Treatment of Wine To Reduce Hazing
Publisher’s note: This article is the second installment in a four-part series for winemakers by Nick Smith with the Enology Laboratory at the University of Minnesota. To read the previous article see: Controlling Heat Instability in Wine
The previous article discussed protein chemistry and how ionic interactions between proteins and hydrogen bonding affect protein solubility. The presence of heat susceptible proteins in wine accounts for the haze risk. To reduce the risk, the proteins need to be removed from solution. The common approach to removing problematic proteins from solution is by adding a negatively charged fining agent. The typical fining agent used for this purpose is bentonite.
Bentonite is a clay powder that is hydrated in water and added to wine. A weight to volume slurry of 5% bentonite in water is considered hydrated. When hydrated, bentonite has a large surface area with a negative charge. Its hydration also releases cations into solution.
The interaction of the cations with certain amino acids can reduce proteins’ interaction with water (hydrogen bonding) and allow the protein to settle out of solution. Positively charged proteins further absorb onto the negatively charged surface of the bentonite. After the bentonite settles, the wine is racked and then filtered to remove residual bentonite. Prolonged aging on bentonite lees is not recommended.
Bentonite not only removes proteins, but can also affect other wine components. The tricky part of bentonite fining, or any fining for that matter, is adding just enough of the fining agent to achieve the desired result.
Bentonite fining of wine has a reputation of lowering wine quality. Some winemakers may therefore hesitate to apply bentonite because they believe it will impact wine quality. Indeed, various studies and reference texts indicate a potential negative impact due to bentonite fining. Conversely, many other studies have indicated that using bentonite under common addition ranges does not noticeably impact wine. One aspect of fining is certain however: Long term bentonite contact with wine is not desirable and should be avoided.
In the end, wine is complex. The impact of wine treatments depends on many factors. Everything from the vintage, location, and harvest date determine wine composition and how the wine will react to fining.
Various theories exist on when to add bentonite. Bentonite can be added at crush; just prior to fermentation. The addition of bentonite during the initial settling of the must may eliminate later cellar operations. However, protein stability cannot be readily predicted at the pre-fermentation stage and a later addition may be required. Wines that are slated for lees contact should not be fined early.
Lees contact can also help improve protein stability in wine and may reduce the amount of bentonite needed. Early fermentation additions of bentonite therefore should be limited to wines meant to be consumed young and will not have lees aging.
The next article, which will appear in June, will focus on methods for determining heat stability in wine.