To: Regional Vintners
From: Bruce Zoecklein
Subject: Maturity Evaluation
The days of determining when to harvest by simply going out to the vineyard with a refractometer have passed. Today, overall ripeness evaluation involves much more than an analysis of Brix, titratable acidity and pH. Years ago, progressive winemakers abandoned the harvest-by-numbers routine.
Determining when to harvest is the most important grape growing/winemaking decision. From that point forward, the parameters of quality are set. Any manipulation done at the winery can have only a limited positive impact on the resultant wine.
Optimum maturity is a balance between the confluence of desirable aroma/flavor and structural compounds. This is a greater challenge in warmer wine growing regions such as Virginia, where excessive maturity can produce alcohol levels which negatively impact varietal aroma intensity. This is an important reason to know the conversion rate of sugar to alcohol from a particular vineyard.
Theoretically, a given weight of fermentable sugar should yield 51.1 % alcohol by weight. The actual alcohol yield is generally less than theoretical. In the past winemakers used the conversion factor of 0.55. However, the actual conversion rate can vary from 0.54 to 0.62. These differences are the result of variety, season, maturity level and fermentation temperature. Due to the importance of alcohol on wine structural balance and perception of varietal character, it is essential to know your conversion of sugar to alcohol.
In Virginia, it is common to see the sugars increase at a greater rate than varietal aroma/flavor. Seasonal variations may have a significant impact that is uncontrollable, but by properly managing canopy vigor and maintaining vine balance, it may be possible to achieve aroma/flavor maturity at relatively low sugar concentrations. Frequently, the more dense the canopy, the greater is the rise in sugar vs. aroma/flavor. The shorter the bloom period, the more even the fruit set and the greater will be the uniformity of aroma/flavor within a cluster.
Prior to the harvest season, each grower and vintner should carefully review fruit maturity gauges and procedures. A publication on grape sampling and fruit maturity evaluations is posted on our web site which outlines a number of considerations. This is available at www.fst.vt.edu/zoecklein/index.html or www.vtwines.info. From the home page click extension, then extension publications on-line, then Vintners Corner Vol 16. No. 1 Grape Sampling and Fruit Maturity.
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
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293