Fermentation broth components influence droplet coalescence and hinder advanced biofuel recovery during fermentation

Abstract

Developments in synthetic biology enabled the microbial production of long chain hydrocarbons, which can be used as advanced biofuels in aviation or transportation. Currently, these fuels are not economically competitive due to their production costs. The current process offers room for improvement: by utilizing lignocellulosic feedstock, increasing microbial yields, and using cheaper process technology. Gravity separation is an example of the latter, for which droplet growth by coalescence is crucial. The aim of this study was to study the effect of fermentation broth components on droplet coalescence. Droplet coalescence was measured using two setups: a microfluidic chip and regular laboratory scale stirred vessel (2 L). Some fermentation broth components had a large impact on droplet coalescence. Especially components present in hydrolysed cellulosic biomass and mannoproteins from the yeast cell wall retard coalescence. To achieve a technically feasible gravity separation that can be integrated with the fermentation, the negative effects of these components on coalescence should be minimized. This could be achieved by redesign of the fermentation medium or adjusting the fermentation conditions, aiming to minimize the release of surface active components by the microorganisms. This way, another step can be made towards economically feasible advanced biofuel production. ical de-emulsifier [9]. These steps are currently required because straightforward coalescence does not occur due to substances in the fermentation broth. In this complex mixture, a wide range of substances is capable of hindering coalescence [7], and this problem could become even more prominent when using hydrolysed cellulosic biomass as a feedstock [10], leading to an even more complex composition of the fermentation broth. Gravity separation would be a cheaper alternative method to achieve phase separation compared to centrifugation. This method would be feasible when the droplets grow sufficiently in size by coalescence. Additional advantages of gravity separation are that it could be integrated with the fermentation to achieve continuous product removal and that it enables cell recycle, both contributing to lower production costs (Heeres, A. S., Cuellar, M. C., Van der Wielen, L. A. M., Integrating fermentation and separation for advanced biofuel production. 10th European Symposium on Biochemical Engineering Sciences, Lille, France 2014).The main parameter determining the separation rate in gravity separation, represented by the droplet rise velocity (v d ), is the droplet diameter (d d ), with a quadratic dependency on the phase separation rate, as is shown by Stokes' law for the motion of a single droplet (Eq. 1). Furthermore, the difference in density of the oil droplets (ρ o ) and the continuous aqueous phase (ρ w ), the viscosity (η) and the gravitational constant (g) play a role in the droplet rise velocity:(1) This equation is valid for the rise of a single droplet, but the influence of the parameters extents to more concentrated sy...

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