Small-angle neutron scattering reveals basis for composition dependence of gel behaviour in oleic acid - sodium oleate oleogels

Abstract

The relationship between gel strength and structure on the micro- and nano-scale has been investigated for oleogels prepared in a range of triglyceride (TAG) oils (sunflower, olive, medium chain triglyceride) using mixtures of oleic acid (OA) and sodium oleate (SO) as gelling agents. Microscopy indicated a reduction in crystal size with increasing SO content. Gel strength increased with SO concentration but, for a constant SO concentration, decreased with the addition of OA. Small-angle neutron scattering (SANS) and ultra-SANS was measured from the gels formed in the TAG oils, as well as in hexadecane oil, the latter employing solvent contrast variation, to study the structures formed in more detail. In both the TAG oils and in hexadecane, SO formed lamellar crystalline structures whose low q scattering was consistent with mass fractal-like behaviour (for 0:1 and 1:8 compositions). Further OA addition (1:4–2:1) resulted in the simultaneous presence of inverse micellar structures. We hypothesise that the negative effect of OA on gel strength is due to the partial dissolution of SO by OA and the loss of gel-mediating SO-based lamellar crystals. The variation in OA:SO ratio is demonstrated to provide control over mechanical properties via large-scale structure formation while the tunability of such gel properties using this mechanism potentially provides an alternative route to the use of solid fats for structuring food.