Introduction
The list of potential oil structurants, substituting triglyceride
(TAG)-based high-melting hardstock fats to create semi-solid lipid
phases, is long . Among those, waxes are promising candidates to find
industrial application. They can be applied without chemical
modification and are available in sufficient quantities against
reasonable procurement costs . Their gelation mechanism is affiliated to
the formation of three-dimensional networks of crystals with a high
oil-binding capacity . Even though many natural waxes already achieved
GRAS status , their current application in the food industry is
predominantly as a coating agent. Nonetheless, this prior use may
benefit future consumer acceptance as ingredient in other food products.
For this reason, the potential of natural waxes as gelling agent in
liquid oils has been extensively studied by several authors, e.g. . Due
to many varying internal and external factors and lacking
standardization of characterization methods, a comparison of different
studies and thus general characterization of wax based oleogels remains
difficult. Previous studies focused on the influence of external factors
such as cooling rate , shear or ultrasound . Among the internal factors,
the effect of the concentration on the gel properties is understood the
best. Above the critical gelling concentration, the gel strength and
thermal properties show systematic behavior with increasing wax
concentration . Further, it was shown that variation of the oil type can
induce different crystal morphologies or changes of thermal behavior of
wax-based oleogels.
It is, however, generally acknowledged that the composition of the waxes
is the most important parameter determining the wax crystal morphologies
and, hence, the properties of a wax-based oleogel . The chemical
composition of waxes varies strongly, depending on the origin, the
growth conditions and the extraction and purification processes.
Waxes considered for application in food products are, among others, are
e.g., beeswax (BWX), candelilla wax (CLX), carnauba wax (CRX), rice bran
wax (RBX) and sunflower wax (SFX). These are predominantly composed of
long-chained alkyl esters, the so-called wax esters (WE), n-alkanes
(HC), free fatty acids (FA) and free fatty alcohols (FaOH). In most
natural waxes, WEs are the main component and consequently govern the
crystallization and gelling behavior . A waxes composition varies in the
content of the different molecular species – WE, FA, FaOH, HC – but
also within the makeup of each species- Within the WE fraction,
different waxes have a specific chain length distribution, covering an
overall range from 34 to 64 C-atoms.
The WE content of different natural waxes and the total carbon number
(CN) distribution of their WE fraction are gathered in Tab. 1 .
As the data, compiled from different sources ( reveals, RBX and SFX are
the most homogeneous waxes with a WE content above 90 % (w/w).
Typically, oleogel formation is based on low quantities of a high
melting material. This approach intrinsically has the risk to deliver
structures with dissolution characteristics or structural properties
with adverse organoleptic perceptions. To overcome this unfavorable
sensory and manipulate functional product characteristics, mixtures of
different waxes have been studied . The studies basically showed that
properties can be manipulated but also revealed that the control of the
changes induced is far from trivial. The results indicated that the
thermal and viscoelastic properties of blends of two different waxes not
necessarily develop as a linear combination of the individual mono-wax
gels. This should not surprise as the molecular composition of the
high-melting fraction within a lipid phase is decisive for the
crystallization characteristics and, hence, the microstructure. DSC data
has shown that the formation of mixed crystals and the presence of
multiple solid phases is very sensitive to the chain length distribution
(quality) of the WE fraction.
To successfully apply wax-based oleogels in foods, the understanding of
the effects of external and internal factors is an indispensable
prerequisite. Currently, an a priori definition of compositional and
process parameters enabling the manipulation of the resulting product
properties is not possible. Similar to the design of functional fat
phases, also called hardstock fats, unravelling the solidification and
structuring behavior of waxes on a molecular level therefore appears to
be a meaningful target. To this end, the aforementioned studies
regarding internal and external factors are certainly very helpful but
do not allow to relate the molecular composition of the waxes and the
structure and functionality of their gels. Earlier works describe the
thermal and crystallographic behavior of pure WEs. Hitherto, only one
publication deals with the properties of pure WEs as oleogelators
studying the thermal, viscoelastic and microstructural properties of
different symmetric (CN 28; 32; 36; 40) and asymmetric (FaOH_FA:
18_14; 18_16; 18_20; 18_22) WEs in high oleic safflower oil . The
results suggest a systematic behavior, depending on the WE composition
and structure.
Therefore, this manuscript focuses on the WE-fraction of natural waxes
and their quality. In particular, the CN distribution and the symmetry
of the molecule, respectively the chain length of FA and FaOH moieties,
are of concern. For this purpose, a systematic analysis of the
information available on WE crystallization is performed and own
experimental data on pure WEs and oleogels based on pure WEs are
discussed. Further, the behavior of WEs in oleogels composed of a single
WE as structurant or a binary mixture of two WEs of different quality is
investigated. The gels are formed with medium-chained triglyceride (MCT)
oil, a simply composed TAG-oil, not taking chemical reactions at regular
treatment. Therefore, it exhibits a low level of polar components, whose
impact on oleogel was shown e.g. by Scharfe et al. .