Endogenous lipids: disregarded co-passengers shaping the emulsifying and interfacial properties of pea and lupin protein ingredients

Sustainable incentives foster the use of plant-based ingredients to stabilize emulsions, which display promising technological properties. Yet, their functionality, for instance in terms of solubility, is far from optimal. The underlying reasons remain unclear, partly because comprehensive characterization of the physicochemical properties of such ingredients is lacking.
In-depth analysis of the composition of pea and lupin protein isolates and concentrates revealed a large part of non-proteinaceous compounds, notably lipids, constituting 3.8 to 11.7 wt.% (d.m.) of the powders. Over half of these lipids were phospholipids, which may have a pivotal impact regarding interfacial properties. A high-pressure homogenization (HPH) treatment was applied to aqueous suspensions of the ingredients. It broke down undispersed powder grains, enhanced protein solubility and released endogenous submicron lipid structures. The interplay between these endogenous lipids and proteins was investigated regarding emulsifying and interfacial properties. Oil-in-water emulsions (10 wt.% oil) were prepared using HPH to reach droplet diameters around 2.5 µm. Protein surface load, composition measurements, and detailed microscopic investigations suggested the formation of different interfacial films and an inner competition between proteins and phospholipids for interfacial adsorption. Dilatational interfacial rheology assays were performed to further explore this competition and the associated interfacial structures (oil-water interface). Kinetic adsorption studies revealed the dynamic architecture of the interfaces evolving through time. While proteins initially prevailed at the interface, phospholipids played a crucial role in reaching high surface pressures (around 20 mN/m). Oscillatory deformations of the interface were conducted and analyzed by Lissajous plots (Figure) providing insights into complex structural arrangements within the film. Atomic force microscopy images confirmed the formation of mixed interfacial films where lipid domains coexisted with protein aggregates.
These new insights are key to understanding the interactions of phospholipids with proteins to stabilize food emulsions and facilitate food product’s rational formulation with such sustainable, yet complex ingredients.