Enology Notes

Enology Notes #25 August 15, 2001

To: Virginia Vintners

From: Bruce Zoecklein

Subject: Maximizing Aroma/Flavor, Fermentation with Bentonite, the Power of LEES

Maximizing aroma/flavor.

To help avoid stripping, and to aid in enhancing white wine aroma/flavor, juice should be clarified with pectinolytic enzymes, rather than bentonite. Juice from clean sound fruit will contain a limited colloids concentration, which should settle quickly with the use of enzymes. Additionally, research has shown that wines produced by enzyme settling vs. bentonite have a greater aroma/flavor intensity.

Remember, the addition of bentonite to juice may aid in juice clarification, but will not have a practical influence on ultimate wine stability. If you are not doing barrel fermentation and extended lees contact, consider fermentation with bentonite.

Fermentation with Bentonite

Post-fermentation additions of bentonite, especially those exceeding 48 g/hL (4 lbs/1,000 gal), may strip wine flavor and body. Further, it may impart an earthy character to the wine. Some winemakers choose to ferment settled juice in contact with bentonite to aid protein stability, and to eliminate or reduce the amount of bentonite needed to stabilize the wine. The procedure for fermentation of white juice in contact with bentonite is as follows:

1. Settle juice to remove non-soluble solids by refrigeration and/or with pectenolytic enzymes (a high solids level could foul the bentonite utilized during fermentation and reduce overall efficiency). Add the desired quantity of bentonite in-line while racking into the fermentor.

2. Add yeast nutrients (see Vol. 14, No. 2 of the Vintner's Corner posted on my web site) and any needed sugar and/or acid where allowed.

3. Add yeast inoculum to juice surface.

A yeast nutrient addition is required when fermenting in the presence of bentonite. Bentonite may deplete must assimilable nitrogen due to electrostatic binding and adsorption.

The quantity of bentonite required is determined empirically or analytically. Many winemakers fermenting in contact with bentonite simply add about 24 g/hL (2 lbs/1,000 gal). Methods for predicting specific bentonite levels needed in the juice for subsequent wine stabilization are available (see Zoecklein et al., 1995).

The difficulties encountered with the use of bentonite to obtain protein stability have lead to examination of other techniques and materials including the influence of yeast lees.

The power of LEES.

During sur lie storage, yeast components such as cell wall polysaccharides and particularly mannoproteins are released into the wine. These macromolecules can positively influence structural integration, phenols (including tannins), body, aroma, oxygen buffering and wine stability (both protein and bitartrate). The storage of white wines on lees increases the protein stability, thus reducing the bentonite requirement. This increase is due to mannoproteins released by yeast cell walls during autolysis.

Yeast-derived macromolecules provide a sense of sweetness as a result of binding with wood phenols and organic acids, aiding in the harmony of a wine's structural elements. The natural fining that occurs contributes to reducing the yellow tones in whites and helps to protect against oxidation of certain fruit aroma compounds.

Consider utilization of light, ‘clean lees' to enhance structural integration. Some use lees to increase the complexity of tank-stored wines. If you have such an interest but are concerned with the potential of reductive tones, use lees that have been in barrels for two months or more. Such lees possess all of the desirable features, but are much less likely to cause reductive problems. Using ‘barrel aged' lees is particularly important if you intend to store sur lie in tanks greater than about 1000 gallons. The low oxygen concentration at the bottom of such tanks can create problems. Naturally, a careful sensory evaluation of the lees should be conducted.