Biotransformation of Citrus Waste-I: Production of Biofuel and Valuable Compounds by Fermentation
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processes Review Biotransformation of Citrus Waste-I: Production of Biofuel and Valuable Compounds by Fermentation Neelima Mahato 1 , Kavita Sharma 1,2,† , Mukty Sinha 3,† , Archana Dhyani 4,†, Brajesh Pathak 5,†, Hyeji Jang 1,†, Seorin Park 1,†, Srinath Pashikanti 6,† and Sunghun Cho 1,* 1 School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Korea; [email protected] (N.M.); [email protected] (K.S.); [email protected] (H.J.); [email protected] (S.P.) 2 Department of Chemistry, Idaho State University, Pocatello, ID 83209, USA 3 Department of Medical Devices, National Institute of Pharmaceutical Education and Research, Ahmedabad, Palej, Gandhinagar 382 355, India; [email protected] 4 Department of Applied Sciences, School of Engineering, University of Petroleum and Energy Studies, Dehradun, Uttarakhand 248 007, India; [email protected] 5 Department of Chemistry, Harishchandra PG College, Varanasi 221 005, India; [email protected] 6 Biomedical and Pharmaceutical Sciences, College of Pharmacy, Kasiska Division of Health Sciences, Pocatello, ID 83209, USA; [email protected] * Correspondence: [email protected]; Tel.:+82-53-810-2535 † Contributed equally as second author. Abstract: Citrus is the largest grown fruit crop on the globe with an annual production of ~110–124 million tons. Approximately, 45–55% of the whole fruit post-processing is generally dis- carded as waste by the food processing industries. The waste is a huge problem to the environment in terms of land and water pollution along with displeasure from aesthetic viewpoint and spread of diseases owing to its huge content of fermentable sugars. The waste can be utilized as a raw material feedstock for producing a number of valuable chemicals and products, such as bioethanol, biogas, Citation: Mahato, N.; Sharma, K.; Sinha, M.; Dhyani, A.; Pathak, B.; bio-oil, organic acids, enzymes, and so on. The production of these chemicals from waste biomass Jang, H.; Park, S.; Pashikanti, S.; Cho, gives an inexpensive alternative to the harsh chemicals used during industrial synthesis processes S. Biotransformation of Citrus as well as the possibility of controlling pollution from the waste discarded to the environment. The Waste-I: Production of Biofuel and derived chemicals can be further utilized in the production of industrially important chemicals, Valuable Compounds by as solvents and building blocks of newer chemicals. Furthermore, organic acids, pectin, enzymes, Fermentation. Processes 2021, 9, 220. prebiotics, etc., derived from citrus wastes have an edge over their synthetic counterparts in practical https://doi.org/10.3390/pr9020220 applications in the food processing and pharmaceutical industries. Received: 14 December 2020 Keywords: citrus waste; bio-waste; biofuel; bioethanol; biogas; biotransformation; fermentation Accepted: 19 January 2021 Published: 25 January 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- The annual citrus production in the tropical and subtropical regions on the planet iations. exceeds 120 million tons. A significant portion of the fruit ~40–60% is non-edible and discarded as waste. This waste biomass includes peels, pulp and pith residue, and seeds. The pulp and pith residue contain large contents of fermentable sugars, but the peels contain huge amounts of limonene and bioactive molecules which inhibit proper fermentation processes [1–3]. This causes bad odor and pollution to the land, air and water (Figure1)[ 4]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Processes 2021, 9, 220. https://doi.org/10.3390/pr9020220 https://www.mdpi.com/journal/processes Processes 2021, 9, x FOR PEER REVIEW 1 of 49 Processes 2021, 9, 220 2 of 49 FigureFigure 1. PollutionPollution and and deterioration deterioration of of the the environment caused by dumping of citrus waste [[4].4]. Reproduced with permission from Neelima et al., Foods, 2019, pp. 8, 8, 521–579. 521–579. CitrusCitrus processing processing wastes wastes co containntain huge huge amounts amounts of of moisture moisture (75–85%) rendering rendering it it difficultdifficult to to dry. dry. Most Most of of the the citrus citrus wastes wastes are are dried dried to to transform into into cattle feed pellets whichwhich requires requires high high co consumptionnsumption of of energy. energy. The drying process not only only makes makes the the waste waste remediationremediation expensive expensive but but also also releases releases larg largee quantities of of volati volatilele organic compounds (VOCs),(VOCs), sulfursulfur dioxide, dioxide, nitrogen nitrogen oxides, oxides, MeOH, MeOH, formaldehydes, formaldehydes, carbon dioxide, carbon suspended dioxide, suspendedparticulate matter,particulate and matter, other compounds and other orcompounds potentially hazardousor potentially air pollutantshazardous from air pollutantsdecomposition from ofdecomposition organic compounds of organic present comp inounds the biomass present in in addition the biomass to burning in addition of fuel toor burning natural gasof fuel utilized or natural for the gas drying utilized process. for the It hasdrying been process. reported It thathas thebeen volatile reported organic that thecompounds volatile organic in the citruscompounds waste amountsin the citrus to 9000–18,000waste amounts tons/year to 9000–18,000 [5,6]. The tons/year environment [5,6]. Theprotection environment agencies protection in various agencies countries in are va nowrious taking countries stricter are measures now taking to abolish stricter air measurespollution to caused abolish by air the pollution drying process caused [ 7by–10 the]. drying process [7–10]. InIn pastpast fewfew decades, decades, extensive extensive investigations investigations have have been focusedbeen focused on inventing on inventing efficient efficienttechniques techniques to extract to bioactive extract bioactive compounds compounds from natural from resources. natural resources. We have reviewedWe have extensively the various extraction and purification procedures and applications of the reviewed extensively the various extraction and purification procedures and applications bioactive compounds obtained from citrus wastes in our previous publications [1–4,11–13]. of the bioactive compounds obtained from citrus wastes in our previous publications [1– The citrus waste (after extraction of bioactive molecules and limonene) can be further 4,11–13]. The citrus waste (after extraction of bioactive molecules and limonene) can be utilized to produce ethanol, biogas, fuels, and biosorbents via biotransformation. The further utilized to produce ethanol, biogas, fuels, and biosorbents via biotransformation. latter includes microbial fermentation, and/or physico-chemical processes [14–18]. Citrus The latter includes microbial fermentation, and/or physico-chemical processes [14–18]. biomass, due to its large production worldwide, forms a renewable, sustainable and Citrus biomass, due to its large production worldwide, forms a renewable, sustainable economic feedstock for obtaining eco-friendly fuel. The carbohydrates and fermentable and economic feedstock for obtaining eco-friendly fuel. The carbohydrates and sugars in the citrus waste are processed to produce biogas and ethanol. The biofuels can fermentable sugars in the citrus waste are processed to produce biogas and ethanol. The reduce carbon dioxide emission by 80% compared to gasoline or petroleum fuel [19–21]. biofuels can reduce carbon dioxide emission by 80% compared to gasoline or petroleum Citrus waste contains ~70% carbohydrates, and capable of producing ~1.2 billion liters fuelof ethanol [19–21]. (or Citrus 300 million waste gallons)contains on~70% a global carbohydrates, scale [22]. Inand the capabl recente of years, producing ethanol ~1.2 has billionbeen explored liters of ethanol as an alternative (or 300 million to gasoline gallons) fuel, on a and global thus, scale demand [22]. In has the also recent increased years, ethanolmanifolds. has been Furthermore, explored theas an usage alternative of methyl to gasoline tertiary butylfuel, and ether thus, (MTBE—an demand additivehas also increasedto the petroleum manifolds. fuel) Furthermore, is rapidly declining the usage because of methyl it releases tertiary harmful butyl ether pollutants (MTBE—an upon additivecombustion to the which petroleum are carcinogenic fuel) is rapidly in nature declining [23]. It because can be replacedit releases by harmful ethanol. pollutants However, uponthe ethanol combustion addition which to gasoline are carcinogenic is not advantageous in nature in[23]. every It can case. be Ethanol replaced is hygroscopicby ethanol. However,and may absorbthe ethanol moisture addition in hot to and gasoline humid is climates not advantageous prevalent in in tropical every case. and subtropicalEthanol is Processes 2021, 9, 220 3 of 49 regions and result in corrosion of automobile internal engine parts. On the other hand, in the countries where cold climate is prevalent, the moisture level in the atmosphere is very low, addition of ethanol to gasoline fuel has not been observed to cause any significant harm to the engine or internal automobile parts. In addition, ethanol is used as disinfectant and organic solvent, in clinical purposes