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.
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.
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