Impact of Isolation Method on the Antioxidant Activity of Rapeseed Meal Phenolics SATU VUORELA,* ,† ANNE S. MEYER, ‡ AND MARINA HEINONEN † Department of Applied Chemistry and Microbiology, Division of Food Chemistry, University of Helsinki, P.O. Box 27, 00014 University of Helsinki, Finland Rapeseed meal is the byproduct of the rapeseed deoiling process. Among oilseed plants, rapeseed contains the greatest amount of phenolic compounds. In this study, the rapeseed phenolics were isolated with aqueous methanol, aqueous ethanol, hot water, and enzymatically with ferulic acid esterase. These isolates were tested for radical scavenging and for liposome and low-density lipoprotein (LDL) model systems. The radical scavenging activities of all isolates were >60% at a concentration of 1.5 mg/mL. In the liposome model system the formation of hexanal was inhibited by all rapeseed meal isolates by >90% and the formation of conjugated diene hydroperoxides by >80% (8.4 µg/mL concentration). All rapeseed meal isolates also inhibited oxidation of LDL particles by >90% inhibition (4.2 µg/mL concentration). Isolation of rapeseed meal phenolics with either water or enzyme is a very suitable method devoid of organic solvents. Thus, rapeseed meal phenolics constitute an interesting source for food and cosmetic applications with antioxidant effect. KEYWORDS: Rapeseed phenolics; antioxidants; enzyme-assisted extraction; oxidation model systems INTRODUCTION Rapeseed meal is the byproduct of the rapeseed deoiling process. It contains 40% proteins, and the amino acid content has a high nutritive value (1). Thus, rapeseed meal is commonly used as feed (2). Rapeseed meal also has a high content of fiber and several minerals such as calcium, magnesium, zinc, and copper. It contains some vitamins and other bioactive com- pounds such as R-tocopherol, several B vitamins, and choline, which make the meal nutritionally very valuable. The amount of residual oil in rapeseed meal depends on the processing method and varies typically between 1 and 10% (2-4). Rapeseed, especially rapeseed meal, is rich in phenolic com- pounds. According to Nowak et al. (5), among oilseed plants, rapeseed contains the greatest amount of phenolic compounds. The most significant phenolic compounds in rapeseed are sinapic acid derivatives such as sinapine [the choline ester of sinapic acid (constitutes ∼80% of the total phenolic compounds) (Figure 1)]. Sinapic acid in rapeseed can also exist as a glucosidic ester, glucopyranosyl sinapate (6). Only a small part, <16%, of sinapic acid exists as free sinapic acid (Figure 1) (7). Most of the phenolic compounds remain in the meal when the oil is pressed from the seeds (8). Typically, the amount of sinapic acid derivatives in rapeseed meal varies between 6390 and 18370 µg/g depending on the oilseed plant variety and the oil processing method (7). In crude rapeseed oil there are some phenolic compounds present, with the most abundant phenolic compound being a newly identified phenolic compound, vinyl- syringol, in amounts of 245-700 µg/g (Figure 1)(8, 9). The amounts of sinapic acid and sinapine were 16 and 19 µg/g, respectively, in crude rapeseed oil. During the oil refining process, the amount of phenolic compounds decreases signifi- cantly with only up to a 25 µg/g amount of total phenolics found in commercially refined rapeseed oil (8). Rapeseed phenolics are usually extracted from rapeseed with different organic solvents such as aqueous methanol (70%), methanol, or acetone (8, 9, 11). For compositional analysis, the extract is then hydrolyzed for releasing sinapic acid from its esters, because sinapine is not available as a commercial standard. Alkaline hydrolysis is the most often used procedure for releasing the phenolic acids, but enzymatic hydrolysis can also be employed. According to our previous study ferulic acid esterase and Ultraflo L, a -glucanase with side activities, were as effective as sodium hydroxide in hydrolyzing rapeseed phenolic esters (8). Rapeseed phenolic compounds are potent antioxidants in various environments relevant to food, cosmetic, and pharma- ceutical preparations. Nowak et al. (6) found that rapeseed phenolic compounds, especially sinapic acid, were active in inhibiting the oxidation of emulsions. According to Wana- sundara et al. (12) the antioxidant activity of different rapeseed phenolic fractions was lower than the activity of crude ethanolic extract due to synergism between different phenolics in a -carotene-linoleate model system. The most active rapeseed meal phenolic fraction contained several classes of phenolic compounds including phenolic acids, flavones, and flavonols. Koski et al. (9) fractionated crude rapeseed oil and found the vinylsyringol-containing fraction to be the most effective * Author to whom correspondence should be addressed (telephone +358 9 191 59796; fax +358 9 191 58475; e-mail satu.vuorela@helsinki.fi). † University of Helsinki. ‡ Biocentrum, Technical University of Denmark. 8202 J. Agric. Food Chem. 2004, 52, 8202-8207 10.1021/jf0487046 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/02/2004