Available at: https://digitalcommons.calpoly.edu/theses/1903
Date of Award
MS in Agriculture - Food Science and Nutrition
Food Science and Nutrition
More than 16,000 tons of olive cake was produced in the United States in 2017. Olive cake is a by-product of olive oil extraction, which has limited animal feed potential, and poses an environmental threat when landfilled due to its high organic load and polyphenol content. This residue has potential for biofuel (bioethanol and biomethane) production because it is rich in polysaccharides such as pectin, hemicellulose, and cellulose. Yet, olive cake contains olive stones that can impede its conversion to biofuel. Therefore, two methods of destoning, centrifugation and screening by horizontal screw press, were first compared. Both methods removed an equal percentage of stones (95%), but centrifugation partitioned the majority (57 – 79%) of digestible solids (olive pulp) with the stones. Then, two strategies were compared to maximize both biomethane and bioethanol production; enzymatic conversion of insoluble to soluble carbohydrates and destoning by screening. After 30 days of anaerobic digestion at 35 °C, both the enzymatically pretreated and the destoned olive cakes produced similar amounts of methane (~295 mL CH4/g VS), 42% more than the control (209.5 mL CH4/g VS). The biogas produced was composed of 60-70% methane. A comparison of biomethane yields with a broad range of agricultural residues demonstrated olive cake’s suitability for biomethane production. The digestate, residue from the anaerobic digestion, have high Kjeldahl nitrogen content (3.6%, db) and low polyphenol concentration (0.02 mg GAE/g), which then qualify it as an ingredient for soil amendment. Ethanol production investigations showed that after 3 days of fermentation at 32 °C, only the destoned and enzymatically pretreated olive cake produced ethanol (1.3 mg/mL). Acetic acid, an inhibitor of ethanol production, was present in all samples broth, suggesting microbial contamination was present.
These results provide evidence that olive cake can be diverted from landfills to be converted into a biofuel. Sustainable pretreatments such as destoning and enzymatic pretreatment increase biomethane yield. The digestate created from the anaerobic digestion of olive cake can be used as a soil amendment, adding further value to olive cake.