Bentonite Fining (Clay — Protein Removal in White Wine)

Bentonite is a naturally occurring aluminosilicate clay formed from the decomposition of volcanic ash, classified as a smectite of the montmorillonite type. Its layered structure consists of an octahedral alumina layer sandwiched between two tetrahedral silica layers. Substitution of aluminum ions with other cations in these layers generates a net negative surface charge, which is neutralized by exchangeable cations, primarily sodium, calcium, and magnesium, located between the layers and on the outer surface. In winemaking, food-grade bentonite is used as a fining agent to remove heat-unstable proteins that would otherwise cause haze and cloudiness in white wines.

• Composed of at least 75% montmorillonite; the active surface charge arises from isomorphic substitution within the clay lattice
• Three OIV-recognized types for winemaking: natural sodium bentonite, natural calcium bentonite, and activated calcium bentonite (sodium carbonate-modified)
• Sodium bentonites swell more extensively and bind proteins more efficiently; calcium bentonites flocculate better and compact lees more tightly
• Sold in granular or powder form and must always be hydrated in water, never directly in wine or acidic solution, before addition to a tank

Bentonite removes proteins through electrostatic adsorption. When hydrated, the clay's lattice structure opens and its exchangeable sodium or calcium cations become reactive, giving the clay an overall negative charge. The negatively charged clay platelets then attract positively charged proteins, which carry a net positive charge at typical wine pH values of approximately 3.0 to 3.6. The primary haze-forming proteins in white wine are grape pathogenesis-related proteins, specifically chitinases and thaumatin-like proteins (TLPs), which aggregate irreversibly under heat or prolonged storage to form visible haze. Bentonite binds these proteins and settles them out of suspension, typically over 24 to 72 hours. Because bentonite is not a selective adsorbent, it may also co-adsorb desirable compounds including volatile esters and phenolics, which is why dosage precision is critical.

• Proteins are positively charged at wine pH (~3.0 to 3.6), making electrostatic binding to the negatively charged clay efficient at typical winemaking pH
• Grape chitinases are considered the primary driver of heat-induced haze, with thaumatin-like proteins also implicated; both are targeted by bentonite
• Bentonite is non-selective: research shows it can reduce volatile esters, key thiols in Sauvignon Blanc, and phenolic compounds alongside targeted proteins
• Protein removal efficiency correlates positively with the clay's sodium-to-calcium ratio, cation exchange capacity, and swelling capacity

Bentonite can be applied at multiple stages of production, including juice settling, during fermentation, and in the pre-bottling phase. The most common approach is post-fermentation addition, after the completion of alcoholic fermentation, when the wine's protein load is fully expressed and stable. Bench trials are essential to determine the minimum effective dose, as blanket application risks over-fining. The protein stability test most commonly used is the heat test, where a fined sample is heated at 80 degrees Celsius for six hours and assessed for residual turbidity. Bentonite must always be hydrated in water for at least several hours before addition; swelling in acidic solutions or wine directly can reduce efficacy by up to 50%. Research has shown that fining during fermentation can reduce the total dose required, though this approach carries trade-offs for aroma retention.

• Post-fermentation timing is most common; fining the final wine blend after all proteinaceous additions have been completed is the standard protocol
• Pre-bottling application, typically 4 to 8 weeks before bottling, allows adequate settling and racking before filtration
• The heat stability test (80 degrees Celsius for six hours) is the standard method for confirming protein stabilization after bentonite addition
• Fining during fermentation can reduce required dosage by up to 16 to 21% but may affect varietal aroma compounds, particularly thiols in Sauvignon Blanc

Bentonite's binding action is not fully selective. While it effectively removes haze-forming proteins, research confirms that volatile compounds, particularly esters and key varietal thiols in Sauvignon Blanc, can be significantly reduced even at moderate dosages. Studies at standard protein-stabilizing rates have shown no significant sensory differences in some varieties, such as Chardonnay, while aromatic varieties like Gewurztraminer show more measurable impacts. The guiding principle recognized across the industry is that the lower the dose used, the better the final organoleptic properties of the wine. Precise bench trials are essential to identify the minimum effective dose, balancing protein stability against flavor and texture preservation.

• Volatile esters and thiols (the key aromatic drivers in Sauvignon Blanc) are among the most susceptible compounds to bentonite adsorption
• Aromatic varieties such as Gewurztraminer are more sensitive to bentonite's secondary effects than neutral varieties like Chardonnay
• Excess bentonite can impart an earthy, freshly laundered aroma if applied at very high rates; this is a recognized risk in the technical literature
• Anthocyanins are positively charged at wine pH and can bind to bentonite, which is one reason the agent is rarely applied to red wines

Bentonite remains the most widely adopted and effective material for protein stabilization globally, but its use has economic and quality costs that have driven substantial research into alternatives. The hidden costs of bentonite fining, including wine volume losses and quality impacts, have been estimated to cost the global wine industry around one billion dollars annually. Winemakers in aromatic regions, including Alsace and Germany's Riesling zones, apply bentonite with particular care given the sensitivity of terpene-driven aromas. Alternatives being studied and, in some cases, adopted include plant-based proteins (pea, potato), chitosan, carboxymethyl cellulose, and flash pasteurization. Most of these alternatives remain at the research stage or are not yet authorized under all major regulatory frameworks.

• High-protein varieties and warm vintages typically require higher bentonite doses; cool-climate, low-protein wines may achieve stability at minimal rates
• Natural wine producers frequently decline to use any fining agents, accepting some risk of protein instability as part of their minimal-intervention philosophy
• Chitosan and plant-based protein fining agents are authorized in some jurisdictions and show promise for selective protein removal with reduced aromatic impact
• Fermenting on bentonite is practiced for high-protein white varietals where early protein reduction simplifies later processing and preserves aromatic freshness

Bentonite is an authorized oenological compound for use in winemaking in the European Union under Commission Delegated Regulation (EU) 2019/934, which sets out the conditions and limits of authorized oenological practices based on OIV recommendations. Its purpose under this framework is the prevention of protein and copper hazes. The substances used must comply with the prescriptions of the International Oenological Codex. In the United States, bentonite is permitted by the TTB (Alcohol and Tobacco Tax and Trade Bureau) as a clarifying agent. As a mineral-derived processing aid with no animal components, bentonite is considered vegan-compatible and is accepted by most organic and biodynamic certification bodies, though some natural wine producers avoid all fining agents on principle.

• Approved under EU Commission Delegated Regulation (EU) 2019/934 for the prevention of protein and copper hazes in must and wine
• Authorized by the TTB in the United States as a clarifying and fining agent for wine production
• Mineral-based and allergen-free; suitable for vegan-labeled wines and accepted by most organic and biodynamic certification schemes
• Environmental concerns related to open-pit mining and bentonite lees disposal have driven industry interest in lower-dose protocols and non-clay alternatives