< Vintner's Corner
Vintner's Corner

Vol. 13, No. 2 March - April, 1998

Bruce Zoecklein

Department of Food Science and Technology


Blacksburg, VA 24061-0418, E-mail: bzoeckle@vt.edu

Table of Contents

I. Effect of Cold Soak and Alcohol Concentration at Dejuicing on Cabernet

Sauvignon glycosides 1

II. Homestead Food and Wine Spectacular 4

III. ASEV-ES Annual Meeting and Sparkling Wine 4

IV. ASEV-ES Membership Renewal/Application 4

There is a growing recognition of the significance of the products of glycoside hydrolysis to varietal wine aroma and flavor.

The chemical composition of grape glycosides and the hydrolytic mechanisms by which these non-volatile precursors can liberate aroma volatiles has received increased attention. The sensory significance of glycoside hydrolysis to varietal grape aroma has established providing justification and rationalization for their quantification.

Heather McMahon is a graduate student working in my laboratory. The following is a review of the influence of cold soak and alcohol content at dejuicing on Cabernet Sauvignon glycosides which is authored by Heather.

I. The effects of cold soak and alcohol concen-tration at dejuicing on Cabernet Sauvignon glycocides and clycoside fractions.

Wine quality is dependent on grape aroma and flavor compounds which are, in part, bound to sugars. This connection is a chemical linkage known as a glycosidic bond. Glycosidic precursors are non-volatile compounds which remain odorless and flavorless until hydrolyzed. Consequently, research concerning tech-niques to increase extraction and hydrolysis of glycosides may lead to enhanced wine quality.

Few publications have studied the effects of processing on glycosides, although there are numerous publications concerning phenolic compounds. Since many phenols are glycosides, some of the observed effects of phenols are applicable to glycosides. Phenols affect red wine aroma, color, astringency and bitterness. Because of their structural and color contributions, phenolic content can be correlated to wine quality. Anthocyanins and procyanidins polymers (tannins) are two major types of phenolic compounds. Anthocyanins impart color while tannins provide astringency and structure. Phenolic compounds may undergo qualitative and quantitative changes through vine management, and be subsequently extracted through various processing techniques. Maceration releases grape phenols, giving wine color and tannic structure (1).

Cold Soak

During cold soak, crushed grapes are stored at a low temperature prior to fermentation. This prefermen-tation skin contact increases phenolic extraction and enhances color. The absence of alcohol allows the formation of anthocyanin/phenol complexes which stabilize wine color (2). Storage at lower temperatures (4C) has been shown to give darker, less bitter wines than at higher temperatures (10C) (3).

Lee et al., (4) illustrated a linear relationship between fermentation temperature (15-33C) and the extraction of anthocyanins. Low fermentation temperature (25C) increases fermentation aromas, and results in a fruity, aromatic wine (2). A wine with higher tannin levels occurs after fermentation at an elevated temperature (30C) (5).

Processing techniques may result in significant sensory differences in wines. In a recent study conduc-ted by the Enology and Grape Chemistry Group at Virginia Tech, cold soak (prefermentation skin contact) and alcohol content at press were monitored to determine effects on Cabernet Sauvignon glycosides. Cold soak treated musts were held for 5 days at 10 C prior to inoculation with a cultured strain of Saccharomyces cerevisiae. Control musts were inoculated immediately after crush.

Total, phenolic-free, and red-free glycosides were estimated by the quantification of glycosyl-glucose (G-G). Although phenolic compounds are structurally important to wine, particularly to color, there are other glycosides which are important aroma and flavor precursors. In order to isolate the compounds which possibly have a greater impact on aroma and flavor, the phenolic portion was eliminated. The remaining glycosides were quantified and termed "phenolic-free" glycosides. The term "red-free" was used to denote all glycosides except those with anthocyanins. The analysis procedures were developed by Dr. Pat Williams and his colleagues at the Australian Wine Research Institute (AWRI).

Monitoring glycosides throughout fermentation can provide an understanding of the effects of processing variations (Fig. 1). All three of the glycoside fractions exhibit similar behaviour; glycosides increase from fermentation initiation until they reach a maximum concentration. This maximum concentration is reached approximately at press (when it occurs well before dryness), when additional glycosides are extracted from the skin into the must. Glycoside concentration slowly decreases throughout the final days of fermentation and continues to decline during aging. This decline may be the result of glycoside hydrolysis resulting in the liberation of free aroma/flavor volatiles. Comparing glycoside levels from initial to the maximum concentration, and maximum concentrations to those at completion of fermentation may provide insight into the effects of processing on these important secondary metabolites.

At the completion of the cold soak in our investigation, total glycosides increased by 78%, red-free glycosides by 68%, and phenolic-free glycosides by 106%. At the completion of fermentation, cold soak wines had a higher intensity (A420nm+A520nm), higher total anthocyanins, and lower hue (A420nm/A520nm) than the controls. Therefore, there was a larger ratio of pigments (A520nm) to colorless compounds (A420nm) in the cold soak compared to the non cold soak wines. The cold soak also had higher glycoside extraction (Fig. 2) than the non cold soak replicates; total and red-free glycosides increased by 10%, while the phenolic-free portion increased by 60%.

When the concentration of glycosides in the finished wine was compared with the maximum concentration of glycosides during processing, there was a greater reduction with the cold soak wines (Fig. 3). The difference was greater in phenolic-free glyco-sides by 28%, total glycosides by 47%, and red-free glycosides by 100%. This reduction in glycoside concentration may be indicative of hydrolysis. Aroma and flavor compounds are, in part, released when a sugar is removed from the glycoside.

Alcohol Level at Press

Although the majority of anthocyanins are extracted within the first 7 days of fermentation, tannin extraction occurs throughout the skin-contact period (6). In general, delayed pressing results in an increase in phenolic extraction, while dejuicing prior to dryness may provide low astringency, low total phenols, and a light, floral wine (7). Extended skin contact time gives complexity and color stability due to higher tannin levels. Tannins stabilize anthocyanins by forming polymeric complexes. However, these complexes may be hydrolyzed in the presence of ethanol (5). Conversely, solubilization of phenols by ethanol may lead to color enhancement (5).

In our study, the early press was dejuiced at 10% sugar while the late press wines were dejuiced at dryness. At the completion of fermentation, the early press wines had 25% more total and red-free glycosides than the late press. However, when the concentration of glycosides at fermentation completion was compared with the maximum concentration of glycosides (at press) in both wines, there was a larger difference in the late press wines (Fig. 4). The difference was a 8% higher phenolic-free glycoside content and a 48% higher red-free glycoside content. The most significant difference was in the total glycosides which was three times higher. Therefore, the lower concentration of glycosides (total and red-free) in the late press wines may be indicative of hydrolysis and possibly the liberation of aroma and flavor compounds.

Pressing at dryness resulted in diminished color intensity and anthocyanins and elevated hue. The lower concentration of anthocyanins in the late press wines might be explained by elevated hydrolysis of anthocyanin-polymeric complexes. These results provide no indication of the effects of long term storage on color stability.

This preliminary investigation provides insight to the effects of various processing techniques on the extraction of grape glycosides, and in part, important aroma and flavor precursors. This information could lead to manipulation of aroma and flavor compounds to achieve desired product characteristics.

1. Ribereau-Gayon, J. Peynaud, E., Ribereau-Gayon, P., Sudraud, P. 1976. Sciences et techniques du vin. Tome 3-Vinifications, transformations du vin. Traite d-oenologie.

2. Zoecklein, B.W., Fugelsang, K.C., Gump, B.H., and Nury, F.S. 1996. Wine Analysis and Production. Chapman & Hall, New York, NY.

3. Heatherbell, D., Dicey, M., Goldsworthy, S., and Vanhanen, L. 1997. Effect of pre-fermentation cold maceration on the composition, colour, and flavor or Pinot Nior wine. New Zealand Society for Viticulture and Oenology Symposium.

4. Lee, T.H., Hin, M.W., Ough, C.S., and Berg, H.W. 1977. Effect of fermentation variables on the color and sensory quality of Pinot Noir wines. Proceedings of the third Australain Wine Industry Technical Conference, Albany. Pg. 77.

5. Ribereau-Gayon, P., and Glories, Y. 1987. Phenolics in grapes and wines. In: Proceedings of the Sixth Australian Wine Industry Technical Conference. Pp. 247-256.

6. Berg, H.W., and Akiyoski, M. 1965. The effect of contact time of juice with pomace on the colour and tannin content of red wines. American Journal of Enology and Viticulture 7:84-90.

7. Zoecklein, B.W. 1991. An overview of maceration during red winemaking. Wine Industry Journal. Pp. 265-267.

II. Homestead Food and Wine Spectacular

The Homestead, in Hot Springs, Virginia, is conducting its fourth annual Food & Wine Spectacular April 17-19, 1998. This event features culinary delights prepared by world-renowned chefs and a selection of wines from Virginia, California, Washington, Oregon and Maryland. For additional information, contact the Homestead.

III. ASEV-ES Meeting and Sparkling Wine Symposium

The annual meeting of the American Society for Enology and Viticulture, Eastern Section is scheduled for July 22-24, 1998 at the Crowne Plaza Hotel in Grand Rapids, Michigan. An international symposium on sparkling wines - Issues in Sparkling Wine Production, will be held in conjunction with the meeting. Further details to follow.

IV. ASEV-ES Membership Renewal/Application


By supporting the American Society for Enology and Viticulture (ASEV) Eastern Section with you membership you are not just promoting research on Eastern wine and grape issues, but supporting and awarding students studying in these fields. (Heather McMahon a student in the Enology-Grape Chemistry Group at VPI-SU is a current scholarship recipient.) ASEV/ES is a two-way bridge between academia and industry. Your support of that bridge contributes to the vitality of our thriving Eastern wine industry. In addition, membership in the Eastern Section provides you with the opportunity to attend the annual conference, which presents grape and wine issues from an Eastern perspective. Eastern Section Membership is only $15.00.

Attached is a Special Membership Renewal/Application Form for Eastern Section status only. If you have already sent in your dues for the 97-98 fiscal year (Nov. 1), thank you for your support. If you are interested in upgrading your membership beyond just the Eastern Section to a full membership in the national ASEV, applications are available upon request. Full membership in ASEV entitles you to receive the American Journal of Enology & Viticulture.