Zhang, Hui
(2020)
Rice bran wax oleogel in an oil-in-water emulsion system.
PhD thesis, University of Nottingham.
Abstract
In this thesis, the application of rice bran wax (RBW) oleogel into oil-in-water (o/w) emulsion systems was investigated in terms of the selection of an emulsifier to stabilise RBW emulsions, the morphology and thermal properties of RBW crystals, the influence of additives on properties of RBW crystals and emulsion droplets, as well as the in vitro lipid digestibility.
Three emulsifiers, including Tween 20, Sodium caseinate and gum Arabic, were investigated to stabilise 1% (wt% of the lipid phase) RBW o/w emulsion. All emulsions were 10% (wt% of the total weight) o/w emulsions and produced with a high shear mixer at the temperature above the melting point of RBW. Emulsions were characterised in terms of emulsion droplet size distribution and droplet shape. Morphology and thermal properties of RBW crystals in o/w emulsions were studied using polarised light microscopy and differential scanning calorimetry (DSC). After that, two additives including Span 60 and lecithin were used to influence the crystallisation of RBW at o/w interface. The in-vitro lipid digestibility of o/w emulsions containing RBW and/or lecithin was studied by using a pH-stat method to evaluate the effect of RBW concentrations or the solid interfaces on the extent of lipolysis.
Results indicated that 5% gum Arabic was capable of stabilising (up to 10%) RBW o/w emulsions against partial coalescence and coalescence during storage at ambient temperature (~20 °C) for four weeks. In emulsion droplets, RBW showed needle-like crystals, which sizes are not influenced by the concentrations of RBW and cooling methods, but affected by the emulsion droplet size.
With the addition of Span 60 or lecithin in the lipid phase before emulsification, RBW crystals size decreased with increasing concentration of additives. For the thermal properties, the melting peak temperatures were not influenced, while the crystallisation peak temperatures were affected and this effect was dependant on the concentration of additives. In emulsions containing 3% (wt% to the oil phase) Span60 and 5% (wt% to the oil phase) RBW, ellipse-shaped and partial coalesced droplets were observed. Also, the size of needle-like crystals decreased. In addition, in emulsions containing 1% (wt% to the oil phase) lecithin and 5% RBW, crystals at droplet surface were observed but few needle-like crystals observed. The in-vitro lipolysis study of o/w emulsions showed that the presence of RBW (up to 10%) had little influence on the extent of lipolysis, compared with emulsions only containing sunflower oil in the lipid phase. Emulsions containing both 1% lecithin and 5% RBW had a significantly lower extent of lipolysis, compared with emulsions containing 5% only RBW or 1% lecithin.
These results showed a possible formulation and method to produce stable wax-based oleogel-in-water emulsions, and broadened the understanding of factors that influence the size, shape and thermal property of wax crystals in o/w emulsions. In addition, the in-vitro lipolysis study demonstrates that the use of RBW oleogel in o/w emulsion could be a promising way to delay lipolysis.
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