The behaviour of sunflower oleosomes at the interfaces.

Oleosomes are particles equipped with a sophisticated membrane, comprising a continuous monolayer of phospholipids and hydrophobic proteins, which covers the triglyceride core and grants them extreme physical and chemical stability. The noteworthy qualities of oleosomes have attracted strong interest for their incorporation in emulsion formulations; however, little is known about their emulsifying properties and their behaviour on interfaces. For these reasons, oleosomes were isolated from sunflower seeds (96.2 wt% oil, 3.1 wt% protein) and used as an emulsifier for the stabilization of O/W and W/O interfaces. In both cases, oleosomes showed high interfacial and emulsifying activity. Individual oleosome particles had a broad size distribution from 0.4 to 10.0 µm and it was observed that the membrane of the larger oleosomes (>1-5 µm) was disrupted and its fractions participated in the newly formed interface. Oleosomes with a smaller diameter (<1 µm) seemed to have survived the applied mild emulsification step as a great number of them could be observed both in the bulk of the emulsions and on the interface of the emulsion droplets. This phenomenon was more pronounced for the W/O interface where oleosomes were absorbed intact in a manner similar to a Pickering mechanism. However, when the triglycerides were removed from the core of oleosomes in order to focus more on the effect of the membrane, the remaining material formed sub-micron spherical particles, which clearly acted as Pickering stabilisers. These findings showcase the intriguing behaviour of oleosomes upon emulsification, especially the crucial role of their membrane. The study demonstrates relevance for applications where immiscible liquid phases are present.

Covalent Bonding of Chlorogenic Acid Induces Structural Modifications on Sunflower Proteins.

Proteins and phenols coexist in the confined space of plant cells leading to reactions between them, which result in new covalently bonded complex molecules. This kind of reactions has been widely observed during storage and processing of plant materials. However, the nature of the new complex molecules and their physicochemical properties are largely unknown. Therefore, we investigated the structural characteristics of covalently bonded complexes between sunflower protein isolate (SFPI, protein content 85 wt %) and the dominant phenol in the confined space of a sunflower seed cell (chlorogenic acid, CGA). It was shown that the efficiency of bond formation goes through a maximum as a function of the SFPI:CGA ratio. Moreover, the bonding of CGA with proteins resulted in changes in the secondary and tertiary structure of the protein. It was also shown that the phenol bound strongly to the protein, which resulted in new crosslinks between the polypeptide chains. As a result, secondary structures like α-helices and ß-sheets diminished, which in turn resulted in more disordered domains and a subsequent modification of the tertiary structure of the proteins. These findings are relevant for establishing future protocols for extraction of high-quality proteins and phenols when utilizing plant material and offer insight into the impact of processing that these ingredients endure.

Aqueous foams stabilized by chitin nanocrystals.

The aim of the present study was to explore the potential use of chitin nanocrystals, as colloidal rod-like particles, to stabilize aqueous foams. Chitin nanocrystals (ChN) were prepared by acid hydrolysis of crude chitin and foams were generated mainly by sonicating the respective dispersions. The foamability of the chitin nanocrystals was evaluated and the resulting foams were assessed for their stability, in terms of foam volume reduction and serum release patterns, during storage. Additionally, the samples were studied with light scattering and optical microscopy in order to explore the bubble size distribution and morphology of the foam. Nanocrystal concentration and charge density was varied to alter the packing of the crystals at the interface. At low concentrations of ChNs, foams were stable against coalescence and disproportionation for a period of three hours, whereas at higher concentrations, the foams were stable for several days. The enhanced stability of foams prepared with ChNs, compared to surfactant-stabilized foams, can be mainly attributed to the irreversible adsorption of the ChNs at the air-water interface, thereby providing Pickering stabilization. Both foam volume and stability of the foam were increased with an increase in ChNs concentration, and at pH values around the chitin's pKa (pH 7.0). Under these conditions, the ChNs show minimal electrostatic repulsion and therefore a higher packing of the nanocrystals is promoted. Moreover, decreased electrostatic repulsion enhances network formation between the ChNs in the aqueous films, thereby providing additional stability by gel formation. Overall, ChNs were proven to be effective in stabilizing foams, and may be useful in the design of Pickering-stabilized food grade foams.