Protein Stability and Bentonite Fining in White Wines

Problem Definition

White wines contain grape-derived proteins (primarily pathogenesis-related proteins: thaumatin-like proteins and chitinases) that can denature and aggregate when wines are exposed to elevated temperatures, causing visible haze. Protein haze is a commercial fault requiring preventive treatment before bottling.

The problem is particularly acute in:

Aromatic whites with high protein retention
Wines processed at cold temperatures (protein denaturation delayed)
Wines from Botrytis-affected fruit (additional proteins)
White wines intended for warm-climate markets or transport

Technical Context

Protein Chemistry

Grape proteins in wine:

Molecular weight: 20-30 kDa (most haze-forming)
Isoelectric point: pH 4.5-6.0 (positively charged at wine pH)
Concentration: 10-300 mg/L depending on variety, vintage, processing

Primary haze-forming proteins:

Thaumatin-like proteins (TLPs): Most abundant; stable to heat alone but aggregate with other factors
Chitinases: Less stable; denature at lower temperatures

Denaturation mechanism: Heat exposure (>20°C, increasing with temperature) causes protein unfolding. Unfolded proteins aggregate and interact with other wine components (polyphenols, polysaccharides) to form visible haze or precipitate.

Factors affecting haze:

Temperature (threshold ~20°C; accelerates above 50°C)
pH (higher pH reduces protein solubility)
Ionic strength
Phenolic content (increases aggregation)
Presence of wine matrix components

Stability Testing

Heat test (standard method):

Heat wine sample to 80°C for 2 hours (or 80°C for 6 hours for more stringent)
Cool to room temperature
Chill to 4°C for 4 hours
Measure turbidity (NTU)

Stability criteria:

Stable: <2 NTU increase
Marginal: 2-5 NTU increase
Unstable: >5 NTU increase

Tannin addition test: Adding tannic acid intensifies protein precipitation; provides faster indication but less predictive for all conditions.

Bentotest (quick screening): Proprietary reagent provides rapid indication; not substitute for heat test.

Varietal Protein Content

Variety	Typical Protein Level	Bentonite Requirement
Sauvignon Blanc	High	50-100 g/hL
Gewürztraminer	High	50-100 g/hL
Muscats	High	40-80 g/hL
Chardonnay	Moderate	30-60 g/hL
Riesling	Low-Moderate	20-50 g/hL

Values are indicative; actual requirements vary by vintage, vineyard, and processing.

Options and Interventions

Bentonite Fining

Mechanism: Bentonite (montmorillonite clay) carries negative surface charge; binds positively charged proteins electrostatically. Protein-bentonite complex settles out.

Bentonite types:

Sodium bentonite: High swelling capacity; effective; requires more preparation
Calcium bentonite: Lower swelling; less effective; easier preparation
Activated bentonite: Enhanced surface area; more efficient

Preparation:

Hydrate bentonite in water (10:1 water:bentonite ratio) for 12-24 hours
Blend until smooth slurry
Add to wine with mixing
Allow to settle (24-72 hours)
Rack or filter

Dosage determination:

Conduct bench trial with incremental additions (20, 40, 60, 80, 100 g/hL)
Heat-test each treatment
Select minimum dose achieving stability

Timing options:

Juice (pre-fermentation): High protein removal; minimal sensory impact; may remove nutrients
Post-fermentation: Standard timing; full wine assessment possible
Pre-bottling: Emergency treatment; shorter settling time

Alternatives to Bentonite

Zirconium oxide:

Selective protein adsorption
Regenerable; reusable
Lower wine losses than bentonite
Not permitted in all jurisdictions

Flash pasteurization:

Heat-denature proteins in-line
Filter precipitated protein
May affect wine character
Energy-intensive

Haze-preventing enzymes (aspartic proteases):

Hydrolyze haze-forming proteins
In development; not widely available
May have off-target effects

Ultrafiltration:

Removes proteins by size exclusion
Also removes desirable macromolecules
Quality trade-off significant

Combined Treatments

Bentonite + enzyme (pectinase, glucanase):

Enzyme addition improves clarification
May reduce bentonite requirement
Common in Botrytis-affected wines

Bentonite + silica gel (kieselsol):

Sequential addition
Improves settling; reduces bentonite dose
Common in Germany

Trade-offs and Risks

Over-fining:

Excessive bentonite strips texture, body, and aromatics
Reduces wine quality significantly
Always use minimum effective dose

Under-fining:

Protein haze in bottle
Consumer complaints; returns
Commercial disaster

Bentonite lees volume:

Bentonite creates significant lees volume (5-10% wine loss at high doses)
Economic impact on premium wines
Rotary vacuum filtration can recover wine from lees

Timing trade-offs:

Juice fining: Lower sensory impact but less accurate (fermentation changes protein profile)
Post-fermentation: Most accurate but higher sensory impact

Quality impact:

Bentonite may remove positive aroma compounds (indirectly, via protein-aroma complexes)
Some producers accept slight instability risk for premium lots
Consumer tolerance varies by market

Practical Implications

Variety-specific considerations:

Sauvignon Blanc: High protein content typical. Marlborough GI producers routinely fine at 60-100 g/hL. Thiols may be affected at high bentonite doses.
Chardonnay: Moderate protein. Chablis AOC unoaked styles require stability for extended aging. Barrel-fermented wines have lower protein (precipitation on lees).
Gewürztraminer: High protein and high aromatic content. Alsace AOC producers balance stability against aromatic preservation. Minimal bentonite preferred.
Viognier: High protein with delicate aromatics. Juice fining may preserve aroma better than post-fermentation.

Appellation-specific implications:

Marlborough GI: Export market demands pristine stability. Comprehensive heat testing standard. High bentonite usage accepted as necessary.
Alsace AOC: Premium Gewürztraminer and Riesling require balanced approach. Some producers accept marginal stability for Grand Cru wines.
Chablis AOC: Precision and clarity define style. Complete stability required. Modern filtration and fining standard.

References

Waters, E.J., Alexander, G., Muhlack, R., Pocock, K.F., Colby, C., O’Neill, B.K., Høj, P.B., & Jones, P. (2005). “Preventing Protein Haze in Bottled White Wine.” American Journal of Enology and Viticulture, 56(4), 324-330. AJEV Link
Pocock, K.F., & Waters, E.J. (2006). “Protein Haze in Bottled White Wines: How Well Do Stability Tests and Bentonite Fining Trials Predict Haze Formation During Storage and Transport?” Australian Journal of Grape and Wine Research, 12, 188-196. DOI: 10.1111/j.1755-0238.2006.tb00058.x
Ribéreau-Gayon, P., Glories, Y., Maujean, A., & Dubourdieu, D. (2006). Handbook of Enology, Volume 2: The Chemistry of Wine Stabilization and Treatments (2nd ed.). Wiley. Publisher Link
Marangon, M., Van Sluyter, S.C., Waters, E.J., & Menz, R.I. (2014). “Structure of Haze Forming Proteins in White Wines: Vitis vinifera Thaumatin-Like Proteins.” PLoS ONE, 9(12), e113757. DOI: 10.1371/journal.pone.0113757