RESUMO
The comparative study between the mixing behavior of two binary mixtures of cocoa butter (CB)/tristearin (TS) and cocoa butter (CB)/coconut oil (CO) was investigated by using differential scanning calorimetry (DSC). The DSC profile for CB/TS blends resulted in a monotectic temperature-concentration (T-X) phase diagram, whereas a phase diagram of eutectic type was observed for CB/CO blends at 65 wt % of CO and 35 wt % CB; this suggests that the eutectic crystal can be formed when the saturated fat (blue = CO) is smaller in size compared to monounsaturated fat (orange = CB), whereas, for similar and larger size (red = TS) to CB, phase separation under crystallization is likely to occur (as shown in the graphical abstract). In order to understand the interaction between the binary systems, the profile of the phase diagram was fitted with Bragg-Williams approximation for estimation of the nonideality mixing parameter. Moreover, the morphology of the two different systems by polarized light microscopy (PLM) also depicted the variations in phase behavior by showing a significant change in CB morphology from spherulitic, grainy to granular and needlelike after the addition of TS and CO, respectively. Our findings emphasize the fundamental understanding of the interaction of bulk fat/fat and fat/oil system.
RESUMO
HYPOTHESIS: Oleosomes are stabilized by a complex outer phospholipid-protein-layer. To improve understanding of its structure and stabilization mechanism, this shell has to be studied in extracellular native conditions. This should be possible by SANS using contrast variation. Oleosomes are expected to be highly temperature stable, with molecular changes occurring first in the protein shell. Direct measurements of changes in the shell structure are also important for processing methods, e.g. encapsulation. EXPERIMENTS: Extracted soybean oleosomes were studied directly and after encapsulation with pectin by SANS using contrast variation. In order to determine structure and size, a shell model of oleosomes was developed. The method was tested against a simple phospholipid-stabilized emulsion. The oleosomes' temperature stability was investigated by performing SANS at elevated temperatures. FINDINGS: Size (Rgâ¯=â¯1380â¯Å) and shell thickness of native and encapsulated oleosomes have been determined. This is the first report measuring the shell thickness of oleosomes directly. For native oleosomes, a shell of 9â¯nm thickness surrounds the oil core, corresponding to a layer of phospholipids and proteins. Up to 90⯰C, no structural change was observed, confirming the oleosomes' high temperature stability. Successful coavervation of oleosomes was shown by an increase in shell thickness of 10â¯nm after electrostatic deposition of pectin.
Assuntos
Glycine max/química , Gotículas Lipídicas/química , Difração de Nêutrons/métodos , Emulsões/química , Tamanho da Partícula , Fosfolipídeos/análise , Espalhamento a Baixo Ângulo , TemperaturaRESUMO
Lipid storage in plants is achieved among all plant species by formation of oleosomes, enclosing oil (triacylglycerides) in small subcellular droplets. Seeds are rich in this pre-emulsified oil to provide a sufficient energy reservoir for growing. The triacylglyceride core of the oleosomes is surrounded by a phospholipid monolayer containing densely packed proteins called oleosins. They are anchored in the triacylglycerides core with a hydrophobic domain, while the hydrophilic termini remain on the surface. These specialized proteins are expressed during seed development and maturation. Particularly, they play a major role in the stabilization and function of oleosomes. To better understand the importance of oleosins for oleosome stabilization, enzymatic digestion of oleosins was performed. This made it possible to compare and correlate changes in the molecular structure of oleosins and changing macroscopic properties of oleosomes. Tryptic digestion cleaves the hydrophilic part of the oleosins, which is accompanied by a loss of secondary structures as evidenced by Fourier-transform infrared and sum frequency generation spectra. After digestion, the ability of oleosins to stabilize oil-water or air-water interfaces was lost. The surface charge and the associated aggregation behavior of oleosomes are governed by interactions typical of proteins before digestion and by interactions typical of phospholipids after digestion.
Assuntos
Proteínas de Plantas/química , Ar , Dicroísmo Circular , Emulsões/química , Helianthus/metabolismo , Concentração de Íons de Hidrogênio , Óleos de Plantas/química , Proteínas de Plantas/metabolismo , Glycine max/metabolismo , Espectroscopia de Infravermelho com Transformada de Fourier , Água/químicaRESUMO
The combination of different gelling and nongelling hydrocolloids is known to yield complex systems with a wide range of mechanical properties. Here, the influence of the nongelling hydrocolloids sodium-alginate and xanthan on the gelation of agarose is investigated. The two polyelectrolytes differ significantly in their flexibility, leading to opposing effects on the thermomechanical properties of the resulting composite gels. The network structure of the agarose as well as viscoelasticity, gelling temperature, and thermal stability of the gels are altered. These properties are investigated by strain and temperature dependent oscillatory rheological measurements as well as confocal laser scanning microscopy. A phenomenological model to describe the network formation of agarose in the presence of alginate or xanthan respectively is presented.