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.