Effect of limonene on batch anaerobic digestion of citrus peel waste

doi:10.1016/j.bej.2015.12.011

Biochemical Engineering Journal

Volume 109, 15 May 2016, Pages 9-18

https://doi.org/10.1016/j.bej.2015.12.011 Get rights and content

Highlights

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BMP of citrus waste (peel, pulp and rotten fruit) was 354–398 L kg_VS⁻¹.
•
Grinding the citrus waste did not improve BMP but slowed the kinetics.
•
IC₅₀ of limonene was 423 mg kg⁻¹ in batch anaerobic digestion of cellulose.
•
Inhibition of anaerobic digestion process by limonene was reversible.
•
IC₅₀ value increased to 669 mg kg⁻¹ for a second load of limonene, indicating that some biomass adaptation may occur.

Abstract

The objective of this study was to analyze the anaerobic digestion process inhibition by limonene, the main component of citrus essential oils (CEO) present in citrus peel.

The biochemical methane potential (BMP) values of the citrus waste tested (orange peel, mandarin peel, mandarin pulp and rotten fruit) were 354–398 L_CH4 kg_VS⁻¹. Grinding the orange peel (2.5 g_limonene L⁻¹) did not influence the BMP values, but slowed the kinetics, due to the increased availability of CEO caused by the grinding.

The effect of (R)-limonene (0–3000 mg L⁻¹) on the batch anaerobic digestion of microcrystalline cellulose was also assessed. The half maximal inhibitory concentration, IC₅₀, was 423 mg kg⁻¹ in an initial run and 669 mg kg⁻¹ in a second run of batch experiments. The methane course and IC₅₀ values indicate that there are reversible inhibition and biomass activity recovery during the anaerobic digestion process, despite the non-reversible antimicrobial mechanism described in the literature for limonene to date.

Introduction

Citrus waste, peel and pressed pulp, is generated during the juice manufacturing process. Another source of waste is the fruit discarded for commercial reasons or due to production limiting regulations. The amount of waste generated varies, since it is a fraction of the total amount of fruit produced and processed. The processing of citrus fruit for juice extraction produces around 500 tons of waste per 1000 tons of fruit processed [1], [2]. The percentage of fruit discarded as a result of commercial or regulatory issues ranges from 2% to 10%. In the region of Valencia (eastern Spain), 3.5 million tons of citrus fruit was produced in the production year 2010–2011. Of this, around 0.4 million tons (11% of the total production) was reported as losses. Another 0.4 million tons was industrially processed to make juice, which would produce around 0.2 million tons of waste [3].

The characteristics of citrus waste (pH 3–4, water content 80–90%, high organic matter content) constrain the alternatives for its management or valorization. The use of citrus waste as livestock feed is possible due to its nutritive value (similar to that of sugar beet pulp), which is the result of its high carbohydrate content, its significant proportion of cell wall components and its low level of lignification [4]. However, the market for this material is becoming saturated. Composting citrus waste is a complex process due to its low pH, the presence of essential oils that inhibit the composting process and the fast rate of biodegradation of the waste, which can cause anaerobiosis problems. Thermal treatment alternatives (incineration, gasification or pyrolysis) are not feasible for citrus waste due to its high humidity; neither is disposal in landfill sites. The manufacture of bioethanol from citrus waste, although technically possible, requires a very large investment [5] and is not as energy efficient as methane (biogas) production through anaerobic digestion: while methane production can yield more than 700 kWh per ton of waste, the energy yield of the bioethanol production process is only around 300 kWh per ton [6].

Given this situation, anaerobic digestion is a technically feasible, environmentally friendly and energy efficient process for the valorization of citrus waste. However, citrus essential oil (CEO) can inhibit this bioprocess; as observed in several studies of the inhibitory effect of limonene (the major component of CEO) on anaerobic digestion of citrus waste [7], [8], [9], [10], [11], [12], [13]. Nevertheless, there is no consensus regarding the inhibitory concentration of this compound. While we have found no references to the determination of the IC₅₀ inhibitory concentration of citrus waste, inhibitory doses of limonene in semi-continuous anaerobic digestion are reported to vary from 24 mg L⁻¹ d⁻¹ to 75 mg L⁻¹ d⁻¹ [7], [8], [9]. This range of values could be due to the different conditions applied in the experiments (the operating conditions of the digester, use of additives, different varieties of fruit, etc.). In addition, the studies observed that minority components of essential oils also seem to have an inhibitory effect, since CEO inhibition is stronger than inhibition by commercial limonene. More recent work on the anaerobic digestion of citrus waste did not aim to find the inhibitory concentration of limonene and therefore no information on this is provided [10], [11], [12], [13].

The CEO antimicrobial mechanism is based on cell malfunction and lysis. Once dissolved in the aqueous medium, the CEO accumulates in the membrane of the microorganisms and changes its structure. The membrane fluidity changes, becomes more permeable and finally leakage of the cell contents occurs [14]. We found no studies on the possible adaptation of the microorganisms responsible for the anaerobic digestion to CEO.

The objectives of this study were: (i) to determine the biodegradability of citrus waste, (ii) to study the inhibitory effect of limonene on the anaerobic digestion of citrus waste and (iii) to characterize this inhibition and the possible adaptation of the biomass.

Section snippets

Materials and methods

The anaerobic digestion of citrus waste was assessed using different approaches. The biochemical methane potential (BMP) of the most common citrus waste was determined by testing the following materials: orange peel (OP1, OP2), mandarin peel (MP), mandarin pulp (MPU) and rotten mandarin (MR).

The effect of the limonene naturally present in the orange peel, as the main component of CEO, on the batch anaerobic digestion was assessed by digesting orange peel (OP2) that contained a known

Results and discussion

The origin and chemical characteristics of the citrus waste used in the BMP tests are summarized in Table 2.

Conclusions

The limonene in citrus peel has an inhibitory effect on anaerobic digestion for concentrations higher than 200 mg kg⁻¹ of limonene in the digester. This inhibitory effect seems to be due to cymene, which is produced from limonene during the anaerobic digestion.

Grinding the citrus peel releases the limonene into the medium and increases its inhibitory effect. Toxicity is observed after biodegradation of the limonene, which is attributed to its biotransformation into other inhibitory compounds.

The

Acknowledgements

Some of the results of this study were obtained within research on the project PS-120000-2007-6, cofinanced by the Spanish Ministry of Economy and Competitiveness.

The authors thank the laboratory and pilot plant technicians of AINIA Centro Tecnológico for their help with the experiments and Dr. Antonio Eduardo Palomares Gimeno of the Polytechnic University of Valencia (Spain) for help reviewing this paper.

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Orange peel waste (OPW) and sewage sludge (SS) valorization for volatile fatty acids (VFAs) production from anaerobic co-fermentation are attractive and feasible. The highest VFAs reached 11996.3 mg COD/L within 10 d at the mass ratio (TS/TS) of 1:1, which was approximately 30-fold of that in sole SS fermentation. The OPW provided plenty of organic substrates and facilitated the fermentation processes by disintegrating SS structure and inhibiting methanogenesis due to the abundant limonene. Also, the OPW feeds reshaped the microbial community and enriched fermentative bacteria, especially those saccharolytic ones (i.e. Prevotella-7). The key genes involved in membrane transport (i.e. ptsG), glycolysis (i.e. pgk), pyruvate metabolism (i.e. ace), and fatty acid biosynthesis (i.e. accA), which are associated with VFAs biosynthesis, were up-regulated in OPW/SS reactors. Overall, it was the increase in bioavailable organic matter and functional microorganisms, and the simultaneous enhancement of metabolic activity that improved the efficient VFAs production.

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Regular articleEffect of limonene on batch anaerobic digestion of citrus peel waste

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusions

Acknowledgements

Bioresour. Technol.

Bioresour. Technol.

Waste Manag.

Biol. Wastes

Proc. Biochem.

Bioresour. Technol.

Waste Manag.

Energy

Food Chem. Toxicol.

Bioresour. Technol.

Syst. Appl. Microbiol.

FEMS Microbiol. Lett.

Removal of peel oil from citrus peel press liquors before anaerobic digestion

Environ. Technol. Lett.

Regular article
Effect of limonene on batch anaerobic digestion of citrus peel waste