces, tropical fruit waste, methane production

Introduction

The increasing global energy demand, coupled with the issues of climate change and environmental degradation, is driving the transition to renewable energy sources. Among existing renewable energy sources, biogas is a promising solution. Biogas is produced through an anaerobic digestion (AD) process. This process not only produces methane (the only marketable gas) but also effectively processes organic waste, thus aligning with the principles of a circular economy (Emmanuel et al 2024). Efforts to meet energy needs based on waste processing to produce biogas in Indonesia are highly relevant because the abundant availability of biomass can be used as a raw material or biogas substrate (Marendra et al 2019). Currently, the biomass commonly used as a substrate is dairy cow feces, because dairy cows are capable of producing large amounts of feces. A dairy cow with an average weight of 300 kg can produce 15 kg of feces/day and 12 liters of urine/day (Siswati dan Rizal, 2017). Improperly managed dairy cow feces will cause environmental pollution and the release of methane gas that contributes to global warming through the greenhouse gas effect (Liu et al 2025). However, the use of dairy cow feces as a mono or single substrate results in low methane production and is therefore less efficient. One effort used to increase methane production is by combining the substrate with other organic materials (co-substrate) (Sibiya et al 2017). Substrate combination is the process of mixing two or more substrates to increase the biogas production, one of which uses fruit wastes (Ngabala and Emmanuel, 2024).

In addition to the livestock sector, Indonesia is also a tropical country that is a major producer of various fruits. Based on data from the Central Bureau of Statistics (BPS, 2022), fruit production in Indonesia reached 28.52 million tons. This high fruit production is accompanied by high fruit waste production. Fruit waste consists of peels, seeds and other parts originating from market, industrial and even household activities. Fruit waste contains easily biodegradable organic matter and has the potential to be used as a biogas substrate (Marendra et al 2019). Anaerobic digestion (AD) is the best biological treatment for fruit waste, as it can convert organic matter into a renewable energy source with the help of microorganisms in oxygen-depleted environments (Arifan et al 2022; Gebresilasie et al 2025). Although research on co-substrates of dairy cow feces (DCF) and fruit waste has been widely conducted, there has been no specific research exploring the use of DCF in combination with various types of tropical fruit waste (TFW), especially in Indonesia. The characteristics of each TFW have not been widely explored in the international literature compared to those of subtropical fruit waste, thus representing an excellent research gap to be filled. Furthermore, the different organic materials contained in TFW are thought to influence the success of the AD process, which in turn influences biogas production. Therefore, exploratory research on the use of various types of TFW is essential. The biogas production process using AD is more suitable for processing TFW due to its high water content. However, there are challenges in utilizing TFW as a substrate, including low waste pH and the accumulation of VFA concentrations that affect methanogenic activity (Ngabala and Emmanuel, 2024).

Based on the background above, this study aims to determine the effect of using DCF combined with various TFWs in Indonesia. Specifically, the TFWs used are banana, mango, pineapple, orange, durian, snake fruit, papaya, avocado, rambutan and jackfruit, which are the 10 fruits with the highest production in Indonesia (BPS, 2022). This research will be a significant solution in Indonesia, especially in the simultaneous processing of two wastes, namely livestock waste (DCF) and agricultural waste (TFW). The evaluation of the success of the AD process is carried out through analysis of methane production and other variables in the digested slurry. The results of this study are expected to provide baseline data for planning renewable energy production based on waste processing, designing a small-scale biogas production system, as well as providing practical implications for organic waste management to create renewable energy sources in Indonesia.

Materials and methods

Inoculum dan substrate

The inoculum was prepared using fresh lactating DCF from the Dairy Cattle Barn of the Faculty of Animal and Agricultural Sciences, Diponegoro University, diluted with water (1:1.65) and fermented for approximately 2 weeks anaerobically. The inoculum was then filtered through nylon cloth to obtain the liquid fraction (Sutaryo et al 2012). The substrate consisted of dairy cow feces from the same dairy cows as the inoculum. The TFW was obtained from various traditional markets around Semarang City, Central Java, Indonesia. There were 10 types of waste from 10 fruits, with the highest production in Indonesia, according to BPS (2022), namely pineapple, rambutan, banana, durian, avocado, jackfruit, snake fruit, orange, mango and papaya. The waste used was whole fruit waste consisting of the skin, seeds and crown (depending on the fruit type), which was then dried and ground into flour. The total solids (TS) and volatile solids (VS) content of the inoculum, DCF and mixed substrates are presented in Table 1.

Experimental set-up

The digester used was a 500-mL batch digester. This study used 36 batch digesters, consisting of 10 treatments with combinations of DCF and TFW substrates, a mono-substrate DCF as a control and a digester that contained only inoculum as a blank (methane production reduction factor), each with three replications. The AD process was carried out by adding 200 mL of inoculum to the digester, then DCF and TFW (± 5 g), which was a mixture of DCF and TFW in a proportion of 50:50% (Abeng et al 2024). The digester was then closed with a rubber, claimed using aluminum crimps, followed by a 2-minute nitrogen flushing process and incubated in an incubator at 37°C for 90 days, as shown in Photo 1. (Møller et al 2004; Purwasih et al 2024).

Photo 1. Digester incubation process in an incubator with a temperature of 37°C(a); Measuring methane volume using the liquid displacement method(b)

Analytical methods

Tropical fruit waste was subjected to proximate analysis, including moisture content, ash content, crude fat, crude fiber, crude protein and carbohydrates, using the standard method (AOAC, 2005). Moisture content was analyzed using an oven at 105°C for 7 hours, followed by ashing at 550°C for 4 hours to measure ash content. Fat and crude protein were analyzed using the Soxhlet extraction and Kjeldahl methods, respectively, while crude fiber and carbohydrate were analyzed using gravimetric and volumetric methods.

Methane production was measured on days 3, 7, 12, 20, 30, 45, 60, 75 and 90 at 10:00 A.M. (Purwasih et al 2024) and calculated cumulatively. Biogas produced from the AD process was flowed into a 500 mL glass bottle containing 4% NaOH (Merck®, Cat. No. 1064981000) and then stored in a Tedlar gas bag (Hedetech-Dupont, China) with a volume of 1 L in Figure 1(b). Methane production was measured using the liquid displacement method in Figure 1(c) (Sutaryo et al 2020). Volatile fatty acids (VFAs) were analyzed with a Clarus® 680 gas chromatograph, PerkinElmer. Total ammonia nitrogen (TAN) was analyzed using photometric kits (cat. no. 1.00683.0001) NOVA 60 A Spectroquant®. The pH measurements were carried out with a pH meter (Ohaus® ST300). VFA, TAN and pH analysis were carried out on the digested slurry on the 90^th day. The collected data were analyzed using Analysis of Variance (ANOVA) at a significance level of 5%. If there was a significant difference (p<0.05), then it was continued with Duncan's Multiple Range Test (DMRT) analysis (Gomez and Gomez, 2007).

Results and discussion

Characteristics of Tropical Fruit Waste

Characterization of TFW is crucial for determining the success of the AD process. The appearance of TFW and TFW flour is shown in Photo 2. The results of the proximate analysis are presented in Table 2. Significant variations in substrate composition were observed. The organic matter composition of the substrate influenced methane production.

Photo 2 shows that the pre-research process successfully converted heterogeneous fruit waste into a more homogeneous flour. The flouring process is a physical pre-treatment carried out to increase efficiency, preserve nutrients and improve the quality of the final product. It is also an important step in standardizing samples (Deng et al 2019; Larrosa and Otero, 2021; Santos et al 2022). Research conducted by Soldano et al (2021) found that the flouring process of wheat waste can increase methane production. Fruit waste has a complex and dense polymer structure. The flouring process increases the surface area to surface area ratio, increasing the contact area between the substrate and the microbes, allowing more microbes to work and making the decomposition process more efficient. The flouring process also directly accelerates the hydrolysis process. The smaller particle size allows the enzymes secreted by the microbes to penetrate and break down the waste structure (Yang et al 2023).

Photo 2. Appearance of TFW and TFW Meal

This research successfully identified that snake fruit, pineapple, durian, mango and avocado were rich in carbohydrates, as much as 70-77% (Table 2). Carbohydrates are the ideal organic material for methane production because they are easily hydrolyzed into simple sugars by microorganisms. This process efficiently provides a substrate for the acidogenesis stage and ultimately methanogenesis. Cirne et al (2007) stated that hydrolyzed carbohydrates are rapidly converted into simple sugars, which are then converted into VFAs during AD. However, carbohydrate availability is influenced by crude fiber. The snake fruit and papaya had high crude fiber contents, at 40.38% and 35.48%, respectively. Crude fiber consists of lignin and cellulose, forming a sturdy structure and resistant to microbial enzymatic degradation and is a major limiting factor (Jeihanipour et al 2011). Lignin is a very complex compound to break down. Due to the presence of lignin, the hydrolysis stage is slow. It will reduce the overall rate of methane production. Meanwhile, cellulose plays a crucial role in biogas production because it is converted into glucose and then utilized in methanogenesis (Arifan et al 2022).

Meanwhile, substrates with high fat and protein contents were papaya and rambutan. Theoretically, fat has the highest methane production potential per unit mass compared to carbohydrates and protein, but the degradation of fat into long-chain fatty acids (LCFAs) can be a limiting factor and its accumulation can inhibit methanogenic activity. Duan et al (2023) stated that LCFA are significant intermediates in AD. LCFA toxicity occurs at high concentrations because they can interfere with coupled β-oxidation by increasing bubble separation from anaerobic granules and reducing bubble coalescence, which affects interfacial interactions necessary for efficient methane production.

The high protein content of papaya waste is an important organic material that provides essential nitrogen for cell synthesis and microbial population growth in the digester. However, high protein content in the substrate can lead to a decrease in the C/N ratio. During the decomposition process, proteins are hydrolyzed into amino acids and deaminated to release ammonia. Free ammonia (NH₃) is toxic to methanogens because it can easily diffuse across cell membranes and cause changes in intracellular pH, which can inhibit or even stop the methane production process altogether (92.1%) (Li et al 2015). Meanwhile, substrates containing high ash content (banana) cannot be converted into biogas. Therefore, the use of TFW as a biogas substrate has enormous potential but also presents challenges.

Methane production

Cumulative methane production from the anaerobic co-digestion process of dairy cow feces (DCF) and various types of tropical fruit waste (TFW) over 90 days is presented in Table 3 and Figure 1. The results show that the addition of TFW as a co-substrate for DCF significantly (p<0.05) increased methane production compared to the control. However, the type of TFW gave various effects on the amount of methane produced.

Methane production on day 30

Methane production on day 30 showed a significant difference (p<0.05), with a production range of 140.00 ± 20.00 mL/g-VS _added to 517.07 ± 128.26 mL/g-VS_added. The DCF Durian treatment produced the highest methane production and was significantly different (p<0.05) compared to the control and other treatments. High methane production was also achieved by DCF Banana and DCF Snake fruit, which produced 474.02 ± 117.06 mL/g-VS _added and 431.65 ± 81.35 mL/g-VS_added, respectively. In contrast, other treatments, such as DCF Mango (159.93 ± 6.61 mL/g-VS_added), DCF Pineapple (148.59 ± 30.39 mL/g-VS_added), DCF Orange (142.23 ± 3.99 mL/g-VS_added) and DCF Jackfruit (152.40 ± 26.24 mL/g-VS_added), showed methane production that was not significantly different (p>0.05) from the DCF control (140.00 ± 20.00 mL/g-VS_added).

The high methane production in the initial stages of the anaerobic process in several treatments was thought to be related to the organic matter content, particularly carbohydrates (sugars), in the substrate used. For example, durian had a high carbohydrate content of 74.63% (Table 3), which can be utilized by microorganisms in the hydrolysis and acidogenesis processes, thus influencing methane production. Meanwhile, the banana had 56.62% carbohydrates and 8.04% protein, which can also be utilized by microorganisms. Snake fruit contained a very high carbohydrate content (77.75%), but its crude fiber was also high (40.38%). It was suspected that the non-fiber carbohydrate fraction had an impact on methane production at the beginning of the anaerobic phase. In contrast, the DCF Mango, DCF Pineapple, DCF Orange and DCF Jackfruit treatments produced low methane production, which indicates that the addition of fruit wastes did not provide a significant increase in methane production, which was thought to be caused by the crude fiber content in each TFW (Table 3). The higher the crude fiber content in the substrate, the lower the methane production (Luthfi et al 2018), but other factors also influence, namely the presence of inhibitor compounds such as limonene and phenolic compounds, which are toxic to methanogens (Millati et al 2018; Kurniawan et al 2018).

Methane production on day 60

Methane production on day 60 showed a significant increase (p<0.05) in each treatment, indicating that the decomposition process was still ongoing. The DCF_Durian (600.27 ± 136.50 mL/g-VS_added) and DCF Banana (539.31 ± 155.57 mL/g-VS_added) treatments had the highest methane production compared to the control and other treatments. It indicates that the decomposition of more complex organic material (such as cellulose and hemicellulose) has begun, thus affecting methane production. In contrast, the DCF Orange treatment showed the lowest methane production (149.21 ± 5.24 mL/g-VS_added), although not significantly different (p>0.05). The DCF Mango (172.42 ± 13.91 mL/g-VS_added), DCF Pineapple (162.17 ± 31.85 mL/g-VS_added), DCF Jackfruit (159.61 ± 24.21 mL/g-VS_added) and the control (170.00 ± 34.64 mL/g-VS_added) showed the lowest methane production.

Although the orange had a high organic content, methane production from orange waste was low, possibly due to the inhibition in the anaerobic process. Orange peel contains D-limonene, a terpenoid compound with strong antimicrobial properties. It is suspected that the inhibition mechanism is cytotoxic, where its lipophilic nature allows this compound to penetrate and damage the integrity of the lipid cell membrane of microbes, especially methanogenic archaea, which are very sensitive to inhibitory factors. This damage to the cell membrane will disrupt ion gradients and essential metabolic functions, which ultimately suppress or even stop methane production activity. This is in accordance with the research of Millati et al (2018) and Kurniawan et al (2018), who used orange waste as a substrate. The results showed that the main component of orange waste, namely limonene, was proven to inhibit the AD process. This inhibition process caused the accumulation of VFAs. The antimicrobial properties of limonene also specifically affected methanogens, thus affecting the low methane production. Other treatments, such as DCF Pineapple and DCF Jackfruit, also had low methane production, allegedly due to the lignocellulose content, which is difficult to degrade. According to Sierra et al (2008) and Li et al (2019), lignin forms a very strong protective barrier and will block the access of enzymes secreted by microbes to cellulose and hemicellulose, making it difficult for enzymes to access and break down polysaccharides. Labatut et al (2011) and Raposo et al (2012) also stated that substrates with high lignin content generally show low methane production, so that a pre-treatment process is needed.

Methane Production on day 90

Methane production at the end of the observation period, on Day 90 (Figure 1), confirmed the trend observed in the previous observations on Day 30 and Day 60. Methane production from the DCF_Durian treatment (619.08 ± 142.15 mL/g-VS_added) was the highest and significantly different (p<0.05) compared to the control and other treatments, except for the DCF Banana treatment (560.59 ± 157.70 mL/g-VS_added). This supports previous research conducted by (Muenmee and Prasertboonyai, 2021), which stated that durian peel has high potential to be used as a mono-substrate (34.93 ± 1.30 mL/g-VS_added) or co-substrate (36.50 ± 0.01 to 65.77 ± 0.16 mL/g-VS_added). A previous study (Nathoa et al 2014) also revealed that the use of banana peel as a biogas substrate is capable of producing methane of 38.1 ± 8 to 251.3 ± 10 mL/g-VS_added.

Figure 1. Cumulative methane production (mL/g-VSadded) from the combination of DCF and various TFWs

The treatments of DCF Snake fruit, DCF Papaya, DCF Avocado and DCF Rambutan produced methane in the medium group, namely 458.39 ± 79.13, 358.27 ± 16.68, 318.78 ± 24.03, 314.60 ± 74.07 mL/g-VS_added, respectively. Meanwhile, the combination of DCF Orange (154.01 ± 5.90 mL/g-VS_added), DCF Jackfruit (165.55 ± 25.47 mL/g-VS_added) and DCF Pineapple (172.78 ± 31.57 mL/g-VS_added) was the treatment with the lowest production and did not show a significant difference (p>0.05) with the control (185.00 ± 39.05 mL/g-VS_added) and DCF Mango (184.50 ± 20.95 mL/g-VS_added). Strengthening the results obtained on the 30 ^th and 60^th days, the low methane production was thought to be related to several factors, including high organic acid production, the presence of secondary metabolite compounds, nutritional imbalance, low digestibility, high lignocellulose conditions and acidic conditions that will have a negative impact on the AD process. Jeong et al (2009) stated that the AD process of fruit waste (such as orange and pineapple peels) produces significant organic acid production, which can lower the pH of the final substrate. A low pH (4.5-4.7) can inhibit methanogenic bacteria, thereby reducing methane production. Avena et al (2023) reported that the use of pineapple waste has the potential to create acidic conditions that are unfavorable for methanogens. Meanwhile, mango waste is suspected of not having an ideal C/N ratio, thus affecting the efficiency of the AD process (Musser et al 2025).

According to Figure 1, the highest methane production was achieved by the DCF_Durian, followed by DCF Banana. Conversely, the lowest methane production was achieved by DCF combined with orange, jackfruit and pineapple wastes. Figure 1 depicts the methane production curve, from the rapid phase to the peak and then the slope. In the initial phase, from day 0 to day 20, methane production in the DCF Durian, DCF Banana and DCF Snake fruit treatments showed a sharp slope, indicating a significant increase in methane production. Meanwhile, from day 20 to day 60, the methane production rates among treatments became more pronounced. The cumulative methane production curves for the DCF_Durian and DCF Banana treatments continued to increase, although the slopes became more gentle. In contrast, the DCF_Orange treatment showed a slight increase, but it was below the control curve. This supported the results of previous research stating that an inhibition process suppresses microbial activity so that methane production is almost completely stopped. Meanwhile, treatments using substrates with a high lignocellulose content, such as pineapple and jackfruit, experienced a gradual increase, supporting the hypothesis that the degradation process is very slow due to the resistant lignocellulose structure.

Variables in digested slurry

The addition of TFW as a co-substrate for DCF significantly (p<0.05) affected the volatile fatty acid (VFA) concentration, total ammonia nitrogen (TAN) concentration and pH, as shown in Table 5. However, TFW types had varying effects on each observed variable.

VFA concentration

The addition of TFW as a co-substrate for DCF significantly (p<0.05) affected VFA concentration (Table 4). The control (DCF alone) showed the lowest VFA concentration (0.26 ± 0.01 mM) compared to the other treatments. The addition of all TFW types increased VFA concentration compared to the control. The highest VFA concentrations were observed in the DCF Papaya (80.23 ± 0.05 mM), DCF Pineapple (76.51 ± 0.01 mM), DCF Orange (69.95 ± 0.06 mM) and DCF Jackfruit (65.26 ± 0.04 mM). Conversely, the DCF Banana and DCF Rambutan treatments had VFA concentrations of 1.77 ± 0.04 mM and 2.66 ± 0.01 mM, respectively, showing relatively low VFA concentrations, although still significantly higher than the control.

VFA is an important intermediate product for the AD process and is formed during the acidogenesis and acetogenesis stages before being converted by methanogens to methane. Low and stable VFA concentrations indicate a balance between the rate of VFA production by acidogenic bacteria and the rate of VFA consumption by methanogens (Amani et al 2011; Wagner et al 2017). This occurred in the control, where the methanogenesis process was thought to run stably and efficiently because the VFA formed was directly consumed by methanogens. Conversely, the increase in VFA concentrations in all co-digestion treatments was a normal phenomenon because fruit waste was a highly degradable carbon source. Fruits such as papaya, pineapple, orange and jackfruit were rich in simple sugars, thought to trigger hydrolysis activity and acidogenesis to occur rapidly and cause VFA accumulation. As explained by Lukitawesa et al (2020), a very rapid VFA production rate, if not balanced by methanogen consumption, will have a negative impact, namely an imbalance in the process or an excessive organic load that ultimately leads to VFA accumulation.

The combination of DCF and pineapple, orange and jackfruit wastes was thought to have a low initial pH, inhibiting methanogen activity from the beginning of the AD process. This was confirmed by the cessation of methane production on the 30^th day. As a result, the formed VFAs were not consumed and continued to accumulate. Conversely, fruit waste containing more complex carbohydrates, such as bananas, had a more controlled degradation rate, allowing methanogens to adapt and consume the produced VFAs, thus preventing organic overload.

TAN concentration

The addition of TFW as a co-substrate for DCF significantly (p<0.05) affected TAN concentration, with a concentration range of 63.33 ± 5.77 mg/L to 140.00 ± 20.00 mg/L. The lowest TAN concentration was found in the DCF_Avocado treatment and the highest TAN concentration was found in the DCF_Papaya treatment. The low concentration of TAN in the DCF_Avocado treatment reflected its substrate composition (Table 3). Avocado waste had a low protein content (4.69%) in the AD process. TAN is produced by nitrogenous compounds, with protein as the main precursor. Proteolytic microorganisms break down proteins into amino acids, which then undergo deamination to release ammonia (NH₃) into the digester, which dissolves in water to form ammonium ions (NH₄⁺) and is collectively measured as TAN. Thus, the limited protein content of the substrate will affect the production of ammonia. This was different from the DCF Papaya (140.00 ± 20.00 mg/L) and DC Banana (133.33 ± 30.55 mg/L) treatments, which had high protein content (16.3% and 8.04%), which also affected the concentration of TAN. However, the success of the AD process does not depend only on the protein content of the substrate. There are organic materials and other factors that also have an influence. Hence, even though the TAN concentration of DCF Avocado was low, it can still produce methane.

pH value

The addition of TFW as a co-substrate for DCF significantly (p<0.05) affected the pH value. pH is an important indicator of the stability of the AD process, with values ranging from 4.87 ± 0.02 to 7.70 ± 0.01. The DCF Pineapple (4.87 ± 0.02), DCF Orange (5.10 ± 0.01) and DCF Jackfruit (4.93 ± 0.05) treatments had the lowest pH values compared to the control and other treatments. Conversely, the DCF Banana (7.60 ± 0.03) and the control (7.70 ± 0.01) treatments had the highest pH values, which were not significantly different from the DCF Durian (7.33 ± 0.04) and DCF Snake fruit (7.43 ± 0.08) treatments. According to (Azevedo et al 2023), pH is one of the most important factors in influencing the environmental stability of the AD process, namely regulating enzymatic reactions and microbial activity. Although in general microorganisms prefer a neutral pH (6.8-7.2), each stage of the AD process has a different optimal pH (Dai et al 2015; Neshat et al 2017; Srisowmeya et al 2020). The initial stages, namely hydrolysis, acidogenesis and acetogenesis, have an optimal pH of 5.5 to 6.5, while at the end, namely methanogenesis, requires a neutral optimal pH of 7.0 (Yao et al 2017). According to Sawyerr et al (2019) and Azevedo et al (2023), one of the challenges in utilizing fruit waste as a biogas substrate is that naturally some fruit waste has an acidic pH of 3.20 to 5.60, which causes acidification of the reactor to inhibit or even stop the AD process. Gebresilasie et al (2025) stated that one of the main challenges of AD using fruit waste is the rapid acidification process, caused by low pH and high VFA production, which inhibits methane production. This was confirmed by the VFA concentrations in DCF pineapple, DCF Jackfruit and DCF Orange.

These pH values supported methane production in each treatment (Table 4). The DCF Banana, DCF Durian and DCF Snake fruit treatments were at optimal pH conditions. It indicates that the digestion process in the digester was balanced, where the rate of VFA production by acidogenic bacteria was balanced by the rate of VFA consumption by methanogenic archaea. This balance prevented acid accumulation and maintained a conducive environment for continuous methane production. On the other hand, low or even cessation of methane production occurred in the DCF pineapple, DCF Jackfruit and DCF Orange treatments, as evidenced by the low pH values, indicating acidification. Acidification conditions will inhibit or even kill methanogens. Methanogens are microorganisms responsible for methane production, which are very sensitive to changes in pH (Kim et al 2004; Li et al 2014).

Conclusion

Methane production from AD varied depending on the combination of DCF and TFW. The combination of DCF with durian, banana, snake fruit, papaya, avocado and rambutan waste significantly (p<0.05) increased methane production compared to the control by 70.06–234.64%. Among the treatment combinations, the DCF Durian treatment, followed by the DCF Banana treatment, showed the highest methane production (619.08 ± 142.15 and 560.59 ± 157.70 mL/g-VS_added, respectively). Meanwhile, mango, pineapple, orange and jackfruit waste did not show significant differences (p<0.05) from the control, possibly due to inhibiting factors and failure of the AD process. These results indicate that the combination of DCF and TFW is an effective strategy for increasing biogas production, but the selection of the type of fruit waste as a substrate is a very important and crucial factor. The use of a combination of DCF with durian and banana waste was most recommended because it produced the highest methane production.

Acknowledgement

This study was supported by Diponegoro University (UNDIP) through the International Post Doc, World-Class University Program 2025.

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