High concentrations of dried sorghum stalks as a biomass feedstock for single cell oil production by Rhodosporidium toruloides.

Leonidas Matsakas,Nemailla Bonturi,Everson Alves Miranda,Paul Christakopoulos,Ulrika Rova

doi:10.1186/s13068-014-0190-y

Leonidas Matsakas, Nemailla Bonturi

Open Access

https://doi.org/10.1186/s13068-014-0190-y

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Abstract

BackgroundEnvironmental crisis and concerns for energy security have made the research for renewable fuels that will substitute the usage of fossil fuels an important priority. Biodiesel is a potential substitute for petroleum, but its feasibility is hindered by the utilization of edible vegetable oil as raw material, which is responsible for a large fraction of the production cost and fosters the food versus fuel competition. Microbial oils are an interesting alternative as they do not compete with food production, and low cost renewable materials could serve as raw materials during cultivation of microorganisms. Sweet sorghum is an excellent candidate as substrate for microbial oil production, as it possesses high photosynthetic activity yielding high amounts of soluble and insoluble carbohydrates, and does not require high fertilization and irrigation rates.ResultsInitially the ability of sweet sorghum to fully support yeast growth, both as a carbon and nitrogen source was evaluated. It was found that addition of an external nitrogen source had a negative impact on single cell oil (SCO) production yields, which has a positive effect on the process economics. Subsequently the effect of the presence of a distinct saccharification step on SCO was examined. The presence of an enzymatic saccharification step prior to SCO production improved the production of SCO, especially in high solid concentrations. Removal of solids was also investigated and its positive effect on SCO production was also demonstrated. When juice from 20% w/w enzymatically liquefied sweet sorghum was used as the raw material, SCO production was 13.77 g/L. To the best of our knowledge this is one of the highest SCO titers reported in the literature when renewable raw materials were utilized.ConclusionsThe use of sweet sorghum at high solid concentrations as a feedstock for the efficient production of SCO by Rhodosporidium toruloides was demonstrated. Moreover, addition of enzymes not only led to liquefaction of sweet sorghum and permitted liquid fermentation, but also enhanced lipid production by 85.1% and 15.9% when dried stalks or stalk juice was used, respectively.

Highlights

Environmental crisis and concerns for energy security have made the research for renewable fuels that will substitute the usage of fossil fuels an important priority
Effect of nitrogen addition on single cell oil production Sweet sorghum stalks contain high amounts of soluble carbohydrates and insoluble carbohydrates, which could be converted to monomeric sugars and used by content resulted in inhibition of growth, probably due to inefficient air-transfer properties of the high solids mash
When a distinct enzymatic saccharification step was used, this resulted in efficient growth of yeast even at 16% w/w sweet sorghum content, and higher single cell oil (SCO) production yields for all solids contents, underscoring the great importance of saccharification

Summary

Introduction

Environmental crisis and concerns for energy security have made the research for renewable fuels that will substitute the usage of fossil fuels an important priority. Unlike electricity production, which has several renewable options available, the transportation sector is usually limited to diesel and gasoline, with ethanol and Biodiesel is a renewable, non-toxic, and biodegradable fuel [3] It is produced by the transesterification of triacylglycerols (TAGs) from vegetable oils and animal fats into fatty acid methyl or ethyl esters (FAMEs), for which edible plant oils (sunflower, rapeseed, and soybean) have been the main raw material. It has become evident that biofuel production should be based on non-food crops, agro-industrial wastes, and renewable resources to avoid direct competition with food production [4] These concerns have turned the efforts of researchers to Matsakas et al Biotechnology for Biofuels (2015) 8:6 finding alternative sources of TAGs, such as microbial oils, so called single cell oils (SCOs), for the production of biodiesel. In order to be classified as oleaginous, the microorganism should have the ability to accumulate at least 20% of the total dry cell mass as lipids [5], and can be a bacterium, yeast, filamentous fungus, or algae [6]

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