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A mixture of ceramide (CER) and lecithin (LEC) at specific ratios was capable of forming oleogels in sunflower oil triggered by adding a trace amount of water. It was noted that the addition of water at different temperatures (TW) resulted in different gelation behaviors and microstructures. To better illuminate the assembly mechanism at different TW, samples with water added at different TW (20 °C, 45 °C, 70 °C and 95 °C) were prepared. The viscoelastic properties, microstructures, and the crystal packing of these samples were investigated. It was observed that all samples prepared at TW of 20 °C and 95 °C formed gels, while most samples prepared at TW of 45 °C and 70 °C were too weak to form gels. Gels prepared at 95 °C were stronger but more fragile in texture compared to gels produced at 20 °C. The crystal morphology of gels drastically changed with TW. Spindle-shaped crystals were observed in gels prepared at low TW (20 °C), while gels prepared at high TW (95 °C) exhibited a network with packed oil droplets stabilized by lamellar shells together with fibrillar crystals in the bulk phase. X-ray diffractograms showed a different reflection peak (d-spacing of 14.5 Å) in gel prepared at 20 °C, compared to the d-spacing in oleogels with a single gelator (13.14 Å and 15.33 Å, respectively, for CER and LEC). Gel prepared at 95 °C showed two long-spacing characteristic peaks, which correspond to the characteristic peaks of CER gel (∼13 Å) and LEC gel (∼12 Å). Fourier transform infrared spectroscopy results indicated that the different gelation behaviors at different TW were mainly caused by vibrational changes in the amide bond of CER. Our hypothesized assembly mechanism can be concluded as: increasing TW resulted in the conversion of CER and LEC crystallization from co-assembly (TW = 20 °C) to self-sorting by individual gelators (TW = 95 °C). In this study, novel water-induced oleogels were produced by manipulating TW, and such information further assists the rational design of lipid-based healthy fat products.

Ceramide (CER) is a highly effective organogelator with numerous health implications. Lecithin (LEC) is often used as a crystal modifier to improve gel properties. It was found in this study that in the presence of a small amount of water, CER and LEC self-assembled at specific ratios and form oleogels in sunflower oil (SFO), and the structure of which differed from any of the mono-component gel structures. To the best of our knowledge, this is the first water-induced multi-component oleogelation reported to date. To clarify when gelation occurs, a phase diagram of the four-component system SFO/CER/LEC/water was established and characterized. In the gel region, confocal laser scanning microscopy revealed that water was evenly distributed in the self-assembled microstructure. To clarify the vital role of water in formation of stable gel, a series of samples with varying ratios in CER and LEC with and without water were explored. The thermal properties, viscoelastic properties, microstructures, as well as crystal packing were investigated. With water added, the microscopic structures of CER/LEC gels changed from three-dimensional dendritic crystals to two-dimensional rod-like or platelet-like crystals, which are more effective at structuring liquid oil. A different reflection was observed (d-spacing of 14.9 Å), suggesting the rearrangement in crystal packing of ceramide and lecithin molecules, compared to the d-spacing in oleogels with single gelator (13.1 Å and 15.3 Å, respectively for ceramide and lecithin). Fourier transform infrared spectroscopy results indicated the presence of interaction between water and gelators, which confirmed the importance of water in self-assembly of this mixed-gelator system. Water-induced conversion of oleogelation provides alternatives in producing hard-stock fat replacers as well as possibilities to incorporate water-soluble nutrients into oil-based products.