1. Introduction
Solid lipid particles are promising carrier systems for lipophilic
bioactive compounds because of their high compatibility with the solid
lipids used as carrier material (Santos et al., 2019). These particles
are required for the development of innovative cosmetic, food and
pharmaceutical products (Panigrahi et al., 2019); therefore, it is
necessary to overcome their potential disadvantages, such as
insufficient load capacity and polymorphic transitions during storage
that lead to the expulsion of the bioactive compound (Poovi et al.,
2019).
One strategy to avoid the disadvantages of solid lipid particles is the
use of solid-liquid lipid mixtures as carrier material in order to alter
the crystalline lattice (Santos et al., 2019). For this purpose, beeswax
(a solid lipid), which is a complex organic lipid-based material used as
a carrier material in particle formation, could be mixed with edible
oils (liquid lipids). Some of these edible oils are rich in bioactive
compounds that could improve the bioactivity of the formed particles.
For example, avocado oil is used in the cosmetic industry due to its
anti-inflammatory and skin hydration power (Tan, 2019; Rydlewski et al.,
2020), and Brazil nut oil could positively influence the modulation of
the immune system and organic responses to inflammatory and
hypolipidemic processes (Santos et al., 2012).
The formation of solid lipid particles can be achieved using different
technologies, among which the supercritical CO2technology has advantages such as operation at moderate temperature and
use of CO2 as a green solvent. This technology involves
some techniques like gas antisolvent precipitation (GAS), particles from
gas saturated solution (PGSS), and rapid expansion of supercritical
solution (RESS) (Spilimbergo et al., 2006). The GAS process is based on
the supersaturation of a liquid solution by dissolving supercritical
fluid in which initially the solute (immiscible in supercritical
CO2) is solubilized in an organic solvent that must be
completely miscible with the supercritical fluid. This then proceeds to
produce a volumetric expansion of the liquid solution together with the
gradual addition of supercritical fluid to the solution that results in
the precipitation of the solute of interest. PGSS is used for materials
with relatively low melting temperatures, such as polymers and lipids.
In this technique the supercritical CO2 dissolves in a
melted lipid or plasticized polymer, and the rapid expansion of the
solution through a nozzle results in precipitated particles (Akbari et
al., 2020). In the RESS process, supercritical CO2 is
used as a means to extract the target material at high pressure;
subsequently, the supercritical solution is depressurized to achieve
high supersaturation for the generation of fine particles (Yang et al.,
2020). Neither RESS nor PGSS uses organic solvents. Supercritical fluid
technology has several advantages such as dry particles (water-free and
solvent-free, except GAS that isn’t solvent-free), process simplicity,
low processing cost and a wide range of products due to the modulation
of the supercritical fluid density by changing pressure, thus easy
separation and recovery of solvent and antisolvent can be performed by a
depressurization step (Akbari et al., 2020).
In this context, the design of the particle formation process requires
knowledge of the behavior of the carrier material in supercritical
CO2 to define process parameters for verifying their
economic and practical feasibility (Spilimbergo et al., 2006).
Accordingly, measurements of melting temperature (Tm)
and solidification temperature (Ts) of the lipid mixture
at fixed pressures is necessary in defining the process temperature.
Temperature is a key process parameter because it must be high enough to
keep the lipid mixture in liquid state during processing, but low enough
to allow the use of thermally labile bioactive compounds (Ciftci and
Temelli, 2014). Furthermore, measurements of volumetric expansion,
solubility of supercritical CO2 on lipids and lipid
mixtures and solubility of the lipid mixtures in supercritical
CO2 at the process temperature is important in
explaining particle characteristics such as morphology and size when
formed at different pressures. There are behavioral studies in
supercritical CO2 for many solid and liquid lipids.
Nevertheless, there is no experimental data available in the literature
about beeswax-edible oils mixtures.
Thus, in this investigation a behavioral study that covered the
measurement of Tm and Ts,
Ve, solubility of supercritical CO2, and
solubility of lipid mixtures of beeswax with avocado oil or Brazil nut
oil in supercritical CO2 at fixed pressures was carried
out.