Climate impact of yeast oil from fast-growing perennial biomass (willow)

Global demand for fats is predicted to increase in coming decades. Production of vegetable oil causes environmental issues, and further agricultural expansion could compromise sensitive and valuable habitats and substantially increase greenhouse gas emissions. Oleaginous yeasts can convert lignocellulosic hydrolysate into an oil that is similar in composition to rapeseed oil and could provide an alternative to agricultural expansion. In this study, an energy and climate assessment was performed for a system where different varieties of willow (Salix spp.) were assumed to be grown on fallow land and used as feedstock for yeast oil production. The effects of biomass growth and soil organic carbon sequestration by six different commercial varieties of Salix on the climate impact of the system were also assessed. The results showed that production of 1 tonne of yeast oil required 18.1 GJ in fossil primary energy demand and 15.2 GJ in process electricity, while 18.5 GJ of biomethane and 6.5 GJ of excess power were generated simultaneously. Global warming potential of the oil was on average 1.86 kg CO2-eq kg(-1) and was substantially lower when carbon sequestration in soil by Salix was included in the analysis. Similar trends in net climate impact in terms of temperature change were observed. These findings indicate that edible oil can be produced from Salix feedstock via the novel conversion pathway of yeast fermentation, with considerable climate benefits. However, varietal differences in Salix, especially regarding soil carbon sequestration potential, can have a strong influence on the net climate impact of the yeast oil, highlighting the importance of including varietal differences when assessing climate impacts from a systems perspective.