Shear and dilatational rheological properties of vegetable proteins at the air/water interface

Many food products are multiphase systems and their structural and textural properties are determined by the amount and the characteristics of the different ingredients making up the system. They are created by homogenization of two or more immiscible phases, to form emulsions or foams. Proteins are widely used to stabilize the interfaces of these types of system because they have high nutritional value and they are able to adsorb quickly at the interface decreasing the interfacial tension and improving the mechanical resistance. Recently, proteins extracted from vegetable seeds have been receiving much attention from the food industry, thanks to their good environmental sustainability and healthy composition, but the scientific literature is lacking in information on the surface activity and emulsifying ability of these proteins. In this work, the surface properties of three different proteins extracted from vegetable sources (soy, hemp and brown rice seeds) were investigated using interfacial shear and dilatational kinematics. Linear viscoelastic properties were evaluated using small amplitude oscillatory tests (about 10%Delta V/V and 2% for dilatational and shear deformation, respectively); in addition, stress relaxation after quick expansion/compression and shear creep were also used to obtain information about the viscoelasticity and the characteristic time of relaxation of the interfacial film. The three proteins displayed excellent interfacial properties similar among them. Brown rice protein yields stronger interfacial layers whereas soy is more liable to structure the interface and hemp exhibits intermediate properties. Obtained results demonstrate that brown rice and hemp proteins could be used to obtain food multiphase systems, like foams, without the addition of surfactants or animal proteins, thanks to their interfacial characteristics, and they can yield a high nutritional content thanks to their amino acid profile.