Batch and Flow Ultrasound-Assisted Extraction of Grape Stalks: Process Intensification Design up to a Multi-Kilo Scale

Abstract

Nowadays, approximately 1 billion kg/y of grape stalks, with a remarkable polyphenols content, are produced worldwide. In this paper, the extraction process intensification of polyphenols in water was achieved under ultrasound-assisted recovery, focusing on kinetics and scaling-up factors. Immersion and cup-horn systems were exploited as acoustic cavitation sources, and the total phenolic content (TPC) was chosen to assess the process efficiency. The kinetics were evaluated by Peleg's hyperbolic model, and the effect of both the initial feedstock granulometry and ultrasound size-reduction were determined. The results were compared with conventional extraction methods (data analysis by ANOVA). The best polyphenols yield was obtained after 45 min of sonication, giving between 29.71 and 31.89 mg/g (gallic acid equivalents over the dry matter). The extracts were characterized using HPLC-DAD, UPLC-ESI-MS/MS, DPPH^• assay (2,2-diphenyl-1-picrylhydrazyl), TEAC assay (Trolox equivalent antioxidant capacity), and proanthocyanidin content determination. The flow-mode extraction procedure of grape stalks (2 kg) was carried out in a 15 L reactor. A semi-industrial decanter unit and a bag-filter were the keys units of the downstream operations. The resulting particle-free solution underwent nanofiltration on a membrane pilot skid, providing a final polyphenols-enriched stream concentrated up to 355.91%, as shown by the antioxidant activity and TPC.

Keywords: antioxidant activity; extraction kinetics; flow system; grape stalks; kilo-scale procedure; polyphenols; process intensification; ultrasound-assisted extraction.

Figure A1

Linearization, I-horn; (A): RT; (B): 45 °C.

Figure A2

Linearization, C-horn.

Figure A3

HPLC-DAD chromatograms of l-UAE (H₂O, C-horn, 45 min, 45 °C; first and second on the left, 280 and 340 nm, respectively) and conventional hydroalcoholic extracts (EtOH/H₂O 60:40, 23 min, 95 °C, reflux; first and second on the right, 280 and 340 nm, respectively) recorded with method Pol (HPLC method Pol) or recorded with method Cat at 280 nm (third on the left, l-UAE, and third on the right, conv.)

Figure A4

Linearization, S-UAE.

Figure 1

Recirculating flow system. Left: set-up scheme (1: mixed tank, 2: ultrasound (US) flow-through cell); right: facility picture.

Figure 2

I-horn UAE at different temperatures: experimental values and the Peleg model.

Figure 3

C-horn UAE: experimental values and Peleg model.

Figure 4

Electrospray ionization (ESI)^- total ion chromatograms obtained from UPLC-ESI-MS/MS analyses; (A) l-UAE (water, cup-horn, 45 min, 45 °C); (B) conventional hydroalcoholic extracts (EtOH/H₂O 60:40, 23 min, 95 °C, reflux).

Figure 5

S-UAE: experimental values and Peleg model.

Figure 6

Lab-scale/scale-up extraction curve comparison at 45 °C.

Figure 7

Lab-scale/scale-up kinetic-parameter comparison.

Figure 8

NF concentration trend, counter-pressures reported.

Figure 9

Mass-balance report.