Interfacial and foaming properties of soluble lupin protein isolates: Effect of pH

Abstract

Lupins are rich in proteins (∼40%) and fibers (∼40%), and have been widely used for animal feed, but limitedly for human consumption due to the presence of alkaloids. Due to their high protein content, lupins have a high potential to be used as protein sources in diverse food manufacturing processes, including emulsion and foam stabilization. However, the interfacial properties of lupin protein isolates (LPI), especially their performance at different pH values, are not well understood so far. Here, we systematically investigated the air-water interfacial and foaming properties of the soluble fraction of LPI at pH 7.0, pH 6.0, pH 4.0, and pH 3.5. We observed that LPI at pH 4.0 (LPI-4) and pH 3.5 (LPI-3.5) adsorbed faster at the air-water interface than at pH 7.0 (LPI-7) and pH 6.0 (LPI-6) due to a smaller particle size and higher surface hydrophobicity of LPI-4 and LPI-3.5. This resulted in a higher foam overrun of LPI-4 and LPI-3.5 (300% and 331%, respectively). The air-water interfaces formed by LPI at different pH values were dramatically different in terms of interfacial structure and mechanical properties. LPI-4 and LPI-3.5 formed stiff and solid-like interfaces, leading to higher foam stability. In contrast, for LPI-7 there were clearly more protein aggregates at the interface, and this structure was weaker and less stretchable in response to shear and dilatational deformation, causing lower foam stability. LPI-6 also showed weaker air-water interfaces and thus formed less stable foams. Overall, LPI at acid pH (3.5 and 4) has a better performance in foam stabilization than at a pH close to neutral (6 and 7). Our results suggest that LPI can be potentially used as a plant-based clean-label additive in the production of foam-based food products in acidic environments.