Enology Notes

Enology Notes #62, October 11, 2002

To: Regional Wine Producers

From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech

Subject: Red Wine Phenolic Management, continued; Mechanical Harvester Demonstration

Red Wine Phenolic Management, continued. There remain questions about tannin management and ways to secure flavor depth and quality. The challenge is to produce red wines with satisfactory depth of flavor, and tannins which are supple and provide a desirable mouth-feel, mid-palate plumpness, without excessive astringency.

In previous editions of Enology Notes, I have reported several studies we are conducting involving tannin management, including micro-oxygenation and delestage. This continues a discussion from the previous edition (Enology Notes # 61) on this subject.

Tannin levels are quantitatively and qualitatively impacted by a number of vineyard and winemaking parameters. We know, for example, that under-ripe or immature tannins can result from excessive crop loads, late season moisture and too much canopy shading. It appears that late season moisture deficit is important to maximize tannin maturity, just as it may be important in maximizing varietal aroma/flavor.

It has been demonstrated that Shiraz fruit undergoes an engustment or increase in free volatiles only after the berry begins to shrivel (see Enology Notes # 50). This shriveling is the result of cap stem closure which prevents water incorporation into the fruit and, therefore, allows berry dehydration.

It is likely that this same mechanism occurs with other varieties. This is highly important for aroma/flavor. Equally important is the fact that engustment appears to coincide with full tannin maturation and maximum tannin extractablity. What is the relationship between your fruit maturity at harvest and phenol extractability?

The importance of tannin extractablity was discussed in a previous edition (Enology Notes # 60). Using gentle punch-down with fruit which has mature, extractable phenols produces wines with fine-grained tannins. That is exactly why this cap management procedure has become so popular.

However, with some varieties, including Merlot, punching down can produce wines which are lighter in mid-palate body than may be desired. The most common approach to solving this problem in Virginia is the use of wood and/or tannin additions to the fermentation vessel (see Enology Notes # 41) and/or the use of extended post-fermentation maceration.

What is the relationship between your cap management program and wine structure, including mid-palate body?

We have been evaluating a third approach, one used occasionally in Bordeaux, involving post-fermentation thermal treatment. Heating can have a dramatic impact on the wine structure/texture. Pre-dejuicing heat treatment increases the extraction of components including anthocyanin glycosides from the pomace. Additionally, monomeric or free anthocyanins are incorporated with other phenols as a result of polymerization. Such polymerization can impact wine color stability and palate structure/texture.

We have evaluated fruit maturity and processing methods to determine the extent of polymerization by measuring three classes of pigments. These three classes are separated based on bisulfite bleaching and protein precipitation, and consist of monomeric anthocyanins, small polymeric pigments and large polymeric pigments (Adam et al., 2001).

Using a post-fermentation maceration temperature gradient system ( 24 degrees C with a 3 degree increase per day) large polymeric pigments increased by 208%, and small polymeric pigments by 41%, as a result of monomeric anthocyanin incorporation with other phenols. The reduction in free anthocyanin pigments as a result of the association with small and large polymeric pigments was off-set by the increased extraction of pigments from the skin. The increase in the large polymeric pigments may indicate changes in the phenol structure similar to those found in aged wines. This increased polymerization also aids long-term color stability.

Additional details regarding this study, and others involving phenol management, will be provided at the Virginia Vineyard Association winter meeting scheduled for February 13-15, Charlottesville, VA, and during the Phenol Management and Measurement Short Course, January 16 (see Enology Notes # 61 for details).

Mechanical Harvester Demonstration. There will be a demonstration of the Euro-Machines, Inc. Braud model 2720 over the row, self propelled mechanical harvester at White Hall vineyards on Monday, October 14, beginning at approximately 9:00 am. Anyone interested is welcome to come and observe.

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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu