Multifunctionality of rapeseed meal protein isolates prepared by sequential isoelectric precipitation

Rapeseed meal is a byproduct of the oil-producing industry with an underestimated application. Mainly, it is used as an inexpensive protein-rich ingredient in feed formulation but in limited quantity. Alternatively, it could be used as a source for the preparation of protein isolates for the food industry. Most plant protein isolates are prepared by isoelectric precipitation at a single pH point in the acidic pH area, where they exhibit the lowest solubility, limiting their functionality and subsequent application. In the current study, two protein isolates, PI2.5-8.5 or PI10.5-2.5, were obtained from industrial rapeseed meal after treatment with aqueous ethanol solution. The alkaline-extracted proteins were sequentially precipitated by two different modes, from pH 10.5 to 2.5, and vise-versa, from 2.5 to 8.5, with an increment of 1 pH unit. The protein isolates, comprised of a mixture of proteins, differed by biochemical composition, protein profile, and solubility, which was evaluated at two levels of NaCl concentrations, 0.03 and 0.25 M, and a broad pH range from 2 to 8.5. The supplementation of PI10.5-2.5 with 0.25M NaCl enhanced the protein solubility to 56.11% at pH 4.5 and 94.26% at pH 6.5. The solubility of PI2.5–8.5 was lower but still almost 10-fold higher than the negligible solubility (2.80%) of the rapeseed meal protein isolate prepared by isoelectric precipitation at pH 4.5. The preparation approach influenced both the functional and antioxidant properties of the isolates. The PI10.5-2.5 exhibited higher water and oil-absorption capacities than PI2.5-8.5 reaching 2.68 g H2O/g sample and 2.36 g oil/g sample, respectively. The emulsion stability of the PI2.5-8.5, 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, which reached 93.87%. For the PI10.5-2.5, decreases in the emulsion stability were observed at pH 8 (85.71%) and pH 10 (53.15%). In the entire concentration range, 0.2% to 1.0%, the PI10.5-2.5 exhibited a higher scavenging ability on 2,2‑diphenyl‑1‑picryl hydrazyl (DPPH) and hydroxyl radicals than PI2.5-8.5 as evaluated by DPPH and 2-deoxyribose assays, respectively. At the highest concentration studied, 1.0%, the neutralization of DPPH radicals by PI10.5-2 reached half of that exhibited by synthetic antioxidant butylhydroxytoluene (82.65%). At the same concentration, the inhibition of hydroxyl radicals by PI10.5-2 (71.25%) was close to that achieved by mannitol (75.62%) which was used as a positive control. Being fairly soluble in water and with good emulsifying properties, the PI2.5-8.5 and PI10.5-2.5 would have a broader scope of application as multifunctional ingredients with antioxidant properties as an add.