Protein Stability in White Wine — Heat Testing and Bentonite Fining

Protein haze is a white, amorphous cloudiness that forms in bottled white and rosé wine when grape-derived proteins denature, aggregate, and scatter light. The proteins responsible are pathogenesis-related proteins, primarily thaumatin-like proteins (TLPs) and chitinases, which survive fermentation intact because of their resistance to the low pH and proteolytic conditions of winemaking. These proteins remain soluble at bottling but can slowly unfold and aggregate during warm storage or transport, forming visible haze or sediment. The problem is cosmetic rather than a health or safety concern, but consumers and trade buyers regard it as a serious quality fault.

• TLPs and chitinases are the dominant haze-forming proteins; other minor contributors include beta-glucanases and grape ripening-related proteins such as GRIP22 and GRIP32
• Non-protein factors including polyphenols, sulfates, pH, ethanol, and ionic strength all influence the rate and extent of protein aggregation
• White wine limpidity is considered an essential sensory quality parameter; any visible haze typically renders a wine commercially unsaleable

The heat stability test is the winemaking industry's primary tool for predicting whether a white wine will form protein haze during its shelf life. A filtered wine sample is heated at 80°C for 2 hours, then allowed to cool to room temperature for at least 3 hours before turbidity is measured with a nephelometer or turbidimeter. A change in nephelometric turbidity units (NTU) of less than 2.0 between the heated and unheated control is the most widely accepted pass threshold, though some laboratories apply the more stringent criterion of less than 0.5 NTU used by the Australian Wine Research Institute. The test was historically run for 6 hours at 80°C, but research has confirmed that 2 hours produces equally reliable bentonite dosage predictions for short to medium term storage.

• Wine must be filtered to 0.45 microns before heating to prevent existing particles from seeding additional haze formation and distorting the result
• A minimum cooling period of 3 hours at room temperature is required for aggregated proteins to form measurable haze; rapid chilling on ice gives misleading results
• Commercial reagent-based tests such as Bentotest and Proteotest provide rapid qualitative screening but tend to overestimate instability compared to the heat test, leading to unnecessary over-fining

Bentonite is a volcanic montmorillonite clay that, when hydrated in water, expands and develops a strongly negative surface charge. At the typical pH of white wine, haze-forming proteins carry a net positive charge, causing them to bind electrostatically to bentonite. The protein-clay complexes then settle to the bottom of the tank and are removed by racking. Bentonite can be added at several stages: before fermentation to settle juice lees, during fermentation to preserve aromatics while removing proteins, or post-fermentation as a pre-bottling fining. Post-fermentation addition is the most common stage, and bench trials at a range of doses are conducted to find the minimum effective rate, typically between 25 and 100 g/hL of wine.

• Sodium bentonite is more effective at protein removal but produces voluminous, fluffy lees that cause significant wine volume loss; calcium bentonite produces more compact lees but requires higher addition rates
• Bentonite added during fermentation can reduce total doses required but has been shown to significantly reduce varietal thiols in Sauvignon Blanc, a particular concern for aromatic white wines
• Excessive bentonite addition can strip aroma compounds and phenolics; using the minimum effective dose identified by bench trials is essential for preserving wine quality

Bentonite fining has long been considered to have minimal sensory impact, but research has confirmed meaningful quality trade-offs, particularly for certain varieties. Varietal thiols, the compounds responsible for the grapefruit and passionfruit aromatics in Sauvignon Blanc, are significantly reduced by bentonite treatment, especially when added during fermentation. Bentonite also removes phenolic compounds that contribute to mouthfeel and texture. These losses are dose-dependent and variety-specific; aromatically neutral varieties such as Pinot Grigio show fewer perceptible effects than thiol-rich varieties like Sauvignon Blanc and Gewürztraminer. This reality drives research into alternatives that can achieve protein stability with less collateral impact on wine composition.

• Bentonite addition during fermentation causes greater thiol loss than post-fermentation fining; winemakers of aromatic whites often prefer to fine the finished wine at the lowest effective dose
• Bentonite fining can also reduce foam quality in sparkling wine by removing grape proteins that contribute to foam stability and persistence
• Artisanal and natural wine producers frequently decline bentonite fining, accepting minor haze risk in exchange for minimal intervention; this is more viable for domestic markets with reliable temperature-controlled storage

Protein stability is most critical for white wines destined for long-distance export, retail environments with variable temperature control, or markets in warm climates. Temperature fluctuations during shipping, particularly on long ocean routes or through equatorial regions, accelerate protein aggregation and haze development in unstable wines. High-volume producers of Marlborough Sauvignon Blanc, northeastern Italian Pinot Grigio, and Australian Riesling routinely ensure protein stability as a non-negotiable pre-bottling step. Conversely, producers of premium barrel-fermented whites sold into temperature-controlled fine wine channels sometimes accept minor haze risk in exchange for avoiding bentonite's potential quality impact.

• High-protein grape varieties, particularly Sauvignon Blanc and Gewürztraminer, carry the greatest natural haze risk and almost universally require fining before bottling for commercial release
• pH changes from acidification or blending can alter protein stability; the heat test must be rerun on the final, blended wine before bottling to confirm stability under actual conditions
• Certified organic and biodynamic producers often seek alternatives to bentonite for philosophical reasons, driving uptake of newer techniques such as flash pasteurization with protease enzymes

The limitations of bentonite, including wine volume loss, aroma stripping, and significant environmental waste from lees disposal, have driven sustained research into alternative stabilization methods. The most commercially advanced is flash pasteurization combined with the food-grade protease Aspergillopepsin (AGP), which unfolds haze-forming proteins through brief heating at 75°C for one minute and then degrades them enzymatically; this combination achieves 80 to 90 percent total protein reduction and has been approved for commercial use in Australia and New Zealand. Zirconium oxide pellets, carrageenan, and engineered yeast strains with elevated cell wall chitin levels have also shown promise in laboratory and pilot-scale trials.

• The AGP protease and flash pasteurization combination has been shown to produce protein-stable wines with no significant sensory differences from untreated controls in triangle tests
• Zirconium oxide can remove TLPs and chitinases by around 90 percent when used during fermentation and has the advantage of being regenerable, reducing material costs
• Yeast strains with high cell wall chitin naturally bind and remove grape chitinase, offering the possibility of building protein stability into fermentation without any post-fermentation additions