Effect of Electric Fields on Probiotic Cells: Modulation of the Encapsulation, Drying, and Viability

Probiotic cells are microorganisms that have health-promoting properties, which could be exerted providing that the cells are viable. Maintaining their viability, however, is challenging, due to their sensitivity to several factors, such as the pH, the presence of oxygen, and the processing conditions (i.e., heating, freezing).

This Ph.D. thesis elucidates different approaches to enhance the viability of probiotic cells by the utilization of electric fields. A literature review on selected studies is initially introduced, regarding the effect of the electric field on the stimulation, the manipulation of surface properties, and the electrophoretic movement of probiotic cells and other bacteria.

The first experimental part of this Ph.D. thesis explicates the effect of DC electric field to encapsulate surface-charged and hydrophobic probiotic cells (rod-shaped Bifidobacterium animalis subsp. lactis–BIFIDO, and coccus-shaped Streptococcus thermophilus–ST44) within maltodextrin microcapsules, using electrospray processing. The generated electrostatic forces between the negatively surface-charged probiotic cells and the applied negative polarity on the electrospray nozzle, allowed to control the location of the cells towards the core of the electrosprayed microcapsules. The organization of the cells affected the evaporation of the solvent (water), and subsequently the glass transition temperature (Tg) of the electrosprayed microcapsules, as well as the viability of probiotic cells. The utilization of auxiliary ring-shaped electrodes between the nozzle and the collector, enhanced the electric field strength and controlled the deposition of the capsules on the collector. Numerical simulation, through Finite Element Method (FEM), shed light to the effects of the additional ring-electrodes on the electric field strength and potential distribution, revealing a locally stronger electric field (in the proximity of the collector), that enhanced solvent evaporation, and contributed to higher glass transition temperatures of the microcapsules.

In addition to maltodextrin, non-water-soluble compounds, such as ethyl cellulose, were used for the encapsulation of BIFIDO probiotic cells in electrosprayed core–shell microcapsules. Different core compounds (concentrated BIFIDO, BIFIDO–maltodextrin and BIFIDO–glycerol) were tested, with ethyl cellulose (ETC) as a shell material. The ETC microcapsules exhibited relatively low water activity (a_w below 0.20) and high BIFIDO viability (10⁹–10¹¹ CFU/g). The electrosprayed microcapsules that contained BIFIDO–glycerol in the core, demonstrated a loss in viable cells of barely 3 log CFU/g, while the non-encapsulated BIFIDO lost approximately 7.57 log CFU/g. Even though the shell matrix was prepared using solvents that typically significantly decrease the viability of probiotics, the results of this study evidently demonstrated that the viability of BIFIDO can be extended by encapsulating within core-shell ETC electrosprayed capsules.

Since drying and moisture content are crucial for the long-term storage stability of probiotics, the effect of the ionic wind on the electrohydrodynamic drying (EHD) of probiotics was examined in another study of this thesis. Several parameters, such as the polarity and voltage of the electric field, the collector type, the surrounding temperature and gas, as well as the use of excipient, were found to affect the EHD drying process. BIFIDO were dried by the EHD drying and freeze-drying process, the comparison of which uncovered that the survival of cells and the water evaporation rates were similar, confirming the potential of EHD drying as an alternative drying technology for probiotic cells.

Probiotic cells’ long-term stability can also be induced through cell aggregation. The stimulation of probiotic cell aggregation by the application of DC electric fields and combined electric field with standing acoustic waves (SAW), was evaluated in an additional study of this thesis. The sole application of acoustic waves did not trigger any cell aggregation; however, the DC electric field induced the polarization and aggregation of the BIFIDO cells. The aggregation was accelerated (~6 times) when applying simultaneously DC electric field and SAW, due to the accumulation of the cells at the acoustic pressure nodes, followed by the aggregation of the polarized cells by dielectrophoresis (DEP). The synergistic effect of DC electric field and SAW not only facilitated the high aggregation of the cells, but also considerably enhanced the hydrophobicity of BIFIDO cells, without compromising their viability or altering their surface charges.

The protective effect of lecithin phospholipids’ coating on the viability of BIFIDO in aqueous media was also investigated. It was demonstrated by FTIR and Raman spectroscopy, that the BIFIDO–lecithin interactions are hydrophobic. Moreover, the effect of lecithin concentration on BIFIDO surface properties (surface charge, hydrophobicity) was also assessed and correlated with the long-term stability of probiotics.

Notwithstanding the differences of the explored methods, the aim was consistent; to maintain the probiotic cells viable during processing and storage. In summary, the utilization of electric fields significantly modulated the encapsulation, drying, and aggregation of the probiotic cells, subsequently enhancing their viability, as did the
employment of coating materials.