Plant proteins often have inferior functional properties compared to animal-based proteins with respect to emulsion and foam stabilization because their larger and more rigid complex structures affect their ability to form a cohesive and stiff interfacial layer. To enhance film formation at liquid interfaces, molecular properties can be changed by using reducing agents such as dithiothreitol (DTT) to cleave disulfide bonds. Here, we explore this strategy for lentil legumin, one of the main proteins in lentils for which its structure is held together by covalent and noncovalent bonds, particularly disulfide bonds, giving it a rigid structure. Legumin was modified with different DTT concentrations (5, 50, and 100 mM) to obtain different degrees of modification. Disulfide bond cleavage in legumin led to increased surface hydrophobicity, resulting in enhanced adsorption kinetics, while particle size and surface charge remained similar to those of native legumin. Native and modified legumin were studied for their interfacial rheology at both oil-water and air-water interfaces. The interface formed by the modified legumin was stiffer than that of native legumin at both interfaces. The increased stiffness of the oil-water interface was attributed to the formation of denser interfacial layers rather than to stronger in-plane protein-protein interactions. In contrast, the stiffer air-water interface was associated with both stronger in-plane protein interactions and formation of denser interfaces. This was also confirmed by visualization of the interfacial film at the air-water interface, where the modified legumin formed denser and more cohesive networks than native legumin. Lastly, emulsion formation and stability were studied to assess whether structural changes following legumin modification affect stability on the macroscopic scale. Both native and modified legumin displayed stable emulsions against droplet flocculation and coalescence for 14 days, owing to the droplets having sufficient surface charge. The increased interfacial stiffness did not have a significant effect on stability.
Plant-based proteins and their use as natural emulsifiers to form and stabilize oil-in-water emulsions have greatly raised attention in the past decade. This study investigated proteins extracted from tannin-free and tannin-rich sorghum flours for their ability to produce emulsions that remain stable over storage time and under different pH conditions. Sorghum protein-based emulsions had small mean droplet sizes, ranging between 0.3 and 2.5 μm, with the largest droplets observed for tannin-rich samples. Optical and confocal fluorescence microscopy revealed differences when protein suspensions had been subjected to a high-pressure homogenization pretreatment prior to emulsification, in comparison with non-pretreated ones. Protein surface load values ranged from 2.5 to 4.0 mg/m2, with tannin-free samples presenting the highest values. Interfacial protein composition encompassed β-, γ-, and α-kafirin, also with some dimers and oligomers for one of the tannin-rich samples. Emulsions produced with tannin-free samples were more stable to flocculation compared to those produced with tannin-rich samples. For all emulsions, creaming was observed for pHs equal to or lower than 6.0. For the tannin-rich samples at pH 3.0, optical microscopy revealed a very peculiar morphology of droplet-containing flocs, with oil droplets “budding” at the surface of the flocs containing insoluble protein extract material. Overall, even though emulsion stability to flocculation and creaming was limited, sorghum proteins were able to produce emulsions that did not exhibit oiling-off for 28 days.
