Optimisation of bacterial release from a stable microfluidic-generated water-in-oil-in-water emulsion |
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| Authors: | Nur Suaidah Mohd Isa Hani El Kadri Daniele Vigolo Konstantinos Gkatzionis |
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| Affiliation: | Faculty of Fisheries and Food Science, Universiti Malaysia Terengganu, 21030 Kuala Terengganu Terengganu Malaysia ; School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT UK ; School of Biomedical Engineering, University of Sydney, NSW 2006 Australia.; Department of Food Science and Nutrition, School of the Environment, University of the Aegean, Metropolite Ioakeim 2, 81400 Myrina Lemnos Greece, |
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| Abstract: | Application of droplet microfluidics for the encapsulation of bacteria in water-in-oil-in-water (W/O/W) emulsion allows for production of monodisperse droplets with controllable size. In this study the release of bacteria from W/O/W emulsion, the effect of the double emulsion structure on bacterial growth and metabolic activity, and the stability and mechanism of bacterial release were investigated. W/O/W emulsions were formed using a double flow-focusing junction microfluidic device under controlled pressure to produce droplets of approximately 100 μm in diameter containing an inner aqueous phase (W1) of about 40–50 μm in diameter. GFP-labelled Escherichia coli (E. coli-GFP) bacteria were encapsulated within the W1 droplets and the stability of emulsions was studied by monitoring droplet size and creaming behaviour. The double emulsions were stabilised using a hydrophilic (Tween 80) and a lipophilic surfactant (polyglycerol polyricinoleate) and were destabilised by altering the osmotic balance, adding NaCl either in the inner W1 phase (hypo-osmotic) or outer W2 phase (hyper-osmotic). The release of E. coli-GFP was monitored by plating on agar whereby the colony form unit (CFU) of the released bacteria was determined while fluorescent microscopy was employed to observe the mechanism of release from the droplets. The release of E. coli-GFP was significantly increased with higher concentrations of NaCl and lower amounts of Tween 80. Microscopic observation revealed a two-step mechanism for the release of bacteria: double W/O/W emulsion droplet splitting to release W1 droplets forming a secondary double emulsion followed by the collapse of W1 droplets to release E. coli-GFP into the continuous aqueous phase. Encapsulation enhanced viability and metabolic activity. Nutrients can cross the oil layer. Bacterial release increased while emulsion stability decreased at high osmotic pressure and low surfactant concentration. Two-step release mechanism observed. |
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