Integrated approach to grape stalks valorization: sustainable recovery of bioactive compounds and biofuel production

Abstract

Grape stalks are a byproduct of the winemaking process and represent a valuable and inexpensive source of bioactive compounds. While their direct use in whole bunch fermentation is known [1, 2], the majority of grape stalks are discarded, posing environmental and economic challenges. Notably, this byproduct contains a diverse array of extractable polyphenolic compounds, including phenolic acids, flavanols, flavonols, and condensed tannins [3]. Among polyphenols, quercetin-3-glucuronide, catechin, caftaric acid, and astilbin have been identified [4]. However, the high lignocellulosic content in grape stalks limits the direct extraction of a large portion of the polyphenolic component just by conventional methods. Lignin, a major structural component, consists in fact of a complex network of polymerized phenolic units. Structural analysis of grapes lignin revealed a predominance of β-O-4′ structures, with moderate amounts of β-5′, β-β, β-1′, 5-5′, and 4-O-5′ structures. The condensation degree in grape stalks is higher than that of other lignin from other agricultural residues. Grape stalks lignin was shown to contain on average a 3:71:26 molar proportion of p-hydroxyphenyl, guaiacyl, and syringyl phenolic units [5]. Several strategies have been investigated to yield a partial or even total breakdown of the lignocellulosic fraction, with subsequent extraction of the depolymerization products, providing an inexhaustible source of phenolic compounds from a practically inexpensive source material.

In this contribution, we are presenting preliminary results from an integrated approach based on tuning hydrothermal carbonization (HTC) conditions to facilitate a partial breakdown of lignin, then to increase the efficiency of extraction of polyphenolic compounds. HTC was further tested to investigate the application of grape stalks residues for producing hydrochar, also testing the process for the valorization of unextracted residues as biofuels, contributing to a fully circular bioeconomy. Our approach highlights the potential for grape stalks applications, demonstrating a sustainable approach to repurposing this underutilized resource.

References

[1] Wimalasiri, P. M., Olejar, K. J., Harrison, R., Hider, R., & Tian, B. (2022). Whole bunch fermentation and the use of grape stems: Effect on phenolic and volatile aroma composition of Vitis vinifera cv. Pinot Noir wine. Aust. J. Grape Wine Res., 28(3), 395-406

[2] Wimalasiri, P. M., Harrison, R., Olejar, K. J., Hider, R., & Tian, B. (2023). Colour characterisation of two‐year‐old Pinot noir wines by UV-Vis spectrophotometry and tristimulus colourimetry (CIELab): Effect of whole bunch or grape stems addition. Int. J. Food Sci.& Technol., 58(3), 1176-1185

[3] Souquet, J. M., Labarbe, B., Le Guernevé, C., Cheynier, V., & Moutounet, M. (2000). Phenolic composition of grape stems. J. Agr. Food Chem., 48(4), 1076-1080.

[4] Esparza, I., Moler, J. A., Arteta, M., Jiménez-Moreno, N., & Ancín-Azpilicueta, C. (2021). Phenolic composition of grape stems from different Spanish varieties and vintages. Biomol., 11(8), 1221

[5] Prozil, S. O., Evtuguin, D. V., Silva, A. M., & Lopes, L. P. (2014). Structural characterization of lignin from grape stalks (Vitis vinifera L.). J. Agr. Food Chem., 62(24), 5420-5428