SC-CO2 Extraction of guayule biomass (Parthenium argentatum) - yield and selectivity towards valuable co-products, lipids and terpenics

Guayule (Parthenium argentatum) a shrub native to Mexico, is now being acclimated in Europe as a potential crop. The main extractible fractions are polyisoprene (similar to Hevea rubber) for producing tires and allergy-free gloves and resin (GRe). This work deals with valorising this last fraction, after water-based extraction of the latex. To date GRe has not been extensively investigated, in spite of containing useful compounds, and there is a need for a green and selective extraction process, which is the aim of present work. First, resin -a mixture of lipids and terpenics-was analysed with focus on fatty acids and other carboxylic acid containing classes. Then the influence of extraction conditions was investigated, under "soft" conditions with supercritical carbon dioxide (SC-CO2). Investigated parameters act on extraction efficiency (yield, selectivity) towards valuable fractions. Our results do show that SC-CO2 alone cannot extract substantial amounts of resin. As co-solvent, ethanol -a benign one-and acetone, this last being the most common solvent for that purpose, here also used alone as reference, under pressurized extractions conditions (ASE). Under SC-CO2, ethanol allows the highest resin yield (12.1%-dw biomass dry weight) compared to acetone (7.0%-dw) and to the reference ASE-acetone method (5.2%-dw). Found suitable conditions with SC-CO2-ethanol are affordable: flow rate 34 gCO(2)/min and 3.0mL/min of co-solvent, temperature 45 degrees C, pressure 300 bar. Saponification was used for separating resins components before analysis. Aromatic carboxylic acids (cinnamic, panisic) initially linked to a sesquiterpene, are the major components found in G-Re from the biomass (branches) using the reference ASE method. But these are even more prominent in SC-CO2-ethanol extracts from bagasse (68.7%), compared to SC-CO2-acetone (51.1%) which allows a lower selectivity. The unsaponifiable fraction shows a low dependency on extraction temperature with SC-CO2-ethanol, playing in favor of preserving extract value, guayulins (sesquiterpenes) and argentatins (triperpenes) being known anti-insect and fungistatic agents for wood preservation. Minor resin components are fatty acids containing lipids C18: 2, C18: 3, C16: 0. SC-CO2 and ethanol, which can be easily recycled, are highly suitable for extracting high-value natural compounds, given their low environmental impact. This work provides the first detailed insight for selecting appropriate parameters and their variation range, in view of further optimizing a "green" process for extracting above compounds.