  Citation: Georgiev, R.; Kalaydzhiev, H.; Ivanova, P.; Silva, C.L.M.; Chalova, V.I. Multifunctionality of Rapeseed Meal Protein Isolates Prepared by Sequential Isoelectric Precipitation. Foods 2022, 11, 541. https://doi.org/10.3390/ foods11040541 Academic Editor: Maria Hayes Received: 30 December 2021 Accepted: 5 February 2022 Published: 14 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). foods Article Multifunctionality of Rapeseed Meal Protein Isolates Prepared by Sequential Isoelectric Precipitation Radoslav Georgiev 1 , Hristo Kalaydzhiev 2 , Petya Ivanova 1 , Cristina L. M. Silva 3 and Vesela I. Chalova 1, * 1 Department of Biochemistry and Molecular Biology, University of Food Technologies, 26 Maritsa Blvd, 4002 Plovdiv, Bulgaria; racho95@abv.bg (R.G.); petia_ivanova_georgieva@abv.bg (P.I.) 2 Department of Analytical Chemistry and Physicochemistry, University of Food Technologies, 26 Maritsa Blvd, 4002 Plovdiv, Bulgaria; hristo.kalaydzhiev@yahoo.com 3 CBQF-Centro de Biotecnologia e Química Fina–Laboratório Associado, Escola Superior de Biotecnologia, Universidade Católica Portuguesa, Rua Diogo Botelho 1327, 4169-005 Porto, Portugal; clsilva@porto.ucp.pt * Correspondence: veselachalova@gmail.com; Tel.: +359-32-603-855 Abstract: Rapeseed meal is a by-product of the oil-producing industry with a currently underesti- mated 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 se- quentially 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. Keywords: rapeseed; sequential isoelectric precipitation; multifunctionality; protein isolates; antioxidant capacity 1. Introduction Rapeseed oil is a valuable commodity widely used in the food industry [1] or other industrial sectors to produce biofuel, paper, textile, plastics, lubricants, and surfactants [2,3]. It is also used in agriculture for dust masking in swine barns [4] or as a biopesticide [5]. The enhanced demand for this vegetable oil worldwide results in high amounts of rapeseed meal as a by-product, which reached 38.8 million tons in 2018 [6] and 40 million tons in 2020 [7]. Currently, rapeseed meal has an underestimated application. Mainly, it is used as an inexpensive protein-rich ingredient in feed formulation [8,9]. However, the presence of substantial amounts of glucosinolates, tannins, and fibres limits the inclusion of the meal because of their negative effect on animal growth and physiology [10]. Alternatively, rapeseed meal could be used as a source for preparing protein iso- lates/concentrates. Recently, there has been an increasing interest in plant proteins. The global demand for protein is driven by various factors such as population growth, increases in urbanization and ageing, and altered customers’ preferences which are recently directed Foods 2022, 11, 541. https://doi.org/10.3390/foods11040541 https://www.mdpi.com/journal/foods