Enology Notes #104, August 3, 2005
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech
1. Volatile Sulfur Compounds Workshop slide show posted. The slides from my presentation in New Zealand last November and from our recently-concluded workshop on volatile sulfur compounds are posted at www.vtwines.info and can be found in our On-line Publications and Current Topics section.
2. Bleeding and Fermentation. Many conduct tank bleeding in order to help increase the body and structural depth of red wines. Several have expressed concern that some juices produced by bleeding do not ferment to dryness, have a tendency toward reductive notes, and/or are difficult to get to complete MLF.
As discussed in the recently-concluded workshop on volatile sulfur compounds, amino acids are impacted by fruit maturity and are not equally distributed in the grape berry.
For example, with mature Cabernet Sauvignon, about 8.5% of the total are in the seeds, 15% in the skins and 77% in the pulp. It would seem that the separation of the pulp juice from the skins as occurs with bleeding would not have a large quantitative effect on juice that is removed. However, there is a significant qualitative influence.
The two amino acids present in the greatest concentration in the fruit are proline and arginine. Proline cannot be used by the yeast, while arginine can. Indeed, because it has four atoms of N per molecule (three of which are believed used by yeasts), arginine is a very good source of fermentable N.
The ratio of these two amino acids is impacted by maturity. Arginine accumulation in the fruit begins before veraison, continues to increase, plateaus, and declines. Proline, on the other hand, increases late in the season and continues to increase with increased hang-time.
High levels of proline are, therefore, associated with increased maturity. High proline is also associated with plant stress. The reduction in the arginine to proline ratio can be significant and can negatively impact the amount of fermentable nitrogen available to the yeast.
The ratio of arginine to proline is much greater in the skins than the pulp. In other words, pulp juice taken off in bleeding has a relatively high concentration of proline (approximately 55%) which cannot be used by the yeast, and a small concentration of the more potent amino acid arginine and others needed to carry out a healthy fermentation. The lower incidence of incomplete fermentation in red compared with white musts supports the concept that the slow release of nitrogen from grape skins during fermentation is important.
Wines produced by bleeding should be tested separately for fermentable N and likely given a higher concentration of supplemental fermentable N - higher than is required to ferment the juice remaining in contact with the skins. This will help assure ‘clean’ fermentations which go to completion.
If you desire to have your blush or rosé wines undergo malolactic fermentation, I would also suggest that you add a commercial MLF supplement at dryness and before bacterial inoculation.
As discussed at the workshop, all musts should be evaluated for fermentable N. A simple method involves the formol titration (see the Wine/Enology – Grape Chemistry Group website: www.vtwines.info, under On-line Publications and Current Topics section).
Also as discussed at the volatile sulfur compound workshop, all wines should undergo sulfite screening prior to bottling. See Enology Notes and Zoecklein et al. (1999).
3. Current Topics in Fermentation Workshop. We will conduct a pre-harvest workshop on current fermentation topics, and a sensory evaluation of our training systems research wines on Monday, August 22nd beginning at 1:00 p.m. at Horton Vineyards.
Invited speakers to the meeting include:
- Lisa Van de Water, Director, Pacific Rim Oenology
- Patricia Roca, Technical Manager, Vinotec, Chile
Lisa and Patricia will discuss the latest practical information on conducting healthy fermentations.
I will present our Traminette, Viognier and Cabernet Franc wines produced from our training system trials: VSP, GDC and Smart-Dyson (Up vs. Down) from 2003 and 2004.
Registration: To reserve a spot, send an email message to Terry Rakestraw, at with the word "Pre-harvest" in the subject line.
Cost: $25 per person. The complete registration fee is due NO LATER THAN August 18, 2005. Checks are to be written payable to Virginia Tech Foundation and mailed to Terry Rakestraw, Department of Food Science and Technology (0418), Virginia Tech, Blacksburg, VA 24061. Course fee is non-refundable.
4. Berry Weight and Maturity Evaluation. Any measure of grape maturation should correlate to wine aroma/flavor potential. In a number of issues of the Vintner's Corner available on the Web at www.vtwines.info, I have suggested the importance of using berry weight to help determine the progress of fruit maturation, and aid in establishing consistent and optimum wine styles.
Many of the secondary metabolites (aroma/flavor and phenolic compounds) are located in the skins. Therefore, a change in berry size (measured easily by weight) can and should influence winemaking decisions. Berry weight should also influence fruit maturity decisions.
The earlier the estimation of average berry weights, the more time the winemaker has to evaluate the crop load, make adjustments, and plan for the season. There is a relationship between berry weight at veraison and berry weight at maturity.
For Syrah, McCarthy (1997) determined that relationship to be the following: y = 1.35x + 0.53, where y = the berry weight at 23 Brix and x = the berry weight at about 5 Brix. This relationship will differ by cultivar and site, but can easily be determined by collecting veraison and harvest samples for several seasons.
Changes in berry weight confound the measurement of degrees Brix, and are yet another reason why Brix must not be the sole monitor of fruit maturation. Research indicates that the maximum rate of production of aroma/flavor compounds in the fruit occurs at about the time when the berry stops importing water from the phloem or shortly thereafter. Therefore, maximum aroma/flavor occurs sometime after the berry reaches maximum weight in most instances. This is the reason why Syrah producers, for example, monitor the extent of berry shriveling.
Work conducted in Australia (Mike McCarthy, 2001) has determined the optimum weight for this variety at harvest for maximum concentration of secondary metabolites to be about 1.2g per berry. The decline in berry weight is more closely related to the time from flowering than to degrees Brix. The evaluation of berry weight is an important tool in stylistic winemaking - use it.
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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: http://www.vtwines.info/
Phone: (540) 231-5325
Fax: (540) 231-9293