Power generation from cheese whey using enzymatic fuel cell
Introduction
Cheese whey is an organic waste of the cheese manufacturing after process of milk casein removed in the dairy industry which produces large volumes of waste (Zhou et al., 2019). The worldwide production of cheese whey was estimated to approximately 160 million tones, with a 2% increase per year (Rama et al., 2019). Cheese whey has been reported to cause environmental pollution as a waste disposal in dairy industry process due to its high organic contents, in which lactose, 75% of the cheese whey solids, is one of the most polluting by-product (Veeravalli and Mathews, 2018). Without the prior treatments, these organic contents can cause enormous pollution problems in ecosystem (Ahmada et al., 2019). However, the waste water treatment of cheese whey also has problems, which produce soluble microbial products (SMP) and exteracellular polymeric substance (EPS). (Sepehri and Sarrafzadeh, 2019). And these substances cause membrane fouling and increase chemical oxygen demand (COD) during the treatment process (Yamanouchi et al., 2017). Thus, the new biotechnological alternatives have been researched to utilize lactose of cheese whey via microbial and enzymatic processes to obtain bioproduct, such as chemicals (Juodeikiene et al., 2016), plastics (Domingos et al., 2018) and fuels (Carota et al., 2017), with potential applications to reduce the environmental pollution. Recently, several organic wastes have been attempting to use as substrates in biofuel cell applications. The organic matter such as micro algae (Lee et al., 2016), sewage sludge (Guan et al., 2019) and food waste (Moharir and Tembhukar, 2018) have been successfully demonstrated to generate the clean electricity production via biotechnology in fuel cell system.
Biofuel cell could be defined as a device for chemical conversion to electrical energy, which briefly classified as microbial fuel cell (MFC) and enzymatic fuel cells (EFC) (Nasar and Perveen, 2019). In MFC system, a continuous maintenance of whole living cells for their sustenance has been required for catalytic oxidation reactions (Slate et al., 2019). However, EFC is based on reactions of redox-active enzyme, such as glucose oxidase and laccases, to generate electron power without maintenance (Xiao et al., 2019). When the EFC system was employed, there are some advantages such as its utilization of renewable biocatalysts, abundant green resources, high energy density generation, and biocompatible working system. In addition, the miniaturization of EFC made its feasibility to apply for wearable, potable and implantable devices. In recent research, the flexible electrode has been reported to apply in EFC for skin-based fuel (Bandodkar et al., 2017). To improve the power density and stability, supercapacitor has also utilized for EFC in recent researche (Hou and Liu, 2017). Therefore, the EFC system is thought to be a promising technology for various industrial applications. Furthermore, selection of proper enzyme and establishment of an EFC system with efficient performance have been investigated to overcome the utilization challenge. Several enzymes including cellobiose dehydrogenase (CDH), hexose oxidase, and glucooligosaccharide oxidase have been utilized lactose as substrate. Of various enzymes, CDH has attracted attention due to its outstanding electrochemical property.
Cellobiose dehydrogenase (CDH, EC 1.1.99.18) is a kind of unique anodic enzyme, due to enabling the production of bioelectrical energy from β-1,4-linked di-and oligosaccharide such as cellulose, cellobiose, cellodextrins or lactose (Nyanhongo et al., 2017). This enzyme has a unique structure consisting of a catalytically active flavin adenine dinucleotide (FAD) containing dehydrogenase domain (DHCDH) and heme b containing cytochrome domain (CYTCDH) connected via a peptide linker region (Harada et al., 2017). In the enzyme catalytic reaction, electrons from di-/mono- saccharides donated to the FAD cofactor of DHCDH and converted FAD into its fully reduced state. The electrons were transferred to heme b cofactor of the CYTCDH by internal electron transfer (IET), which occurred in the absence of any electron acceptor (Tavahodi et al., 2017). Due to its outstanding abilities of electron transfer, CDH from different microorganisms has been widely used in electrochemical applications of EFC (Zafar et al., 2019) and biosensor (Bollella et al., 2017a).
In this study, cheese whey was used as a fuel for power generation, which was firstly demonstrated in EFC using CDH. For the efficient electron generation, CDH from Phanerochate chrysosporium (PcCDH) was immobilized on the electrode of EFC system for enzyme reaction with lactose from cheese whey. The modified electrode was characterized and the EFC performance with various kinds of substrates, enzyme concentrations and reaction conditions was investigated to achieve high performance of power generation from cheese whey.
Section snippets
Chemicals
The chitosan, cobalt (II) chloride hexahydrate, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), graphite, potassium ferricyanide (III), potassium chloride, lactose and whey powder (from bovine milk), were purchased from Sigma-Aldrich Co. (USA).
Preparation of enzymes
Cellobiose dehydrogenase (EC 1.1.99.18) from Phanerochate chrysosporium KCCM60256 was expressed in Pichia pastoris X-33 cells (PcCDH), and purified based on a previous study (Choi et al., 2016). The
Electrochemical characterization of modified electrode
To generate power, an EFC system was established with immobilized PcCDH on its anode electrode, and lactose from cheese whey was employed as substrate for the enzyme reaction. The effect of modified electrode and substrates for this EFC system was investigated, respectively. For an efficient electron transfer, the electrode was modified with a mediator structured as GO/Co/chitosan. To evaluate electron transfer properties of the bare and modified electrode surface, the electrochemical impedance
Conclusions
This study has been firstly demonstrated the cheese whey from dairy manufacture to generate electron power using an EFC system with CDH. The power density of this successful demonstration was 1,839 μW/cm2 when cheese whey directly used as fuel. Additionally, the maximum power density of this EFC was achieved 2,973 μW/cm2 with the optimal conditions of enzyme concentration (47.07 mg/ml) and initial lactose concentration (100 mM) at pH 4.5. These high performances of power generation indicated
Author contribution
Han Suk Choi: Methodology, Formal analysis, Investigation, Writing - Original Draft, Visualization, Data Curation. Xiaoguang Yang: Formal analysis, Validation. Dong Sup Kim: Formal analysis, Software. Ji Hyun Yang: Software. Sung Ok Han: Validation. Chulhwan Park: Writing - Review & Editing, Supervision. Seung Wook Kim: Conceptualization, Writing - Review & Editing, Project administration, Funding acquisition, Supervision.
Declaration of competing interest
There is no conflict of interest.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2014R1A2A2A01007321 and NRF-2019R1A2C1006793) and Kwangwoon University (2020).
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