Enology Notes

Enology Notes #23 July 25, 2001

To: Virginia Vintners

From: Bruce Zoecklein

Subject: Berry Weight and Maturity Evaluation, Delestage Research Review, Microoxygenation Research Review, Laboratory Certification Program

Berry Weight and Maturity Evaluation. Any measure of grape maturation should correlate to wine aroma/flavor potential. In a number of issues of my news journal, the Vintner's Corner, 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 sites 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. Frequently, towards the end to the season there is an increase in Brix. This may not be due to the production and transport of sugar but as a result of simple dehydration. Between the maximum berry weight and harvest there can be as much as a 30% reduction in weight in a warm climate such as Virginia. 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 McCarty, 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.

Delestage Research Review. For several seasons we have evaluated a modification of the traditional rack and return procedures as a means of modifying wine structure, aroma and flavor. In editions of the Vintner's Corner, I have discussed some of our work conducted at White Hall Vineyards with Brad McCarthy and Karen Vaillant. More than half of the red wine tannins are extracted from the seeds during fermentation. Seed tannins are chemically different from skin tannins, being both more bitter and astringent.

Anthocyanin pigments in grapes and wines are available in several forms. They are available as monomers or unbound compounds, and as those associated with other phenols, either small polymeric pigments (SPP) or large polymeric pigments (LPP). The SPP along with monomeric anthocyanins are extracted from the skin during winemaking. During winemaking the anthocyanin monomers and SPP's are incorporated into the LPP. The result of this incorporation is to change the palate structure of the wine and to help stabilize red wine color.

Our Delestage research from the 2000 season suggests that this procedure reduced the tannin concentration by 35%, reduced the monomeric anthocyanins by 15% and increased the polymeric pigments by a 59% compared to conventional fermentation lots. The LPP pigments are mainly formed during the winemaking and aging process as a result of phenol polymerization and incorporation of monomeric anthocyanins. The significant difference in the large polymeric pigment concentration, coupled with the reduction of seed tannins helps to explain the sensory differences between Delestage-produced and conventional punch-down fermentations. The Delestage-produced wines are more fruit forward and have a richer yet supple and integrated tannin structure. Due to the higher percentage of LPP it is assumed that the color stability of the Delestage wines will be greater as the wines age.

If you would like additional information of this procedure look at my web site under newsjournals or contact my office.

Microoxygenation Research Review. This research was begun at Horton Cellars and is tied in with a larger study involving California State University-Fresno, Havens Cellars, BV Napa, Sutter Home and Sebastiani Vineyards using wines produced with several different cultivars. A general discussion of microoxygenation was provided in my newsjournal The Vintner's Corner (Vol. 15 No. 2) and is available on my web site .

All wines produced last season have been chemically and sensorially evaluated by one of our students, Patrick Sullivan. Pat looked at the following sensory aroma/flavor attributes: Fruity, Green Tannin, Grit, Off Aroma, Oxidation, Tannin Plushness and Vegetative. These aroma/flavor and textural parameters were evaluated compared to control wines using Principal Component Analysis.

Approximately 88% of the variability of the data (difference between control and treatment) is the result of two principal components, fruitiness and tannins plushness. In addition to the increased fruitiness and suppleness of the tannins, vegetative characteristics were substantially reduced by treatment. One of my students, Peter Hartman, has developed an analysis tool for measuring the compounds which are responsible for vegetative aromas. These compounds are present in wines in very low concentrations, ppt.

Our research will continue this season looking at methods to optimize this exciting new technology.

Laboratory Certification Program. The next phase of our laboratory certification program will begin this week with the Formol analysis for fermentable nitrogen. If you signed up for this program and completed phase 1, you will be receiving the nitrogen test sample within 10 days.

My hard copy newsjoural, the Vintner's Corner is available by subscription (along with Dr. Tony Wolf's Viticulture Notes) and is posted on my web site at: www.fst.vt.edu/zoecklein.index.html (see Extension section on the top row of buttons).

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