Background: Secoisolariciresinol diglucoside (SDG) is a natural antioxidant generally extracted from flaxseed, which is one of the most important oil crops in China, the by-product of the flaxseed oil, i.e., flaxseed meal still contains a lot of lignans. However, flaxseed meal is generally treated as waste, resulting in a huge waste of resources.
Objective: To establish an efficient and convenient method for extraction and purification of lignans from flaxseed meal.
Methods: First, we used response surface methodology (RSM) to optimize the extraction conditions of the ultrasonic-assisted (UA) aqueous two-phase system (ATPS), and we obtained the purified extracts by macroporous resin purification (MRP). Second, the antioxidant ability of the extracts was studied in vitro.
Results: The best extraction conditions obtained were as follows: 9.0% (w/w) sodium hydroxide; 30.0% (w/w) isopropanol; extraction time, 39 min; liquid-to-solid ratio, 52.0 mL/g; ultrasonic wave, 560 W; and extraction temperature, 40°C. Under the optimal conditions, the purity of crude extracts (SDG-APTS-C) reached 21.5%. The desorption conditions of MRP were as follows: eluting 3 BV with ultrapure water, and then eluting with 25% ethanol at 2 BV (bed volume)/h to collect eluents. The purified extracts (SDG-ATPS-P) had a purity quotient of 73.9%, which was 52.4% higher than that of SDG-ATPS-C. Additionally, experiments conducted revealed that SDG-ATPS-C and SDG-ATPS-P could effectively remove DPPH (2,2-Diphenyl-1-picrylhydrazyl), ABTS （2,-Azinobis-3-ethylbenzthiazoline-6-sulphonate）, and hydroxyl free radicals in vitro.
Conclusions: The method was validated for extracting SDG from flaxseed meal, thus achieving the reuse of flaxseed meal.
Highlights: This research provides some references for the application of UAATPS combined with MRP in natural products.
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