An optimized microwave-assisted extraction (MAE) method was developed for extracting secoisolariciresinol diglucoside (SDG) from flaxseed. This paper presents the optimization of factors for maximizing the extraction yield of SDG. This work was conducted using the experimental domain identified in a previous study by means of screening designs, that is, samples of 1 g defatted flaxseed meal (DFM) were extracted with 50 ml NaOH of concentration of 0.5-1 M, at microwave power levels of 60-360 W, for 3-9 min, with the microwave power applied intermittently (power on 30 s/min) and continuously (power on 60 s/min). The MAE of SDG was maximized when 1 g DFM was extracted with 50 ml 0.5 M NaOH, at 135 W, for 3 min in intermittent power mode (power on 30 s/min). The optimized MAE achieved a 6% increase in the extraction yield (21.45 mg SDG per gram DFM) as opposed to a direct hydrolysis method (20.22 mg SDG per gram DFM). The MAE of SDG was governed by the microwave-NaOH interaction, which had a curvilinear dependence on the microwave power level, and linear dependence on the NaOH concentration. The microwave-induced effects accounted for a 10% increase in the SDG extraction yield (21.45 mg SDG per gram DFM) as opposed to a microwaveless control method (19.45 mg SDG per gram DFM). The optimized MAE method has good repeatability, a 97% recovery of the target compound; it is fast and efficient and can be used for precise quantification of SDG in flaxseed.

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This paper presents the first part of a study that aims at developing an optimized microwave-assisted method for extracting the lignan secoisolariciresinol diglucoside from defatted flaxseed meal. Two-level fractional factorial designs were used for screening the following factors: the microwave power level (30-360 W), the time of residence in the microwave cavity (1-25 min), the concentration of NaOH (0.25-1 M), and the mode of microwave power application (power on 30 and 60 s/min). The experimental domain had to be adjusted after each screening in order to focus the future optimization study within the factors' ranges likely to contain the optimal extraction conditions.

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