Isoprenoids and alkanes produced and secreted by microorganisms are emerging as an alternative biofuel for diesel and jet fuel replacements. In a similar way as for other bioprocesses comprising an organic liquid phase, the presence of microorganisms, medium composition, and process conditions may result in emulsion formation during fermentation, hindering product recovery. At the same time, a low-cost production process overcoming this challenge is required to make these advanced biofuels a feasible alternative. We review the main mechanisms and causes of emulsion formation during fermentation, because a better understanding on the microscale can give insights into how to improve large-scale processes and the process technology options that can address these challenges. © 2014 Elsevier Ltd.324221229Demirbas, A., Political, economic and environmental impacts of biofuels: a review (2009) Appl. Energy, 86, pp. S108-S117Ro, D.-K., Production of the antimalarial drug precursor artemisinic acid in engineered yeast (2006) Nature, 440, pp. 940-943Schalk, M., Firmenich, S.A., Method for producing beta-santalene, US20110281257 A1Wang, C., Microbial production of farnesol (FOH): current states and beyond (2011) Process Biochem., 46, pp. 1221-1229Renniger, N.S., McPhee, D.J., Amyris Biotechnologies. Fuel compositions comprising farnesane and farnesane derivatives and method of making and using same, US7399323 B2Westfall, P.J., Gardner, T.S., Industrial fermentation of renewable diesel fuels (2011) Curr. Opin. Biotechnol., 22, pp. 344-350Peralta-Yahya, P.P., Identification and microbial production of a terpene-based advanced biofuel (2011) Nat. Commun., 2, p. 483Warui, D.M., Detection of formate, rather than carbon monoxide, as the stoichiometric coproduct in conversion of fatty aldehydes to alkanes by a cyanobacterial aldehyde decarbonylase (2011) J. Am. Chem. Soc., 133, pp. 3316-3319Howard, T.P., Synthesis of customized petroleum-replica fuel molecules by targeted modification of free fatty acid pools in Escherichia coli (2013) Proc. Natl. Acad. Sci. U.S.A., 110, pp. 7636-7641Schirmer, A., Microbial biosynthesis of alkanes (2010) Science, 329, pp. 559-562Domínguez de María, P., Recent developments in the biotechnological production of hydrocarbons: paving the way for bio-based platform chemicals (2011) ChemSusChem, 4, pp. 327-329Tabur, P., Dorin, G., Amyris Biotechnologies, , Method for purifying bio-organic compounds from fermentation broth containing surfactants by temperature-induced phase inversion, US20120040396Neu, T.R., Significance of bacterial surface-active compounds in interaction of bacteria with interfaces (1996) Microbiol. Rev., 60, pp. 151-166Pedetta, A., Phenanthrene degradation and strategies to improve its bioavailability to microorganisms isolated from brackish sediments (2013) Int. Biodeterior. Biodegrad., 84, pp. 161-167Banat, I.M., Microbial biosurfactants production, applications and future potential (2010) Appl. Microbiol. Biotechnol., 87, pp. 427-444Jain, R.M., Isolation and structural characterization of biosurfactant produced by an alkaliphilic bacterium Cronobacter sakazakii isolated from oil contaminated wastewater (2012) Carbohydr. Polym., 87, pp. 2320-2326Janek, T., Identification and characterization of biosurfactants produced by the Arctic bacterium Pseudomonas putida BD2 (2013) Colloids Surf. B: Biointerfaces, 110, pp. 379-386Berton, C., Quantification of unadsorbed protein and surfactant emulsifiers in oil-in-water emulsions (2011) J. Colloid Interface Sci., 354, pp. 739-748Mukherjee, A.K., Das, K., Microbial surfactants and their potential applications: an overview (2010) Biosurfactants, pp. 54-64. , Springer, R. Sen (Ed.)(1987) Biosurfactants and Biotechnology, , Marcel Dekker, N. Kosaric (Ed.)Healy, M.G., Microbial production of biosurfactants (1996) Resour. Conserv. Recycl., 18, pp. 41-57Cameotra, S.S., Makkar, R.S., Recent applications of biosurfactants as biological and immunological molecules (2004) Curr. Opin. Microbiol., 7, pp. 262-266Benincasa, M., Chemical structure, surface properties and biological activities of the biosurfactant produced by Pseudomonas aeruginosa LBI from soapstock (2004) Antonie Van Leeuwenhoek, 85, pp. 1-8Lovaglio, R.B., Rhamnolipid emulsifying activity and emulsion stability: pH rules (2011) Colloids Surf. B: Biointerfaces, 85, pp. 301-305Prieto, L.M., The production of rhamnolipid by a Pseudomonas aeruginosa strain isolated from a southern coastal zone in Brazil (2008) Chemosphere, 71, pp. 1781-1785Pornsunthorntawee, O., Structural and physicochemical characterization of crude biosurfactant produced by Pseudomonas aeruginosa SP4 isolated from petroleum-contaminated soil (2008) Bioresour. Technol., 99, pp. 1589-1595Treichel, H., A review on microbial lipases production (2010) Food Bioprocess Technol., 3, pp. 182-196Lienemann, M., Structure-function relationships in hydrophobins: probing the role of charged side chains (2013) Appl. Environ. Microbiol., 79, pp. 5533-5538Song, C.-S., Molecular behavior of a microbial lipopeptide monolayer at the air-water interface (2007) Colloid Surf. A, 302, pp. 82-87Deleu, M., Interfacial and emulsifying properties of lipopeptides from Bacillus subtilis (1999) Colloid Surf. A, 152, pp. 3-10Rosenberg, E., Ron, E.Z., High- and low-molecular-mass microbial surfactants (1999) Appl. Microbiol. Biotechnol., 52, pp. 154-162Peypoux, F., Recent trends in the biochemistry of surfactin (1999) Appl. Microbiol. Biotechnol., 51, pp. 553-563Lang, S., Biological amphiphiles (microbial biosurfactants) (2002) Curr. Opin. Colloid Interface Sci., 7, pp. 12-20Vater, J., Matrix-assisted laser desorption ionization-time of flight mass spectrometry of lipopeptide biosurfactants in whole cells and culture filtrates of Bacillus subtilis C-1 isolated from petroleum sludge (2002) Appl. Environ. Microbiol., 68, pp. 6210-6219Mirtallo, J.M., State of the art review: intravenous fat emulsions: current applications, safety profile, and clinical implications (2010) Ann. Pharmacother., 44, pp. 688-700Knoth, A., Stability of water-in-oil-emulsions containing phosphatidylcholine-depleted lecithin (2005) Food Hydrocolloid, 19, pp. 635-640Cameron, D.R., The mannoprotein of Saccharomyces cerevisiae is an effective bioemulsifier (1988) Appl. Environ. Microbiol., 54, pp. 1420-1425Barriga, J.A.T., Components of the bioemulsifier from S. cerevisiae (1999) Enzyme Microb. Technol., 25, pp. 96-102Rajakylä, E., Paloposki, M., Determination of sugars (and betaine) in molasses by high-performance liquid chromatography: comparison of the results with those obtained by the classical lane-eynon method (1983) J. Chromatogr. A, 282, pp. 595-602Almeida, J.R.M., Increased tolerance and conversion of inhibitors in lignocellulosic hydrolysates by Saccharomyces cerevisiae (2007) J. Chem. Technol. Biotechnol., 82, pp. 340-349Mohebali, G., Stabilization of water/gas oil emulsions by desulfurizing cells of Gordonia alkanivorans RIPI90A (2007) Microbiology, 153, pp. 1573-1581Dorobantu, L.S., Stabilization of oil-water emulsions by hydrophobic bacteria (2004) Appl. Environ. Microbiol., 70, pp. 6333-6336Binks, B.P., Particles as surfactants - similarities and differences (2002) Curr. Opin. Colloid Interface Sci., 7, pp. 21-41Dickinson, E., Use of nanoparticles and microparticles in the formation and stabilization of food emulsions (2012) Trends Food Sci. Technol., 24, pp. 4-12Chevalier, Y., Bolzinger, M.-A., Emulsions stabilized with solid nanoparticles: Pickering emulsions (2013) Colloid Surf. A, 439, pp. 23-34Huang, X., Separation and characterization of effective demulsifying substances from surface of Alcaligenes sp. S-XJ-1 and its application in water-in-kerosene emulsion (2013) Bioresour. Technol., 139, pp. 257-264Akartuna, I., Stabilization of oil-in-water emulsions by colloidal particles modified with short amphiphiles (2008) Langmuir, 24, pp. 7161-7168Ly, M.H., Importance of bacterial surface properties to control the stability of emulsions (2006) Int. J. Food Microbiol., 112, pp. 26-34Poortinga, A.T., Electric double layer interactions in bacterial adhesion to surfaces (2002) Surf. Sci. Rep., 47, pp. 1-32Marinova, K.G., Charging of oil-water interfaces due to spontaneous adsorption of hydroxyl ions (1996) Langmuir, 12, pp. 2045-2051McClements, D.J., Protein-stabilized emulsions (2004) Curr. Opin. Colloid Interface Sci., 9, pp. 305-313Olofsson, K., A short review on SSF-an interesting process option for ethanol production from lignocellulosic feedstocks (2008) Biotechnol. Biofuels, 1, pp. 1-14Reuß, M., Viscosity of yeast suspensions (1979) Eur. J. Appl. Microbiol. Biotechnol., 8, pp. 167-175McClements, D.J., (2005) Food Emulsions, Principles, Practices, and Techniques, , CRC PressWalstra, P., Principles of emulsion formation (1993) Chem. Eng. Sci., 48, pp. 333-349Desai, J.D., Banat, I.M., Microbial production of surfactants and their commercial potential (1997) Microbiol. Mol. Biol. Rev., 61, pp. 47-64Sareen, S.S., Coalescence in fibrous beds (1966) AIChE J., 12, pp. 1045-1050Hlavacek, M., Break-up of oil-in-water emulsions induced by permeation through a microfiltration membrane (1995) J. Membr. Sci., 102, pp. 1-7van Reis, R., Zydney, A., Membrane separations in biotechnology (2001) Curr. Opin. Biotechnol., 12, pp. 208-211Schlieper, L., Liquid-liquid phase separation in gravity settler with inclined plates (2004) AIChE J., 50, pp. 802-811Laleh, A.P., Design and CFD studies of multiphase separators-a review (2012) Can. J. Chem. Eng., 90, pp. 1547-1561Grace, R., Commercial emulsion breaking (1992) Emulsions, pp. 313-339. , American Chemical Society, L.L. Schramm (Ed.)Young, G.A.B., Oil-water separation using hydrocyclones: an experimental search for optimum dimensions (1994) J. Petrol. Sci. Eng., 11, pp. 37-50Meldrum, N., Hydrocyclones: a solution to produced-water treatment (1988) SPE Prod. Eng., 3, pp. 669-676Yu, L.Q., World Intellectual Property Organization. Oil/water/biocatalyst three phase separation process, US5772901Rubio, J., Overview of flotation as a wastewater treatment technique (2002) Miner. Eng., 15, pp. 139-155Al-Shamrani, A.A., Destabilisation of oil-water emulsions and separation by dissolved air flotation (2002) Water Res., 36, pp. 1503-1512Schroën, C.G.P.H., Woodley, J.M., Membrane separation for downstream processing of aqueous-organic bioconversions (1997) Biotechnol. Prog., 13, pp. 276-283Schmid, A., Developments toward large-scale bacterial bioprocesses in the presence of bulk amounts of organic solvents (1998) Extremophiles, 2, pp. 249-256Brandenbusch, C., Efficient phase separation and product recovery in organic-aqueous bioprocessing using supercritical carbon dioxide (2010) Biotechnol. Bioeng., 107, pp. 642-651Leppchen, K., Microbial de-emulsification: a highly efficient procedure for the extractive workup of whole-cell biotransformations (2006) Org. Process Res. Dev., 10, pp. 1119-1125Wilde, P., Proteins and emulsifiers at liquid interfaces (2004) Adv. Colloid Interface Sci., pp. 63-71Prins, A., van't Riet, K., Proteins and surface effects in fermentation: foam, antifoam and mass transfer (1987) Trends Biotechnol., 5, pp. 296-301Choi, O.K., Enhancement of phase separation by the addition of de-emulsifiers to three-phase (diesel oil/biocatalyst/aqueous phase) emulsion in diesel biodesulfurization (2003) Biotechnol. Lett., 25, pp. 73-77Prasad, K., Cost cutting strategies in downstream processing industry (2012) Downstream Process Technology: A New Horizon in Biotechnology, pp. 32-39. , PHI Learning Pvt, K. Prasad (Ed.)Rude, M.A., Schirmer, A., New microbial fuels: a biotech perspective (2009) Curr. Opin. Biotechnol., 12, pp. 274-281Bullis, K., Why Amyris is focusing on moisturizers, not fuel, for now (2012) MIT Technology Review, , http://www.technologyreview.com/news/427890/why-amyris-is-focusing-on-moisturizers-not-fuel-for-now/, MIT PressCuellar, M.C., Large-scale production of diesel-like biofuels - process design as an inherent part of microorganism development (2013) Biotechnol. J., 8, pp. 682-689Efe, C., (2007) Technical and Economical Feasibility of Production of Ethanol from Sugar Cane and Sugar Cane Bagasse, , http://repository.tudelft.nl/view/ir/uuid:5f3b7381-0da3-4d26-b334-9b4856ecacda/, B-Basic ReportTaylor, P., Ostwald ripening in emulsions (1998) Adv. Colloid Interface Sci., 75, pp. 107-163Arima, K., Surfactin, a crystalline peptide lipid surfactant produced by Bacillus subtilis: isolation, characterization and its inhibition of fibrin clot formation (1968) Biochem. Biophys. Res. Commun., 31, pp. 488-494Large, K.P., The effect of agitation rate on lipid utilisation and clavulanic acid production in Streptomyces clavuligerus (1998) J. Biotechnol., 63, pp. 111-119Galindo, E., Study of drop and bubble sizes in a simulated mycelial fermentation broth of up to four phases (2000) Biotechnol. Bioeng., 69, pp. 213-221Clarke, K.G., Correia, L.D.C., Oxygen transfer in hydrocarbon-aqueous dispersions and its applicability to alkane bioprocesses: a review (2008) Biochem. Eng. J., 39, pp. 405-429Erler, S., Oil/water and pre-emulsified oil/water (PIT) dispersions in a stirred vessel: implications for fermentations (2003) Biotechnol. Bioeng., 82, pp. 543-551Ron, E., Rosenberg, E., Role of biosurfactants (2010) Handbook of Hydrocarbon and Lipid Microbiology, pp. 2515-2518. , Springer-Verlag, K.N. Timmis (Ed.)Abbasnezhad, H., Influence of adhesion on aerobic biodegradation and bioremediation of liquid hydrocarbons (2011) Appl. Microbiol. Biotechnol., 92, pp. 653-675Lawniczak, L., Contributions of biosurfactants to natural or induced bioremediation (2013) Appl. Microbiol. Biotechnol., 97, pp. 2327-2339Babich, I.V., Moulijn, J.A., Science and technology of novel processes for deep desulfurization of oil refinery streams: a review (2003) Fuel, 82, pp. 607-631Monticello, D.J., Biodesulfurization and the upgrading of petroleum distillates (2000) Curr. Opin. Biotechnol., 11, pp. 540-546Lye, G.J., Woodley, J.M., Application of in situ product-removal techniques to biocatalytic processes (1999) Trends Biotechnol., 17, pp. 395-402Straathof, A.J.J., Auxiliary phase guidelines for microbial biotransformations of toxic substrate into toxic product (2003) Biotechnol. Prog., 19, pp. 755-762Laane, C., Rules for optimization of biocatalysis in organic solvents (1987) Biotechnol. Bioeng., 30, pp. 81-87