Pickering emulsions stabilized by
bacteria acting as particle emulsifiers
are new platforms for microbial transformations of hydrophobic chemicals.
However, their high stability often hampers demulsification during
downstream processing. Since the existing methods (like addition of
surfactants) to demulsify bacteria-stabilized Pickering emulsions
have negative effects, new practical methods need to be developed.
Here, using chemically modified fumed silica particles with different
hydrophobicity, the demulsification of W/O Pickering emulsions stabilized
by Mycobacterium neoaurum whole cells
was first studied. The binary particle-stabilized emulsions exhibited
phase inversion and dewatering induced by the coalescence of W/O emulsions
or creaming of O/W emulsions. The silica particle hydrophobicity and
concentration were the important parameters influencing the emulsion
type, droplet morphology, and dewatering rate. The highest dewatering
rate and largest droplet size were obtained at the inversion point
from W/O to O/W. Confocal microscopy showed that no interaction between
the bacteria and silica particles existed and the silica particle
adsorption at the interface induced the detachment of bacteria from
the interface, revealing that there was competitive adsorption between
the binary particles at the interface. Based on these results, we
suggested that the average hydrophobicity of the binary particles
at the interface would determine the emulsion type and stability.
Finally, this strategy was successfully applied to the demulsification
of the Pickering emulsion formed during microbial transformation of
sterols. Overall, this study provides a new strategy to demulsify
Pickering emulsions by addition of another particle emulsifier. This
is also the first example of separation of products as well as organic
phases after microbial transformation in Pickering emulsions.
Biocatalysis, which can be performed by whole cells and isolated enzymes, has become a topic of public interest for its potential use in the chemical industry in manufacturing, monitoring, and waste management [...]
Microbial transformation of hydrophobic phytosterols into the pharmaceutical steroid precursors AD (androst-4-ene-3, 17-dione) and ADD (androst-4-diene-3, 17-dione) in a water–plant oil two-phase system by Mycolicibacterium neoaurum is a paradigm of interfacial biocatalysis in Pickering emulsions stabilized by bacterial cells. In the present work, phase inversion of Pickering emulsions—i.e., Pickering emulsions turning from water-in-oil (W/O) emulsions into oil-in-water (O/W) ones—was observed during microbial transformation in the presence of high concentrations of crystal phytosterols. It was found that there is a correlation relationship between the phase behaviors of Pickering emulsions and the biocatalytic activity of utilizing M. neoaurum as a whole-cell catalyst. Efficient microbial transformation under the high crystal phytosterol loadings was achieved due to the formation of O/W emulsions where interfacial biocatalysis took place. Under the optimal conditions (volume ratio of soybean oil to water: 15:35 mL, phytosterols concentration in the soybean oil: 80 g/L, glucose as co-substrate in the aqueous culture medium: 10 g/L), the concentrations of AD and ADD reached 4.8 g/L based on the whole broth (16 g/L based on the oil phase) after microbial transformation for 9 days.
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