Stearyl Stearate: Synthesis, Food Coating Use & Phase Behaviour

Stearyl Stearate is a plant-based waxy ester used in cosmetics and personal care products for its emollient, thickening, and skin-conditioning properties. It softens and smooths the skin and hair while helping retain moisture. It also improves the surface texture as well as the texture of the products to which it is added. Giving body and structure to creams and sticks, this ingredient improves the sensory feel and user experience. Stearyl Stearate appears as white waxy flakes in its raw form and gives a rich, velvety feel to beauty products. Being non-greasy yet hydrating, makes it suitable for dry skin and hair.

Synthesis of Stearyl Stearate from stearic acid of buffalo fat

Fruits and vegetables are known for their high nutritional benefits as they contain many valuable chemical compounds such as citric acid, ascorbic and other organic acids, sugars, minerals, vitamins, carotenoids, and phenolic compounds. Some of these compounds have physiological properties, including anti-inflammatory, anti-allergic, antimicrobial, vasodilating, anticoagulant, heart disease prevention, and antioxidant. The purpose of this study was to prepare a new hydrophobic coating film with a waxy texture, in order to be able for fruit coating, and to control the water permeability through the fruit epidermis, thus enhancing the shelf life of the freshly harvested fruits. And it was achieved via preparation of stearyl stearate, due to absence of any unsaturation center with in the very long hydrocarbon chain. In addition, buffalo fat can be considered as a byproduct, and it well known by its high concentration of stearic acid. Stearyl stearate is a waxy ester formed by combining between stearic acid and stearyl alcohol, stearic acid was previously extracted individually in pure form, while stearyl alcohol was prepared by reducing the corresponding acid.[1]

During the ripening process, the soluble solids concentrations increased generally due to hydrolysis of polysaccharide to maintain the respiration rate. Whereas in stored tomatoes, slight increases in the TDS level are associated with a decrease in tomato weight resulting from losing some water. Tomatoes coated with stearyl stearate film (2.00% w/v diethyl ether) showed minimum changes in the total soluble solids content compared to that coated with chitosan at the same concentration, and control sample. The low cost, synthesized, hydrophobic stearyl stearate is characterized with its waxy texture. Moreover, it is formed from a combination of two naturally occurring components. So, it is suitable be considered as a safe food coating compound. And as freshly harvested tomatoes are usually fast spoiled, it was used as a typical example to be coated with a film (2.0% w/v in diethyl ether) of the pervious prepared compound. Thus, the shelf life of tomatoes was extended as this hydrophobic film could control the water permeability through tomato epidermis, thus controlling most of the physiological changes. In addition, this film showed complete safety up to 25 g/kg mice weight. The use of stearyl stearate in coating films is a promising technique that can be used in food applications to increase the shelf life of fast-corruption vegetables and fruits.

Phase behaviour of oleanolic acid/stearyl stearate binary mixtures

The perpendicular orientation of SA molecules to the interface in the SA rich microdomains favours a similar orientation of OLA molecules in the OLA rich phase, with the carboxylic groups of neighbour molecules involved in hydrogen bonds. In this work, the above methodology was extended to the study of phase behaviour of the OLA/stearyl stearate (SS) binary system. Fatty esters of cuticular waxes  are long chain molecules, also present in pharmaceutical or food formulations. This study reports DSC measurements to analyze the 3D phase behaviour of OLA/SS mixtures. Because the working temperatures are limited by the decomposition temperature of SS, which is below the melting temperature of OLA, the effect of OLA in mixtures is deduced through its impact on the thermal behaviour of Stearyl Stearate, namely broadening of the transitions, melting point depression and crystallization behaviour. Complementary techniques were used to characterize the phase behaviour of the mixtures: hot-stage polarized optical microscopy (HSPOM), X-ray powder diffraction (XRD) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. A tentative “non-equilibrium” phase diagram is established, based on the combined information. The behaviour of OLA/SS mixed system investigated at the air–water interface adds valuable information on the orientation and packing of molecules and their tendency to self segregate in 2D, or in 3D after the monolayer collapse.[2]

The three-dimensional phase behaviour of oleanolic acid mixed with stearyl stearate, based on the information of various physical techniques, points to a eutectic type phase diagram with the eutectic composition close to pure SS, and immiscibility in the solid state. Above the melting temperature of SS, and below the liquidus curve, OLA is only slightly soluble in the SS liquid phase that coexists with solid OLA. Pure OLA, previously dissolved in chloroform, although mainly amorphous, has a tendency to crystallize at room temperature. The thermal treatment at high temperature favours OLA crystallization in needle shaped crystals. In SS/OLA mixtures it was found that there is a mutual influence on the ability of both components to crystallize. OLA preserves the tendency to form a crystalline phase on heating, but a very distinct morphology from the needle shaped crystals of pure OLA appears in mixtures with Stearyl Stearate. This new morphology, spherulitic structures, likely reflects the influence of SS on OLA crystallization. XRD confirmed that the presence of SS favours OLA crystallinity in samples thermally treated at high temperatures. DRIFT analysis clearly indicated that the H bonds involving the OLA carbonyl groups increase in the presence of Stearyl Stearate. Reciprocally, the influence of OLA on SS crystallization was also observed, that is, the crystallinity degree of SS rich mixtures decreases with the OLA content. Based on DRIFT analysis, the absence of specific SS–OLA interactions in bulk is compatible with immiscibility, although a mutual influence in the crystallization has been observed. XRD revealed that OLA molecules disrupt more the SS lamellar structure than the short space between chains.

References

[1]Soliman HM, Zahran HA. Synthesis of a new hydrophobic coating film from stearic acid of buffalo fat. Sci Rep. 2022 Nov 2;12(1):18465. doi: 10.1038/s41598-022-23003-4. PMID: 36323708; PMCID: PMC9630542.

[2]Teixeira, Ana C T et al. “Phase behaviour of oleanolic acid/stearyl stearate binary mixtures in bulk and at the air-water interface.” Chemistry and physics of lipids vol. 160,1 (2009): 45-57. doi:10.1016/j.chemphyslip.2009.04.001

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