Enology Notes

Enology Notes #32 October 30, 2001

To: Regional Vintners and Prospective Vintners

From: Bruce Zoecklein

Subject: Late harvest wine production, the 2001 Season, Retaining Aroma and Flavor, Proceedings Available, Juice and Wine Analysis Short Course.

Late Harvest Wine Production. This season we have the luxury of producing a wider array of late harvest style wines. High sugar concentration can inhibit yeast growth, due to osmotic pressure, possibly resulting in slow fermentations and sticking. Butske and Dukes (1998) suggested that fermentable nitrogen be adjusted depending upon the degrees Brix of the fruit:

21 Brix-200 mg/L N
23 Brix -250 mg/L N
25 Brix- 300 mg/L N
27 Brix- 350 mg/L N

These levels may be minium concentrations, in that they assume no additional stress, beyond osmotic pressure, which could inhibit fermentation. It should be noted that nutrient supplementation should be done in stages, with the addition of a propriety product, followed by DAP after several degrees Brix fermentation has occurred.

The fermentable N equivalent of several proprietary compounds and DAP is provided in Enology Notes # 26, posted on-line.

Factors and conditions to produce a healthy fermentation were discussed in detail in two relatively recent issues of my newsjournal, the Vintner's Corner Vol. 14, No 2 and Vol. 14, No. 4, 1999. These are posted on-line at www.fst.vt.edu/zoecklein/index.html.

The 2001 Season. As a generalization, the 2001 season appears to be one of the best ever. Why? For the most part, we have had a climate during the last stage of fruit maturity that is favorable to the production of secondary plant metabolites (aroma, flavor and phenolic compounds). We are seeing fruit with rich, ripe aroma and flavor potential, with excellent color, and mature tannins which will provide wines with both suppleness and structural carpentry or depth.

Climate vs. variety is a critical issue for our industry-what to plant, where, and how to define the ideal environmental and viticultural elements for individual grape varieties and wine styles. Most agree that the growing conditions (particularly soil moisture and temperature) during the last month of maturity is critical to quality of the vintage.

The temperatures which help to define the vintage during the last month of maturity include average daily mean, average monthly highest maximum, average monthly highest maximum this year minius the average maximum (Gladstones, 1992) . Clearly, this year the average mean temperatures at most sites were cooler than the long term averages by 5 degree F, or more.

It appears that many grape-derived aroma/flavor compounds are in a constant cycle of formation, accumulation, and conjugation with sugars into odorless forms. There is evidence that cooler growing/ripening temperatures result in more aromatic fruit. Cooler temperatures (to a point) may result in a wider range of grape volatile compounds retained.

The size of the pool of free grape volatiles may vary directly with their rate of formation, which is a function of temperature. There may also be a qualitative effect. Different varieties have different volatile components influencing their varietal expression. These are likely to be formed and degraded at different temperatures.

The differences in grape volatile compounds and other secondary metabolites may be one important reason for the apparent difference of opinion on the importance of cool night temperatures or the diurnal range. It certainly seems evident that for some varieties grown in certain regions, significant differences in the day and nighttime temperature may not be that important.

Below normal dry weather from September through October likely also contributed to the quality of the vintage in at least two ways, lower berry size and the increased production of aroma/flavor precursors. Low berry size effects the concentration of secondary metabolites, due to their high concentration in the skins. We are measuring grape-derived glycosides, in part aroma/flavor precursors. There is evidence that limited soil moisture during the final month of fruit maturity increases the concentration of these important compounds by as much as 20%.

Retaining Aroma/Flavor. This season each winemaker should be able to produce quality wines out of the fermentor. Retaining that quality is the next important step. Premium producers have an established HACCP (hazard analysis critical control point) plan to help assure and maintain the quality of the vintage (see Enology Notes # 22 and the Vintner's Corner newsjournal Vol. 15, No. 1,2 Jan/Feb and March/April, 2000, posted at: www.fst.vt.edu/zoecklein/index.html).

For wines other than those stored sur lie and designed for long term aging, check the bitartate and protein stability soon and carefully. If required, these two stabilization procedures should be conducted at one time. Remember, as little as 2 pounds/1000 gallons (24 grams/hL) of bentonite can strip wine aroma and body. Because of the differences in protein (quantitative and qualitative) from one vintage to the next, protein stability testing is required. Note the discussion of this subject in Enology Notes # 11, posted on the Enology-Grape Chemistry Group web site.

Also remember, young wines are much more forgiving of the action of protein fining agents than are older wines. This may be an important issue for low phenol, delicate whites. Evaluate carefully and fine early.

Winery Planning and Design Workshop Proceedings Available. Proceedings of the Winery Planning and Design Workshop conducted in July are available. The 104 page proceedings covers establishing a business plan, and winery design considerations, including gravity flow, winery tank selection, sanitation, etc. Send $45 payable to Dr. Bruce Zoecklein, Foundation Account, Department of Food Science and Technology, Virginia Tech (0418), Blacksburg, VA 24061. Proceeds are used to support our enology graduate education efforts.

Juice and Wine Analysis Short Course. A two-day, hands-on analysis short course is scheduled for January 8 and 9, 2002. For details, see the Events part of the Home section on the Enology-Grape Chemistry Group web site.

Subscription to Enology Notes. All past Enology Notes and Vintner's Corner newsjournals are posted on the Enology-Grape Chemistry Group's web site at: www.fst.vt.edu/zoecklein/index.html. Enology Notes are different in content from this subscription-based Vintner's Corner newsjoural. To be added to the Enology Notes list serve send an email message to bzoeckle@vt.edu with the word ADD in the subject line.

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.fst.vt.edu/zoecklein/index.html

Phone: (540) 231-5325
Fax: (540) 231-9293
E-mail: bzoeckle@vt.edu