Effect of membrane filtration and direct steam injection on mildly refined rapeseed protein solubility, air-water interfacial and foaming properties

Rapeseed is an upcoming source of alternative proteins and extensive processing is necessary to utilise these proteins as functional ingredients. A risk here is the potential alteration of proteins upon processing, such as aggregation, thus affecting final ingredient's functional properties. Therefore, we aim to evaluate processing methods that are considered ‘mild’ for the proteins to retain native proteins with good techno-functionality. We evaluate the impact of two upcoming processes: 1) membrane filtration (5 kDa) to remove small solutes and 2) short but high-temperature heating using direct steam injection (DSI, 4 s at 115 °C) to ensure microbial safety. These extracts were studied for their protein composition, size and hydrophobicity. We, for instance, show the presence of cruciferin and napin in the extracts, which are the two main rapeseed protein families. Membrane filtration was suitable to remove phenols and non-protein solutes (e.g. carbohydrates and minerals), thereby increasing the protein content of the protein powders from 45.1 to 63.4% (w/w). DSI led to about three times lower protein solubility (from ∼44% to ∼16%) due to aggregation. These aggregates were mainly formed by cruciferin proteins, while napins remained soluble. As a result, the soluble napin proteins dominated the air-water interface and foam stabilisation. At the same time, cruciferin proteins were more dominant in the non-heated extracts, as they were in a soluble non-aggregated state. We showed an improved foamability of about 10% after heating but a 30–40% decrease in foam stability. A final finding was the impact of non-protein solutes, which vastly decreased the interfacial stiffness, leading to substantially less stable foams compared to membrane-filtered samples. In this work, we demonstrate how crucial processing steps, such as heating and filtration, impact (protein) composition, molecular and functional properties, which are crucial insights in designing protein extraction processes to obtain functional protein ingredients.