Multifunctionality of Rapeseed Meal Protein Isolates Prepared by Sequential Isoelectric Precipitation

Rapeseed meal is a by-product of the oil-producing industry with a currently underestimated application. Two protein isolates, PI(2.5–8.5) or PI(10.5–2.5), were obtained from industrial rapeseed meal after treatment with an aqueous ethanol solution. The alkaline-extracted proteins were sequentially precipitated by two different modes, from pH 10.5 to 2.5, and vice versa, from 2.5 to 8.5, with a step of 1 pH unit. The preparation approach influenced both the functional and antioxidant properties of the isolates. The PI(10.5–2.5) exhibited higher water and oil absorption capacities than PI(2.5–8.5), reaching 2.68 g H(2)O/g sample and 2.36 g oil/g sample, respectively. The emulsion stability of the PI(2.5–8.5), evaluated after heating at 80 °C, was either 100% or close to 100% for all pH values studied (from 2 to 10), except for pH 6 where it reached 93.87%. For the PI(10.5–2.5), decreases in the emulsion stability were observed at pH 8 (85.71%) and pH 10 (53.15%). In the entire concentration range, the PI(10.5–2.5) exhibited a higher scavenging ability on 2,2-diphenyl-1-picryl hydrazyl (DPPH) and hydroxyl radicals than PI(2.5–8.5) as evaluated by DPPH and 2-deoxyribose assays, respectively. At the highest concentration studied, 1.0%, the neutralization of DPPH radicals by PI(10.5–2) reached half of that exhibited by synthetic antioxidant butylhydroxytoluene (82.65%). At the same concentration, the inhibition of hydroxyl radicals by PI(10.5–2) (71.25%) was close to that achieved by mannitol (75.62%), which was used as a positive control. Established antioxidant capacities add value to the protein isolates that can thus be used as both emulsifiers and antioxidants.