Enology Notes


To: Virginia Vintners and Prospective Vintners

From: Bruce Zoecklein

Subject: High pH and high TA, The power of macromolecules

High pH and high TA. This season some of you processed fruit with high TA's and high pH's-how is such fruit chemistry possible? Grape pH and TA are linked to environmental conditions (ambient temperatures, canopy temperatures, soil moisture, soil oxygen etc.) and potassium concentration.

A plant enzyme system (hydrogen potassium ATPase) regulates the uptake of soil potassium (K+) into the vine. Potassium ions are exchanged one-for-one with hydrogen ions(H+). The pH is a logarithmic measure of the hydrogen ions present. The greater the potassium concentration, the fewer the hydrogen ions and the higher the fruit pH.

This season we had optimum conditions for this enzyme system, cool and wet. As a result, we saw an increase in fruit potassium and the elevation in pH, even at relatively low Brix values. In most grape growing seasons in Virginia, the warm day and night temperatures greatly reduce the TA as a result of malic acid respiration. This season, because of the relatively cool weather, the malic acid remained relatively high. Naturally, site variation can influence enzyme activity. There are also significant differences in the hydrogen potassium ATPase activity among cultivars.

Substantial increases in pH as a result of cold soaking and/or skin contact are the result of the high potassium loads in the fruit. Those with high pH and TA wines must contend with how to produce a structurally integrated product. Some sur lie involvement may be helpful.

The power of macromolecules. 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. The interest in lees utilization goes beyond barrel-fermented Chardonnay.

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 posses 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.

Proper utilization of lees may be an important stylistic tool for reds as well as white wines. After fermentation, red wines are frequently settled and barreled. Transfer of light lees into barrels is a tool some use to add structure and complexity to reds. A disadvantage of such a practice is the loss of some red color, not a problem this season.

For most reds, I recommend some settling post-fermentation, aeration when racking off the heavy lees, estimating the non-soluble solids content (lees), then mixing while barreling down to obtain an equal distribution.

Anyone interested in a simple procedure for determining the lees content or more information on the subjects of fruit pH/TA, structural integration and/or sur lie storage can contact my office.