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Mesophilic batch anaerobic digestion from fruit fragments
Renewable Energy xxx (2016) 1e7
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Mesophilic batch anaerobic digestion from fruit fragments Adhitya Pitara Sanjaya a, Muhammad Nur Cahyanto b, Ria Millati b, * a b
Department of Food Science and Technology, Faculty of Agriculture, Sebelas Maret University, Surakarta, 57126, Indonesia Department of Food and Agricultural Product Technology, Faculty of Agricultural Technology, Universitas Gadjah Mada, Yogyakarta, 55281, Indonesia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2015 Received in revised form 14 February 2016 Accepted 19 February 2016 Available online xxx
Fresh ripe and rotten fruits including oranges, mangosteen, bananas, and rambutan were separated into its fragments, i.e., peel, pulp, and seed in order to determine the rates and yield of their conversion into methane. Methane production from each of the components of the fruit was carried out under mesophilic conditions (35 C) using 120 ml-glass serum bottles during 60 days of incubation. The effectiveness of the anaerobic digestion was expressed using the value of digestibility. The level of methane yield from the tested fruit fractions was in the order of seed > pulp > peel. The methane yields from the seed, pulp, and peel were in the range of 504.11 ± 21.15 to 657.89 ± 63.58 ml CH4/g VS, 287.89 ± 38.79 to 468.91 ± 27.62 ml CH4/g VS, and 0.00 ± 0.00 to 202.75 ± 40.86 ml CH4/g VS, respectively. The highest digestibility was obtained from the anaerobic digestion of the seed of mangosteen, which was 99.3% and 99.4% from the fresh ripe and rotten mangosteen, respectively. The lowest digestibility was obtained from the mangosteen peel, which was 0.00%. The chemical composition, the presence of flavor compounds, and the physical structure of the fruit fragments affect the methane production. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Methane Fruit fraction Orange Mangosteen Banana Rambutan
1. Introduction World production and trade of fresh ripe fruit have been growing steadily in the last decade. Food and Agricultural Organization of the United Nation (FAO UN) reported that in 2010, the global fresh ripe fruit production was 775 million tons. This number was an increase of 177 million tons over the 2000 base periods [1]. However, around 6.8% of the fruits were wasted during the postharvest handling process, distribution, and consumption. Fruit is a perishable material and is easily degraded by microorganisms. Mechanical damage, physiological deterioration, and excessive ripeness can all accelerate biological degradation [2,3]. The most common fruit waste disposal practice, particularly in developing countries, is land filling, since it is cheap, easy, and needs little capital investment. However, landfill produces leachate, attracts vectors (e.g. insects, rodents, and birds) and emits greenhouse gases [4,5], which all hinder the future of landfill practices due to the environmental concerns. Fruits have high moisture and organic contents and are readily biodegradable, therefore, anaerobic digestion is considered as a suitable method for fruit waste treatment [6,7]. Furthermore,
* Corresponding author. E-mail address: [email protected] (R. Millati).
anaerobic digestion represents an affordable low cost and lowtechnology system to supply biogas as a clean energy source [8,9]. Previous studies have shown that fruit wastes can be converted into biogas with reasonably high methane yields. Scano [2] reported that the methane yield from the fruit and vegetable waste that had 8.7% total solid (TS) and 86% volatile solid (VS) was 0.43 Nm3/kg VS. In another study, the methane yields from sapodilla and pomegranate fruits were 0.327 m3/kg VS and 0.342 m3/kg VS, respectively [10]. In this study, four tropical fruits, e.g. oranges, mangosteen, bananas, and rambutan were used as feedstocks for the individually anaerobic digestion to determine the methane yield potential of each fruit. Oranges and bananas are among the major fruits in the world. FAO UN reported that in 2010, the world production of oranges and bananas reached 68.3 million tons and 106 million tons, respectively [1]. Mangosteen and rambutan are typically tropical fruits, which are produced in plenty of amounts in Indonesia. Mangosteen is a tropical evergreen tree and its edible portion is only about 40% of the whole fruit and the rest is peel [11]. Rambutan is a bright-red oval fruit, which has a seed, soft hairy peel, and a translucent aril. According to the Central Bureau Statistics of Indonesia, the productions of mangosteen and rambutan in Indonesia in 2010 were 84,538 tons and 522,852 tons, respectively [12]. In anaerobic digestion process of fruits, characteristics of the substrate, e.g., nutritional value, flavor compound, and physical
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properties of the fruits affect the high value production of methane [13]. In general, fruits have three major fractions, i.e., seed, pulp, and peel. Every single fraction of fruit has a different characteristic. For example, seed, as an ovary in the fruit development, has a high fat, protein, and carbohydrate content. On the other hand, peel contains fiber, e.g., pectin and cellulose. Pulp also contains high carbohydrate, vitamins, minerals, fiber, etc. Due to their nutritional value, seed, peel, and pulp of fruits can be used as a potential substrate for anaerobic digestion to produce methane. Besides having the high nutritional value, which can be converted into methane using anaerobic digestion, another challenge comes from the substrate characteristics, i.e., flavor compound and physical properties. In nature, most plants are equipped with either physical or chemical defense against degradation by microbia. The physical barrier is the inherent properties of a native cell wall, which is very rigid and complex, making it resistant from enzymatic attack. It involves a sophisticated and integrated defense system combining crystalline cellulose in microfibrils, heteropolysaccharides, and lignin. This is in the case of a lignocellulosic substrate. Lignocellulosic material and lignin are degraded by anaerobic digestion slowly and incompletely [14,15]. Furthermore, several studies showed that some flavor compounds can inhibit the anaerobic digestion process [16,17]. Due to the complexities of the substrate, the fruits were fractionated into seed, pulp, and peel in order to investigate the methane production from each fraction. 2. Materials and methods 2.1. The fruits Fresh ripe and rotten fruits, including “Pontianak” orange (Citrus nobilis Lour. Var. microcarpa Hassk.), mangosteen (Garcinia mangostana L.), bananas (Musa paradisiaca L.), and rambutan (Nephelium lappaceum L.) var. Binjai were used as substrates for the anaerobic digestion. The fruits were obtained from a local market in Yogyakarta. Both the fresh ripe and rotten fruits were peeled and separated into three parts, i.e., peel, pulp, and seed. Rotten fruits were prepared by incubating fresh ripe fruits in open space at room temperature for 4e7 days. The definition of fresh ripe fruits used in this work was fruits that were edible and had bright peel color. Whereas, the definition of rotten fruits is fruits that had damaged peel, moldy, bruised pulp, and easily crushed. Each part of the fruit was chopped and further homogenized using a blender. Thereafter, the samples were put in plastic bags and stored at 20 C before use. A summary of the chemical compositions of the fresh ripe and rotten fruits is presented in Table 1.
according to the methods no. 971.28 and no. 940.26 of AOAC by drying the samples using a convection drying oven at 105 C to achieve a constant weight, and burning the samples at 575 C [19]. Fat content was determined according to the AOAC method no.963.15. The fat content was extracted from the fruits by petroleum ether as a non-polar solution using a soxhlet extractor and then dried at 105 C to achieve constant weight [19]. Total nitrogen content was determined according to the Micro-Kjedahl method. The determination of the total nitrogen content was followed by hydrolysis using sulfuric acid and a catalyzer (Na2SO4 and HgO (20:1)), distillation, and titration using 0.02 N HCl [19]. Hemicellulose and cellulose were determined according to the method described by Chesson using the thermogravimetric method [20]. In this method, dried blended fruits were hydrolyzed in a two-step acid hydrolysis process with 1 N sulfuric acid to hydrolyze the hemicellulose and then 72% sulfuric acid to hydrolyze the cellulose. After each hydrolysis step, the samples were filtered and dried at 105 C to achieve a constant weight. In accordance with McKay et al. regarding modification [21], the total sugar and soluble starch were determined by hydrolyzing the samples using 6% HCl to get simple sugars. Then, the simple sugar was reacted with cupric sulfate and ammonium sulfate to form a sugar-copper ammonium sulfate complex. The absorbance of standard sugars in the working reagent was read at 310 nm. Methane and carbon dioxide production and composition were analyzed using a gas chromatograph (Shimadzu GC-2010, Japan) equipped with a capillary column (RT-Q bond, 30 m, 0.32 nm ID, 10 mm DF, Restek Corporation, U.S.A.), and a thermal conductivity detector (TCD) (Shimadzu, Japan) with an inject temperature of 100 C. The carrier gas used was helium, operated with a flow rate of 1.5 ml/min at 50 C using a split ratio of 20. The initial methane production rate was measured as the mean of methane production per day during the first ten days. Analysis of Variance (ANOVA) with a significance level of 0.05 was used to analyze the significance of the means differences of each sample. This test was performed using the Statistical Package for the Social Sciences version 19 (SPSS). 3. Results and discussion Four tropical fruits were selected as feedstocks for the individual anaerobic digestion, under mesophilic conditions during 60 days of incubation. In this study, the fruits were fractionated into its three components, i.e., peel, pulp, and seed in order to investigate the characteristic of each component and its effect on the methane production. Due to the different chemical and structure of each fraction, the methane production varied significantly.
2.2. Anaerobic digestion 3.1. Methane production of seed, pulp, and peel of fruits Anaerobic digestion was carried out in batch operations using 120 ml-glass serum bottles under mesophilic conditions (35 C), according to a method previously described by Hansen [18]. Active inoculum was obtained from the biogas digester (the Agricultural Training, Research and Development Station (ATRD), Universitas Gadjah Mada, Yogyakarta, Indonesia). Each bottle contained 0.60 g VS (Volatile Solid) of inoculum and 0.15 g VS of fresh ripe or rotten fruits and an addition of distilled water up to 30 ml. Hence, the ratio of organic content and volume is 0.025 g VS/ml. Blank samples were also prepared containing deionized water and inoculum. All these experiments were performed in triplicate, and the accumulated methane production was determined during 60 days. 2.3. Analytical methods The total solid, volatile solid, and ash content were determined
3.1.1. Oranges The methane production profile of the oranges is shown in Fig. 1a,b and Table 2. The highest initial methane production rates for both the fresh ripe and the rotten oranges were achieved by anaerobic digestion of the seeds, i.e., 37.59 ± 3.48 ml CH4/g VS/day and 42.79 ± 2.57 ml CH4/g VS/day, respectively. The initial methane production rate from the pulp was 27.37 ± 2.79 ml CH4/g VS/day from the rotten orange and 21.93 ± 7.06 ml CH4/g VS/day from the fresh ripe orange. Meanwhile, the methane production from the orange peels both fresh ripe and rotten, was very low, compared to that from the seeds and the pulp. No methane was produced during the first 10 days of the digestion. Methane began to be produced after 20 days of digestion (Fig. 1a,b). As shown in the figures, the methane production on the last days of incubation had reached almost a constant level, which means that the maximum methane
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Table 1 Composition of organic materials in the fresh ripe and rotten fruits. Type of fruit
Fraction
Total solid (%)
VS (%)
Hemicellulose (%)
R.Orange
Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Pulp Peel Pulp Peel Seed Pulp Peel Seed Pulp Peel
37.5 7.6 26.2 37.1 9.9 24.4 24.0 12.7 41.5 34.9 15.3 34.5 18.0 18.3 22.9 10.6 50.3 9.4 23.6 62.0 18.6 35.1
96.1 92.3 95.8 95.7 94.6 96.5 95.1 93.5 95.5 97.3 98.9 96.4 91.1 89.4 95.4 85.4 98.6 94.0 97.6 99.2 99.6 98.3
18.5 0.0 4.0 19.1 6.9 16.5 3.2 7.7 15.3 14.8 4.7 14.4 27.1 11.8 32.6 28.6 33.0 12.3 19.2 31.0 9.9 19.2
F.Orange
R.Mangosteen
F.Mangosteen
R.Banana F.Banana R.Rambutan
F.Rambutan
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.1 0.0 3.5 0.4 0.0 0.3 0.3 0.2 1.4 3.0 0.1 0.5 0.6 1.0 0.7 0.6 0.1 0.1 0.4 1.0 0.0 0.0
Cellulose (%) 25.9 30.7 29.9 25.5 11.2 4.8 18.7 9.7 22.5 24.9 3.5 23.5 8.4 9.8 3.6 13.8 8.4 11.8 17.0 12.5 12.7 17.5
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.2 4.5 4.0 0.2 0.3 0.4 0.0 0.2 0.3 0.6 0.3 0.3 0.4 0.9 0.3 0.1 0.8 0.2 0.0 0.9 0.2 0.2
Fat (%) 36.2 2.7 2.2 31.7 0.3 1.6 22.2 3.7 2.4 29.2 0.8 2.4 1.0 6.3 0.3 3.1 29.3 3.8 2.4 27.1 0.5 0.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Total sugar (%) 0.0 0.2 0.2 1.8 0.0 0.0 0.1 0.0 0.0 0.5 0.0 0.0 0.0 0.2 0.0 0.0 0.1 0.2 0.0 0.3 0.0 0.1
3.6 19.5 29.8 4.8 15.1 30.6 6.3 21.0 4.3 8.5 24.4 4.8 19.5 15.5 18.8 6.8 6.7 23.3 9.1 7.6 23.9 9.3
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.4 1.1 0.4 0.0 5.0 0.1 1.8 0.4 0.2 1.8 0.1 0.1 0.4 0.6 0.3 0.3 0.0 0.0 0.2 0.7 0.4
Soluble starch (%) 13.1 20.2 24.7 7.4 31.5 22.1 10.8 22.8 4.1 5.1 41.8 4.1 27.1 25.7 25.7 40.6 17.6 39.8 26.3 19.0 42.3 19.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1.0 1.1 0.3 0.1 1.4 1.0 1.1 0.4 0.0 0.4 2.5 0.1 1.0 0.5 0.0 3.5 0.2 0.0 1.4 0.8 0.5 1.1
F ¼ fresh ripe; R ¼ rotten. The results listed in this table are based on dry weight, except total solid content.
production had been obtained. Based on Fig. 1aeb, the technical digestion time T90 (the time needed to produce 90% of the maximum methane production) of the seeds, pulp, and peel occurred within 30e37 days. The technical digestion time is a useful information to determine the hydraulic retention time (HRT) for continuous anaerobic digestion of the corresponding substrate [22]. The methane yield from the different fractions of the oranges varied (Table 2). The digestion of the seeds from the fresh ripe and rotten oranges gave the highest methane yields, which were 580.71 ± 27.00 ml CH4/g VS and 657.89 ± 63.58 ml CH4/g VS, respectively. However, the lowest methane yields of 71.60 ± 32.64 ml CH4/g VS and 48.23 ± 6.01 ml CH4/g VS were obtained from the peels of the fresh ripe and rotten oranges, respectively. The digestibility was determined in order to analyze the potential methane production from each fruit fraction. The digestibility expresses the digestion capability of the material to be converted into methane. The seeds and pulp of the fresh ripe and rotten oranges showed a high percentage digestibility of more than 90% (Table 2). Furthermore, the peels showed the lowest values for the digestibility, which were 26.3% from the rotten orange and 29.5% from the fresh ripe orange. This corresponds to the considerably low methane production from the orange peels during the digestion process. The reason for this could be that oranges contain several flavor compounds such as myrcene, car-3-ene, a-pinene, octanol, which have inhibitory effects on anaerobic digestion [23]. Furthermore, limonene, a flavor compound in the orange peel oil, inhibited anaerobic digestion process and caused an ultimate failure of the process at a concentration of 400 mL/L in a mesophilic continuous system [24]. The methane content of both the fresh ripe and the rotten oranges from all the components was at relatively the same level (Fig. 2a,b). The corresponding values were 54.2e61.5% (v/v) of methane (Table 2).
3.1.2. Mangosteen The profiles of methane production from the mangosteen fractions, i.e., peel, pulp, and seed are presented in Fig. 1c,d and Table 2. The highest initial methane production rates were obtained from the seeds of the fresh ripe and rotten mangosteen, which were
30.75 ± 3.15 ml CH4/g VS/day and 34.53 ± 4.64 ml CH4/g VS/day, respectively. The initial methane production rates from the pulp of the fresh ripe and rotten mangosteen were 28.41 ± 11.06 ml CH4/g VS/day and 31.17 ± 18.09 ml CH4/g VS/day, respectively. The methane production from the rotten pulp was increased significantly after the 15th day of digestion, while the fresh ripe pulp mangosteen was increased steadily (Fig. 1 c,d). It can be seen in the figures that the technical digestion time (T90) of the seeds and pulp of the fresh ripe and rotten mangosteen occurred within 25e30 and 30e44 days, respectively. Meanwhile, no methane production was produced from the peels during the digestion process. The result indicates that there is an inhibition effect in the methane production of mangosteen peel. Suchitra and Wanapat [25] reported that the methane production in the rumen was significantly reduced when the feed of dairy cows was supplemented with mangosteen peel. It was mentioned that the inhibition of methane production was caused by the presence of tannins and saphonine in the mangosteen peel. To substantiate the complexity, mangosteen peel is a lignocellulosic material [26], which contains hemicellulose, cellulose, and lignin. Table 1 shows that hemicellulose and cellulose contents in the fresh ripe and rotten mangosteen peels were among the highest compared to that of the other fruits' fractions. Although lignocellulosic materials are biodegradable, they are difficult to be digested due to its rigid structure [27,28]. The level of the methane yields from the different fractions of the mangosteen is shown in Table 2. The highest methane yields were obtained by anaerobic digestion of the seeds, which were 551.65 ml CH4/g VS from the fresh ripe mangosteen and 535.12 ± 35.82 ml CH4/g VS from the rotten mangosteen. The methane yields from the fresh ripe and rotten pulp were significantly different. The methane yield from the fresh ripe pulp was lower than that of the rotten pulp, which were 297.77 ± 40.17 ml CH4/g VS and 468.91 ± 27.62 ml CH4/g VS, respectively. Furthermore, the highest values of the digestibility were achieved from the digestion of the seeds, which were 99.4% from the rotten mangosteen and 99.3% from the fresh ripe mangosteen. The biogas produced from anaerobic digestion of all the fractions of fresh ripe and rotten mangosteen consisted of 55.2e62.3% (v/v) methane and
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Fig. 1. Accumulated methane production from seed ( ), pulp (C), and peel ( ) of rotten orange (a), fresh ripe orange (b), rotten mangosteen (c), fresh ripe mangosteen (d), rotten banana (e), fresh ripe banana (f), rotten rambutan (g), and fresh ripe rambutan (h) in 35 C batch digester for 60 days.
35.1e42.7% (v/v) carbon dioxide (Fig. 2c,d and Table 2). 3.1.3. Bananas In this study, the banana peels and the pulp were used as substrates in the anaerobic digestion. The methane production from the fresh ripe and rotten banana peels and pulp is shown in Fig. 1e,f and Table 2. The profile of the methane production from the fresh ripe banana was very similar to that of the rotten banana. The initial methane production rates from the peels and the pulp were at relatively the same level. There was no significant difference between the initial methane production rates from both the fresh ripe and the rotten bananas. The initial production rates from the peels and pulp were 22.22 ± 3.29 ml CH4/g VS/day and 19.71 ± 3.76 ml CH4/g VS/day from the rotten bananas, respectively, and 24.18 ± 1.11 ml CH4/g VS/day and 22.28 ± 3.59 ml CH4/g VS/day from the fresh ripe bananas, respectively. Fig. 1e,f shows that the technical digestion time (T90) for methane production of the peels and pulp occurred within 37e44 days. The methane yields from the fresh ripe and rotten bananas were at relatively the same level. The methane yields from the peels and
pulp of the fresh ripe bananas were 342.25 ± 54.79 ml CH4/g VS and 320.64 ± 9.52 ml CH4/g VS, respectively, while the methane yields from the peels and pulp of the rotten bananas were 331.65 ± 20.45 ml CH4/g VS and 351.49 ± 17.30 ml CH4/g VS, respectively. The biogas from all the fractions of fresh ripe and rotten bananas consisted of 56.4e58.3% (v/v) methane content and 40.5e41.6% (v/v) carbon dioxide content (Fig. 2e,f and Table 2). 3.1.4. Rambutan The profile of the methane production from the fresh ripe and rotten rambutan is shown in Fig. 1g,h and Table 2. The highest initial methane production rate was achieved by using the seeds. The corresponding values were 31.55 ± 0.51 ml CH4/g VS/day for the rotten rambutan and 33.91 ± 3.77 ml CH4/g VS/day for the fresh ripe rambutan. The initial methane production rates for the fresh ripe and rotten pulp were relatively in the same level, which were 31.54 ± 4.72 ml CH4/g VS/day and 21.68 ± 2.57 ml CH4/g VS/day, respectively. The peels from the fresh ripe and rotten rambutan showed the lowest initial methane production rates, which were 16.58 ± 1.94 ml CH4/g VS/day and 21.68 ± 2.57 ml CH4/g VS/day,
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Table 2 Process performance and methane production profile of the fresh ripe and rotten fruits at 35 C during 60 days of incubation. Type of fruit
Fraction
Methane production profile Initial rate (ml CH4/g VS/day) ± stda
R.Orange
F.Orange
R.Mangosteen
F.Mangosteen
R.Banana F.Banana R.Rambutan
F.Rambutan
Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Pulp Peel Pulp Peel Seed Pulp Peel Seed Pulp Peel
42.79 27.37 0.00 37.59 21.93 4.16 34.53 31.17 13.92 30.75 28.41 15.15 19.71 22.22 22.28 24.18 31.55 29.54 21.68 33.91 31.54 16.58
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
2.57 2.79 0.00 3.48 7.06 3.69 4.64 1.09 1.32 3.15 1.06 3.92 3.76 3.29 3.59 1.11 0.51 0.15 2.57 3.77 4.72 1.94
Methane yield (ml CH4/g VS) ± stda 657.89 311.71 48.23 580.71 287.89 71.60 535.12 468.91 0.00 551.65 297.77 0.00 351.49 331.65 320.64 342.25 504.11 322.82 126.30 527.05 309.30 202.75
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
63.58 10.03 6.01 27.00 38.79 32.64 144.54 260.49 0.00 19.79 40.14 0.00 17.30 20.45 9.52 54.79 21.15 20.39 44.29 29.04 13.59 40.89
% CH4 (v/v)
% CO2 (v/v)
pH
61.2 57.6 55.2 58.3 61.5 54.2 55.2 62.3 0.0 51.7 61.5 0.0 56.4 58.3 56.6 57.1 58.8 55.3 58.8 57.6 61.3 55.3
38.5 40.7 41.7 39.5 37.4 42.2 42.7 35.9 0.0 36.0 35.1 0.0 41.0 40.5 41.1 41.6 40.1 42.8 40.7 40.2 38.2 42.5
7.29 7.34 7.34 7.33 7.28 7.33 7.13 7.12 7.15 7.16 7.25 7.18 7.29 7.24 7.18 7.15 7.27 7.22 7.04 7.24 7.19 6.82
Digestibility (%)b 92.1 96.0 26.3 96.8 96.0 29.5 99.4 88.8 0.0 99.3 88.7 0.0 95.8 97.9 79.8 90.4 82.6 77.5 37.4 79.2 83.8 69.9
F ¼ fresh ripe; R ¼ rotten. a Std ¼ standard deviation. Yield of methane production from fruit b Digestibility ¼ Methane 100%: production from theoretical value
respectively. The methane production from the fresh ripe and rotten peels was significantly different. After 20 days of digestion, the production of methane from the rotten peel decreased until the end of the digestion; as a result, the methane production from the rotten peel was considerably low. Meanwhile, the methane production from the fresh ripe peels increased after 20 days of digestion. The low methane production from the rotten peel indicates that the anaerobic digestion process is inhibited. The inhibition could be attributed to the phenolic components that are present in rambutan peels [29]. Furthermore, as shown in Fig. 1geh, the technical digestion time (T90) of the seeds, pulp, and peels of fresh ripe and rotten rambutan occurred within 37e44, 44e51, 44e51 days, respectively. The highest methane yield from the rambutan was obtained from the seeds of the fresh ripe and rotten rambutan, which were 527.05 ± 29.04 ml CH4/g VS and 504.11 ± 21.15 ml CH4/g VS, respectively. The peels of rambutan had the lowest methane yield than that of the other fractions. The digestibility from the seeds of the fresh ripe and rotten rambutan and the pulp of the fresh ripe and rotten rambutan were relatively the same level. The lowest digestibility was achieved from the peels of the rotten rambutan, which was 37.4%. The biogas from all the fractions of fresh ripe and rotten rambutan consisted of 55.3e61.3% (v/v) of methane content and 38.2e42.8% (v/v) of carbon dioxide (Fig. 2g,h and Table 2). The results of the anaerobic digestion of the fruits'fractions in this work showed that the seeds of fresh ripe and rotten fruits gave a higher methane production and methane yield than that of the pulp and peels. This could be explained by the fact that the seeds had a higher fat content than both the pulp and the peels did. Fat is a macromolecule consisting of carbon, hydrogen, and oxygen atoms. The existence of carbon, hydrogen, and oxygen atoms make fat a suitable substrate for anaerobic digestion process. Fat was considered as a potential substrate for anaerobic digestion as the methane yield from fat was higher compared to that of protein or carbohydrate [30,31]. In this work, the seeds that contained fat of 22.2e36.2% from the total chemical composition (Table 1) contribute to 50e60% of the theoretical methane production (data
not shown). Kafle et al. [32] reported that substrates with high fat and low carbohydrate contents produced higher methane yields than that of substrates with high carbohydrate and low fat contents. Meanwhile, the production of methane from the pulp was higher than that of the peels. This result could be related to the total sugar and soluble starch contents that were higher in the pulp than that of the peels. The peel of oranges, mangosteen, and rotten rambutan produced low or even no methane production. The low methane production from the peels could be attributed to the presence of inhibitory flavor compounds and the complexity of the cellulose and hemicellulose structure of the peels. If we sum up the methane yields from the seeds, pulp, and peels of the rotten fruits in Table 2; orange, mangosteen, and rambutan yielded methane almost in the same level whereas rotten banana produced the lowest methane yield. From the summed methane yields of the fresh ripe fruits' fractions, the fresh ripe rambutan produced a higher methane yield compared to that of the fresh ripe of orange, which was then followed by mangosteen. Similar to the results from the rotten fruits, the fresh ripe banana produced the lowest methane yield. The results showed that higher methane yields were produced from the fruits that had seeds. As aforementioned, seeds contain fat that can be used as substrate to produce high methane yield. Since banana is a fruit that does not have seeds, both the fresh ripe and rotten bananas produced the lowest methane yields among the other fruits. 3.2. Effect of fruit ripening condition on methane production During the ripening process in fruits, a number of physical and chemical changes occur. The changes can be different for each fruit. Almost all fruits change on their external colors and texture during the ripening process. The external color changing is caused by the degradation of fruit pigments such as lycopene, carotenes, xanthophylls, and chlorophylls. The color changes have been used as a rough guide to determine the stage of ripeness. As fruits normally soften progressively during ripening [33], their texture also
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Fig. 2. Methane contents for methane produced from seed (B), pulp (C), and peel ( ) of rotten orange (a), fresh ripe orange (b), rotten mangosteen (c), fresh ripe mangosteen (d), rotten banana (e), fresh ripe banana (f), rotten rambutan (g), and fresh ripe rambutan (h).
undergoes changes. In this work, the fresh ripe and rotten fruits were used in order to evaluate the effect of fruit ripening conditions on methane production. The pulp of the rotten fruits generated higher methane yields compared to that of the pulp of the fresh ripe fruits (Table 2). In Table 1, the soluble starch content in the pulp of the fresh ripe fruits was higher than that of the rotten fruits. On the other hand, the total sugar in the pulp of the rotten fruits was higher than that of the fresh ripe fruits. During the ripening process, starch is completely hydrolyzed to simple sugars [33]. Simple sugars are more easily degraded and converted to methane than starch. Accordingly, higher methane yields from the pulp of the rotten fruits can be expected. In general, the seeds and peels contain high cellulose and hemicellulose (Table 1). Cellulose and hemicellulose are recalcitrant materials that are hard to be degraded by biological process. Due to the rigid structure of cellulose and hemicellulose, the seeds and peels might not been totally degraded into simple sugars
during the ripening and decay processes. As a result, the methane yields from the seeds and peels of the fresh ripe and rotten fruits were almost similar (Table 2). 4. Conclusion This study has demonstrated the differences in the methane production of each fruit fraction. Except for the peels from oranges, mangosteen, rotten rambutan, the other fruit fractions studied in this work could be converted into methane with reasonable yields. The methane yields from the seeds, pulp, and peel were in the range of 202.75 ± 40.86 to 657.89 ± 63.58 ml CH4/g VS. However, the anaerobic digestion from the peels of oranges, mangosteen, and rotten rambutan resulted in a low or even no methane production. The methane production from the seeds was higher than that from the pulp and the peels, while the methane production from the pulp was higher than that from the peels. Methane production from the fruits can be affected by the chemical composition, the presence
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of flavor compounds, and the physical structure of the fruits. The chemical composition and the structure of fruits' fractions change during the ripening process. The changes influence the level of methane production. Acknowledgment The authors wish to thank The Swedish International Development Agency (SIDA, project number 348-2013-8918) for providing financial support for this work. References [1] FAOSTAT, Food and Agriculture Organization of The United Nations Statistics Division, Online: 2015 http://faostat3.fao.org. [2] E.A. Scano, C. Asquer, A. Pistis, L. Ortu, V. Demontis, D. Cocco, Biogas from anaerobic digestion of fruit and vegetable wastes: experimental results on pilot-scale and preliminary performance evaluation of a full-scale power plant, Energy Convers. Manage. 77 (2014) 22e30. [3] S.K. Chattopadhyay, Handling, Transportation and Storage of Fruits and Vegetables, Global Media, Delhi, India, 2007. [4] M. Aljaradin, K.M. Persson, Environmental impact of municipal solid waste landfills in semi-arid climates e case study e Jordan, Open Waste Manage. J. 5 (2012) 28e39. [5] M. El-Fadel, A.N. Findikakis, J.O. Leckie, Environmental impacts of solid waste land filling, Environ. Manage. 50 (1997) 1e25. [6] H. Bouallagui, R.B. Cheikh, L. Marouani, M. Hamdi, Mesophilic biogas production from fruit and vegetable waste in a tubular digester, Bioresour. Technol. 86 (2003) 85e89. ~ a, P. Parameswaran, D.W. Kang, M. Canul-Chan, R. Krajmalnik[7] E.I. Garcia-Pen Brown, Anaerobic digestion and co-digestion processes of vegetable and fruit residues: process and microbial ecology, Bioresour. Technol. 102 (2011) 9447e9455. [8] A. Khalid, M. Arshad, M. Anjum, T. Mahmood, L. Dawson, The anaerobic digestion of solid organic waste, Waste Manage. 31 (8) (2011) 1737e1744. [9] Santosh Yadvika, T.R. Sreekrishnan, S. Kohli, V. Rana, Enhancement of biogas production from solid substrates using different techniqueseea review, Bioresour. Technol. 95 (2004) 1e10. [10] V.N. Gunaseelan, Biochemical methane potential of fruits and vegetable solid waste feedstocks, Biomass Bioenergy 26 (2004) 389e399. [11] Y. Chen, B. Huang, M. Huang, B. Cai, On the preparation and characterization of activated carbon from mangosteen shell, J. Taiwan Inst. Chem. Eng. 42 (2011) 837e842. [12] Central Bureau Statistics of Indonesia. Produksi buah-buahan dan sayuran tahunan di Indonesia, Online: 2015 http://www.bps.go.id/site/pilihdata. [13] D. Deublein, A. Steinhauser, Biogas from Waste and Renewable Resources, €rlenbach, Germany, 2008. WILEY-VCH Vewrlag GmbH & Co. KgaA., Mo [14] M.J. Taherzadeh, K. Karimi, Pretreatment of lignocellulosic waste to improve ethanol and biogas production: a review, Int. J. Mol. Sci. 9 (2008) 1621e1651. [15] D. Brown, J. Shi, Y. Li, Comparison of solid-state to liquid anaerobic digestion of lignocellulosic feedstocks for biogas production, Bioresour. Technol. 124
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(2012) 379e386. [16] H. Yanti, R. Wikandari, R. Millati, C. Niklasson, M.J. Taherzadeh, Effect of ester compounds on biogas production: beneficial or detrimental? Energy Sci. Eng. 2 (1) (2014) 22e30. [17] R. Wikandari, S. Youngsukkasem, R. Millati, M.J. Taherzadeh, Performance of semi-continuous membrane bioreactor in biogas production from toxic feedstock containing D-limonene, Bioresour. Technol. 170 (2014) 350e355. [18] T.L. Hansen, J.E. Schmidt, I. Angelidaki, E. Marca, J.C. Jansen, H. Mosbæk, T.H. Christensen, Method for determination of methane potentials of solid organic waste, Waste Manage. 24 (2004) 393e400. [19] AOAC, Official Methods of Analysis of AOAC International, eighteenth ed., AOAC International, Maryland, 2006. [20] A. Chesson, Acidogenic fermentation of lignocellulose-acid yield and conversion of components, Biotechnol. Bioeng. 23 (1981) 2167e2170. [21] D.B. McKay, G.P. Tanneq, D.J. MacClean, K.J. Scott, Detection of polyols and sugars by cuprammonium ion in the presence of strong base, Anal. Biochem. 165 (1987) 392e398. [22] G.K. Kafle, S.H. Kim, K.I. Sung, Ensiling of fish industry waste for biogas production: a lab scale evaluation of biochemical methane potential (BMP) and kinetics, Bioresour. Technol. 127 (2013) 326e336. [23] R. Wikandari, S. Gudipudi, I. Pandiyan, R. Millati, M.J. Taherzadeh, Inhibitory effects of fruit flavors on methane production during anaerobic digestion, Bioresour. Technol. 145 (2013) 188e192. [24] E. Mizuki, T. Akao, T. Saruwatari, Inhibitory effect of Citrus unshu peel on anaerobic digestion, Biol. Wastes 33 (3) (1990) 161e168. [25] K. Suchitra, M. Wanapat, Effects of mangosteen (Garcinia mangostana) peel and sunflower and coconut oil supplementation on rumen fermentation, milk yield and milk composition in lactating dairy cows, Livest. Res. Rural. Dev. 20 (Suppl.) (2008). [26] B.T.H. Guan, P.A. Latif, T.Y.H. Yap, Physical preparation of activated carbon from sugarcane bagasse and corn husk and its physical and chemical characteristics, Int. J. Eng. Res. Sci. Tech 2 (3) (2013) 1e14. €nsson, C. Martín, Pretreatment of lignocellulose: formation of inhibitory [27] L.J. Jo by-products and strategies for minimizing their effects, a review, Bioresour. Technol. 199 (2016) 103e112. [28] M.M. Kabir, C. Niklasson, M.J. Taherzadeh, I.S. Horv ath, Biogas production from lignocelluloses by N-methylmorpholine-N-oxide (NMMO) pretreatment: effects of recovery and reuse of NMMO, Short Communication, Bioresour. Technol. 161 (2014) 446e450. [29] N. Thitilertdecha, A. Teerawutgulrag, N. Rakariyatham, Antioxidant and antibacterial activities of Nephelium lappaceum L. extracts, LWT Food Sci. Technol. 41 (10) (2008) 2029e2035. € rnsson, M.M. Alves, B. Mattiasson, Anaerobic [30] D.G. Cirne, X. Paloumet, L. Bjo digestion of lipid-rich wastedeffects of lipid concentration, Renew. Energy 32 (2007) 965e975. [31] S. Lansing, J.F. Martin, R.B. Botero, T. Nogueira da Silva, E. Dias da Silva, Methane production in low-cost, unheated, plug-flow digesters treating swine manure and used cooking grease, Bioresour. Technol. 101 (2010) 4362e4370. [32] G.K. Kafle, S.H. Kim, Effects of chemical compositions and ensiling on the biogas productivity and degradation rates of agricultural and food processing by-products, Bioresour. Technol. 142 (2013) 553e561. [33] A.K. Thompson, Fruit and Vegetables: Harvesting, Handling and Storage, Blackwell Publishing Ltd, Oxford, United Kingdom, 2003.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Mesophilic batch anaerobic digestion from fruit fragments Adhitya Pitara Sanjaya a, Muhammad Nur Cahyanto b, Ria Millati b, * a b
Department of Food Science and Technology, Faculty of Agriculture, Sebelas Maret University, Surakarta, 57126, Indonesia Department of Food and Agricultural Product Technology, Faculty of Agricultural Technology, Universitas Gadjah Mada, Yogyakarta, 55281, Indonesia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2015 Received in revised form 14 February 2016 Accepted 19 February 2016 Available online xxx
Fresh ripe and rotten fruits including oranges, mangosteen, bananas, and rambutan were separated into its fragments, i.e., peel, pulp, and seed in order to determine the rates and yield of their conversion into methane. Methane production from each of the components of the fruit was carried out under mesophilic conditions (35 C) using 120 ml-glass serum bottles during 60 days of incubation. The effectiveness of the anaerobic digestion was expressed using the value of digestibility. The level of methane yield from the tested fruit fractions was in the order of seed > pulp > peel. The methane yields from the seed, pulp, and peel were in the range of 504.11 ± 21.15 to 657.89 ± 63.58 ml CH4/g VS, 287.89 ± 38.79 to 468.91 ± 27.62 ml CH4/g VS, and 0.00 ± 0.00 to 202.75 ± 40.86 ml CH4/g VS, respectively. The highest digestibility was obtained from the anaerobic digestion of the seed of mangosteen, which was 99.3% and 99.4% from the fresh ripe and rotten mangosteen, respectively. The lowest digestibility was obtained from the mangosteen peel, which was 0.00%. The chemical composition, the presence of flavor compounds, and the physical structure of the fruit fragments affect the methane production. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Methane Fruit fraction Orange Mangosteen Banana Rambutan
1. Introduction World production and trade of fresh ripe fruit have been growing steadily in the last decade. Food and Agricultural Organization of the United Nation (FAO UN) reported that in 2010, the global fresh ripe fruit production was 775 million tons. This number was an increase of 177 million tons over the 2000 base periods [1]. However, around 6.8% of the fruits were wasted during the postharvest handling process, distribution, and consumption. Fruit is a perishable material and is easily degraded by microorganisms. Mechanical damage, physiological deterioration, and excessive ripeness can all accelerate biological degradation [2,3]. The most common fruit waste disposal practice, particularly in developing countries, is land filling, since it is cheap, easy, and needs little capital investment. However, landfill produces leachate, attracts vectors (e.g. insects, rodents, and birds) and emits greenhouse gases [4,5], which all hinder the future of landfill practices due to the environmental concerns. Fruits have high moisture and organic contents and are readily biodegradable, therefore, anaerobic digestion is considered as a suitable method for fruit waste treatment [6,7]. Furthermore,
* Corresponding author. E-mail address: [email protected] (R. Millati).
anaerobic digestion represents an affordable low cost and lowtechnology system to supply biogas as a clean energy source [8,9]. Previous studies have shown that fruit wastes can be converted into biogas with reasonably high methane yields. Scano [2] reported that the methane yield from the fruit and vegetable waste that had 8.7% total solid (TS) and 86% volatile solid (VS) was 0.43 Nm3/kg VS. In another study, the methane yields from sapodilla and pomegranate fruits were 0.327 m3/kg VS and 0.342 m3/kg VS, respectively [10]. In this study, four tropical fruits, e.g. oranges, mangosteen, bananas, and rambutan were used as feedstocks for the individually anaerobic digestion to determine the methane yield potential of each fruit. Oranges and bananas are among the major fruits in the world. FAO UN reported that in 2010, the world production of oranges and bananas reached 68.3 million tons and 106 million tons, respectively [1]. Mangosteen and rambutan are typically tropical fruits, which are produced in plenty of amounts in Indonesia. Mangosteen is a tropical evergreen tree and its edible portion is only about 40% of the whole fruit and the rest is peel [11]. Rambutan is a bright-red oval fruit, which has a seed, soft hairy peel, and a translucent aril. According to the Central Bureau Statistics of Indonesia, the productions of mangosteen and rambutan in Indonesia in 2010 were 84,538 tons and 522,852 tons, respectively [12]. In anaerobic digestion process of fruits, characteristics of the substrate, e.g., nutritional value, flavor compound, and physical
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properties of the fruits affect the high value production of methane [13]. In general, fruits have three major fractions, i.e., seed, pulp, and peel. Every single fraction of fruit has a different characteristic. For example, seed, as an ovary in the fruit development, has a high fat, protein, and carbohydrate content. On the other hand, peel contains fiber, e.g., pectin and cellulose. Pulp also contains high carbohydrate, vitamins, minerals, fiber, etc. Due to their nutritional value, seed, peel, and pulp of fruits can be used as a potential substrate for anaerobic digestion to produce methane. Besides having the high nutritional value, which can be converted into methane using anaerobic digestion, another challenge comes from the substrate characteristics, i.e., flavor compound and physical properties. In nature, most plants are equipped with either physical or chemical defense against degradation by microbia. The physical barrier is the inherent properties of a native cell wall, which is very rigid and complex, making it resistant from enzymatic attack. It involves a sophisticated and integrated defense system combining crystalline cellulose in microfibrils, heteropolysaccharides, and lignin. This is in the case of a lignocellulosic substrate. Lignocellulosic material and lignin are degraded by anaerobic digestion slowly and incompletely [14,15]. Furthermore, several studies showed that some flavor compounds can inhibit the anaerobic digestion process [16,17]. Due to the complexities of the substrate, the fruits were fractionated into seed, pulp, and peel in order to investigate the methane production from each fraction. 2. Materials and methods 2.1. The fruits Fresh ripe and rotten fruits, including “Pontianak” orange (Citrus nobilis Lour. Var. microcarpa Hassk.), mangosteen (Garcinia mangostana L.), bananas (Musa paradisiaca L.), and rambutan (Nephelium lappaceum L.) var. Binjai were used as substrates for the anaerobic digestion. The fruits were obtained from a local market in Yogyakarta. Both the fresh ripe and rotten fruits were peeled and separated into three parts, i.e., peel, pulp, and seed. Rotten fruits were prepared by incubating fresh ripe fruits in open space at room temperature for 4e7 days. The definition of fresh ripe fruits used in this work was fruits that were edible and had bright peel color. Whereas, the definition of rotten fruits is fruits that had damaged peel, moldy, bruised pulp, and easily crushed. Each part of the fruit was chopped and further homogenized using a blender. Thereafter, the samples were put in plastic bags and stored at 20 C before use. A summary of the chemical compositions of the fresh ripe and rotten fruits is presented in Table 1.
according to the methods no. 971.28 and no. 940.26 of AOAC by drying the samples using a convection drying oven at 105 C to achieve a constant weight, and burning the samples at 575 C [19]. Fat content was determined according to the AOAC method no.963.15. The fat content was extracted from the fruits by petroleum ether as a non-polar solution using a soxhlet extractor and then dried at 105 C to achieve constant weight [19]. Total nitrogen content was determined according to the Micro-Kjedahl method. The determination of the total nitrogen content was followed by hydrolysis using sulfuric acid and a catalyzer (Na2SO4 and HgO (20:1)), distillation, and titration using 0.02 N HCl [19]. Hemicellulose and cellulose were determined according to the method described by Chesson using the thermogravimetric method [20]. In this method, dried blended fruits were hydrolyzed in a two-step acid hydrolysis process with 1 N sulfuric acid to hydrolyze the hemicellulose and then 72% sulfuric acid to hydrolyze the cellulose. After each hydrolysis step, the samples were filtered and dried at 105 C to achieve a constant weight. In accordance with McKay et al. regarding modification [21], the total sugar and soluble starch were determined by hydrolyzing the samples using 6% HCl to get simple sugars. Then, the simple sugar was reacted with cupric sulfate and ammonium sulfate to form a sugar-copper ammonium sulfate complex. The absorbance of standard sugars in the working reagent was read at 310 nm. Methane and carbon dioxide production and composition were analyzed using a gas chromatograph (Shimadzu GC-2010, Japan) equipped with a capillary column (RT-Q bond, 30 m, 0.32 nm ID, 10 mm DF, Restek Corporation, U.S.A.), and a thermal conductivity detector (TCD) (Shimadzu, Japan) with an inject temperature of 100 C. The carrier gas used was helium, operated with a flow rate of 1.5 ml/min at 50 C using a split ratio of 20. The initial methane production rate was measured as the mean of methane production per day during the first ten days. Analysis of Variance (ANOVA) with a significance level of 0.05 was used to analyze the significance of the means differences of each sample. This test was performed using the Statistical Package for the Social Sciences version 19 (SPSS). 3. Results and discussion Four tropical fruits were selected as feedstocks for the individual anaerobic digestion, under mesophilic conditions during 60 days of incubation. In this study, the fruits were fractionated into its three components, i.e., peel, pulp, and seed in order to investigate the characteristic of each component and its effect on the methane production. Due to the different chemical and structure of each fraction, the methane production varied significantly.
2.2. Anaerobic digestion 3.1. Methane production of seed, pulp, and peel of fruits Anaerobic digestion was carried out in batch operations using 120 ml-glass serum bottles under mesophilic conditions (35 C), according to a method previously described by Hansen [18]. Active inoculum was obtained from the biogas digester (the Agricultural Training, Research and Development Station (ATRD), Universitas Gadjah Mada, Yogyakarta, Indonesia). Each bottle contained 0.60 g VS (Volatile Solid) of inoculum and 0.15 g VS of fresh ripe or rotten fruits and an addition of distilled water up to 30 ml. Hence, the ratio of organic content and volume is 0.025 g VS/ml. Blank samples were also prepared containing deionized water and inoculum. All these experiments were performed in triplicate, and the accumulated methane production was determined during 60 days. 2.3. Analytical methods The total solid, volatile solid, and ash content were determined
3.1.1. Oranges The methane production profile of the oranges is shown in Fig. 1a,b and Table 2. The highest initial methane production rates for both the fresh ripe and the rotten oranges were achieved by anaerobic digestion of the seeds, i.e., 37.59 ± 3.48 ml CH4/g VS/day and 42.79 ± 2.57 ml CH4/g VS/day, respectively. The initial methane production rate from the pulp was 27.37 ± 2.79 ml CH4/g VS/day from the rotten orange and 21.93 ± 7.06 ml CH4/g VS/day from the fresh ripe orange. Meanwhile, the methane production from the orange peels both fresh ripe and rotten, was very low, compared to that from the seeds and the pulp. No methane was produced during the first 10 days of the digestion. Methane began to be produced after 20 days of digestion (Fig. 1a,b). As shown in the figures, the methane production on the last days of incubation had reached almost a constant level, which means that the maximum methane
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Table 1 Composition of organic materials in the fresh ripe and rotten fruits. Type of fruit
Fraction
Total solid (%)
VS (%)
Hemicellulose (%)
R.Orange
Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Pulp Peel Pulp Peel Seed Pulp Peel Seed Pulp Peel
37.5 7.6 26.2 37.1 9.9 24.4 24.0 12.7 41.5 34.9 15.3 34.5 18.0 18.3 22.9 10.6 50.3 9.4 23.6 62.0 18.6 35.1
96.1 92.3 95.8 95.7 94.6 96.5 95.1 93.5 95.5 97.3 98.9 96.4 91.1 89.4 95.4 85.4 98.6 94.0 97.6 99.2 99.6 98.3
18.5 0.0 4.0 19.1 6.9 16.5 3.2 7.7 15.3 14.8 4.7 14.4 27.1 11.8 32.6 28.6 33.0 12.3 19.2 31.0 9.9 19.2
F.Orange
R.Mangosteen
F.Mangosteen
R.Banana F.Banana R.Rambutan
F.Rambutan
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.1 0.0 3.5 0.4 0.0 0.3 0.3 0.2 1.4 3.0 0.1 0.5 0.6 1.0 0.7 0.6 0.1 0.1 0.4 1.0 0.0 0.0
Cellulose (%) 25.9 30.7 29.9 25.5 11.2 4.8 18.7 9.7 22.5 24.9 3.5 23.5 8.4 9.8 3.6 13.8 8.4 11.8 17.0 12.5 12.7 17.5
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.2 4.5 4.0 0.2 0.3 0.4 0.0 0.2 0.3 0.6 0.3 0.3 0.4 0.9 0.3 0.1 0.8 0.2 0.0 0.9 0.2 0.2
Fat (%) 36.2 2.7 2.2 31.7 0.3 1.6 22.2 3.7 2.4 29.2 0.8 2.4 1.0 6.3 0.3 3.1 29.3 3.8 2.4 27.1 0.5 0.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Total sugar (%) 0.0 0.2 0.2 1.8 0.0 0.0 0.1 0.0 0.0 0.5 0.0 0.0 0.0 0.2 0.0 0.0 0.1 0.2 0.0 0.3 0.0 0.1
3.6 19.5 29.8 4.8 15.1 30.6 6.3 21.0 4.3 8.5 24.4 4.8 19.5 15.5 18.8 6.8 6.7 23.3 9.1 7.6 23.9 9.3
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.4 1.1 0.4 0.0 5.0 0.1 1.8 0.4 0.2 1.8 0.1 0.1 0.4 0.6 0.3 0.3 0.0 0.0 0.2 0.7 0.4
Soluble starch (%) 13.1 20.2 24.7 7.4 31.5 22.1 10.8 22.8 4.1 5.1 41.8 4.1 27.1 25.7 25.7 40.6 17.6 39.8 26.3 19.0 42.3 19.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1.0 1.1 0.3 0.1 1.4 1.0 1.1 0.4 0.0 0.4 2.5 0.1 1.0 0.5 0.0 3.5 0.2 0.0 1.4 0.8 0.5 1.1
F ¼ fresh ripe; R ¼ rotten. The results listed in this table are based on dry weight, except total solid content.
production had been obtained. Based on Fig. 1aeb, the technical digestion time T90 (the time needed to produce 90% of the maximum methane production) of the seeds, pulp, and peel occurred within 30e37 days. The technical digestion time is a useful information to determine the hydraulic retention time (HRT) for continuous anaerobic digestion of the corresponding substrate [22]. The methane yield from the different fractions of the oranges varied (Table 2). The digestion of the seeds from the fresh ripe and rotten oranges gave the highest methane yields, which were 580.71 ± 27.00 ml CH4/g VS and 657.89 ± 63.58 ml CH4/g VS, respectively. However, the lowest methane yields of 71.60 ± 32.64 ml CH4/g VS and 48.23 ± 6.01 ml CH4/g VS were obtained from the peels of the fresh ripe and rotten oranges, respectively. The digestibility was determined in order to analyze the potential methane production from each fruit fraction. The digestibility expresses the digestion capability of the material to be converted into methane. The seeds and pulp of the fresh ripe and rotten oranges showed a high percentage digestibility of more than 90% (Table 2). Furthermore, the peels showed the lowest values for the digestibility, which were 26.3% from the rotten orange and 29.5% from the fresh ripe orange. This corresponds to the considerably low methane production from the orange peels during the digestion process. The reason for this could be that oranges contain several flavor compounds such as myrcene, car-3-ene, a-pinene, octanol, which have inhibitory effects on anaerobic digestion [23]. Furthermore, limonene, a flavor compound in the orange peel oil, inhibited anaerobic digestion process and caused an ultimate failure of the process at a concentration of 400 mL/L in a mesophilic continuous system [24]. The methane content of both the fresh ripe and the rotten oranges from all the components was at relatively the same level (Fig. 2a,b). The corresponding values were 54.2e61.5% (v/v) of methane (Table 2).
3.1.2. Mangosteen The profiles of methane production from the mangosteen fractions, i.e., peel, pulp, and seed are presented in Fig. 1c,d and Table 2. The highest initial methane production rates were obtained from the seeds of the fresh ripe and rotten mangosteen, which were
30.75 ± 3.15 ml CH4/g VS/day and 34.53 ± 4.64 ml CH4/g VS/day, respectively. The initial methane production rates from the pulp of the fresh ripe and rotten mangosteen were 28.41 ± 11.06 ml CH4/g VS/day and 31.17 ± 18.09 ml CH4/g VS/day, respectively. The methane production from the rotten pulp was increased significantly after the 15th day of digestion, while the fresh ripe pulp mangosteen was increased steadily (Fig. 1 c,d). It can be seen in the figures that the technical digestion time (T90) of the seeds and pulp of the fresh ripe and rotten mangosteen occurred within 25e30 and 30e44 days, respectively. Meanwhile, no methane production was produced from the peels during the digestion process. The result indicates that there is an inhibition effect in the methane production of mangosteen peel. Suchitra and Wanapat [25] reported that the methane production in the rumen was significantly reduced when the feed of dairy cows was supplemented with mangosteen peel. It was mentioned that the inhibition of methane production was caused by the presence of tannins and saphonine in the mangosteen peel. To substantiate the complexity, mangosteen peel is a lignocellulosic material [26], which contains hemicellulose, cellulose, and lignin. Table 1 shows that hemicellulose and cellulose contents in the fresh ripe and rotten mangosteen peels were among the highest compared to that of the other fruits' fractions. Although lignocellulosic materials are biodegradable, they are difficult to be digested due to its rigid structure [27,28]. The level of the methane yields from the different fractions of the mangosteen is shown in Table 2. The highest methane yields were obtained by anaerobic digestion of the seeds, which were 551.65 ml CH4/g VS from the fresh ripe mangosteen and 535.12 ± 35.82 ml CH4/g VS from the rotten mangosteen. The methane yields from the fresh ripe and rotten pulp were significantly different. The methane yield from the fresh ripe pulp was lower than that of the rotten pulp, which were 297.77 ± 40.17 ml CH4/g VS and 468.91 ± 27.62 ml CH4/g VS, respectively. Furthermore, the highest values of the digestibility were achieved from the digestion of the seeds, which were 99.4% from the rotten mangosteen and 99.3% from the fresh ripe mangosteen. The biogas produced from anaerobic digestion of all the fractions of fresh ripe and rotten mangosteen consisted of 55.2e62.3% (v/v) methane and
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Fig. 1. Accumulated methane production from seed ( ), pulp (C), and peel ( ) of rotten orange (a), fresh ripe orange (b), rotten mangosteen (c), fresh ripe mangosteen (d), rotten banana (e), fresh ripe banana (f), rotten rambutan (g), and fresh ripe rambutan (h) in 35 C batch digester for 60 days.
35.1e42.7% (v/v) carbon dioxide (Fig. 2c,d and Table 2). 3.1.3. Bananas In this study, the banana peels and the pulp were used as substrates in the anaerobic digestion. The methane production from the fresh ripe and rotten banana peels and pulp is shown in Fig. 1e,f and Table 2. The profile of the methane production from the fresh ripe banana was very similar to that of the rotten banana. The initial methane production rates from the peels and the pulp were at relatively the same level. There was no significant difference between the initial methane production rates from both the fresh ripe and the rotten bananas. The initial production rates from the peels and pulp were 22.22 ± 3.29 ml CH4/g VS/day and 19.71 ± 3.76 ml CH4/g VS/day from the rotten bananas, respectively, and 24.18 ± 1.11 ml CH4/g VS/day and 22.28 ± 3.59 ml CH4/g VS/day from the fresh ripe bananas, respectively. Fig. 1e,f shows that the technical digestion time (T90) for methane production of the peels and pulp occurred within 37e44 days. The methane yields from the fresh ripe and rotten bananas were at relatively the same level. The methane yields from the peels and
pulp of the fresh ripe bananas were 342.25 ± 54.79 ml CH4/g VS and 320.64 ± 9.52 ml CH4/g VS, respectively, while the methane yields from the peels and pulp of the rotten bananas were 331.65 ± 20.45 ml CH4/g VS and 351.49 ± 17.30 ml CH4/g VS, respectively. The biogas from all the fractions of fresh ripe and rotten bananas consisted of 56.4e58.3% (v/v) methane content and 40.5e41.6% (v/v) carbon dioxide content (Fig. 2e,f and Table 2). 3.1.4. Rambutan The profile of the methane production from the fresh ripe and rotten rambutan is shown in Fig. 1g,h and Table 2. The highest initial methane production rate was achieved by using the seeds. The corresponding values were 31.55 ± 0.51 ml CH4/g VS/day for the rotten rambutan and 33.91 ± 3.77 ml CH4/g VS/day for the fresh ripe rambutan. The initial methane production rates for the fresh ripe and rotten pulp were relatively in the same level, which were 31.54 ± 4.72 ml CH4/g VS/day and 21.68 ± 2.57 ml CH4/g VS/day, respectively. The peels from the fresh ripe and rotten rambutan showed the lowest initial methane production rates, which were 16.58 ± 1.94 ml CH4/g VS/day and 21.68 ± 2.57 ml CH4/g VS/day,
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Table 2 Process performance and methane production profile of the fresh ripe and rotten fruits at 35 C during 60 days of incubation. Type of fruit
Fraction
Methane production profile Initial rate (ml CH4/g VS/day) ± stda
R.Orange
F.Orange
R.Mangosteen
F.Mangosteen
R.Banana F.Banana R.Rambutan
F.Rambutan
Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Seed Pulp Peel Pulp Peel Pulp Peel Seed Pulp Peel Seed Pulp Peel
42.79 27.37 0.00 37.59 21.93 4.16 34.53 31.17 13.92 30.75 28.41 15.15 19.71 22.22 22.28 24.18 31.55 29.54 21.68 33.91 31.54 16.58
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
2.57 2.79 0.00 3.48 7.06 3.69 4.64 1.09 1.32 3.15 1.06 3.92 3.76 3.29 3.59 1.11 0.51 0.15 2.57 3.77 4.72 1.94
Methane yield (ml CH4/g VS) ± stda 657.89 311.71 48.23 580.71 287.89 71.60 535.12 468.91 0.00 551.65 297.77 0.00 351.49 331.65 320.64 342.25 504.11 322.82 126.30 527.05 309.30 202.75
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
63.58 10.03 6.01 27.00 38.79 32.64 144.54 260.49 0.00 19.79 40.14 0.00 17.30 20.45 9.52 54.79 21.15 20.39 44.29 29.04 13.59 40.89
% CH4 (v/v)
% CO2 (v/v)
pH
61.2 57.6 55.2 58.3 61.5 54.2 55.2 62.3 0.0 51.7 61.5 0.0 56.4 58.3 56.6 57.1 58.8 55.3 58.8 57.6 61.3 55.3
38.5 40.7 41.7 39.5 37.4 42.2 42.7 35.9 0.0 36.0 35.1 0.0 41.0 40.5 41.1 41.6 40.1 42.8 40.7 40.2 38.2 42.5
7.29 7.34 7.34 7.33 7.28 7.33 7.13 7.12 7.15 7.16 7.25 7.18 7.29 7.24 7.18 7.15 7.27 7.22 7.04 7.24 7.19 6.82
Digestibility (%)b 92.1 96.0 26.3 96.8 96.0 29.5 99.4 88.8 0.0 99.3 88.7 0.0 95.8 97.9 79.8 90.4 82.6 77.5 37.4 79.2 83.8 69.9
F ¼ fresh ripe; R ¼ rotten. a Std ¼ standard deviation. Yield of methane production from fruit b Digestibility ¼ Methane 100%: production from theoretical value
respectively. The methane production from the fresh ripe and rotten peels was significantly different. After 20 days of digestion, the production of methane from the rotten peel decreased until the end of the digestion; as a result, the methane production from the rotten peel was considerably low. Meanwhile, the methane production from the fresh ripe peels increased after 20 days of digestion. The low methane production from the rotten peel indicates that the anaerobic digestion process is inhibited. The inhibition could be attributed to the phenolic components that are present in rambutan peels [29]. Furthermore, as shown in Fig. 1geh, the technical digestion time (T90) of the seeds, pulp, and peels of fresh ripe and rotten rambutan occurred within 37e44, 44e51, 44e51 days, respectively. The highest methane yield from the rambutan was obtained from the seeds of the fresh ripe and rotten rambutan, which were 527.05 ± 29.04 ml CH4/g VS and 504.11 ± 21.15 ml CH4/g VS, respectively. The peels of rambutan had the lowest methane yield than that of the other fractions. The digestibility from the seeds of the fresh ripe and rotten rambutan and the pulp of the fresh ripe and rotten rambutan were relatively the same level. The lowest digestibility was achieved from the peels of the rotten rambutan, which was 37.4%. The biogas from all the fractions of fresh ripe and rotten rambutan consisted of 55.3e61.3% (v/v) of methane content and 38.2e42.8% (v/v) of carbon dioxide (Fig. 2g,h and Table 2). The results of the anaerobic digestion of the fruits'fractions in this work showed that the seeds of fresh ripe and rotten fruits gave a higher methane production and methane yield than that of the pulp and peels. This could be explained by the fact that the seeds had a higher fat content than both the pulp and the peels did. Fat is a macromolecule consisting of carbon, hydrogen, and oxygen atoms. The existence of carbon, hydrogen, and oxygen atoms make fat a suitable substrate for anaerobic digestion process. Fat was considered as a potential substrate for anaerobic digestion as the methane yield from fat was higher compared to that of protein or carbohydrate [30,31]. In this work, the seeds that contained fat of 22.2e36.2% from the total chemical composition (Table 1) contribute to 50e60% of the theoretical methane production (data
not shown). Kafle et al. [32] reported that substrates with high fat and low carbohydrate contents produced higher methane yields than that of substrates with high carbohydrate and low fat contents. Meanwhile, the production of methane from the pulp was higher than that of the peels. This result could be related to the total sugar and soluble starch contents that were higher in the pulp than that of the peels. The peel of oranges, mangosteen, and rotten rambutan produced low or even no methane production. The low methane production from the peels could be attributed to the presence of inhibitory flavor compounds and the complexity of the cellulose and hemicellulose structure of the peels. If we sum up the methane yields from the seeds, pulp, and peels of the rotten fruits in Table 2; orange, mangosteen, and rambutan yielded methane almost in the same level whereas rotten banana produced the lowest methane yield. From the summed methane yields of the fresh ripe fruits' fractions, the fresh ripe rambutan produced a higher methane yield compared to that of the fresh ripe of orange, which was then followed by mangosteen. Similar to the results from the rotten fruits, the fresh ripe banana produced the lowest methane yield. The results showed that higher methane yields were produced from the fruits that had seeds. As aforementioned, seeds contain fat that can be used as substrate to produce high methane yield. Since banana is a fruit that does not have seeds, both the fresh ripe and rotten bananas produced the lowest methane yields among the other fruits. 3.2. Effect of fruit ripening condition on methane production During the ripening process in fruits, a number of physical and chemical changes occur. The changes can be different for each fruit. Almost all fruits change on their external colors and texture during the ripening process. The external color changing is caused by the degradation of fruit pigments such as lycopene, carotenes, xanthophylls, and chlorophylls. The color changes have been used as a rough guide to determine the stage of ripeness. As fruits normally soften progressively during ripening [33], their texture also
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A.P. Sanjaya et al. / Renewable Energy xxx (2016) 1e7
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Fig. 2. Methane contents for methane produced from seed (B), pulp (C), and peel ( ) of rotten orange (a), fresh ripe orange (b), rotten mangosteen (c), fresh ripe mangosteen (d), rotten banana (e), fresh ripe banana (f), rotten rambutan (g), and fresh ripe rambutan (h).
undergoes changes. In this work, the fresh ripe and rotten fruits were used in order to evaluate the effect of fruit ripening conditions on methane production. The pulp of the rotten fruits generated higher methane yields compared to that of the pulp of the fresh ripe fruits (Table 2). In Table 1, the soluble starch content in the pulp of the fresh ripe fruits was higher than that of the rotten fruits. On the other hand, the total sugar in the pulp of the rotten fruits was higher than that of the fresh ripe fruits. During the ripening process, starch is completely hydrolyzed to simple sugars [33]. Simple sugars are more easily degraded and converted to methane than starch. Accordingly, higher methane yields from the pulp of the rotten fruits can be expected. In general, the seeds and peels contain high cellulose and hemicellulose (Table 1). Cellulose and hemicellulose are recalcitrant materials that are hard to be degraded by biological process. Due to the rigid structure of cellulose and hemicellulose, the seeds and peels might not been totally degraded into simple sugars
during the ripening and decay processes. As a result, the methane yields from the seeds and peels of the fresh ripe and rotten fruits were almost similar (Table 2). 4. Conclusion This study has demonstrated the differences in the methane production of each fruit fraction. Except for the peels from oranges, mangosteen, rotten rambutan, the other fruit fractions studied in this work could be converted into methane with reasonable yields. The methane yields from the seeds, pulp, and peel were in the range of 202.75 ± 40.86 to 657.89 ± 63.58 ml CH4/g VS. However, the anaerobic digestion from the peels of oranges, mangosteen, and rotten rambutan resulted in a low or even no methane production. The methane production from the seeds was higher than that from the pulp and the peels, while the methane production from the pulp was higher than that from the peels. Methane production from the fruits can be affected by the chemical composition, the presence
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