Abstract
Glycerol is a co-solvent for water extraction that has been shown to be highly effective for obtaining polyphenol extracts under atmospheric conditions. However, its efficacy under subcritical conditions has not yet been studied. We assessed different water-glycerol mixtures (15%, 32.5%, and 50%) in a hot pressurized liquid extraction system (HPLE: 10 MPa) at 90 °C, 120 °C, and 150 °C to obtain extracts of low molecular weight polyphenols from Carménère grape pomace. Under the same extraction conditions, glycerol as a co-solvent achieved significantly higher yields in polyphenols than ethanol. Optimal extraction conditions were 150 °C, with 32.5% glycerol for flavonols and 50% for flavanols, stilbenes, and phenolic acids. Considering gallic acid as a model molecule, computational chemistry calculations were applied to explain some unusual extraction outcomes. Furthermore, glycerol, methanol, ethanol, and ethylene glycol were studied to establish an incipient structure–property relationship. The high extraction yields of gallic acid obtained with water and glycerol solvent mixtures can be explained not only by the additional hydrogen bonds between glycerol and gallic acid as compared with the other alcohols, but also because the third hydroxyl group allows the formation of a three-centered hydrogen bond, which intensifies the strongest glycerol and gallic acid hydrogen bond. The above occurs both in neutral and deprotonated gallic acid. Consequently, glycerol confers to the extraction solvent a higher solvation energy of polyphenols than ethanol.
Highlights
Chile is the largest world producer of Carménère (~95 million L/year) [1], a highly appreciated wine due to its deep red color and low astringency [2]
Five phenolic acids were quantified in the hot pressurized liquid extraction system (HPLE) extracts, where the gallic acid content was significantly higher (~71 μg/gdp) than the others
We developed a sustainable HPLE process using glycerol as an alternative green and low-cost co-solvent for the recovery of specific families of polyphenols from Carménère grape pomace at high temperatures
Summary
Chile is the largest world producer of Carménère (~95 million L/year) [1], a highly appreciated wine due to its deep red color and low astringency [2] These distinctive characteristics are related to its high polyphenol content (240 to 350 mg gallic acid equivalents (GAE)/L) that comes from the skin and seeds of the grapes [2,3]. Skin pomace contains high concentrations of anthocyanins, flavonols, stilbenes, and phenolic acids, while seed pomace is rich in flavanols and phenolic acids [8,9] These compounds are highly demanded for the production of functional ingredients for the food, nutraceutical, and cosmeceutical industries [10]
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