Enology Notes

Enology Notes #58, August 23, 2002

To: Regional Wine Producers

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

Subject: This year's aroma/flavor potential

The production of grapes with desirable concentrations of aroma and flavor at harvest is the result of the convergence of two broad influences. The first is the reproductive process that, through seed development, determines potential berry size. The second influence comes from environmental factors and viticultural management that condition the genetic potential and influence berry growth. This year we have two important conditions heat (and the associated drought stress) and asynchronous berry development.

The production of aroma/flavor is influenced by the temperature during Stage 3, or the final period of fruit maturation. Therefore, it has been suggested that the best variety for a site is one that matches the length of the growing season so that fruit maturation occurs during the portion of the season that is cool, but warm enough to allow the fruit to continue to accumulate aroma/flavor. Site climate has been divided into two general temperature zones, Alpha and Beta.

In Beta zones (most Virginia vineyards) the majority of grapes ripen well before temperatures begin to drop. Specifically, Beta zones are those with a mean temperature above 16EC at the time of ripening for a particular variety. Thus, days and nights are still warm, and attainment of adequate degrees Brix (either by sugar production or dehydration) is not a problem. In many Beta zones, grape varieties with short, moderate and long seasons are grown, but perhaps should not be.

A more negative correlation may exist between yield and quality in Beta vs. Alpha zones. This is an important reason why I have suggested that degrees Brix in our climate is neither a good predictor of maturity or ultimate wine quality.

It is well known that aromas and flavors are best preserved at low temperatures. There is both a quantitive and qualitative effect. In warmer seasons such as this, delicate fruit-estery flavourants would be expected to be diminished.

What to do? Attempt to maximize aroma/flavor potential by minimizing asynchronous development and by avoiding processing which strips aroma/flavor.

Asynchronous development. Uneven ripening may be a problem in some vineyards due to the late spring frost. Such ripening means that the clusters or berries with the desirable aroma/flavor are diluted by those with the undesirable characteristics. This has a very large and negative impact on wine quality (see previous editions of Enology Notes).

The easiest method for evaluation of uneven ripening is to look at the variation in maturity of samples. To be accurate to plus or minus 0.5 degrees Brix, 5, 100 berry samples must be collected. Each of the 5 samples should be processed separately and evaluated.

If the samples were properly collected (see Maturity Analysis for Growers on my web site) the differences among these samples should be less than 5%.

If the differences among samples is greater than desired, the fruit should remain on the vine. Generally, the benefit of the advanced maturity of the undeveloped fruit offsets the potential downside of over maturity of the more ripe fruit.

Processing considerations. Processing cold white fruit, use of enzymes as a clarification aid and/or the possible use of bentonite in the fermentor and the proper fermentation management including nitrogen levels are ways of helping to preserve aroma/flavor.

Pectinolytic enzymes vs. bentonite should be used as a white juice settling aid. These enzymes contain some glycosidases which can increase the aroma/flavor of the finished product.

Avoid using bentonite as a clarification aid.

Use bentonite in the fermentor with grapes that consistently require a high (greater than 24g/hL) post-fermentation addition for protein stability.

Fermentation with bentonite requires the addition of fermentable nitrogen. (See Zoecklein et al., 1995, 1999 or call or write.)

Formol Test for Nitrogen. Two methods of the Formol procedure for fermentable nitrogen that we have developed are posted on the web site under on-line publications. From www.vtwines.info click Extension, then On-Line Publications.

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: http://www.fst.vt.edu/zoecklein/index.html or http://www.vtwines.info/. Enology Notes are slightly different in content from the subscription based Vintner's Corner newsjournal.

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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