Near-critical extraction of pigments and oleoresin from stinging nettle leaves

Chlorophylls and carotenoids were extracted from leaves of stinging nettle (Urtica dioica L.) with supercritical and liquid CO2. The solvent modified with ethanol (0–7 wt.%) was applied at the pressures of 200, 250 and 280 bar and the temperatures of 25, 40 and 60 °C. Extracts were analysed by means of HPLC and RP-HPLC methods. The maximum yields of chlorophyll a, chlorophyll b, β-carotene and lutein obtained with CO2 were 73, 100, 24 and 39 mg/100 g dry matter, respectively. In comparison with the extract obtained using conventional solvents, the maximum yields obtained with CO2 were similar for carotenoids, higher for chlorophyll b and lower for chlorophyll a. Chlorophyll a partially degraded in the extractor to the more soluble pheophytin a; the extent of carotenoid degradation was much lower than that of chlorophylls. The degradation was substantially reduced by addition of a small amount of antioxidant to the solvent. Extraction rates in dependence on extraction conditions were measured both for individual pigments and for oleoresin. A model describing the extraction of a solute that is present in two forms, free and interacting with matrix, was applied to interpret the experimental extraction curves. A loss of volatile substances from the ground nettle leaves and a decrease in β-carotene content from 24 to 18 mg/100 g dry matter was observed during the period of 155 days between the first and last experimental run.