Assessment of NIR spectroscopy for predicting biochemical methane potential of agro-residues – A biorefinery approach
Graphical abstract
Introduction
Agro-residues are biodegradable materials generated in food processing plants, markets, canteens, or restaurants, which currently represent a huge amount of unexploited resources. The Waste Framework Directive (Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008, amended by the Directive (EU) 2018/851) recommends that the Member States must ensure by the end of 2023 that biowaste is separated at source (not mixed with another type of waste). Besides, all countries should take measures to encourage recycling, including anaerobic digestion and composting, to guarantee a high level of environmental protection and give a positive contribution to close the carbon cycle. In this study, agro-residues from the tomato processing industry and wine production will be considered. According to European statistics [1], in 2017, the EU harvested production of fresh tomatoes was 17.4 Mt, and the top 5 countries were: Italy 5.6 Mt, Spain 5.2 Mt, Portugal 1.7 Mt, Poland 0.9 Mt, and the Netherlands 0.9 Mt. In addition to the tomato pomace generated at an industrial level, after harvesting season the proper management of the residues generated in fields and greenhouses is a problem that producers have to face [2]. Moreover, the EU is a major player in the wine market, accounting for 56% of world production (volume) in 2017, with 44% of wine-growing areas in the world [1]. In 2017, the global EU- 28 production of wine grapes was 21.2 Mt. Also, this industry produces a considerable amount of wastewater and pomace (skins, pulp, seeds, and stems of the grapes) that requires also proper management.
According to the waste management hierarchy and circular economy from the EU, the recycling of biodegradable waste must be boosted and landfill disposal reduced. In the literature, it has been highlighted that tomato residues may be valorized as a source of bioactive compounds and antioxidant ingredients [3], namely carotenoids [4], flavor enhancers [5], lycopene [6], carbohydrates [7], and others. Regarding grape pomace, some authors focus on unfermented sugars, polyphenols, alcohol, tannins, pigments, etc. to produce brandy [8], while others feature its valorization as fertilizer [9], phenolic compounds [10], and biogas production [11]. However, in none of these cases, the biorefinery approach has been explored in the literature. Indeed, considering that the biorefinery allows the conversion of agro-residues into diverse biobased products (e.g. value-added compounds) and bioenergy (biofuels, power, heat), it is increasingly important to apply this strategy to intensify value creation [12,13]. In this scope, the European circular economy and Member States programs as well can boost the biorefinery in a near future. In this study, the recovery of polyphenol through solvent extraction followed by biogas production from anaerobic digestion will be explored for both types of agro-residues (tomato industry – rotten tomato, green tomato, and tomato branches – and wine industry – grape pomace). In the literature, some studies highlighted the relevance of obtaining products (e.g. lactic acid, phenolic compounds, etc.) from agro-residues [10] while a few studies integrated this approach with further processing to recover energy [14,15].
Phenolic compound extraction has been increasingly studied due to its beneficial properties as an antioxidant, antimicrobial, anti-inflammatory, antitumor, antiviral, analgesic, and antipyretic [16]. Although several methods can be used to extract these compounds, solid-liquid extraction, ultrasound-assisted extraction, microwave-assisted extraction, and supercritical fluid extraction are the most commonly used. In the present study, solid-liquid extraction with ethanol will be explored.
Anaerobic digestion (AD) is a biological process that converts biodegradable organic matter into biogas (a mixture of methane and carbon dioxide) through complex microbiological stages (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) [17,18]. This process allows not only to recover the remaining energy from the agro-residues after extraction but also to partially stabilize the remaining fraction and divert it from the landfill. In recent years, the utilization of anaerobic digestion to manage residues has been increasing. Indeed, the biogas produced by AD in the EU increased from about 210 PJ in 2010 to 510 PJ in 2015 [19]. In order to design the anaerobic digestion reactor, it is important to characterize the substrates used. The biochemical methane potential (BMP) is one of the key parameters to predict the biodegradability of the substrates and optimize the AD process. BMP corresponds to the maximum quantity of methane (mL CH4 g−1, on a volatile solids basis) potentially produced from substrates in anaerobic conditions [18]. However, the BMP determination at the experimental level is very time-consuming (at least 30 days), hindering waste stock management at an industrial scale [20]. New approaches have been developed to overcome this constraint. Indeed, multivariate regression models and spectroscopy techniques have been explored to predict the composition of the materials. In particular, BMP can be estimated through the elemental composition of the substrates, chemical composition (e.g. carbohydrates, lipids, and proteins), and using multivariate regression models based on near-infrared spectroscopy (NIR) [18,[20], [21], [22]]. Considering all the advantages and drawbacks of the several methods, the NIR spectroscopy has been revealed a fast, cheap, and reliable approach to predict the BMP of substrates [18,20]. Initially, a big effort and work are needed to determine experimentally the BMP of several substrates (more than dozens or hundreds) and acquire NIR spectra of those materials. Then, spectra arrays are analyzed through multivariate statistical methods to develop and calibrate the model. Once the regression model is available, the BMP of a specific substrate may be estimated in a few minutes. For obtaining reliable results, it is important to ensure that the material under analysis is similar to the ones used in the calibration phase. However, to the best of our knowledge, the possibility of using models based on NIR spectroscopy in the biorefinery context was never before considered.
Thus, the main objective of this study is to evaluate the NIR spectroscopy as a reliable method to predict the BMP of raw and processed agro-residues after extracting to recover value-added compounds. As the main scientific contribution of this work, the testing of agro-residues before and after extraction stands out.
Section snippets
Samples and inoculum collection
Distinct agro-residues, from the tomato and wine industry, were collected and prepared for the characterization and the BMP assessment. Ripe tomato in a semi-rotten state unfit for consumption (RT), green (unripe) fruit (GT), and tomato plant (TB) were obtained on a farm from the central region of Portugal. Moreover, a grape pomace (including skins, pulp, seeds, and stems of the fruit) (GP) was also collected in the same region. RT and GT were minced mechanically, and frozen at −18 °C until use
Substrate characteristics
Table 2 summarizes the most relevant characteristics of the raw (RT, GT, TB, GP) and extracted (RTe, GTe, TBe, GPe) materials studied. RT, GT, and GP are acidic substrates, with pH 4.75, 4.00, and 3.91, respectively, while TB is a neutral material (pH 6.82). The natural pH of the residues was not modified with the extraction process. RT and GT are mainly composed of water, with a TS lower than 8%. On the other hand, TB and GP contain much less water, with 71.4% and 76.3% of total solids,
Conclusion
Agro-residues from the tomato and wine industry (rotten tomato, green tomato, tomato plant, and grape pomace) were extracted with ethanol for phenolic compounds recovery. While the grape pomace extract was the richest substrate in total phenolic content, 55.77 mg g−1 (expressed gallic acid equivalents on a dry extract basis), green tomato extract had the lowest concentration, 9.9 mg g−1 dry extract.
Anaerobic digestion of crude and extracted substrates were performed to assess the biochemical
Acknowledgments
This work was financially supported by COMPETE 2020, Fundação o para a Ciência e Tecnologia (FCT, Portugal), through the project “MultiBiorefinery - Multi-purpose strategies for broadband Agro-forest and fisheries by-products: a step forward for a truly integrated biorefinery - POCI-01- 0145-FEDER-016403”. P.V. Almeida acknowledges FCT for the respective fellowships from the MultiBiorefinery project. R. P. Rodrigues acknowledges the fellowship from the project MATIS, Fundo Europeu de
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