White Grape Cell Walls as Fining Agents in Red Wine

A recent study published in the journal Foods investigated white grape pomace cell wall material (from Zalema, Pedro Ximénez, and Moscatel varieties) as a sustainable, allergen-free fining agent for red wine.¹ Led by José Miguel Hernández-Hierro, who is an associate professor at the Universidad de Sevilla (Food Colour & Quality Lab.), the research team used attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and principal component analysis (PCA) revealed polysaccharide-rich structures that interact with phenols and proteins. The findings highlight grape pomace as a promising plant-based alternative, while offering molecular-level insight into fining mechanisms that improve wine stability and sensory quality.

To learn more about the study’s findings and what they mean for wine stability, Spectroscopy sat down with Hernández-Hierro to talk about his team’s work. Part 1 of our conversation focuses on how ATR-FTIR and PCA revealed grape pomace cell wall composition, particularly low-esterified pectins and polysaccharide structures, drives fining performance, with minimal influence from grape variety.

ATR-FTIR combined with PCA was used to characterize the cell wall samples. What were the key spectral markers that distinguished the polysaccharide fractions, and how did PCA help interpret compositional differences across the three grape varieties?

In our analysis, PC1 was dominated by spectral markers for the polysaccharide and phenolic fractions. Key markers included the 1746 cm⁻¹ region, which identifies the degree of pectin esterification. This is a critical factor because lower esterification increases the free carboxyl groups available to interact with wine phenolics. We also identified markers for cellulose, hemicellulose, and arabinogalactans in the 1237–920 cm⁻¹ range, reflecting the complex architecture of the cell wall.

PCA indicated that varietal patterns are almost non-existent. It confirmed that the structural heterogeneity of the pomace samples (initially selected via NIR) is maintained across the varieties in cell wall material.² PCA helped us interpret that the material’s fining potential is driven by its specific compositional architecture, rather than the grape variety itself.

Zalema, Pedro Ximénez, and Moscatel were chosen as the white grape pomace sources. What was the reasoning behind selecting these three specific varieties, and did inter-varietal differences in cell wall composition meaningfully affect fining performance?

We selected Zalema, Pedro Ximénez, and Moscatel because they are the predominant white varieties in Andalusia, where our research is based. By focusing on these local sources, we aimed to support a circular economy approach, transforming abundant winery by-products into high-value oenological tools.

Regarding performance, our study found that inter-varietal differences did not meaningfully affect the fining process. The spectral data of cell wall showed that these materials are representative of the intra-varietal heterogeneity of grape pomace selected via NIR.

Consequently, their efficiency is driven by their specific chemical composition and macromolecular morphology rather than their varietal origin. We plan to further confirm these structural insights in future work through electron microscopy and detailed chemical analysis of the cell wall matrices.

Can you elaborate on the proposed molecular mechanisms, whether hydrogen bonding, hydrophobic interactions, or covalent linkages, that drive these precipitations?

The mechanism is mainly driven by non-covalent interactions, specifically hydrogen bonding, ionic interactions, and hydrophobic associations, between the cell wall's functional groups (such as OH and COO⁻) and wine phenolics.

Low-esterification pectins play a central role by promoting the formation of insoluble macromolecular aggregates that incorporate proteins and then precipitate. We intend to corroborate these spectroscopic findings in future studies through detailed chemical analysis of the cell wall matrices.

Given that the cell wall matrices contained cellulose, hemicellulose, lignin, pectins, and arabinogalactans, did any particular polysaccharide fraction appear to dominate the interaction with wine phenols or proteins, and how was this distinguished analytically?

It is fundamental to highlight that while pectins, and specifically their degree of esterification, have a major contribution to the process, the fining effect is driven by the entire cell wall matrix.

This complex matrix, composed of cellulose, hemicellulose, lignin, pectins, and arabinogalactans, provides a variety of functional groups, primarily OH, C-O-C, and COO⁻, that act together as interaction sites for wine phenolics and proteins. Ultimately, it is the collective architecture and synergy of these different structural polysaccharides that facilitate the formation of insoluble macromolecular aggregates and their subsequent precipitation.

Part 2 of our interview with Hernández-Hierro will focus on fining outcomes, and how the methods used in his study could influence the results.

References

1. Gómez-Pérez, J.; Baca-Bocanegra, B.; Hernández-Hierro, J. M.; et al. White Grape Cell Walls as Fining Agents in Red Wine: Mechanistic Insights from ATR-FTIR Spectroscopy. Foods 2026, 15 (6), 1050. DOI: 10.3390/foods15061050
2. Gómez-Pérez, J.; Baca-Bocanegra, B.; Rodríguez-Pulido, F. J.; et al. Comparative Study of Hyperspectral Imaging and Portable Spectroscopy Techniques for Selecting White Grape Pomace Samples for Fining Agent Production. Microchem. J, 2026, 220, 116601. DOI: 10.1016/j.microc.2025.116601