Nutritional and antioxidant profiling of Algerian rapeseed seeds (Brassica napus L.) and its cold-pressed by-products

This study investigated the "Zitna" variety of rapeseed seeds (Brassica napus L.) cultivated in Algeria and their by-product cakes used for animal feed after cold extraction. Physicochemical parameters, mineral profile, phenolic compounds (extracted with acetone, methanol, ethanol, and water), and antioxidant activity by DPPH free radical scavenging were analysed. The methanolic extract of the Zitna variety were analysed using HPLC. The results showed minor differences in physicochemical composition. The protein content was 3.79 ± 0.01 mg/mL in the seeds and 4.92 ± 0.04 mg/mL in the cakes, reflecting an increased protein concentration in the by-products due to the extraction process. Mineral concentrations were determined using the flame atomic absorption spectroscopy (FAAS) technique, which revealed that cakes had higher levels of essential minerals than the seeds and the most abundant element was potassium (K) with a concentration (27.2 ± 0.8 mg/L). The highest total phenolic content (TPC) was observed in acetone extracts, with seeds having 67.7 ± 4.3 mg GAE/g and cakes 61.3 ± 1.1 mg GAE/g. Methanol extracts showed the highest total flavonoid content (TFC), with seeds containing 23.5 ± 0.6 mg QE/g and cakes 18.5 ± 0.6 mg QE/g. Antioxidant activity, measured by IC50 values, was higher in seeds compared to cakes, with acetone extracts having IC50 values of 6.5 mg/mL for seeds and 14.2 mg/mL for cakes. The negative correlations were detected between IC50 and phenolic contents in seeds (-0.98; -0.97) and cakes (-0.98; -0.990) respectively. The chromatogram of the 'Zitna' rapeseed extract showed that sinapic acid is the major compound, with antioxidant activity enhanced by the novel discovery of gossypin, a flavonoid with antioxidant and anti-inflammatory properties. These findings demonstrate that cold-pressed extraction of Algerian rapeseed seeds preserves and concentrates valuable nutritional and antioxidant properties in the by-products, making them suitable for various industrial applications.