Soy protein isolate (SPI)-dextran mixtures were dry-heated at 60?C for time periods of up to three weeks and, as SDS-PAGE analysis of the incubated mixture showed, a protein-polysaccharide conjugate was formed probably along with protein-protein polymerization products. A significant fraction of the initial protein constituents of the SPI remained unreacted at the end of the three weeks incubation period. The dry-heated mixture exhibited improved emulsifying properties in systems of relatively low oil content (φ=0.10), the improvement depending on incubation time, SPI/dextran weight ratio, or the polysaccharide molecular weight. The incubated mixture was particularly effective in retarding oil droplet flocculation and creaming phenomena, and this is attributed to the adsorption of the conjugate at the droplet surface, through its protein moiety, and the enhancement of steric stabilization forces between neighbouring droplets due to the presence at the droplet surfaces of dextran molecules protruding towards the emulsion continuous phase. Aggregation phenomena and creaming instability, following heating of up to 30min in boiling water of emulsions prepared with the incubated at 60?C SPI-dextran mixture, were not detected. On the other hand when the polysaccharide was not conjugated to protein, extensive heat-induced aggregation took place, the reaction following second order kinetics. The conjugate, in spite of its large size and high hydrophilicity, was adsorbed to a appreciable extent at the emulsion droplet surface in the presence of antagonistically-acting low-molecular weight emulsifiers (Tween, monoglycerides) or proteins (BSA, sodium caseinate). This allowed the conjugate to retain its droplet flocculation-preventing ability and stabilizing action against creaming, in food emulsion systems where a mixture of emulsifiers has to be used. In a model salad dressing emulsion (φ=0.50), prepared with the incubated SPI-dextran mixture, the presence of conjugate modulated both the rheological characteristics of the system as well as its creaming behaviour, the effect depending on incubation time. In the case of the incubated for 1 week mixture, the flocculation of droplets was not extensive, resulting in a system exhibiting liquid-like behaviour. Incubation of the SPI-dextran mixture for three weeks produced an emulsion exhibiting solid-like behaviour, possibly as a result of bridging flocculation effects, which may be attributed to protein particles that “bridge” the gap between neighbouring droplets. Incorporation of xanthan in the dressing produced a system exhibiting the weak-gel properties of the xanthan solutions, irrespective of the presence or absence of the protein-polysaccharide conjugate. The stability of this system against creaming however, was higher in the presence of the conjugate, suggesting that droplet rearrangement and interactions during ageing, depended to some extent on the presence of the dextran molecules at the droplet surface and the repulsive steric forces operating between neighbouring droplets. Finally, incubation of a SPI-NaCMC mixture at 60?C demanded a longer heating time period (up to 5 weeks) to produce a mixture exhibiting improved emulsifying properties compared to SPI-dextran mixtures. This was attributed to the more rigid structure of CMC molecules compared to dextran, which limits molecular movement during the incubation period and results in a slower protein-polysaccharide reaction.
Εθνικό Κέντρο Τεκμηρίωσης και Ηλεκτρονικού Περιεχομένου (ΕΚΤ)
