Waxes are a diverse class of organic compounds that are lipophilic, malleable solids at room temperature. They include higher alkanes and lipids, typically with melting points above about 40oC (104oF), melting to give low viscosity liquids. Waxes are insoluble in water but soluble in organic, nonpolar solvents. Different types of natural waxes are produced by plants and animals and are found in petroleum.
Fig.1 Cetyl palmitate, a typical wax ester.
Waxes are good candidates to be applied in carrier systems as most of them are food-grade and cheap and have high availability. Other advantages include nonsensitivity to environmental stresses (pH and salt), the tunable microstructure, and no changes in structure during the homogenization process. Furthermore, in terms of crystal behavior, wax crystals show characteristics of orthorhombic perpendicular subcell and due to long chains of fatty acids, waxes have a low polymorphic transitions rate, leading to more physical stability.
Naturally occurring waxes like CRW, CLW, and BW has been approved as generally recognized as safe for direct addition to foods and have been used as solid lipid in the formulation of SLN and NLC by some researchers. However, the potential of less explored waxes (PW and orange wax) in the formation of an appropriate solid lipid matrix was also reported.
Oleogels are a subset of colloidal systems known as organogels and are defined as oil-based structures with the physical properties of hydrogels. Waxes have proven to be the most efficient crystalline oleogelators not only due to their ability to induce oleogelation at very low concentrations, but also the improved textural properties and strong OBC of the obtained oleogels. In addition, most waxes are food-grade, low-priced, and widely obtainable. They can convert oil into a gel by creating a crystalline network through the arrangement of n-alkanes or wax esters.
Emulsions as one of the main delivery systems used for encapsulation and controlled delivery of bioactive ingredients and are broadly classified into W/O and O/W types. Pickering emulsions are considerably more stable to coalescence and Ostwald ripening compared to classical surfactant-stabilized counterparts.
Fig.2 Schematic representation of main types of wax-based delivery systems. (Soleimanian, 2020)
It is an ester derived from hexadecanoic acid and hexadecanol. This white waxy solid is the main ingredient of spermaceti, a highly prized wax once found in the skull of sperm whales. Cetyl palmitate is a component of some solid lipid nanoparticles.
It is a wax of the leaves of the carnauba. It is known as the "queen of waxes" and in its pure state, usually comes in the form of hard yellow-brown flakes. It is obtained from the leaves of the carnauba palm by collecting and drying them, beating them to loosen the wax, and then refining and bleaching the wax.
It is a natural wax produced by honey bees of the genus Apis. Beeswax has been used since prehistory as the first plastic, as a lubricant and waterproofing agent, in lost wax casting of metals and glass, as a polish for wood and leather, for making candles, as an ingredient in cosmetics, and as an artistic medium in encaustic painting.
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