Production of biofuel precursor molecules (monocarboxylic acids, biohydrogen) from apple and pumpkin waste through an anaerobic fermentation process

Abstract

Medium-chain fatty acids (MCFA) such as caproic, heptanoic, and caprylic acids are monocarboxylic acids, which can be used as precursor molecules to synthesize biodiesel, bioplastics, antimicrobials, and corrosion inhibitors. Anaerobic bacteria transform ethanol and short-chain fatty acids (SCFA) such as acetate, butyrate, and valerate into MCFA through chain elongation. SCFA and MCFA production were evaluated in this study using two types of waste: apple waste (AW) and pumpkin waste (PW). Two kinds of PW, pumpkin pulp waste (PPW) and pumpkin skin waste (PSW), were used. AW and PW were first converted into a paste using a blender, and then into a leachate (liquid extract obtained from the paste) by adding an anaerobic mixed bacterial culture to the paste in a separate anaerobic chamber. The leachate containing SCFA was further transformed to MCFA using a Clostridium kluyveri augmented, especially enriched anaerobic mixed bacterial culture. The production of SCFA, MCFA and biogas was established for 75 days and analyzed throughout the experiment using high-pressure liquid chromatography (HPLC) and gas chromatography (GC), respectively. Bacteria incubated with AW produced significant amounts of SCFA (7.2 g l⁻¹ of butyric acid) and MCFA (1.2 g l⁻¹ of caproic acid). Bacteria incubated with PPW produced 4.4 g l⁻¹ of butyric acid and 6.1 g l⁻¹ of caproic acid, whereas, bacteria incubated with PSW showed lower amounts of SCFA (1.7 g l⁻¹ of butyric acid) and MCFA (2.5 g l⁻¹ of caproic acid). Clean energy in the form of biohydrogen (H₂) constituted the highest proportion (75%) of the biogas composition.