Synthesis of an homologous series of fatty acid xylitol monoesters and the development of their stabilized oleogels as carriers for β-carotene.

Edible oleogels have recently attracted attention as new fat substitutes and as delivery systems. While thus having great potential, the oleogelators described thus far lack the necessary properties for extended deployment in a broad range of settings. Xylitol fatty acid monoesters have recently been identified as a novel class of oleogelator but little is currently known about how progression through an homologous series of congeners incorporating extended alkyl side-chains impacts on their properties. Accordingly, a series of such esters incorporating 12 to 22 carbons (designated as esters 1-6 based on their side-chain lengths) were prepared enzymatically and subject to extensive spectroscopic and physicochemical characterization. With the exception of ester 5, which features a side-chain derived from oleic acid and exhibits higher solubility in olive oil, all these compounds exhibited a capacity to form stable oleogels at a concentration of 3 wt%, this being lower than most previously studied oleogelators. Furthermore, oleogels formed using sugar esters embodying longer side-chain exhibited enhanced structural intensity and thermal stability, particularly ester 4, which contains a stearic acid side-chain. Oleogels prepared using such esters remained stable for at least 60 days at both 4 and 25 °C and at a concentration of 5 wt%. Such oleogels sequester, when stored at 4 °C, β-carotene with a retention rate exceeding 70 % after 30 days and with the oleogel formed using ester 4 exhibiting the best protective behavior. Similar trends were observed during in vitro digestion studies in terms of the delivery properties of such carrier systems. In particular, oleogels prepared using those esters containing longer saturated side-chain residues performed best and achieving a β-carotene release rate of ca. 75 % in the targeted small intestine. An associated FFA (free fatty acid) release rate of between 75 and 90 % indicates that these stabilized oleogels/lipids can also be effectively digested. Such features serve to emphasize the potential of the title esters to act as new oleogelators with likely utility as functional foods and as controlled-release delivery systems in the food industry.