Changes in Particle Size and Zeta-Potential during In
Vitro Digestion
Changes in particle size and zeta-potential throughout the various
stages of the gastrointestinal (GIT) model are illustrated in Figure 4.
Initially, the MF-prepared emulsion exhibited a significantly smaller
mean particle size, primarily attributable to the different
homogenization method employed. However, as the emulsions progressed
through the oral phase, the particle size of the HPH-prepared emulsion
experienced a notable increase, whereas the MF emulsion exhibited
minimal changes. Subsequent to the gastric phase, the MF emulsion
demonstrated a substantial increase in mean particle diameter,
suggesting a propensity for significant droplet aggregation. This
observation aligns with prior studies (Li et al., 2020), which have
reported that protein-stabilized emulsions tend to aggregate under
gastric conditions. This aggregation can be attributed to factors such
as low pH, hydrolysis of adsorbed proteins, weakening of electrostatic
repulsion, and the occurrence of depletion or bridging flocculation
induced by mucin. The results obtained from confocal microscopy
supported and reinforced these findings. Zeta-potential, a critical
indicator of colloidal suspension stability, was continuously monitored
as the emulsions progressed through the various stages of the GIT model
to assess alterations in interfacial properties. Initially, the
MF-prepared emulsion exhibited a higher absolute value of
zeta-potential, signifying better stability, which aligns with the
previously obtained results. However, following the oral phase, there
was a noticeable decrease in the magnitude of the negative charge on the
MF emulsion. This reduction could potentially be attributed to
electrostatic screening caused by the presence of mineral ions in
simulated saliva or interactions between mucin molecules and the
surfaces of oil droplets.
Further reductions in the absolute value of zeta-potential were observed
as the emulsions encountered simulated stomach conditions. The low pH
and high ionic strength of simulated gastric fluids could have led to
alterations in the electrical properties of the droplets. Subsequently,
after incubation in the small intestine phase, all samples displayed
negative charges. This phenomenon may be attributed to the presence of
anionic species from various types of particles, including undigested
lipids, undigested proteins, micelles, vesicles, and calcium salts.
Notably, the negative charges of the MF-prepared emulsion remained
relatively higher throughout the gastrointestinal model, indicating its
superior stability in the small intestine phase. Therefore, the emulsion
produced by MF demonstrated enhanced stability across the different
stages of the GIT model, highlighting its potential as a robust delivery
system throughout the gastrointestinal tract.