1. Introduction
The development of oleogels has emerged as a new and exciting field of lipid research. For food systems, the incorporation of edible oleogels allow the reduction of saturated fatty acids and the elimination oftrans -fatty acids from the product (Dassanayake et al., 2011; Marangoni and Garti, 2018; Rogers et al., 2014) and in cosmetics the oleogels can be used as vehicles to delivery hydrophobic bioactive compounds (Ferrari and Mondet, 2003; Morales et al., 2009; Perez Nowak, 2011). Within this context, several vegetal waxes have gained considerable attention in the development of oleogels mainly because of their high gelling capacity and gel physical’s properties, some even showing reversible thermomechanical properties. Additionally, the vegetable waxes are easy to obtain at affordable costs and most of them are already approved by the regulatory agencies (Blake et al., 2018).
Although most plant-based waxes are heterogenous mixtures of different components, their gelling capacity is usually associated with the major component that in the case of candelilla wax (CW) is then -alkanes, i.e., 49%–50% n -alkanes with 29–33 carbons with hentriacontane as the n -alkane in major concentration (Grant, 2005; Nippo, 1985; Toro-Vazquez et al., 2007). Therefore, based on the n -alkanes concentration and gelling capacity of organic solvents (Abdallah and Weiss, 2000), our initial publications ascribed the high gelling capacity of the CW to the development of a three-dimensional crystal structure by the molecular self-assembly of the n -alkanes (Chopin-Doroteo et al., 2011; Morales-Rueda et al., 2009; Toro-Vazquez et al., 2007). However, experiments done in our laboratory showed that through a simple treatment extraction to reduce the concentration of long chain esters from the CW, resulted in a significant modification of the crystal habit of the oleogel, and also in a reduction of the gelling capacity and, subsequently, in an increase in the original minimal gelling concentration of CW (Romero Regalado, 2013). These results showed that the interaction among the native components of the CW determines its gelling properties, and the same concept applies to other vegetable waxes (Toro-Vazquez et al., 2023). On the other hand, a more in-deep CW analysis done using gas chromatography coupled with mass spectrophotometry, showed that besidesn -alkanes the CW also has high concentrations of triterpenes, in particular triterpenic alcohols (i.e., ≈23%) and esters of triterpenic alcohols (i.e., ≈2%) (Ortega-Salazar, 2012). Triterpenes are a class of terpenes composed of six isoprene units characterized by a basic steroidal backbone with the C30H48general molecular formula. Triterpenes are commonly present in several vegetables as triterpenoid glycosides or steroids, commonly referred to as saponins (Böttcher and Drusch, 2017; Wojciechowski, 2013). Terpenes and triterpenoid glycosides are compounds with well-known interfacial properties capable of developing foams (i.e., air-water interface activity) and oil-in-water emulsions (i.e., oil-water interface activity) (Liu et al., 2011; Pagureva et al., 2016; Sharma et al., 2023). On the other hand, some pentacyclic triterpenes present in several vegetable waxes (i.e., ursolic acid) have physical properties associated also with the development of organogels (Lu et al., 2019) and recently, it was reported that also are able to develop water-in-oil emulsions stabilized through the Pickering effect (Liu et al., 2022).
Within the previous context and considering that the CW is constituted by components with molecular self-assembly and surface-active properties, this study evaluated the development of structured (i.e., gelled) water-in-oil (W/O) emulsions at room temperature (25°C) just with the use of the CW (i.e., absence of added surfactants). In a recent study, Penagos et al. (2023) developed W/O emulsions at 5°C using mixtures of beeswax and carnauba wax formulated with 20%, 30% and 40% (wt/wt) of water in sunflower oil. Based on contact angle measurement the authors discarded the CW, the berry wax, and the sunflower wax as tentative stabilizers of W/O emulsion. However, in this study no formal W/O emulsions were done to assess the emulsifying capacity of these vegetal waxes (Penagos et al., 2023). On the other hand, García-González et al. (2021) showed, through dynamic interfacial tension measurements, that in the temperature interval of 45°C to 60°C the CW significantly decreased the interfacial tension of safflower oil high in triolein from 26.4 (± 0.9) mN/m to 5.9 (± 0.5) mN/m upon the addition of 3% CW. These authors attributed this surface activity of CW to the adsorption of CW polar compounds (i.e., as fatty acids and triterpenic alcohols and triterpenic esters) on the water-vegetable oil interface (García-González et al., 2021). Although these authors did develop W/O emulsions (10% and 20% of aqueous phase) just with the use of 3% CW, the study made limited discussion regarding the CW emulsion´s microstructure and stability (García-González et al., 2021).
In the present work, we hypothesized that after developing a CW oleogel, particular compounds of the wax (i.e., triterpenic alcohols, esters of triterpenic alcohols, long chain acids and alcohols) remaining in the oil phase, could act as surface-active agents with the capability of emulsifying a water phase forming a W/O emulsion. We considered that the CW oleogel present in the oil phase could provide a stabilizing mechanism for the emulsified phase, tentatively resulting in a structured W/O emulsion. Within this framework, the conditions investigated to develop the W/O emulsions were water to CW oleogel proportions of 40:60, 50:50, and 60:40 (wt:wt). The concentrations of the CW in the oleogels were selected so that, after the addition of water at the corresponding proportion, at each of the water to oleogel ratios studied the CW concentrations in the emulsions were 0.75%, 1.5%, 2.25%, and 3% (wt/wt). The W/O emulsions developed were evaluated for microstructure, water droplet size by NMR, emulsion stability by DSC, and rheological properties after 0 and 20 days of storage at 25°C.