Selective Recovery of Polyphenols from Discarded Blueberries (Vaccinium corymbosum L.) Using Hot Pressurized Liquid Extraction Combined with Isopropanol as an Environmentally Friendly Solvent

Abstract

The use of water–ethanol mixtures in hot pressurized liquid extraction (HPLE) to recover phenolic compounds from agro-industrial waste has been successfully investigated. However, the unresolved challenge of reducing solvent costs associated with the process hinders the scaling of this eco-friendly technology. This study evaluated the use of isopropanol as an alternative, lower-cost solvent for recovering polyphenols from discarded blueberries through the HPLE process. HPLE was carried out using water–isopropanol mixtures (0, 15 and 30%) at 70, 100, and 130 °C. The total polyphenol content (TPC), antioxidant capacity (DPPH and ORAC), glucose and fructose contents, and polyphenol profile of the extracts were determined. HPLE extracts obtained using high isopropanol concentrations (30%) and high temperatures (130 °C) presented the highest TPC (13.57 mg GAE/gdw) and antioxidant capacity (IC50: 9.97 mg/mL, ORAC: 246.47 µmol ET/gdw). Moreover, the use of 30% water–isopropanol resulted in higher yields of polyphenols and removal of reducing sugars compared to atmospheric extraction with water–acetone (60%). The polyphenolic profiles of the extracts showed that flavanols and phenolic acids were more soluble at high concentrations of isopropanol (30%). Contrarily, flavonols and stilbenes were better recovered with 15% isopropanol and pure water. Therefore, isopropanol could be a promising solvent for the selective recovery of different bioactive compounds from discarded blueberries and other agro-industrial residues.

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“…For example, when the ethanol increased from 0% to 30% at 70 • C, it led to a decrease in the levels of fructose and glucose by 34% and 40%, respectively, while when glycerol (from 0% to 30%) was used the fructose and glucose content decreased by 77% and 70% under the same conditions, respectively (Figure 4). Similar behavior was reported by Huaman et al [34], who observed that adding ethanol from 15% to 50% reduces the extraction of reducing sugars (glucose and fructose) by 60%. This phenomenon can be attributed to using intermediate polarity solvents like ethanol during the extraction process, which diminishes the ability of these polar compounds (reducing sugars) to form hydrogen bonds with water molecules [52].…”

“…Pressurized liquid extraction (PLE) is an alternative to conventional extraction methods. Under subcritical conditions, this technique involves the use of pure water or its combinations with solvents like ethanol, glycerol, or isopropanol to enhance the extractability of polyphenols within short processing times (<20 min) and using minimal solvent volumes [31][32][33][34]. PLE is a method conducted under subcritical conditions, wherein foodgrade solvents operate at temperatures and pressures below their critical points; this specific environment enables solvents to retain their liquid form and reduces their dielectric constant, density, viscosity and surface tension, increasing the transfer rates of compounds from the matrix to the solvent [35,36].…”

“…PLE is a method conducted under subcritical conditions, wherein foodgrade solvents operate at temperatures and pressures below their critical points; this specific environment enables solvents to retain their liquid form and reduces their dielectric constant, density, viscosity and surface tension, increasing the transfer rates of compounds from the matrix to the solvent [35,36]. Simultaneously, an increase in temperature increases the kinetic energy of the solvent molecules breaking the matrix [34]. Thus, this process enhances the extractability and solubility of bioactive compounds.…”