Engineering water-induced ceramide/lecithin oleogels: understanding the influence of water added upon pre- and post-nucleation

Abstract

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 (T_W) resulted in different gelation behaviors and microstructures. To better illuminate the assembly mechanism at different T_W, samples with water added at different T_W (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 T_W of 20 °C and 95 °C formed gels, while most samples prepared at T_W 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 T_W. Spindle-shaped crystals were observed in gels prepared at low T_W (20 °C), while gels prepared at high T_W (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 T_W were mainly caused by vibrational changes in the amide bond of CER. Our hypothesized assembly mechanism can be concluded as: increasing T_W resulted in the conversion of CER and LEC crystallization from co-assembly (T_W = 20 °C) to self-sorting by individual gelators (T_W = 95 °C). In this study, novel water-induced oleogels were produced by manipulating T_W, and such information further assists the rational design of lipid-based healthy fat products.