Conversion of Agro-industrial Wastes into Lipids Suitable for Biodiesel Production
Abu, Yousuf (2010) Conversion of Agro-industrial Wastes into Lipids Suitable for Biodiesel Production. [Tesi di dottorato] (Inedito)
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Microorganisms that can accumulate lipids at more than 20% of their dry mass are defined as oleaginous species. The majority of these lipids are triacylglycerol containing long-chain fatty acids, which are comparable to conventional vegetable oils. The recent, increasing interest towards the oleaginous microorganisms is due to the potential use of microbial triglycerides as feedstock for biodiesel production. The oleaginous yeasts used in this thesis work appear to be very promising, due to their versatility, as they allow the use of different kinds of residues as nutrients. In particular, Lipomyces starkeyi is so far one of the best used, as it has been proved to store large amounts of lipids. Lipomyces starkeyi were first grown in the presence of olive oil mill wastewaters (OMW), a medium difficult to process by biological treatments, due to the antimicrobial activities of their phenolic components. We demonstrated that Lipomyces can produce, without external organic supplements, a significant reduction of both the total organic carbon (TOC) and the total phenols content, leading to a significant increase of the germination index. The fatty acid distribution showed a prevalence of oleic acid, demonstrating the potential of L. starkeyi as a source of lipids to be used as a feedstock for the synthesis of II generation biodiesel. The performance of Lipomyces was improved by a preliminary dilution of OMW. Lipomyces were able to grow also in the presence of wastewaters from cheese factory, leading to a satisfactory growth and to a significant reduction of the TOC levels. Cellulosic agricultural residues were also evaluated as feedstock for oleaginous yeasts. Lipomyces starkeyi were first grown in the hydrolysate of tomato wastes, containing mainly peel and seeds, at different nitrogen contents. The yeasts showed a favorable growth, with no need of addition of external nutrients. Hydrolysates of Sorghum and Giant Reed were also studied as nutrients for the Lipomyces starkeyi. The conditions to maximize the lipid yield and the efficiency of the biomass conversion were found in terms of H2SO4 concentration (for the preliminary hydrolysis) and of medium composition (for the yeasts growth). Detoxification of hydrolysate with overlime and activated charcoal was carried out to reduce the concentration of microbial growth inhibitors, improving the growth of the yeasts in the undiluted hydrolysate. In conclusion, the potential of oleaginous yeasts was demonstrated by the satisfactory microbial growth in the presence of different waste materials, and by the favorable composition of the triglycerides. Further studies are ongoing to optimize the preliminary hydrolysis of lignocellulosic materials and the lipid fraction of the yeasts.
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