Enhanced electric field and charge polarity modulate the microencapsulation and stability of electrosprayed probiotic cells (Streptococcus thermophilus, ST44)

Abstract

The effect of the polarity of the direct current electric field on the "organization" of Streptococcus thermophilus (ST44) probiotic cells within electrosprayed maltodextrin microcapsules was investigated. 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. This "organization" of the cells increased the evaporation of the solvent (water) and successively the glass transition temperature (Tg) of the electrosprayed microcapsules. Moreover, the utilization of auxiliary ring-shaped electrodes between the nozzle and the collector, enhanced the electric field strength and contributed further to the increase of the Tg. Numerical simulation, through Finite Element Method (FEM), shed light to the effects of the additional ring-electrode on the electric field strength, potential distribution, and controlled deposition of the capsules on the collector. Furthermore, when the cells were located at the core of the microcapsules their viability was significantly improved for up to 2 weeks of storage at 25 °C and 35% RH, compared to the case where the probiotics were distributed towards the surface. Overall, this study reports a method to manipulate the encapsulation of the surface charged probiotic cells within electrosprayed microcapsules, utilizing the polarity of the electric field and additional ring-electrodes.

Keywords: Electrohydrodynamics; Encapsulation; Lactic acid bacteria; Maltodextrin; Numerical simulation; Streptococcus thermophilus.

Graphical abstract

Fig. 1

Schematic representation of the negative polarity electrospray apparatus inside a polycarbonate chamber, consisting of a syringe pump, a syringe connected to a metal needle, two auxiliary ring-shaped electrodes, and a collecting electrode. High voltage power supplies were connected to the electrospray needle, auxiliary rings and collecting electrode.

Fig. 2

ST44 chaining phenotype observed by Atomic Force Microscopy (AFM). The arrow heads indicate the nascent septum (a), the intermediate septum (b) and the mature septum (c).

Fig. 3

Zeta potential and (%) Hydrophobicity of ST44 in KH₂PO₄ at pH = 6.

Fig. 4

Confocal laser scanning microscopy (CLSM) micrographs showing the distribution of ST44 cells within microcapsules electrosprayed at |20 kV| with different polarities. The edge of the microcapsules is represented by white circles.

Fig. 5

Effect of electrospray polarity, and auxiliary ring-electrodes on the glass transition temperature (Tg) of electrosprayed maltodextrin microcapsules encapsulating ST44.

Fig. 6

Axisymmetric FEM analysis of the electric field strength for electrospray processing (|20 kV|) using negative polarity at the needle (A) Without the auxiliary ring-electrode, (B) With 1 ring-electrode, and (C) With 2 ring-electrodes. The colors represent the electric field strength in V/m (Iso value).

Fig. 7

Electric field strength (A) and Potential (B) at the center line between the needle and the center of the collector when negative polarity was used.

Fig. 8

Axisymmetric FEM analysis of the electric field lines for electrospray processing (|20 kV|) using negative polarity at the needle (A) Without the auxiliary ring-electrode, (B) With 1 ring-electrode, and (C) With 2 ring-electrodes.

Fig. 9

Log loss of viability over time (25 °C and 35 ± 2 %RH) of encapsulated ST44 probiotic cells within electrosprayed maltodextrin microcapsules using different electric field polarities and 2 auxiliary ring-electrodes, and of non-encapsulated (free) ST44 cells.