3. Results and discussion
3.1 Preparation and characterization of algae oil-loaded
nanoemulsion
Algae oil contenting ω-3 LC-PUFA rich in both DHA and EPA
would be a potential alternative for fish oil because of overfishing,
ocean pollution, and low adoption of regular fish consumption
by many people, as well as its use in the vegan diet.
Nevertheless, they show organoleptic constraints and instability
to food processes, storage and ingestion, which has led to
research in the field of bioactive protection and controlled
release. As we all know, β-sitosterol (Sito), as a major type of
phytosterols, is in fact able to self-assemble, and form a threedimensional
crystal network in an oil solution, as clearly seen
by light microscopy (LM) and polarized light micrograph
(PLM) images, which eventually immobilizes the oil liquid for
a structured organogel being 10 wt% sitosterol in algae oil
(Fig. 2A). Thus, it is a major challenge to prepare a water dispersible
emulsion (oil-in-water, O/W) enriched in phytosterols
at room temperature (Fig. S1A†) due to the high melting point
of the sitosterol crystals (∼60–110 °C for hydrated crystals) and
hydrophobic nature (log P = 10.5),24 which commonly induce
sitosterol to flocculate and form water-in-oil (W/O) type emulsion-gels
in the case of a lower surfactant content.25 Hence,
hot-pressure homogenization as one approach was widely used
in O/W emulsion crystalline structuring processing.8,12 Concomitantly,
the sitosterol crystalline structure could orientate
and migrate to the surface of oil droplets due to the presence
of the tetracyclic fused ring skeleton (hydrophobic region) and
hydroxyl group (polar region) in sitosterol crystals with amphiphilic
properties (Fig. S1B†). Unfortunately, hot-pressure homogenization
is not particularly suitable for ω-3 heat-sensitive oil
(e.g., algae oil) because the early stage of oxidation during
emulsification results in a relatively fast oxidation rate during
post-processing and storage. Thus, it is necessarily to take a
strategy to make sure that the ω-3 oil are of good quality
during emulsification.