Physicochemical characterisation of Thymus serpyllum extracts prepared using natural deep eutectic solvents

Abstract

Thymus serpyllum L. (wild thyme) extracts were prepared using 1 g of plant material and three natural deep eutectic solvents (30 mL, malic acid+maltose, lactic acid+menthol, or citric acid+glycerol with 50% of water) in maceration (90 min). The extracts were characterized in terms of total polyphenol content (TPC), total protein content, extraction yield, zeta potential, conductivity, pH, density, surface tension, and viscosity. TPC was the highest in citric acid+glycerol extract (35.06±1.13 mg gallic acid equivalents (GAE)/g of plant material), whereas malic acid+maltose and lactic acid+menthol extracts possessed similar TPC (29.62±1.11 and 28.94±1.15 mg GAE/g, respectively). Total protein content amounted to 5.55±0.20 mg/g in citric acid+glycerol extract, while significantly lower values were determined in malic acid+maltose and lactic acid+menthol extracts (1.22±0.30 and 1.73±0.18 mg/g, respectively). The extraction yield was the highest for citric acid+glycerol extract, 1.57±0.11%, followed by malic acid+maltose and lactic acid+menthol extracts, 1.28±0.10 and 1.27±0.13%, respectively. Zeta potential (absolute value, as a predictor of potential application in water treatment) was low in all extracts (0.86±0.06 mV for citric acid+glycerol, -0.35±0.09 mV for lactic acid+menthol, and -0.17±0.05 mV for malic acid+maltose extract). The conductivity (as a predictor of antioxidant capacity) was in the range of 2.35±0.21 and 2.71±0.29 mS/cm (citric acid+glycerol and malic acid+maltose extracts) to 5.70±0.23 mS/cm (lactic acid+menthol extract). pH ranged from 1.44 in citric acid+glycerol extract to 1.78 and 1.85 in malic acid+maltose and lactic acid+menthol extracts. Density varied from 1.067±0.003 g/mL for lactic acid+menthol extract to 1.169±0.002 and 1.195±0.005 g/mL for citric acid+glycerol and malic acid+maltose extracts. Surface tension was the highest in citric acid+glycerol extract (38.0±0.4 mN/m), followed by malic acid+maltose and lactic acid+menthol extracts (31.5±0.4 and 25.9±0.1 mN/m, respectively). The viscosity of lactic acid+menthol extract was significantly lower (1.83±0.30 mPa•s) in comparison to malic acid+maltose and citric acid+glycerol extracts (6.64±0.15 and 7.84±0.10 mPa•s, respectively). The highest TPC, total proteins, and extraction yield were measured in citric acid+glycerol extract, while lactic acid+menthol extract possessed the highest conductivity and the lowest density, surface tension, and viscosity. Therefore, the constitution of natural deep eutectic solvent should be optimized depending on the future application of T. serpyllum extract.