Mixtures of c-oryzanol and b-sitosterol are able to form transparent organogels in edible oils. Small-angle X-ray scattering was used to elucidate the microstructure of the building blocks of these organogels in sunflower oil. It was found that the plant sterol(ester)s form hollow tubes with a diameter of 7.2 ± 0.1 nm. Tubes prepared with coryzanol-rich structurant show the least bundle aggregation, and can be supercooled during formation most easily. The tubes melt at elevated temperatures, in agreement with the loss of structuring capacity as observed in earlier experiments.Keywords Organogel Á Phytosterol Á Self assembly Á Fibril Á X-ray diffraction (XRD) Á Small-angle X-ray scattering (SAXS) Á Wide-angle X-ray scattering (WAXS) Á Differential scanning calorimetry (DSC)
In this study, water-in-oil emulsions were prepared from water containing different salt concentrations dispersed in an oil phase containing a mixture of β-sitosterol and γ-oryzanol. In pure oil, the β-sitosterol and γ-oryzanol molecules self-assemble into tubular microstructures to produce a firm organogel. However, in the emulsion, the water molecules bind to the β-sitosterol molecules, forming monohydrate crystals that hinder the formation of the tubules and resulting in a weaker emulsion-gel. Addition of salt to the water phase decreases the water activity, thereby suppressing the formation of sitosterol monohydrate crystals even after prolonged storage times (∼1 year). When the emulsions were prepared with less polar oils, the tubular microstructure was promoted, which significantly increased the firmness of the emulsion-gel. The main conclusion of this study is that the formation of oryzanol and sitosterol tubular microstructure in the emulsion can be promoted by reducing the water activity and/or by using oils of low polarity.
The mixture of γ-oryzanol with β-sitosterol forms a network of tubules in edible oil that may serve as an alternative to the network of small crystallites of triglycerides occurring in regular oil structuring. The present experiments demonstrate that the tubules vanish at the melting point of the gel. Moreover, a number of alternative sterols (e.g., ergosterol, stigmasterol, cholesterol, cholestanol) can replace sitosterol in the tubules. The tubule diameter varies between 6.7 and 8.0 nm, the wall thickness between 0.8 and 1.2 nm. The results are consistent with a previously proposed helical ribbon assembly mechanism.
Dispersing micronized fat crystals (MFCs) in oil is a novel route to largely decouple fat crystallisation and network formation and thus to simplify the manufacture of fat-continuous food products. MFCs dispersed in oil form a weak-interaction network organized by crystal aggregates in a continuous net of crystalline nanoplatelets. The rough surface of MFC nanoplatelets hampers stacking into one-dimensional aggregates, which explains the high mass fractal dimensions of the networks formed in MFC dispersions. Applying shear does not have a significant effect on the fractal dimensions of MFC networks, and MFC aggregates in the range of 5-10 μm remain intact. However, shear leads to a significant loss of storage modulus and yield stress over a time frame of an hour. This can be attributed to irreversible disruption of the continuous net of nanoplatelets. Rheo-SAXS revealed that shear releases nanoplatelets from the continuous net, which subsequently align in the shear field and undergo rapid recrystallisation. The release of thin and metastable nanoplatelets from the weak-link network bears relevance for simplified and more effective manufacturing of emulsified food products by effectively decoupling crystallisation, network formation and emulsification.
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