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. .