In 2023, approximately 2.36 million metric tons of olive oil were produced globally. Olive pomace, a byproduct of the flesh and pits left after olive oil extraction, presents environmental challenges when used as landfill due to its high polyphenol and organic contents, or when combusted due to greenhouse gas emissions. Its potential as animal feed is limited, yet it holds promise for methane production via anaerobic digestion (AD), providing a source of renewable energy. However, the highly crystallized polysaccharides in olive pomace, such as cellulose, hemicellulose, and pectin, impede its conversion to methane, and the high polyphenol content inhibits methanogen growth.
To address this, phenolics were extracted from olive pomace, producing a phenolics-extracted olive pomace (PEOP) and a phenolics-rich olive liquid. After further resin-based extraction of phenolics-rich olive liquid, approximately two-thirds of the phenolics were removed, yielding phenolics-extracted olive liquid (PEOL). Enzymatic hydrolysis was conducted on several olive byproduct streams: olive pomace with water, PEOP with PEOL, and PEOP with water, to convert insoluble polysaccharides into reducing sugars that are more readily utilized by methane-producing microorganisms.
Various enzymes, including cellulase, hemicellulase, xylanase, and pectinase, were individually treated with olive pomace to determine the optimal hydrolysis time and enzyme concentrations. Response surface methodology (RSM) identified the optimal enzyme cocktail ratio (1.1% cellulase, db, and 0.9% pectinase, db) for achieving the highest reducing sugar contents (22.3 mg/mL), which had 79.1% increase when compared to the control sample (12.5 mg/mL).
After 19 days of anaerobic digestion at 37 °C, olive samples before phenolics extraction (olive pomace with water) and olive samples after phenolics extraction (phenolics-extracted olive pomace with phenolics-extracted olive liquid), produced similar amounts of methane (~175 mL CH4/g VS). This indicated that in our experimental settings, the phenolics reduction did not significantly impact methane yields.
Carbohydrate profiles may also influence biofuel yields, as hexoses (C6 sugars) are preferred over pentoses (C5 sugars) for end-product production during biotechnical conversion. To explore the effect of carbohydrate profiles on methane production from olive byproducts, two response surface methodology (RSM) coded enzyme cocktail-treated samples with different carbohydrate profiles underwent anaerobic digestion for 19 days at 37 °C, yielding similar amounts of methane (~156 mL CH4/g VS), comparable to the control sample. This suggested that anaerobic digestion can utilize different hexoses and pentoses at similar rates.
These findings demonstrated that olive pomace can be used for biomethane production instead of being landfilled or combusted. While enzymatic hydrolysis increased reducing sugar contents, it did not enhance methane yields. Reducing phenolic contents of 2/3 did not improve biomethane yield, and the impact of greater reduction requires further assessment.
| Identifer | oai:union.ndltd.org:CALPOLY/oai:digitalcommons.calpoly.edu:theses-4572 |
| Date | 01 August 2024 |
| Creators | Zhong, Ningjing |
| Publisher | DigitalCommons@CalPoly |
| Source Sets | California Polytechnic State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Master's Theses |
Page generated in 0.0019 seconds