Production of Biosurfactant by Fermentation with Integral Foam Fractionation

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Abstract

Biosurfactants are naturally occurring amphiphiles with potential for use asalternatives to traditional petrochemical and oleochemical surfactants. The uniqueproperties of biosurfactants, including their biodegradability and tolerance of a wide rangeof temperature and pH, make their use in a range of novel applications attractive. Currentlythe wider ultilisation of biosurfactants is hindered by a lack of economically viableproduction routes, with downstream processing presenting a significant challenge. Thisthesis presents an investigation into the production of HFBII, a hydrophobin protein, usingan adsorptive bubble separation technique called foam fractionation for in situ recovery ofthe biosurfactant.The effects of foaming on the production of HFBII by fermentation wereinvestigated at two different scales. Foaming behaviour was characterised in standardterms of the product enrichment and recovery achieved. Additional specific attention wasgiven to the rate at which foam, product and biomass overflowed from the fermentationsystem in order to assess the utility of foam fractionation for HFBII recovery. HFBII wasexpressed as an extracellular product during fed batch fermentations with a geneticallymodified strain of Saccharomyces cerevisiae, which were carried out with and withoutantifoam. In the presence of antifoam HFBII production is shown to be largely unaffectedby process scale, with similar yields of HFBII on dry matter obtained. More variation inHFBII yield was observed between fermentations without antifoam. In fermentationswithout antifoam a maximum HFBII enrichment in the foam phase of 94.7 was measuredwith an overall enrichment of 54.6 at a recovery of 98.1 %, leaving a residual HFBIIconcentration of 5.3 mg L-1 in the fermenter. It is also shown that uncontrolled foamingreduced the concentration of biomass in the fermenter vessel, affecting total production.This series of fermentation experiments illustrates the potential for the application of foamfractionation for efficient in situ recovery of HFBII, through simultaneous high enrichmentand recovery which are greater than those reported for similar systems.After the suitability of foam fractionation was demonstrated a novel apparatusdesign was developed for continuously recovering extracellular biosurfactants fromfermenters. The design allows for the operating conditions of the foam fractionationprocess, feed rate and airflow rate, to be chosen independently of the fermentationparameters. Optimal conditions can then be established for each process, such as theaeration rate required to meet the biological oxygen demand of the cell population. Therecirculating foam fractionation process was tested on HFBII producing fermentations. It isshown that by using foam fractionation to strip HFBII from fermentation broth in situ theamount of uncontrolled overflowing from the fermenter was greatly reduced from 770.0 gto 44.8 g, compared to fermentations without foam fractionation. Through optimisation ofthe foam column operating conditions the proportion of dry matter retained in thefermenter was increased from 88 % to 95 %, in contrast to a dry matter retention of 66 %for fermentation without the new design. With the integrated foam fractionation process aHFBII recovery of 70 % was achieved at an enrichment of 6.6. This work demonstrates theutility of integrated foam fractionation in minimising uncontrolled foaming in fermenterswhilst recovering an enriched product. This integrated production and separation processhas the potential to facilitate improved biosurfactant production, currently a major barrierto their wider use.

Keyword(s)

antifoam; biosurfactant; fermentation; foam fractionation; hydrophobin; process integration; recovery; separation

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