To: Virginia Vintners and Prospective Vintners
From: Bruce Zoecklein
Subject: Seasonal variation influencing protein stability
This season some white wine varieties are more difficult to protein stabilize than usual. Why? The answer has to do with seasonal variation. The 2000 season was relatively cool and wet. These conditions influenced grape N, both qualitatively and quantitatively. Our analyses of the fermentable nitrogen has demonstrated significant increases in most cultivars this season, compared to last season (as much as a 47% increase).
The major source of protein in wine is the grape. Variety, vintage, maturity, condition of the fruit, pH and processing methodology all influence the protein content of wines. The solubility of wine protein depends primarily on temperature, alcohol level, ionic strength, and pH. Changes in any of these (as may occur with blending, bitartrate stabilization, MLF, etc. ) may affect the potential for protein precipitation.
At a certain pH the molecules of each protein fraction have an equal positive and negative charge. We call this pH the isoelectric point of the protein fraction. Wine proteins are least soluble at this pH or isoelectric point. The greater the difference between the wine pH and the isoelectric point of the protein fraction, the greater the net charge on the protein and the greater the electrostatic binding with fining agents.
If the wine pH is above the isoelectric point of the protein fraction in question, the net charge on the fractio What happens if the wine pH is higher What happens if the wine pH is higher n will be negative, and the protein will bind electrostatically with positively charged fining agents (opposite charges attract, like charges repel). Conversely, if the wine pH is below the the isoelectric point for a particular protein fraction, the net charge on the fraction will be positive. The greater the difference between the wine pH and the isoelectric point of the protein fraction, the greater the net charge on the protein and the greater its binding affinity toward oppositely charged fining agents.
The relationship is illustrated with White Riesling (see table below). Normally, the wine pH is below the isoelectric point of all the protein fractions in this variety. Therefore, with a wine pH of 3.2, 100% of the protein fractions are positively charged and are thus capable of being removed with bentonite, a mainly negatively charged fining agent. Bentonite's strongest binding affinity would be for the protein fraction with an isoelectric point of 7.1, the weakest would be for the fraction with an isoelectric point at 3.6.
Elevated pH values were common this season as discussed in Enology Notes # 6 (previous notes are posted on our web site at www.fst.vt.edu/zoecklein/index.html). What happens in the case of elevated pH values? If the pH of our Riesling is 3.5, and all other conditions were the same, the difference between the wine pH and the isoelectric point of several protein fractions becomes less, and the ability to bind electrostatically with bentonite is also lessened.
If the wine pH was greater then 3.6 than the protein fraction with the isoelectric point at 3.6 would be negatively charged and not easily removed by a fining agent like bentonite which is mainly negatively charged
The charge characteristics of various protein fractions partly explain why, with some wines, protein stability can be achieved only with excessive amounts of bentonite, if at all.
Varieties such as Muscat's, Sauvignon blanc and Gewurztraminer each have multiple protein fractions, some with isoelectric points below and some higher than the wine pH. Therefore, the addition of a single negatively charged fining agent such as bentonite seldom results in protein stability. Negatively charged agents such as Sparkolloid are usually required.
Variations in wine protein fractions and their relationship to the wine pH can vary significantly from one season to the next. This is another example of the need for quality control analyses. What worked last year may not work this season. Do not loose sight of the fact that fining agents such as bentonite are relatively non-selective. Under the proper conditions they will bind wine proteins. They will also bind and remove varietal character and strip body (co-bind with phenols).
I suggest that you evaluate white wines carefully for both protein and bitratrate stability before any stabilization attempts. Do not assume what worked for the 1999 wines will do so for this last vintage. If you have a history of unstable proteins in a certain variety, consider fermentation in contact with bentonite. See Zoecklein et al. (1995) for further details or contact my office.
Isoelectric Points of Protein Fractions in White Riesling Wines.
||Isoelectric Points (pH)||