The impact of high-power ultrasound and microwave on the phenolic acid profile and antioxidant activity of the extract from yellow soybean seeds

The aim of the study was to investigate and compare several extraction protocols like 1) high-power ultrasound probe assisted solvent extraction; 2) microwave assisted solvent extraction; 3) direct acid hydrolysis; 4) direct alkali hydrolysis, and 5) two step extraction consisting of ultrasound or microwave assisted solvent extraction followed by alkaline and acid hydrolysis in terms of efficiency of the extraction of phenolic acids from the yellow soybean seed variety Laura. These extracts were screened for their total phenol content (TPC), and for their antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl radical scavenging (DPPH) assay, as well as on content of some individual phenolic acids. It appeared that the acetone containing mixtures gave much higher TPC than methanol containing ones, but the presence of acid in the methanol solvent significantly improved the extraction of phenolic compounds. To further improve the extraction, an ultrasound lab-scale probe at 20 kHz was used, with 15 and 30% of the maximum amplitude, and the extraction time was varied from 2 to 15 min. Microwave assisted extraction was performed varying the temperature from 55 to 85 °C, microwave power from 25 to 100 W and extraction time from 2 to 10 min. Changes in the content of six phenolic acids were examined: gallic, trans-cinnamic, chlorogenic, caffeic, p-coumaric and ferulic acid. The separation and quantification of phenolic acids was accomplished by high-performance liquid chromatography-diode array detection (DAD) procedure. The results suggested that microwaves contributed to more efficient extraction of phenolic acids from the seed of yellow soybean. The amount of phenolic acids varied from 65.52 μg/g of dry matter (d.m.) for caffeic acid, to 581.84 μg/g d.m. for p-coumaric acid. Both, ultrasound and microwaves contributed to more efficient extraction of total phenol compounds and enhanced antioxidant activity of soybean seed extracts. TPC varied from 12.48 to 18.77 mg GAE/g d.m. and antioxidant activity varied from 244.58 to 345.21 μmol TROLOX eq/g d.m.