Bentonite fining and protein stability

The need for bentonite fining is driven by the propensity of unstable proteins present in the wine to precipitate out in warm conditions and present as a cloudy haze in the bottle. By adding bentonite to the wine, significant amounts of these unstable proteins can be removed, minimizing the potential for haze. There is a down side to bentonite use though as over-fining can lead to flavor stripping. How do we minimize this? By determining the smallest amount of bentonite that can be used to remove the largest amount of protein with the least sensorial impact. 

Protein and Polysaccharide Stability

Clarity has no apparent gustatory impact on wine quality but features prominently in the wine tasting schemes of both the Wine and Spirits Education Trust and the Guild of Sommeliers. The biggest challenges to wine clarity are proteins and polysaccharides, both of which, when unstable, manifest as hazes in the wine: off-white flakes in the case of the former and gelatinous masses in the case of the latter (Table 1, Harbertson).

Wine proteins are derived from grape and yeast sources with their levels dependent on variety, vintage, maturity, fruit condition, pH, and processing methodology (Zoecklein 1988; Butzke). The protein’s ability to retain its stability is determined by its "isoelectric point" plus a number of other factors. Proteins can be either positively or negatively charged based on pH and their isoelectric points are attained when the positive and negative charges of the fractions are equal. They are least soluble (stable) at this point. In addition, the lower the difference between the pH and the isoelectric point, the lower the charge and, hence, the lower the solubility of the protein (Zoecklein 1988). Other factors impinging on solubility include temperature and the “concentration of wine complexing factors” and any one of these factors can cause protein precipitation. It should be noted that yeast proteins are not implicated in haze formation.

The overarching factor in haze formation, however, is heat which causes a denaturation and precipitation of the proteins or formation of insoluble complexes with polysaccharides or polyphenols.

As is the case with proteins, polysaccharides have both grape and yeast components. The grape elements come from healthy and Botrytis-infected fruit and are released by yeasts, lees, and bacteria (AWRI). Most polysaccharides are insoluble in ethanol and, as a result, the population is low in wine. Those that do remain can form a gelatinous mass in the presence of alcohol. As is the case with proteins, yeast-derived polysaccharides do not contribute to haze formation.

The hazes formed by proteins and polysaccharides are irreversible and must be removed. According to Budzke, “most consumers prefer a wine free from unappetizing-looking protein instabilities.”

Bentonite Fining

The approach utilized most frequently within the wine industry to remove unwanted juice/wine components is the use of fining agents. Fining agents are adsorptive or reactive materials which bind to undesirable elements with the denser compounds thus formed precipitating out, leaving the wine much clearer (as well as increasing its filterability). The effectiveness of a fining agent is dependent on (Zoecklein): the agent utilized, the method of preparation and addition, the quantity employed, wine pH, wine metal content, temperature, age of the wine, and previous treatments.

The fining agent that is most widely used to remove protein haze is bentonite, a clay with a layered structure which is added to wine as a clay-water suspension. Bentonite, having a slight negative charge, will bind with positively charged particles (such as proteins) and the complex thus formed will fall to the bottom. Bentonite fining will remove protein fractions with the largest cationic charge and will, especially if conducted simultaneously with bitartrate stabilization efforts, aid in the compaction of lees. The issues associated with bentonite fining are as follows (Butzke; Vincenzi, et al.):

• Flavor stripping
• Potential for oxidative damage if slurry mixing allows air exposure
• Fining before fermentation may lead to a sluggish fermentation due to stripping of certain growth factors
• Creates a solid waste problem
• Long settling times
• Certain quantity of wine lost as bentonite lees.

Polysaccharides are removed from wine by a mixture of enzymes, an approach referred to as the “kitchen-sink method” (Harbertson).The enzymes used in the mixture are cellulose, hemicellulose, protease, pectinase, and beta-glycosidase. These enzymes effectively and simultaneously break down several classes of polysaccharides under specific pH, temperature, and ethanol conditions. The amount of polysaccharides will vary from vintage to vintage, requiring annual experimentation to determine appropriate dosages (Harbertson).

The process generally employed for bentonite fining is to conduct a fining trial wherein some number of samples are treated with regularly increasing doses of bentonite and with one of the samples untreated and held out as a control. The post-fining turbidity of each of these samples is then tested after which they are heated, cooled, and then turbidity-tested again. If the initial fining was sufficient, then the change in turbidity between tests would be less than 2 NTU (Nephelometruc Turbidity Units). If the change is greater than 2 NTU, additional fining trials would be run until the NTU fell below 2 because this shows that enough protein is left in the wine that it would precipitate under the right temperature conditions.  

Assuming that all of the samples pass the turbidity test, the team is then able to conduct sensory evaluations to determine which sample attains protein stabilization with the least sensorial impact and then to use the trial results as the basis for bentonite fining of the production lots.

References

AWRI, Amorphous deposits, www.awri.com.au.

Butzke, Chris, Fining with Bentonite, Purdue Extension FS-53-W, www.extension.purdue.edu.

Harbertson, James F. A Guide to the Fining of Wines, Washington State University Extension Manual EMO16.

Vincenzi, Simone, et al., Removal of Specific Protein Components by Chitin Enhances Protein Stability in a White Wine, AJEV 56(3), 2005.

Zoecklein, Bruce (1988), Protein Stability Determination in Juice and Wine, Virginia Cooperative Extension, Publication 463-015.

©Wine -- Mise en abyme

Appearance as an indicator of wine quality

I recently developed and reported on a framework for the assessment of wine quality, a position at which I had arrived rather tangentially.  I was writing a post on wine balance as a quality measure and, upon concluding, realized that I had not defined quality as a concept or wine quality as an ideal.  Over the course of two subsequent posts I corrected these oversights and then returned to the task of elaborating the components of balance as contributory inputs to wine quality.  My starting point for these discussions, as shown in the quality assessment model below, was fairly "deep in the belly of the beast."  In my future posts on this topic I will be working in a more structured, top-down manner beginning with today's post on appearance as a wine quality marker.

There are two main factors to be considered when visually assessing a wine in terms of quality: clarity and color.

Clarity

According to the WSET® standards, clarity can be categorized as either clear or dull.  Most of the wines available commercially are pretty clear because, especially in the U.S. market, there is a "finickiness" about consuming products that are cloudy, or dull, or impregnated with suspended particles.  The factors which can contribute to dullness in a wine include: cloudiness/haze, bubbles, and suspensions.

Cloudiness

Cloudiness, if not the specific intent of the winemaker (e.g., Scholium Project, selected Gravner wines), could be the result of improper racking/clarification or protein instability.  Most wineries rack their wines from container to container in order to separate it from solid elements that have sedimented out of solution (There is, of course, an oxygen-exposure element to some racking but that is outside the scope of this discussion.).  Intuitively, there will be higher concentrations of large solids sedimenting out of the wine in the earlier stages of storage and then slower sedimentation rates with the passage of time as fewer and fewer large pieces of insoluble material remain in the wine. Some wineries will introduce fining agents such as egg white, bentonite, or isinglass which bind with proteins and fall to the bottom of the containment device.  Some of the flavor elements in the wine exists as solids and some winemakers refuse to fine because of fear of this process diminishing the wine's flavor profile.  If racking and fining does not clear up the wine, the winemaker may resort to filtration.  Again, many purists decry filtering because of fear of "flavor-napping."

A second potential source of cloudiness in wine is protein instability.  According to Dr. Bruce Zoecklein, Enologist at Virginia Tech (and my go-to guy), the major source of protein in wine is the grape and amounts can range between 10 and 275 g/l (Protein Stability Determination in Juice and Wine). White wines have large insoluble proteins which precipitate out of solution imperceptibly.  The more rapid precipitation, which is visible as protein haze, is, according to Dr. Zoecklein, probably the result of protein-flavonol bonding.  Addition of bentonite in the winery can eliminate this protein haze.  Some of the proteins in the solution may precipitate out if the wine bottle is subjected to heat and would manifest as the cloudiness/haziness that would render the wine's quality suspect in the eyes of the consumer.

Secondary Fermentation

If the wine is not a Vinho Verde (young, easy-drinking wine from the Minho region of Portugal), or a designated sparkling wine, it should not contain bubbles.  Bubbles in a still wine is an indication of secondary fermentation in the bottle, a situation that occurs when the wine is bottled while still containing appreciable levels of residual sugar and viable yeast cells.  Under the appropriate conditions, the yeast will begin to ferment the residual sugar (because that is what yeasts do) producing alcohol and releasing carbon dioxide in the process.  Given the anaerobic environment within the bottle, the carbon dioxide dissolves into the wine and will alert to its presence either by pushing the cork up through the capsule, causing the bottle to explode, or manifesting as bubbles when the wine is poured into a glass.

Suspensions

One potential suspension in a wine glass is sediment.  Now sediments are, of course, not harmful if ingested but if encountered in a glass poured in a restaurant the consumer should be teed off.  As wines age, pigments precipitate out and fall to the bottom of the bottle.  Ending up with a mouthful of sediment is an unpleasant feeling and is one of the reasons that older wines should be carefully decanted before being poured into wine glasses.

Another "deposit" which can sometimes be found in wine is tartrate crystals (sometimes called wine diamonds).  Wine contains both tartaric acid and potassium and under cold conditions they combine to form potassium bitartrate crystals.  These are harmless but unattractive to U.S. consumers so winemakers cold stabilize the wine in stainless steel tanks to force the crystals to form in a controlled environment.  The crystals formed in this manner adhere to the sides of the stainless steel tanks and the wine is then poured off leaving the crystals behind.  If a wine is not cold stabilized prior to bottling, tartrate crystals could precipitate out if the wine is subjected to very cold conditions.  Scch crystals, when they occur, are normally found on the underside of the cork or suspended in the wine.

Color

According to the Wine Spectator (Inside Wine: Color), consumers associate darker-hued red wines with greater quality and winemakers are responding to this perception by pursuing viticultural and vinicultural practices that result in wines with the color the customers are demanding.

The color in red wine comes from pigments called anthocyanins which reside in the skin of the berry and provide the red, blue, and purple colors associated with the fruit.  Red varieties such as Syrah, Cabernet Sauvignon, and Petite Sirah have higher anthocyanin concentrations than do varieties such as Pinot Noir and Nebbiolo.  Viticultural techniques such as yield reduction, foliage removal, and control of vegetative growth can yield grapes with darker color while vinicultural practices such as pre-fermentation cold soak, pumping over, andy punching down all seek to extract as much color (tannins and flavor) as possible from the skins (White wines are not generally brought into contact with their skins).

In general, white wines are colorless when young, gaining a brownish tint as it ages.  Red wine should be purple to ruby when young, browning as color sediments out. A young wine with a brown color could be manifesting the effects of oxidation.


© Wine -- The View From Orlando