The influence of different microwave-assisted extraction conditions on polyphenol yield and antioxidant activity of wild thyme

Abstract

Aromatic plants are commercially important and find wide use in food and pharmaceutical industry, because of health benefits related with their antioxidant, antimicrobial, antiinflammatory, cardioprotective, cytotoxic, expectorant, anti-spasmodic and stimulant effects. Most of phytochemicals are impossible to synthesize, therefore the best option is their extraction from natural sources. Microwave-assisted extraction offers a rapid delivery of energy to a total volume of solvent and solid plant material, improving the recovery of polyphenols. In the study, wild thyme herb was evaluated as a source of polyphenols, which were extracted by the application of microwaves instead of the traditional extraction techniques. The objective of the present study was optimization of microwave-assisted extraction process through varying irradiation time (5-180 s), temperature (60-200°C), particle size (0.3, 0.7 and 1.5 mm), solventto-solid ratio (10:1, 20:1, 30:1 and 40:1) and ethanol concentration in the extraction medium (0- 60%). Extraction efficiency was expressed via total polyphenol content (Folin-Ciocalteu method) and antioxidant activity (ABTS radical scavenging method). The statistical analysis (one-way ANOVA and Duncan's post-hoc test) has revealed that irradiation time above 15 s and particle size have not shown statistically significant influence on polyphenol yield, while there were significant differences between extracts obtained by various temperatures and solvent-to-solid ratios. Although the highest polyphenol yield has achieved by using 50% of ethanol, there was no statistically significant difference between ethanol concentration in the range from 30% to 50%. The highest polyphenol yield and antioxidant activity were achived in the extract with particle size of 0.3 mm, 30:1 ratio and 50% ethanol, after 15 s, at 200°C (45.11 mg GAE/L and 27.22 µmol/L Trolox). According to the presented results, microwave-assisted extraction may be used as an efficient and time saving alternative to traditional extraction techniques for extraction of bioactive polyphenolic compounds from wild thyme.