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.