Influence of the presence of monoglyceride on the interfacial properties of soy protein isolate

Abstract

This study focused on the contribution of soy protein isolate (SPI), in the absence or presence of monostearin (ME), to surface and interfacial properties as a function of protein concentration and pH, which is relevant to the physical stability of a variety of food systems. An increase in protein content always yielded a rapid decrease in surface tension followed by an evolution towards an asymptotic value. Addition of ME gave rise to mixed SPI/ME films, although the interface became dominated by SPI above the concentration for interfacial saturation. The relative interfacial shear viscosity of SPI films showed a marked dependence on: aging time, which may be attributed to a reorganisation of protein species at the interface with some penetration of hydrophobic parts into the oil phase; shear forces, which may partially reverse this reorganisation, leading to shear-thickening behaviour; and pH, which is the key factor controlling which SPI species is predominant at the interface. The effect of adding ME also depends on pH, favouring a reinforcement of SPI/ME films only at low pH, at which 3S and 7S fractions are dominant. The results obtained indicate that SPI shows excellent potential to favour stabilisation of air/water and oil/water interfaces in food systems.