A Brief Review of Bentonite and Wine Proteins

By: Denise M. Gardner

Wine protein chemistry is a daunting subject to tackle for a wine novice, and can also be a challenge for those who have been in the industry for years.

Proteins exist in wine in a state of charge: positive, negative, or neutral.  The stability, and charge, of proteins is determined by the protein’s isoelectric point (or pI), in which the net charge of the protein is 0.  Proteins are most unstable at their pI.  Isoelectric points are based on pH, and alterations of pH in the wine will ultimately affect the charge of proteins in the wine.  Alterations to wine chemistry (i.e. cold stabilization, acidification or deacidification, blending, and use of some fining agents) may alter protein stability of a given wine.

[Production Note: Due to the fact that protein stability is based on pH, winemakers should allow a wine to go through cold stabilization before testing and treating for protein stability.  This is because alterations to the wine’s pH and titratable acidity (TA) will occur as a result of cold stabilization.]

Factors that affect protein stability

In general, lower pH wines have less protein stability problems than higher pH wines.  This is a consideration for those producing lower pH (<3.4) wines.  It is possible that the wines may not need bentonite additions.

The only way to evaluate if a wine is a protein stable is through a heat/protein stability test.  For a protocol on protein stability, please refer to Bruce Zoecklein’s “Wine Analysis and Production” book (page 469-473) or via this online protocol.  Most experts recommend following an ethanol-based test for better accuracy affiliated with protein stability.  Additionally, wineries can contract certified wine labs to run protein stability tests for them, in addition to determining a rate for bentonite additions.

Wine that has been tested for protein stability. (Photo from: https://keswickvineyard.wordpress.com/2011/01/)

Wine that has been tested for protein stability. The haze on the left shows that the wine is not protein stable. (Photo from: https://keswickvineyard.wordpress.com/2011/01/)

Tannin content can also affect the stability of wine proteins.  As some tannins bind with proteins, having a higher concentration of tannins in the wine may make the wine more protein stable.  This is one why some wine grape varieties, like Pinot Noir or some hybrids, have more protein instability problems than others.  Additionally, white and rosé wines are more likely to carry protein instability problems as they are typically lower in tannins available to bind to proteins, but other factors contribute to this generality.

Some wine grape varieties may also be high in wine proteins, which dictate a need for more aggressive protein fining treatments.  Gruner Veltliner and native varieties like Concord and Niagara anecdotally tend to have more consistent issues related to protein stability than other varieties.  For Gruner Veltliner, juice treatment with bentonite additions is often recommended.  Native varieties tend to require a treatment with Sparkolloid.

Gruner Veltliner is a high protein variety that often needs a bentonite fining treatment prior to primary fermentation. This aggressive treatment pre-fermentation may be detrimental to other varieties by over-clarifying juice.

Gruner Veltliner is a high protein variety that often needs a bentonite fining treatment prior to primary fermentation. This aggressive treatment pre-fermentation may be detrimental to other varieties by over-clarifying juice.

Be forewarned that protein stability can alter for a given grape variety each vintage year; a wine harvested in one vintage year may be protein stable for that particular year, but not so for a second, consecutive year.  Protein stability varies with vintage season, and should be evaluated on each wine annually.

Why is bentonite used to treat protein instabilities?

Bentonite is a clay-based fining agent used in winemaking to bind and remove some wine proteins through precipitation or acts as a clarifying agent.  Winemakers should note that there is a lot of variation between bentonite products and suppliers, and some products may be more effective compared to others.  Winemakers should also ensure that the bentonite they are using is strictly for wine additions.

Bentonite is negatively charged, and will, therefore, bind with positively charged proteins in wine.  It is possible to treat a wine with bentonite and still have protein stability problems by:

  • A concentration effect in which case all of the proteins in the treated wine have not been bound by bentonite.
  • A charge effect in which negatively charged proteins have not been bound and, therefore, remain in the wine.

In contrast, the fining agent Sparkolloid offers alternatives to winemakers as it is primarily positively charged and will bind to negatively charged proteins.  Winemakers are always recommended to do bench trials before treating a wine with any fining agent.

Like many fining agents, bentonite may have some alternative side effects including aroma/flavor and color stripping.  While it is sometimes preferred to add bentonite without testing a wine for protein stability, winemakers run several risks towards the wine including:

  • No knowledge towards the protein stability of the wine even after treatment of bentonite. (Keep in mind that some wines require 10+ pounds per 1,000 gallon additions of bentonite.)
  • Unnecessary stripping of aroma/flavor composition or wine color when there is no protein instability issue in the wine. Depending on the wine, this can come as a detriment to wine quality.
  • Potential off-flavor development in aroma-neutral varieties.
  • Economic losses for the winery if unnecessary additions of bentonite are made to a wine without protein instabilities (i.e. waste of product). Or a waste of resources if the wine hazes in the bottle even after a bentonite addition was made during production.  If no analytical evaluation to reassure the winery that the addition was successful at stabilizing the wine, the time and cost affiliated with un-bottling, treating, and re-bottling wine can be significant.  [Note: Protein stability tests are not a 100% guarantee, but provide a winemaker with more information than guessing a wine’s protein stability]
  • Unnecessary use of employee time for fining if the wine does not require a bentonite addition.

Practical Directions for Wineries

Wineries should consider routine analysis for protein stability to avoid guesswork affiliated with bentonite additions.  Some wineries may find they are making unnecessary bentonite additions or find a need for making larger bentonite additions.  After a fining treatment, wines should be re-evaluated for protein stability to ensure that the treatment has worked properly.

Bench trials, coded for sensory evaluation.

Bench trials, coded for sensory evaluation.

When adding bentonite to any wine, it is recommended that the fining agent be hydrated in warm, chlorine-free water.  Bentonite additions should be thoroughly mixed into a wine for approximately 15 minutes, and racked after a week’s settling time.  Most wine suppliers will provide preparation and use instructions for wineries unfamiliar with bentonite treatments.

More Practical Information

This article does not touch upon a lot of the theory, chemistry, preparation of bentonite, and wine treatment for protein stability issues.  For more information available online, please consider the following resources:

UC Davis Summary on Protein Stability Issues Retold from AWRI Research

AWRI Power Point Presentation on Protein Stability

Fining with Bentonite from Purdue Extension

Protein Stability Testing via Midwest Wine Press

Resources

“Fining Agents for Use in Wine” from The Wine Lab

Wine Analysis and Production by Bruce W. Zoecklein et al. (ISBN: 0-8342-1701-5)

Winemaking Problems Solved, edited by Christian E. Butzke (ISBN: 978-1-84569-475-3 or 978-1-4398-3416)

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